sandbox-deepseek-flash-2026-05-05-r3

Session ID: ses_207efe028ffeCjCWkDx3lmiJ7w

User

"Read PROMPT.md and SPEC.md at the worktree root, then implement sandbox.py per the spec. Stop when sandbox.py exists at the worktree root and your own quick smoke check passes."

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

The user wants me to read PROMPT.md and SPEC.md, then implement sandbox.py. Let me start by reading those files and understanding the worktree structure.

Tool call: read

Tool call: glob

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Let me read the PROMPT.md and SPEC.md files at the worktree root.

Tool call: read

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Let me analyze the SPEC.md carefully and plan the implementation:

sandbox_run function: Takes command, workspace, image, timeout, network, memory, pids, cpus parameters.
Key behaviors:
- Run command inside ephemeral container of image
- If workspace is a path, bind-mount it r/w at /workspace, set working dir to /workspace
- Container destroyed after command exits (--rm)
- Network: none (default) or bridge
- Resource limits: --memory, --pids-limit, --cpus, --cap-drop=ALL, --security-opt=no-new-privileges
- Use argv list with shell=False. Command passed as sh -c argument inside container
- Wall-clock timeout via subprocess.run(timeout=...). On timeout, exit code 124.
- Stderr and stdout output decoded with errors="replace"
- Output truncated to 50,000 bytes total after construction
- --pull=missing is fine

Return format:

exit=<n>
--- stdout ---
<stdout bytes, decoded>
--- stderr ---
<stderr bytes, decoded>

Formatted rules:
- Each header on its own line terminated by \n
- stdout body if non-empty comes after --- stdout ---\n and ends with \n before --- stderr ---
- stderr body if non-empty comes after --- stderr ---\n. May or may not end with trailing newline
- If body is empty, next header follows directly
Podman/Docker fallback: Try podman first, then docker, then RuntimeError.
CLI: Use argparse, -- separator, default workspace is os.getcwd(), exit code matches container's.
Timeout handling: On timeout, the container is terminated. Exit code 124.

Let me think about the timeout handling more carefully:

subprocess.run(timeout=...) raises TimeoutExpired on timeout
On timeout, we need to terminate the container and return exit=124
We need to kill the subprocess (podman run process) and clean up

But wait - if subprocess.run raises TimeoutExpired, the podman process might still be running, and the container would continue. We should kill it.

Actually, looking at the spec: "Wall-clock timeout enforced via subprocess.run(timeout=...). On timeout, the container is terminated and the returned string indicates a timeout."

And: "exit code 124" indicates timeout.

So on timeout:

Kill the podman/docker process
The container should be cleaned up via --rm
But we might also want to explicitly remove the container

Actually, subprocess.run(timeout=...) will send SIGKILL to the child process when timeout occurs. The child process is podman run, not the container itself. So when podman run is killed, the container should still be running.

Hmm, but --rm means the container is removed when it exits. If podman run is killed, the container might keep running. We might need to handle this.

Actually, let me think again. When subprocess.run times out, it kills the subprocess. The subprocess is podman run --rm .... If we kill podman, the container might still be running. We should kill the container.

But actually, looking at how podman run works: when the podman client process is killed, the container continues running. We need to find and kill the container.

Wait, but timeout in subprocess.run sends SIGKILL to the process. Let me think about what happens with podman run when it receives SIGKILL... It might actually cascade and kill the container. Let me check.

Actually, for Docker, when docker run receives SIGTERM/SIGKILL, it does NOT stop the container. The container keeps running. For Podman, it depends on the version.

So the safest approach is:

In the TimeoutExpired handler, explicitly kill the container
We can use podman stop <container-id> to stop it
Or we can use podman kill <container-id>

But we don't have a container ID... unless we capture it.

Alternative approach: Use podman run --rm and on timeout, we need to track the container. One way is to use the container ID and then podman rm -f it.

Actually, let me think about this differently. We can use podman run --cidfile=<tempfile> to capture the container ID, and on timeout, use podman rm -f $(<tempfile>).

Or we could do something like:

Start the container in detached mode and capture the ID
Attach to it and wait for it to finish
On timeout, kill via podman kill

But this is getting complex. Let me look at what other implementations do.

Actually, I think the simplest approach is:

Use subprocess.run with timeout
On TimeoutExpired, kill the process (which is the podman run process)
The container should stop when the podman process handling it exits

Wait, actually, subprocess.run with timeout=... already kills the subprocess on timeout. The question is whether the container stops.

Let me look at how Docker handles this. When docker run is sent SIGKILL:

Docker's CLI process exits
The container continues running in the background
The --rm flag means "remove when container exits", not "remove when CLI exits"

So the container would keep running. We need to clean it up.

The cleanest approach I think is:

Generate a random container name
Run podman run --name <name> --rm ...
On timeout, run podman kill <name> to forcibly stop the container
Then return exit=124

Let me implement this.

Wait, here's another approach. We could use podman run --rm --attach which has different behavior... No, this is getting too complicated.

Let me think about what the hidden tests will check. The prompt says the timeout test just checks:

python sandbox.py --timeout 2 -- sleep 30 → exit 124, no traceback

So it checks:

Exit code 124
No traceback (no exception, clean output)

So I need to:

Catch TimeoutExpired from subprocess.run
Kill the container
Return the formatted output with exit=124

Let me write the implementation:

#!/usr/bin/env python3
import argparse
import os
import shutil
import subprocess
import sys


def _resolve_runner():
    podman = shutil.which("podman")
    if podman:
        return podman
    docker = shutil.which("docker")
    if docker:
        return docker
    raise RuntimeError("Neither podman nor docker found on PATH")


def _format_output(exit_code, stdout, stderr, truncated=False):
    result = f"exit={exit_code}\n--- stdout ---\n"
    if stdout is not None and stdout != "":
        result += stdout.rstrip("\n") + "\n"
    # Note: actually I should be more careful here.
    # "The stdout body, if non-empty, comes immediately after the --- stdout ---\n line and ends with exactly one \n before the --- stderr --- header."
    # So if stdout is non-empty, it ends with exactly one \n
    # Then --- stderr --- follows
    
    # Actually, let me re-read:
    # - The stdout body, if non-empty, comes immediately after the --- stdout ---\n line and ends with exactly one \n before the --- stderr --- header.
    # - If a body is empty, the next header (or end of string) follows directly after the previous header line — no blank line is inserted.
    
    # So:
    # If stdout is non-empty: "--- stdout ---\n<stdout>\n--- stderr ---\n"
    # If stdout is empty: "--- stdout ---\n--- stderr ---\n"
    
    # And for stderr:
    # "The stderr body, if non-empty, comes immediately after the --- stderr ---\n line. It may or may not end with a trailing newline (preserve whatever the underlying stream produced)."
    # "If a body is empty, the next header (or end of string) follows directly after the previous header line"
    
    # So stderr is just appended directly after "--- stderr ---\n", no trailing \n manipulation
    
    pass

Wait, let me think about this more carefully. Let me re-read the format rules:

Each header on its own line, terminated by \n
stdout body: if non-empty, comes after --- stdout ---\n and ends with exactly one \n before --- stderr --- header
stderr body: if non-empty, comes after --- stderr ---\n. May or may not end with trailing newline - preserve whatever the underlying stream produced
If body is empty, next header follows directly

So algorithm:

f"exit={exit_code}\n--- stdout ---\n" +
(stdout_body if not empty → ensure it ends with exactly one \n; if empty → "")
"--- stderr ---\n" +
(stderr_body if not empty → as-is; if empty → "")

For stdout, I need to strip trailing newlines and add exactly one.

For stderr, I just append as-is.

Wait, but the stdout body needs to preserve its content but end with exactly one \n:

If stdout is "hello", result should be "hello\n"
If stdout is "hello\nworld\n", result should be "hello\nworld\n" (already ends with \n)
If stdout is "hello\nworld\n\n", result should be "hello\nworld\n\n" → hmm, that ends with \n already

Actually "ends with exactly one \n" means we need to ensure there's exactly one trailing newline. So:

Strip trailing newlines → add one.
If empty string → don't add the body at all.

For stderr, preserve as-is.

Let me now think about the output truncation: "Output truncated at 50,000 bytes total after construction (i.e. apply truncation to the final string with the headers in place, not to stdout/stderr separately and not via a proportional split). Slice the tail; do not split mid-byte across a multibyte sequence (decode first, then truncate by characters that re-encode within the cap). Truncation is silent (no error), but a clear marker like ... [truncated] may be appended."

So:

Build the full result string
If it's > 50,000 bytes, truncate
Truncation is by characters (after decoding to avoid splitting multi-byte)
Slice the tail (keep the END, not the beginning)
Optionally append "... [truncated]"

Wait, "Slice the tail" - what does this mean? I think it means to keep the tail (end) of the output. Actually, "Slice the tail" is ambiguous. Let me re-read:

"Slice the tail; do not split mid-byte across a multibyte sequence"

I think "Slice the tail" means truncate from the tail (i.e. slice off the end to make it fit). So keep the beginning, remove from the end. That makes more sense - you build the string, it's too long, so you slice off the tail (end).

Wait, but that could be interpreted differently. "Slice the tail" could mean "keep the tail" (i.e., the last N bytes). But I think the most natural reading is "remove the tail" = truncate.

Actually, looking at this more carefully, I think "slice the tail" means: from the final string, remove enough from the end to fit in 50,000 bytes. So:

If total > 50000 bytes, keep first 50000 bytes (or first N chars that fit)
Optionally append "... [truncated]"

But since it says "Slice the tail" (verb), and "do not split mid-byte", I think it wants us to:

Decode to string first (errors='replace')
Truncate characters from the end until the string fits within 50000 bytes when re-encoded
Optionally add "... [truncated]"

Actually wait - the output is already a string at this point (since we decoded with errors="replace"). So:

Encode the string to bytes
If > 50000 bytes, decode to string, truncate chars from end, re-encode and check
Keep truncating until it fits, or use a simpler approach

A simpler approach: since we know the string, we can truncate it to N characters where N is an approximation, then check the byte length.

Actually the simplest is: while len(encoded) > 50000, remove one character from the end, re-encode, check. But that's expensive for large strings.

Better approach: binary search. Or: estimate, since each char is at most 4 bytes in UTF-8, we can truncate to at most 50000/1 = 50000 ASCII chars, or fewer for multi-byte chars.

Actually let me just do:

if len(result.encode('utf-8')) > MAX_BYTES:
    # Truncate characters from the end until it fits
    while len(result.encode('utf-8')) > MAX_BYTES:
        result = result[:-1]

This is O(n*m) but for a 50K limit it's fine.

Or I could be smarter:

encoded = result.encode('utf-8')
if len(encoded) > MAX_BYTES:
    trunc_marker = '... [truncated]'
    # Decode first MAX_BYTES bytes, then remove chars from end
    result = encoded[:MAX_BYTES].decode('utf-8', errors='ignore')
    # Remove partial characters at the end
    while result and result[-1] == '\ufffd' or ... no, this doesn't work for invalid bytes

Actually the spec says "decode first, then truncate by characters that re-encode within the cap". So:

Work with characters (not bytes)
Truncate characters from the end
Check that the re-encoded version fits within 50K bytes
Slice the tail (i.e. keep the beginning, remove from the end)

if len(result.encode('utf-8')) > MAX_BYTES:
    trunc_marker = '... [truncated]'
    # Reserve space for marker
    max_content = MAX_BYTES - len(trunc_marker.encode('utf-8'))
    # Binary search for the right character count
    lo, hi = 0, len(result)
    while lo < hi:
        mid = (lo + hi + 1) // 2
        if len(result[:mid].encode('utf-8')) <= max_content:
            lo = mid
        else:
            hi = mid - 1
    result = result[:lo] + trunc_marker

Actually this is getting complex. Let me simplify. Since we're truncating the tail, and we can add a marker:

MAX_BYTES = 50000
result_bytes = result.encode('utf-8')
if len(result_bytes) > MAX_BYTES:
    tail = '... [truncated]'
    # Keep as much as fits with the marker
    available = MAX_BYTES - len(tail.encode('utf-8'))
    # Find the longest prefix that fits in 'available' bytes
    # using the approach from the spec: truncate by characters
    truncated = result_bytes[:available].decode('utf-8', errors='ignore')
    # This might still leave a partial multi-byte at the end, but errors='ignore' handles that
    result = truncated + tail

Actually, the spec says "decode first, then truncate by characters that re-encode within the cap". So:

Decode to string first (already done)
Truncate characters from the end
Check re-encoded fits

Let me just go with the simple while loop:

MAX_BYTES = 50000
result_encoded = result.encode('utf-8')
if len(result_encoded) > MAX_BYTES:
    # Truncate by removing characters from the end
    while len(result.encode('utf-8')) > MAX_BYTES:
        result = result[:-1]
    # Result is now <= MAX_BYTES after re-encoding

But the spec says "a clear marker like ... [truncated] may be appended". So we should append it:

MAX_BYTES = 50000
result_encoded = result.encode('utf-8')
if len(result_encoded) > MAX_BYTES:
    trunc_marker = '... [truncated]'
    # Reserve space for marker
    while len((result + trunc_marker).encode('utf-8')) > MAX_BYTES:
        result = result[:-1]
    result += trunc_marker

This is clean and correct. result[:-1] in Python correctly handles multi-byte characters since Python strings are sequences of Unicode code points.

Now let me think about the timeout handling more carefully:

import subprocess
import signal

runner = _resolve_runner()
argv = [runner, 'run', '--rm', '--pull=missing',
        f'--network={network}',
        f'--memory={memory}',
        f'--pids-limit={pids}',
        f'--cpus={cpus}',
        '--cap-drop=ALL',
        '--security-opt=no-new-privileges']

runner = _resolve_runner()
argv = [runner, 'run', '--rm', '--pull=missing',
        f'--network={network}',
        f'--memory={memory}',
        f'--pids-limit={pids}',
        f'--cpus={cpus}',
        '--cap-drop=ALL',
        '--security-opt=no-new-privileges']

if workspace is not None:
    argv.extend(['-v', f'{workspace}:/workspace:rw', '-w', '/workspace'])

argv.extend([image, 'sh', '-c', command])

try:
    proc = subprocess.run(argv, capture_output=True, text=True, timeout=timeout, errors='replace')
    exit_code = proc.returncode
    stdout = proc.stdout
    stderr = proc.stderr
except subprocess.TimeoutExpired as e:
    exit_code = 124
    stdout = e.stdout or ""
    stderr = e.stderr or ""

Wait, but when subprocess.run times out, it kills the subprocess (podman). When podman is killed, does the container keep running? If yes, we need to clean up.

Let me think... For podman run, when the client process is killed, the container continues running. The --rm flag only cleans up when the container exits. So if we just kill podman, the container keeps running.

One approach: generate a container name, use --name to track it, and on timeout, run podman kill <name>.

import uuid

container_name = f"sandbox-{uuid.uuid4().hex[:8]}"
argv = [runner, 'run', '--rm', '--name', container_name, ...]

try:
    ...
except subprocess.TimeoutExpired:
    # Kill the container
    subprocess.run([runner, 'kill', container_name], capture_output=True, timeout=5)
    exit_code = 124
    ...

Wait, but what about the output? On timeout, we might have partial stdout/stderr. The TimeoutExpired exception has stdout and stderr attributes (if capture_output=True was used). But subprocess.run with capture_output=True collects output but the process might be killed before it finishes writing. The TimeoutExpired exception should have the partial output.

Actually, let me check: subprocess.TimeoutExpired has .stdout and .stderr attributes set to whatever was captured so far. Yes.

So the plan is:

Use --name to track the container
On timeout, kill the container explicitly
Use the partial output from the exception

Wait, but there's a subtlety. subprocess.run with capture_output=True and timeout=N will:

Start the process
Collect stdout/stderr
After N seconds, if process hasn't finished, send SIGKILL to the process
Raise TimeoutExpired

After we catch the exception, the podman process is dead (killed by subprocess). But the container is still running. So we need to kill it.

Let me also handle the case where podman kill itself fails (e.g., container already dead).

Now, there's another consideration: when podman kills the container, the --rm flag should clean it up. But actually, since we used --name, podman kill will stop the container and --rm will handle cleanup.

But wait - there's a race condition. subprocess.run sends SIGKILL to podman, and podman might have already started cleanup. Then podman kill might fail. Let's just try and ignore errors.

OK, let me also think about the subprocess.run behavior more carefully. When timeout=N expires, Python sends SIGKILL to the process group. But podman run is just the client - the actual container runtime is separate. So the container processes continue running.

Actually, I just realized: when subprocess.run sends SIGKILL to podman, podman might handle the signal and propagate it to the container. Let me check...

For Docker: docker run is a CLI client. If you kill it, the container keeps running in detached mode even with --rm (the --rm cleanup runs when the container exits naturally).

For Podman: Podman is more integrated. When you Ctrl+C a podman run, it does handle it and stops the container. But SIGKILL can't be caught, so podman run immediately dies and the container might keep running.

Actually wait, subprocess.run with timeout first sends SIGTERM, then after a brief delay sends SIGKILL. Let me check Python's behavior...

From Python docs: "On timeout, it sends SIGKILL to the child on POSIX. If the process doesn't terminate after SIGKILL, a TimeoutExpired exception is raised."

Wait no, that's Popen.communicate(timeout=...). For subprocess.run(timeout=...):

with Popen(...) as process:
    try:
        stdout, stderr = process.communicate(input, timeout=timeout)
    except TimeoutExpired:
        process.kill()
        stdout, stderr = process.communicate()
        raise TimeoutExpired(...)

So yes, subprocess.run with timeout:

Calls process.communicate(timeout=...)
If timeout, calls process.kill() (SIGKILL on POSIX)
Then calls process.communicate() again to get whatever output was captured

So the subprocess is killed with SIGKILL. Podman can't intercept SIGKILL. The container keeps running.

For the container cleanup, I'll use --name pattern.

Actually, wait. Let me reconsider. If we use --cidfile instead of --name, we don't have to worry about name collisions:

import tempfile
import os

cidfile_fd, cidfile_path = tempfile.mkstemp(prefix='sandbox-', suffix='.cid')
os.close(cidfile_fd)

try:
    ...
except subprocess.TimeoutExpired:
    try:
        with open(cidfile_path) as f:
            cid = f.read().strip()
        if cid:
            subprocess.run([runner, 'kill', cid], capture_output=True, timeout=5)
    except:
        pass
finally:
    os.unlink(cidfile_path)

Hmm, but --cidfile might conflict with --rm in some versions. Let me just use --name with a UUID.

Actually, using --name is simple:

container_name = f"sandbox-{uuid.uuid4().hex[:12]}"
argv = [runner, 'run', '--rm', '--name', container_name, ...]

And on timeout:

subprocess.run([runner, 'kill', container_name], capture_output=True, timeout=5)

The --rm flag will clean up the container after it's killed.

OK actually I just realized something. subprocess.run internally does:

with Popen(argv, ...) as process:
    ...
    process.kill()  # on timeout
    process.communicate()

When process.kill() is called, it kills the podman run process. But this might leave the container running. Then we call podman kill <container_name> to clean up.

But actually, there's a problem: when process.kill() kills podman, the container might still be running in the background. The --rm cleanup runs when the container's main process exits. When we call podman kill <name>, it will send SIGKILL to the container's init process (PID 1), causing the container to exit, and then --rm will clean it up.

Let me also consider: what if we use podman run --rm --detach and then podman wait <container> to wait for it to finish? That way we have a clean container ID:

# Start container in detached mode
result = subprocess.run([runner, 'run', '--rm', '--detach', ...], capture_output=True, text=True, timeout=30)
container_id = result.stdout.strip()

# Wait for it to finish, with timeout
try:
    completed = subprocess.run([runner, 'wait', container_id], capture_output=True, timeout=timeout)
    exit_code = int(completed.stdout.strip())
except subprocess.TimeoutExpired:
    subprocess.run([runner, 'kill', container_id], capture_output=True, timeout=5)
    exit_code = 124

# Get logs
logs = subprocess.run([runner, 'logs', container_id], capture_output=True, text=True, timeout=10)
stdout = logs.stdout
stderr = logs.stderr

But this approach has issues:

Need to handle stderr/stdout separately
More complex
Extra commands needed

Hmm actually, podman logs might not separate stdout and stderr. Let me check... Actually podman logs does separate them with different FDs, and subprocess.run with capture_output=True should show them as separate streams.

Wait, podman logs outputs both to stdout by default on some versions. podman logs --compatible?

This is getting complicated. Let me go back to the simpler approach with --name and direct subprocess.run().

Let me also think about: does the timeout need to clean up the container? The smoke test just checks:

python sandbox.py --timeout 2 -- sleep 30 → exit 124, no traceback

It doesn't check whether the container is cleaned up. So maybe for a first pass, we can just let the container leak? No, that's bad practice. Let me clean it up.

Actually, let me think again. In subprocess.run(timeout=...):

It starts the subprocess with Popen
It calls process.communicate(timeout=timeout) which reads stdout/stderr in threads
If timeout, it calls process.kill() which kills the podman process
The podman process is killed (SIGKILL)
The container may or may not be cleaned up by podman

For Docker: docker run --rm + SIGKILL to docker CLI → container continues running, never cleaned up → leak For Podman: Similar behavior.

So we definitely need cleanup. Let me use the --name approach.

Actually, thinking about it more, there's a simpler way. Instead of the complex name tracking, I can just:

Call podman run --detach --rm to start the container and get its ID
Then call podman wait <id> with a timeout to wait for it
Then call podman logs <id> to get output
On timeout, call podman kill <id>

The issue is the error handling. Let me just write the code.

Actually, I'm overthinking this. Let me look at the test descriptions more carefully:

python sandbox.py --timeout 2 -- sleep 30 → exit 124, no traceback

So the test is just a CLI test. The implementation should:

Catch the TimeoutExpired
Return exit=124 with the formatted output

For the library use, the test would be similar:

result = sandbox_run("sleep 30", timeout=2)
assert "exit=124" in result

Let me just write a straightforward implementation. Here's my plan:

#!/usr/bin/env python3
import argparse
import os
import shutil
import subprocess
import sys
import uuid


MAX_BYTES = 50000
TRUNC_MARKER = '... [truncated]'


def _resolve_runner():
    podman = shutil.which("podman")
    if podman:
        return podman
    docker = shutil.which("docker")
    if docker:
        return docker
    raise RuntimeError("Neither podman nor docker found on PATH")


def _truncate(result):
    encoded = result.encode("utf-8")
    if len(encoded) <= MAX_BYTES:
        return result
    result = result[:MAX_BYTES]
    while len(result.encode("utf-8")) > MAX_BYTES - len(TRUNC_MARKER.encode("utf-8")):
        result = result[:-1]
    return result + TRUNC_MARKER


def sandbox_run(
    command: str,
    workspace: str | None = None,
    image: str = "debian:stable-slim",
    timeout: int = 60,
    network: str = "none",
    memory: str = "2g",
    pids: int = 512,
    cpus: float = 2.0,
) -> str:
    runner = _resolve_runner()

    container_name = f"sandbox-{uuid.uuid4().hex[:12]}"

    argv = [
        runner, "run", "--rm", "--pull=missing",
        "--name", container_name,
        f"--network={network}",
        f"--memory={memory}",
        f"--pids-limit={pids}",
        f"--cpus={cpus}",
        "--cap-drop=ALL",
        "--security-opt=no-new-privileges",
    ]

    if workspace is not None:
        argv.extend(["-v", f"{workspace}:/workspace:rw", "-w", "/workspace"])

    argv.extend([image, "sh", "-c", command])

    try:
        proc = subprocess.run(
            argv,
            capture_output=True,
            text=False,  # we want bytes for proper decode
            timeout=timeout,
        )
        exit_code = proc.returncode
        stdout = proc.stdout.decode("utf-8", errors="replace") if proc.stdout else ""
        stderr = proc.stderr.decode("utf-8", errors="replace") if proc.stderr else ""
    except subprocess.TimeoutExpired as e:
        exit_code = 124
        stdout = (e.stdout or b"").decode("utf-8", errors="replace")
        stderr = (e.stderr or b"").decode("utf-8", errors="replace")
        # Clean up the container — it's likely still running
        subprocess.run(
            [runner, "kill", container_name],
            capture_output=True,
            timeout=10,
        )

    # Build formatted output
    result_parts = [f"exit={exit_code}\n--- stdout ---\n"]
    
    if stdout:
        result_parts.append(stdout.rstrip("\n") + "\n")
    
    result_parts.append("--- stderr ---\n")
    
    if stderr:
        result_parts.append(stderr)

    result = "".join(result_parts)
    return _truncate(result)


def main():
    parser = argparse.ArgumentParser(description="Run commands in a sandboxed container")
    parser.add_argument("--image", default="debian:stable-slim")
    parser.add_argument("--timeout", type=int, default=60)
    parser.add_argument("--network", choices=["none", "bridge"], default="none")
    parser.add_argument("--memory", default="2g")
    parser.add_argument("--pids", type=int, default=512)
    parser.add_argument("--cpus", type=float, default=2.0)
    parser.add_argument("--workspace", default=None)
    parser.add_argument("command", nargs=argparse.REMAINDER)

    args = parser.parse_args()
    
    if not args.command:
        parser.error("no command specified")
    
    # Everything after -- is in args.command. Join with space.
    command = " ".join(args.command)
    
    if args.workspace is None:
        args.workspace = os.getcwd()
    
    result = sandbox_run(
        command=command,
        workspace=args.workspace,
        image=args.image,
        timeout=args.timeout,
        network=args.network,
        memory=args.memory,
        pids=args.pids,
        cpus=args.cpus,
    )
    
    sys.stdout.write(result)
    # Extract exit code from result
    for line in result.split("\n"):
        if line.startswith("exit="):
            sys.exit(int(line[5:]))
            break
    
    sys.exit(0)


if __name__ == "__main__":
    main()

Wait, I need to think about the argparse REMAINDER and the -- separator. When using argparse.REMAINDER, argparse handles -- properly - everything after -- goes into the remainder regardless of whether it looks like a flag.

But actually, argparse.REMAINDER behaves a bit differently than expected. Let me think...

Actually, argparse.REMAINDER collects all remaining positional arguments. With -- in the CLI (provided by the shell or argparse), -- causes argparse to stop processing options and treat everything after as positional arguments.

But actually, Python's argparse handles -- automatically. When you use nargs=argparse.REMAINDER, argparse will capture all remaining arguments after option processing.

Let me also think about: does nargs=argparse.REMAINDER respect --? Yes. From Python docs:

argparse.REMAINDER. All the remaining command-line arguments are gathered into a list. This is commonly useful for command-line utilities that dispatch to other command-line utilities

When -- is encountered, argparse stops processing and everything after -- goes to the REMAINDER argument.

Wait, but there might be an issue. If the user runs:

python sandbox.py -- echo hi

argparse will see -- at the end, REMAINDER will get ['echo', 'hi'].

But what if they run:

python sandbox.py --image ubuntu -- echo hi

argparse will see --image as an option, ubuntu as its value, then -- which tells it to stop processing options, and echo hi as remainder. Good.

But wait, does argparse actually know to stop at --? Let me check... Yes, argparse treats -- specially. When it encounters --, it processes it as the end-of-options marker (unless the parser has allow_abbrev=False and something else). Actually, let me double-check: does argparse handle -- correctly with REMAINDER?

From Python docs on argparse:

The parse_args() method normally allows certain command-line abbreviations, and treats '-' (a single dash) and '--' (two dashes) as special.

And:

If the argument string '--' is encountered, it is treated as the end of option arguments, and everything after it is treated as positional arguments.

Yep, argparse natively handles --.

So nargs=argparse.REMAINDER should work here. When the user runs:

python sandbox.py -- echo hi

After argparse sees --, it stops processing options, and REMAINDER grabs ['echo', 'hi'].

Actually wait, I just realized: nargs=argparse.REMAINDER collects ALL remaining args, even those with dashes. But there's a subtlety: does argparse.REMAINDER work with -- properly?

Let me think of an edge case:

python sandbox.py --workspace /tmp --timeout 5 -- echo hi

With nargs=argparse.REMAINDER:

--workspace /tmp: captured as workspace
--timeout 5: captured as timeout
--: end-of-options marker
echo hi: captured in REMAINDER

Yes, this should work.

But what about:

python sandbox.py echo hi

Without --:

REMAINDER sees echo and hi as positional args
Since no options were expecting more values, everything goes to REMAINDER

Wait, no. Let me think again. If we have:

parser.add_argument("command", nargs=argparse.REMAINDER)

And the user runs python sandbox.py echo hi:

echo is not recognized as an option
argparse treats echo as a positional argument
Since REMAINDER captures all remaining, ['echo', 'hi'] is captured

Yes, this works.

But wait, there's a known issue with REMAINDER: it's greedy. From the docs:

The argparse.REMAINDER value tells an argparse.ArgumentParser to capture all remaining command-line arguments as a list. This is similar to the * pattern in some argument parsing libraries, but it doesn't interleave with other arguments.

I think REMAINDER should work fine for our use case.

Actually, I just realized there might be a problem. Let me look at how argparse.REMAINDER interacts with --. When argparse sees --:

Actually, I think there's a subtle difference between Python versions. In some older Python versions, REMAINDER doesn't properly handle --. But in Python 3.10+, it should be fine.

Let me just go with nargs='*' or nargs=argparse.REMAINDER. Both should work.

Wait, nargs='*' captures zero or more positional args as a list. But '*' doesn't handle -- the same way - in fact, with nargs='*', everything after the first positional is captured. The -- handling is the same.

Let me use argparse.REMAINDER for clarity.

OK wait, now I need to reconsider workspace handling. In the CLI:

Default workspace for the CLI is os.getcwd() (so the user's working dir is mounted by default when invoked from the shell).

But in the library:

workspace: str | None = None, # host dir to bind r/w at /workspace; None = no mount

So the library doesn't mount anything by default, but the CLI mounts the current directory by default. That's a difference between library and CLI interfaces. In my main(), I set workspace to os.getcwd() if not provided.

Wait, but what about in the library function itself? If workspace is None, no mount. If workspace is a path, mount it. Let me check if os.getcwd() is a path... yes, it's always a path to the current directory. So if workspace=os.getcwd(), it would mount the current directory.

But there's another subtlety: the cli default workspace vs the cli --workspace flag. The user might want to pass --workspace explicitly to override. My current logic:

if args.workspace is None:
    args.workspace = os.getcwd()

This means: if no --workspace flag, use cwd. If --workspace is explicitly given (even ""), use that. But there's no way to say "I want no mount" from CLI... Actually, looking at the spec, it says "Default workspace for the CLI is os.getcwd()" - there's no way to opt out from CLI. That's fine.

Actually wait, re-reading the PROMPT.md:

workspace=None; No mount

And the CLI defaults to cwd. So the CLI always mounts something (cwd or explicit). There's no way to request no mount from CLI. That's fine per spec.

Now, I'm also wondering: does --workspace accept a value? Yes, I have it as default=None which means if not specified, it's None, and then I set it to cwd.

Let me also think about the nargs=argparse.REMAINDER behavior more carefully.

Actually, I just realized a problem with argparse.REMAINDER and --. Take this command:

python sandbox.py --timeout 2 -- sleep 30

argparse sees:

--timeout → consumes next arg 2
-- → end-of-options marker
sleep → positional (REMAINDER captures it)
30 → positional (REMAINDER captures it)

So args.command = ['sleep', '30']. Good.

But what about:

python sandbox.py --timeout 2 sleep 30

Without --:

--timeout → consumes 2
sleep → positional (REMAINDER captures)
30 → positional (REMAINDER captures)

So args.command = ['sleep', '30']. Still works.

OK, I think the implementation is solid. Let me also think about text=False vs text=True.

Using text=False (the default) means subprocess.run returns bytes. I decode with errors="replace". This gives me control over the decoding.

Alternatively, I could use text=True, errors="replace" and get strings directly. But I need to make sure the timeout case also has the right output format.

Wait, with text=True, the stdout/stderr in the exception are decoded strings. With text=False, they're bytes. Let me use text=False (default) and decode manually. That's more explicit.

Actually, I'll use text=False and decode manually for clarity.

Hmm, but I realize capture_output=True with text=False means the TimeoutExpired exception will have bytes in .stdout and .stderr. Let me handle that.

OK, I think I have a solid implementation. Let me also think about another edge case: what if subprocess.run fails with FileNotFoundError? This would happen if podman/docker is not found. But we already check for this with _resolve_runner().

Wait, what about subprocess.CalledProcessError? subprocess.run only raises this if check=True, which we're not using. So no issue.

Let me also think about the output format more carefully. The format is:

exit=0
--- stdout ---
hi
--- stderr ---

The example shows:

exit=0
--- stdout ---
hi
--- stderr ---

Wait, the example shows:

exit=0
--- stdout ---
hi
--- stderr ---

Let me count newlines:

exit=0\n
--- stdout ---\n
hi\n
--- stderr ---\n

So the final string ends with \n (from --- stderr ---\n). The stderr body is empty, so the next header follows directly.

But wait, the spec says:

The stderr body, if non-empty, comes immediately after the --- stderr ---\n line. It may or may not end with a trailing newline (preserve whatever the underlying stream produced).

So if stderr is non-empty, the output would look like:

exit=0
--- stdout ---
hi
--- stderr ---
some error

Where "some error" may or may not end with \n.

But in the example, stderr is empty:

exit=0
--- stdout ---
hi
--- stderr ---

Trailing \n is present because --- stderr ---\n is the last line.

OK, my formatting code should be:

result = f"exit={exit_code}\n--- stdout ---\n"
if stdout:
    result += stdout.rstrip("\n") + "\n"
result += "--- stderr ---\n"
if stderr:
    result += stderr  # preserve as-is, may or may not end with \n

Then we truncate and return.

Wait, one more thing: the spec says stdout body "ends with exactly one \n before the --- stderr --- header". So if stdout is "hello\nworld\n\n" (trailing multiple newlines), we should output "hello\nworld\n" (exactly one trailing newline). rstrip("\n") + "\n" does this: strips all trailing newlines, then adds exactly one.

And if stdout is empty, we skip the body and go directly to --- stderr ---\n. So the output would be:

exit=0
--- stdout ---
--- stderr ---

Hmm, wait. Let me re-check the spec:

If a body is empty, the next header (or end of string) follows directly after the previous header line — no blank line is inserted.

So if stdout is empty:

exit=<n>
--- stdout ---
--- stderr ---
<stdout or nothing>

Looks like my code handles this. If stdout is empty (falsy), we don't add the body line, so it goes directly to --- stderr ---\n.

One more thing: what about stderr being empty? The spec says:

The stderr body, if non-empty, comes immediately after the --- stderr ---\n line.

And:

If a body is empty, the next header (or end of string) follows directly after the previous header line

So if stderr is empty, nothing follows after --- stderr ---\n. The string ends there.

My code handles this with the if stderr: check.

Actually, wait, there's an edge case I need to consider: what if the spec format example doesn't match exactly? The example shows:

exit=0
--- stdout ---
hi
--- stderr ---

With my code:

result = f"exit=0\n--- stdout ---\n"
stdout = "hi"
result += "hi\n"  # rstrip("\n") + "\n" = "hi\n"
result += "--- stderr ---\n"

Result: "exit=0\n--- stdout ---\nhi\n--- stderr ---\n"

Which when printed:

exit=0
--- stdout ---
hi
--- stderr ---

That matches.

What about the library example:

print(sandbox_run("echo hi"))
# exit=0
# --- stdout ---
# hi
# --- stderr ---

With echo hi, the stdout would be "hi\n". My code: "hi\n".rstrip("\n") + "\n" = "hi\n". Same result. Good.

Now let me think about the truncation one more time. The spec says:

Slice the tail; do not split mid-byte across a multibyte sequence (decode first, then truncate by characters that re-encode within the cap).

"Slice the tail" = remove from the end (i.e., keep the beginning, chop off the tail). So we keep the first N characters that fit in 50K bytes.

def _truncate(result):
    if len(result.encode("utf-8")) <= MAX_BYTES:
        return result
    # Remove chars from end until it fits with marker
    marker = TRUNC_MARKER
    while len((result + marker).encode("utf-8")) > MAX_BYTES:
        result = result[:-1]
    return result + marker

This is O(n*m) but for max 50K it's fine. Actually, the encoding on each iteration is expensive. Let me optimize:

def _truncate(result):
    encoded = result.encode("utf-8")
    if len(encoded) <= MAX_BYTES:
        return result
    # Binary search for the right length
    marker = TRUNC_MARKER
    marker_len = len(marker.encode("utf-8"))
    hi = len(result)
    lo = 0
    while lo < hi:
        mid = (lo + hi + 1) // 2
        candidate = result[:mid]
        if len(candidate.encode("utf-8")) <= MAX_BYTES - marker_len:
            lo = mid
        else:
            hi = mid - 1
    return result[:lo] + marker

This binary search approach is O(log n * n) for encoding, but more efficient than the linear approach. But actually result[:mid].encode("utf-8") is O(mid) each time, so the total is O(n log n). For 50K chars, it's fine either way.

Let me just use a simple bisect loop. Actually, let me just use a simple approach that works:

def _truncate(result):
    encoded = result.encode("utf-8")
    if len(encoded) <= MAX_BYTES:
        return result
    marker = TRUNC_MARKER
    marker_encoded = marker.encode("utf-8")
    available = MAX_BYTES - len(marker_encoded)
    # Truncate character by character until it fits
    decoded = result
    while len(decoded.encode("utf-8")) > available:
        decoded = decoded[:-1]
    return decoded + marker

This could be slow for very large strings truncated to fit 50K from e.g. 500K. But for the typical use case, it's fine.

Actually, a better approach:

def _truncate(result):
    encoded = result.encode("utf-8")
    if len(encoded) <= MAX_BYTES:
        return result
    marker = TRUNC_MARKER
    marker_encoded = marker.encode("utf-8")
    available = MAX_BYTES - len(marker_encoded)
    # Take a byte-slice and decode with errors='ignore' to handle partial multi-byte
    truncated = encoded[:available].decode("utf-8", errors="ignore")
    # May have leftover partial char at end
    while truncated and len((truncated + marker).encode("utf-8")) > MAX_BYTES:
        truncated = truncated[:-1]
    return truncated + marker

This is more efficient because we start with a byte-slice that's at most 50K bytes, then only need to adjust a few characters.

Let me go with this approach. Actually, I think the simplest correct approach is:

def _truncate(result):
    encoded = result.encode("utf-8")
    if len(encoded) <= MAX_BYTES:
        return result
    marker = TRUNC_MARKER
    marker_encoded = marker.encode("utf-8")
    available = MAX_BYTES - len(marker_encoded)
    truncated_bytes = encoded[:available]
    # Decode with ignore to handle any partial multi-byte at the end
    truncated = truncated_bytes.decode("utf-8", errors="ignore")
    return truncated + marker

But wait, errors="ignore" might strip more than just the last partial character. For example, if there are invalid bytes in the middle, they would also be stripped. But since we're working with a string that was already decoded with errors="replace", re-encoding and re-decoding shouldn't introduce new invalid byte sequences (well, replace uses replacement characters \ufffd which are valid UTF-8). So this should be safe.

Actually, let me think again. The original string was decoded from bytes with errors="replace". So any invalid bytes were replaced with the replacement character \ufffd (U+FFFD). When we encode this string to UTF-8, the replacement character becomes 3 bytes: \xef\xbf\xbd. When we take a slice of the encoded bytes and decode with errors="ignore", we might cut in the middle of those 3 bytes. errors="ignore" would skip the partial byte sequence. But we'd lose the whole replacement character, which is fine - it was already a replacement for invalid bytes.

So yes, this approach is safe and efficient.

Wait, but .decode("utf-8", errors="ignore") handles partial multi-byte sequences by ignoring them. Let me verify: if I have b"\xef\xbf" (first two bytes of a 3-byte sequence), .decode("utf-8", errors="ignore") returns "". Good.

And if I have b"\xef\xbf\xbd" (complete replacement char), .decode("utf-8", errors="ignore") returns "\ufffd". Also fine.

OK, I'm confident in this approach. Let me write the final code.

Wait, one more thing: the smoke check. After writing the file, I need to run:

python sandbox.py -- echo hi → exit 0, format matches
python sandbox.py --timeout 2 -- sleep 30 → exit 124, no traceback

Both require podman/docker. Let me check if podman or docker are available.

Actually, looking at the spec: "If podman is not on PATH, fall back to docker. If neither is present, raise a clear RuntimeError."

If neither is available, the smoke checks would fail with RuntimeError. Let me check what's available.

Let me write the implementation now.

One thing I need to consider: the sandbox_run function signature uses str | None (PEP 604 union syntax), which requires Python 3.10+. The spec says Python 3.10+, so this is fine.

Let me also think about the --pull=missing flag. Is --pull=missing supported by both podman and docker?

Docker: --pull=missing is NOT a valid flag. Docker uses --pull=always, --pull=never, or no flag (which defaults to missing). Wait actually, Docker 20.10+ supports --pull=always|missing|never. Let me check... yes, docker run --pull=missing was added in Docker 20.10.
Podman: Podman also supports --pull=missing.

So --pull=missing should work with both.

Actually wait, let me double-check. Docker's --pull flag accepts: always, missing, never, default. And missing means "pull if not present locally". Yes, this is the default behavior for Docker when no --pull flag is given. So explicitly passing --pull=missing is correct.

For Podman, the same flag exists. Good.

OK, I'm ready to write the implementation. Let me also think about one more edge case: what happens when subprocess.run raises an exception other than TimeoutExpired? For example, if the command doesn't exist (which would be caught by the container's shell, not by subprocess), or if there's a permission error with podman. In these cases, I should let the exception propagate or handle it gracefully.

For the spec, it says the function returns the formatted string. If there's an error (e.g., podman not found), _resolve_runner raises RuntimeError. If podman fails (e.g., image not found), subprocess.run returns with a non-zero exit code, and we capture and return it.

Let me also consider: what if the workspace directory doesn't exist? Podman would fail with an error, and that error would be in stderr. We'd capture it and return it in the formatted string. That's the expected behavior.

I think I'm ready. Let me write the implementation.

Wait, there's one more thing: the spec says "The formatted return string is truncated to 50,000 bytes total after construction (i.e. apply truncation to the final string with the headers in place, not to stdout/stderr separately and not via a proportional split)."

So I truncate the FINAL string (with headers and body), not the individual parts. My implementation applies truncation at the end, after building the full string. Good.

But wait: "Slice the tail; do not split mid-byte across a multibyte sequence (decode first, then truncate by characters that re-encode within the cap)."

"Slice the tail" - I think this means "cut off the tail" (keep the head). So we keep the beginning and drop from the end. This is the default behavior of truncation.

But another reading: "Slice the tail" could mean "keep the tail" (the last part). That seems less likely in the context of truncation.

I'll go with "keep the beginning, remove from the end" which is the standard truncation behavior.

OK, let me also handle the stderr trailing newline properly. Looking at the spec:

The stderr body, if non-empty, comes immediately after the --- stderr ---\n line. It may or may not end with a trailing newline (preserve whatever the underlying stream produced).

So for stderr, I just append the raw decoded string as-is. If it ends with \n, it ends with \n. If not, it doesn't. No modification.

For stdout:

The stdout body, if non-empty, comes immediately after the --- stdout ---\n line and ends with exactly one \n before the --- stderr --- header.

So stdout must end with exactly one \n. My rstrip("\n") + "\n" handles this:

"hello" → "hello\n"
"hello\n" → "hello\n"
"hello\n\n" → "hello\n"
"" → "" (if empty, we don't add the body, so the rstrip("\n") + "\n" wouldn't be reached)

Now let me finalize the code. I'll also need to handle the text=False vs text=True issue with capture_output and TimeoutExpired.

When text=False (default):

proc.stdout is bytes
TimeoutExpired.stdout is bytes

When text=True:

proc.stdout is str
TimeoutExpired.stdout is str

I'll use text=False (default) and decode manually with errors="replace".

Actually, I realize there's an issue. If I use text=False, TimeoutExpired.stdout is bytes. But TimeoutExpired with proc.kill() → process.communicate() → the communicate call reads whatever was output. But subprocess.run with timeout does:

with Popen(...) as process:
    try:
        stdout, stderr = process.communicate(input, timeout=timeout)
    except TimeoutExpired:
        process.kill()
        stdout, stderr = process.communicate()
        raise TimeoutExpired(process.args, timeout, output=stdout, stderr=stderr)

Wait, let me check the actual Python source for subprocess.run:

def run(*popenargs, input=None, capture_output=False, timeout=None, check=False, **kwargs):
    if input is not None:
        if kwargs.get('stdin') is not None:
            raise ValueError('stdin and input arguments may not both be used.')
        kwargs['stdin'] = PIPE

    if capture_output:
        if kwargs.get('stdout') is not None or kwargs.get('stderr') is not None:
            raise ValueError('stdout and stderr arguments may not be used with capture_output.')
        kwargs['stdout'] = PIPE
        kwargs['stderr'] = PIPE

    with Popen(*popenargs, **kwargs) as process:
        try:
            stdout, stderr = process.communicate(input, timeout=timeout)
        except TimeoutExpired as exc:
            process.kill()
            stdout, stderr = process.communicate()
            raise TimeoutExpired(process.args, timeout, output=stdout, stderr=stderr)
        except:
            process.kill()
            process.wait()
            raise
        retcode = process.poll()
        if check and retcode:
            raise CalledProcessError(retcode, process.args, output=stdout, stderr=stderr)
    return CompletedProcess(process.args, retcode, stdout, stderr)

So subprocess.run already handles timeout by:

Calling process.kill() (SIGKILL)
Calling process.communicate() to get remaining output
Raising TimeoutExpired with the output

Wait, but there's a subtlety. The process.kill() in run kills the podman/docker subprocess. But the container keeps running (as I discussed). However, the process.communicate() call reads whatever output was collected from the subprocess stdout/stderr pipes.

But when podman is killed, the pipes are closed. So process.communicate() should return the remaining buffered output and then EOF. This should give us the partial stdout/stderr that podman had already received from the container.

But wait, there's a timing issue. When we call process.kill(), podman dies immediately. Any output that was buffered in the container or in podman might not make it to the pipe. Some of it might be lost.

The spec doesn't specifically address partial output on timeout, so capturing whatever we can is fine.

Actually wait, there's another problem. When subprocess.run catches TimeoutExpired from process.communicate(), it calls process.kill() which sends SIGKILL to the podman process. This might leave the container running. Then process.communicate() returns any remaining output. Then subprocess.run re-raises TimeoutExpired.

In my code, I catch this exception and then need to clean up the container. I'll use --name to identify and kill it.

Actually, I think there might be an even simpler approach. When subprocess.run does process.kill() (SIGKILL), and then process.communicate(), eventually process.poll() returns the exit code (which will be -9, since it was killed by SIGKILL). But wait, subprocess.run does NOT return a CompletedProcess on timeout - it raises TimeoutExpired. And after kill+communicate, it re-raises the exception with the partial output.

Hmm, but actually I want to know: does the run function call process.kill() during normal timeout handling? Let me look at the source again:

except TimeoutExpired as exc:
    process.kill()
    stdout, stderr = process.communicate()
    raise TimeoutExpired(process.args, timeout, output=stdout, stderr=stderr)

Yes, it does. So I don't need to kill the subprocess in my handler. But I still need to kill the container.

Let me finalize my implementation and write it.

One more thing about the workspace handling. The spec says:

Library: workspace=None means no mount
CLI: default --workspace is os.getcwd()

But the CLI also allows --workspace /path/to/dir to set it explicitly. My argparse sets default=None and then in main(), if None, set to os.getcwd().

Now, what if someone runs the CLI with --workspace /tmp? Then args.workspace = '/tmp', and sandbox_run mounts /tmp at /workspace.

What if someone wants to run the CLI with NO workspace mount? The spec doesn't provide a way to do this from CLI. That's fine.

OK, I'm done overthinking. Let me write the code.

Actually wait, one more consideration. The spec says:

The -- separator divides flags from the command. Everything after -- is joined with a single space and passed as command.

So command is a string. If the user runs:

python sandbox.py -- ls -la

args.command would be ['ls', '-la'] (or similar), and I join with space: "ls -la". This is then passed to sh -c "ls -la" inside the container. Correct.

But what if the command contains spaces or special characters? For example:

python sandbox.py -- echo "hello world"

The shell (before Python) would parse this. echo "hello world" would have the quotes consumed by the shell. So args.command would be ['echo', 'hello world'] (after shell parsing). Wait, no. Let me think about this...

When the user types:

python sandbox.py -- echo "hello world"

The user's shell (bash/zsh) processes the command line. It sees -- as an argument to python sandbox.py, then echo, then hello world (with quotes removed by the shell). So sys.argv would be:

['sandbox.py', '--', 'echo', 'hello world']

Wait, actually, no. Let me think again. The user's shell passes arguments to Python. When it sees "hello world", it processes the quotes and passes hello world as a single argument. So sys.argv is:

['sandbox.py', '--', 'echo', 'hello world']

(argparse splits -- and the rest). Actually, -- is passed as a separate sys.argv element. Then argparse with REMAINDER ignores -- (it's the end-of-options marker) and captures everything after it. So args.command = ['echo', 'hello world'].

I join with space: "echo hello world". Then sh -c "echo hello world" inside the container executes it. Since sh -c receives a single string argument, the shell inside the container parses echo hello world and runs it. echo receives hello and world as two separate arguments, and prints hello world. Correct!

But what about:

python sandbox.py -- echo 'hello world'

The user's shell processes single quotes, so echo 'hello world' → echo hello world as a single argv element. sys.argv is ['sandbox.py', '--', 'echo', 'hello world']. Same as above. args.command = ['echo', 'hello world']. "echo hello world". sh -c "echo hello world". Output: hello world. Correct.

But what about a more complex case:

python sandbox.py -- sh -c 'echo $HOME'

The user's shell processes this. -- to Python, then sh, then -c, then echo $HOME (with single quotes preserving the literal $HOME). sys.argv is ['sandbox.py', '--', 'sh', '-c', 'echo $HOME'].

After REMAINDER: args.command = ['sh', '-c', 'echo $HOME']. Joined: "sh -c echo $HOME".

Then podman run ... image sh -c "sh -c echo $HOME". The inner sh -c receives echo (part 1) and $HOME (part 2, but since it's quoted, it's not expanded). Actually wait, the podman command is:

podman run ... image sh -c "sh -c echo $HOME"

The outer sh -c receives the string sh -c echo $HOME. The outer shell would parse this as:

sh → command
-c → flag
echo → part 1
$HOME → part 2

But -c only takes one argument, so echo is the command string and $HOME is $0. The inner sh runs echo (which prints nothing meaningful). Hmm, that's not right.

But wait, the user might want to run echo $HOME inside the container. The correct invocation would be:

python sandbox.py -- echo '$HOME'

Or they could just run:

python sandbox.py -- echo $HOME

Since the outer shell expands $HOME before Python sees it, sys.argv would be ['sandbox.py', '--', 'echo', '/home/user']. Then inside the container, echo /home/user would output the (wrong) path. But this is expected behavior - the outer shell expands variables before Python sees them.

If they want to prevent expansion, they'd quote: echo '$HOME'.

OK, I think the joining with space is correct for all normal use cases. The key point is that the command string is passed safely as an arg to sh -c inside the container, with no host-side interpolation.

Alright, let me write the final code now. I'm going to be thorough about the output format.

One last thing: for the text=False approach with capture_output, when TimeoutExpired is raised, the .stdout and .stderr attributes are bytes. I need to decode them with errors="replace".

Let me write the full implementation:

#!/usr/bin/env python3
import argparse
import os
import shutil
import subprocess
import sys
import uuid


_MAX_BYTES = 50000
_TRUNC_MARKER = '... [truncated]'


def _resolve_runner():
    podman = shutil.which("podman")
    if podman:
        return podman
    docker = shutil.which("docker")
    if docker:
        return docker
    raise RuntimeError("Neither podman nor docker found on PATH")


def _truncate(result: str) -> str:
    encoded = result.encode("utf-8")
    if len(encoded) <= _MAX_BYTES:
        return result
    marker = _TRUNC_MARKER
    available = _MAX_BYTES - len(marker.encode("utf-8"))
    truncated = encoded[:available].decode("utf-8", errors="ignore")
    while len((truncated + marker).encode("utf-8")) > _MAX_BYTES:
        truncated = truncated[:-1]
    return truncated + marker


def sandbox_run(
    command: str,
    workspace: str | None = None,
    image: str = "debian:stable-slim",
    timeout: int = 60,
    network: str = "none",
    memory: str = "2g",
    pids: int = 512,
    cpus: float = 2.0,
) -> str:
    runner = _resolve_runner()
    container_name = f"sandbox-{uuid.uuid4().hex[:12]}"

    argv = [
        runner, "run", "--rm", "--pull=missing",
        "--name", container_name,
        f"--network={network}",
        f"--memory={memory}",
        f"--pids-limit={pids}",
        f"--cpus={cpus}",
        "--cap-drop=ALL",
        "--security-opt=no-new-privileges",
    ]

    if workspace is not None:
        argv.extend(["-v", f"{workspace}:/workspace:rw", "-w", "/workspace"])

    argv.extend([image, "sh", "-c", command])

    try:
        proc = subprocess.run(
            argv,
            capture_output=True,
            timeout=timeout,
        )
        exit_code = proc.returncode
        stdout = (proc.stdout or b"").decode("utf-8", errors="replace")
        stderr = (proc.stderr or b"").decode("utf-8", errors="replace")
    except subprocess.TimeoutExpired as e:
        exit_code = 124
        stdout = (e.stdout or b"").decode("utf-8", errors="replace")
        stderr = (e.stderr or b"").decode("utf-8", errors="replace")
        subprocess.run(
            [runner, "kill", container_name],
            capture_output=True,
            timeout=10,
        )

    result = f"exit={exit_code}\n--- stdout ---\n"
    if stdout:
        result += stdout.rstrip("\n") + "\n"
    result += "--- stderr ---\n"
    if stderr:
        result += stderr

    return _truncate(result)


def main():
    parser = argparse.ArgumentParser(
        description="Run commands in a sandboxed container"
    )
    parser.add_argument("--image", default="debian:stable-slim")
    parser.add_argument("--timeout", type=int, default=60)
    parser.add_argument(
        "--network", choices=["none", "bridge"], default="none"
    )
    parser.add_argument("--memory", default="2g")
    parser.add_argument("--pids", type=int, default=512)
    parser.add_argument("--cpus", type=float, default=2.0)
    parser.add_argument("--workspace", default=None)
    parser.add_argument("command", nargs=argparse.REMAINDER)

    args = parser.parse_args()

    if not args.command:
        parser.error("no command specified")

    command = " ".join(args.command)

    if args.workspace is None:
        args.workspace = os.getcwd()

    result = sandbox_run(
        command=command,
        workspace=args.workspace,
        image=args.image,
        timeout=args.timeout,
        network=args.network,
        memory=args.memory,
        pids=args.pids,
        cpus=args.cpus,
    )

    sys.stdout.write(result)
    if not result.endswith("\n"):
        sys.stdout.write("\n")

    for line in result.split("\n"):
        if line.startswith("exit="):
            try:
                sys.exit(int(line[5:]))
            except ValueError:
                sys.exit(1)
    sys.exit(0)


if __name__ == "__main__":
    main()

Wait, I have an issue. When the CLI writes to stdout, the result already ends with a newline (from --- stderr ---\n or from stderr body). So I don't need to add an extra newline. Let me remove that.

Actually, wait. The result might end with a newline or might not, depending on stderr. But since we always have --- stderr ---\n at the end (when stderr is empty) or stderr (which may or may not end with \n), the final result might or might not end with \n.

Hmm, let me think. If stderr is empty, the result is ...--- stderr ---\n. So it ends with \n. If stderr is non-empty and ends with \n, result ends with \n. If stderr is non-empty and doesn't end with \n, result doesn't end with \n.

The spec doesn't require a trailing newline on the final output, but it doesn't forbid it either. But since we want the CLI to be properly composable, ending with a newline is good practice. The format as shown in the example does end with a newline.

Actually, the spec says "Each header is on its own line, terminated by \n". And --- stderr --- is a header. So --- stderr ---\n ends with \n. Then if stderr body is empty, the string ends with \n. If stderr body is non-empty, the string is ...--- stderr ---\n<stderr>.

Hmm, wait. Let me re-read:

The stderr body, if non-empty, comes immediately after the --- stderr ---\n line.

So --- stderr ---\n<stderr> is the format. If stderr ends with \n, the string ends with \n...\n<stderr>\n. If not, \n...\n--- stderr ---\n<stderr>.

The issue is about the print/display. When I do sys.stdout.write(result), it writes the string as-is. If the result ends with \n, the shell prompt appears on the next line. If not, it appears at the end of the last line.

Looking at the example output:

exit=0
--- stdout ---
hi
--- stderr ---

$ echo $?
0

There's a blank line between --- stderr --- and $ echo $? 0. So the result ends with \n (making the shell prompt appear on the next line, with a blank line being just the next line).

Hmm actually, looking more carefully at the example:

exit=0
--- stdout ---
hi
--- stderr ---

$ echo $?
0

There's a newline after --- stderr --- and then the prompt. So yes, the result ends with \n.

Since stderr is empty in this example, --- stderr ---\n is the last thing. So the result ends with \n. sys.stdout.write(result) writes it, and the prompt appears on the next line.

Wait, but the example shows an empty line between --- stderr --- and $ echo $? 0. That's because --- stderr ---\n ends with a newline, and then the shell prompt $ appears on the next line. There's no extra empty line - it just looks that way because of the newline.

OK actually, I think my result format is correct. The --- stderr ---\n at the end (when stderr is empty) provides the trailing newline. When stderr is non-empty and ends with \n, the result also ends with \n. Only when stderr is non-empty and DOESN'T end with \n does the result not end with \n.

Should I always ensure a trailing newline? Looking at the spec format rules:

If a body is empty, the next header (or end of string) follows directly after the previous header line — no blank line is inserted.

For stderr being the last header, if stderr body is empty, the string ends after the --- stderr ---\n header line, which includes a trailing \n. If stderr body is non-empty, the body follows the header line.

The spec doesn't mandate a trailing \n for the final output. The stderr body "may or may not end with a trailing newline (preserve whatever the underlying stream produced)". So we should NOT add a trailing newline if stderr is non-empty and doesn't have one.

But should I add one for the CLI output? The spec doesn't say the CLI should add a final newline. But Python's print() adds one by default. sys.stdout.write() doesn't. The example with CLI shows:

$ python sandbox.py -- echo hi
exit=0
--- stdout ---
hi
--- stderr ---

$ echo $?
0

This output is from running the command in a shell. The shell prompt $ appears after the output. Since the output ends with \n, the prompt is on the next line. So the result does end with \n.

In my code, with stderr empty, the result is "exit=0\n--- stdout ---\nhi\n--- stderr ---\n". This ends with \n. When sys.stdout.write() outputs it, the shell prompt appears on the next line. Correct.

If stderr is non-empty, the result might or might not end with \n. If it doesn't, the shell prompt would appear right after the stderr text. This is desirable behavior (preserving the original output).

OK, I'm going to write the code now. Let me structure it properly.

Actually, wait. I want to double-check one more thing: the exit code extraction from the result in main(). I'm splitting by \n and looking for exit=N. This is fine as long as the exit code line is always the first line, which it is per spec.

But there's an edge case: what if the exit= string appears in stdout or stderr? It shouldn't be in the first line because the format is:

exit=<n>
--- stdout ---
...

So the first line always starts with exit=. My code reads the first line and parses it. This is correct.

But I extract it by splitting the whole result and finding the first line starting with exit=. A simpler approach would be:

first_line = result.split("\n")[0]
if first_line.startswith("exit="):
    sys.exit(int(first_line[5:]))

Actually wait, I should use the exit code from sandbox_run directly, not parse it from the string. Let me restructure sandbox_run to return the string and a separate exit code... No wait, the function signature says it returns str. So I need to parse from the string.

Actually, a cleaner approach: have sandbox_run return the exit code as well. But the spec says the function returns str. So I should stick with returning just the string.

But I can make main() capture the exit code more cleanly. The exit code is always in the first line. Let me just parse it:

exit_match = result.split("\n")[0]
if exit_match.startswith("exit="):
    sys.exit(int(exit_match[5:]))

OK but this is fragile. What if the truncation cuts into the exit= line? The spec says the format must match, and truncation preserves the beginning. So the exit= line is always preserved. Good.

Actually, I realize I should just return the exit code from the function directly for use in main(). Let me refactor:

def sandbox_run(...) -> tuple[str, int]:
    ...
    return formatted_string, exit_code

No wait, the spec says -> str. I need to follow the spec.

Alternative: have main() use the exit code before building the string:

exit_code, result = ... # internal
sys.exit(exit_code)

Let me just add a helper function that returns both:

Actually, let me just parse the exit code from the string. It's simple enough:

result = sandbox_run(...)
sys.stdout.write(result)
try:
    exit_code = int(result.split("\n")[0].removeprefix("exit="))
except (ValueError, IndexError):
    exit_code = 1
sys.exit(exit_code)

Hmm, removeprefix is Python 3.9+. The spec says Python 3.10+, so it's fine.

OK, let me finalize the code and write it.

Let me reconsider the output truncation one more time. The spec says:

"Output truncated at 50,000 bytes total after construction (i.e. apply truncation to the final string with the headers in place, not to stdout/stderr separately and not via a proportional split). Slice the tail; do not split mid-byte across a multibyte sequence (decode first, then truncate by characters that re-encode within the cap). Truncation is silent (no error), but a clear marker like ... [truncated] may be appended."

So:

Build the full string
If > 50,000 bytes, truncate
"Slice the tail" means remove from the end (keep the beginning)
Don't split multi-byte characters
"decode first, then truncate by characters that re-encode within the cap" - this means decode to string (which we already did), truncate characters from the end, then ensure the re-encoded version fits within the cap.
May append "... [truncated]" marker

My approach:

Build full string
Encode to UTF-8 bytes
If len(bytes) > 50000, truncate
Take first available bytes (where available = 50000 - len(marker_bytes))
Decode with errors="ignore" to handle partial multi-byte sequences
Remove chars from end until re-encoded bytes fit within 50000
Append marker

This is correct.

OK, final code:

#!/usr/bin/env python3
import argparse
import os
import shutil
import subprocess
import sys
import uuid


_MAX_BYTES = 50000
_TRUNC_MARKER = "... [truncated]"


def _resolve_runner() -> str:
    podman = shutil.which("podman")
    if podman:
        return podman
    docker = shutil.which("docker")
    if docker:
        return docker
    raise RuntimeError(
        "Neither podman nor docker found on PATH. "
        "Install one of them to use the sandbox."
    )


def _truncate(result: str) -> str:
    encoded = result.encode("utf-8")
    if len(encoded) <= _MAX_BYTES:
        return result
    marker = _TRUNC_MARKER
    available = _MAX_BYTES - len(marker.encode("utf-8"))
    truncated = encoded[:available].decode("utf-8", errors="ignore")
    while len((truncated + marker).encode("utf-8")) > _MAX_BYTES:
        truncated = truncated[:-1]
    return truncated + marker


def sandbox_run(
    command: str,
    workspace: str | None = None,
    image: str = "debian:stable-slim",
    timeout: int = 60,
    network: str = "none",
    memory: str = "2g",
    pids: int = 512,
    cpus: float = 2.0,
) -> str:
    runner = _resolve_runner()
    container_name = f"sandbox-{uuid.uuid4().hex[:12]}"

    argv = [
        runner, "run", "--rm", "--pull=missing",
        "--name", container_name,
        f"--network={network}",
        f"--memory={memory}",
        f"--pids-limit={pids}",
        f"--cpus={cpus}",
        "--cap-drop=ALL",
        "--security-opt=no-new-privileges",
    ]

    if workspace is not None:
        argv.extend(["-v", f"{workspace}:/workspace:rw", "-w", "/workspace"])

    argv.extend([image, "sh", "-c", command])

    try:
        proc = subprocess.run(argv, capture_output=True, timeout=timeout)
        exit_code = proc.returncode
        stdout = (proc.stdout or b"").decode("utf-8", errors="replace")
        stderr = (proc.stderr or b"").decode("utf-8", errors="replace")
    except subprocess.TimeoutExpired as e:
        exit_code = 124
        stdout = (e.stdout or b"").decode("utf-8", errors="replace")
        stderr = (e.stderr or b"").decode("utf-8", errors="replace")
        subprocess.run(
            [runner, "kill", container_name],
            capture_output=True,
            timeout=10,
        )

    result = f"exit={exit_code}\n--- stdout ---\n"
    if stdout:
        result += stdout.rstrip("\n") + "\n"
    result += "--- stderr ---\n"
    if stderr:
        result += stderr

    return _truncate(result)


def main() -> None:
    parser = argparse.ArgumentParser(
        description="Run commands in a sandboxed container"
    )
    parser.add_argument("--image", default="debian:stable-slim")
    parser.add_argument("--timeout", type=int, default=60)
    parser.add_argument("--network", choices=["none", "bridge"], default="none")
    parser.add_argument("--memory", default="2g")
    parser.add_argument("--pids", type=int, default=512)
    parser.add_argument("--cpus", type=float, default=2.0)
    parser.add_argument("--workspace", default=None)
    parser.add_argument("command", nargs=argparse.REMAINDER)

    args = parser.parse_args()

    if not args.command:
        parser.error("no command specified")

    command = " ".join(args.command)

    if args.workspace is None:
        args.workspace = os.getcwd()

    result = sandbox_run(
        command=command,
        workspace=args.workspace,
        image=args.image,
        timeout=args.timeout,
        network=args.network,
        memory=args.memory,
        pids=args.pids,
        cpus=args.cpus,
    )

    sys.stdout.write(result)
    try:
        exit_code = int(result.split("\n")[0].removeprefix("exit="))
    except (ValueError, IndexError):
        exit_code = 1
    sys.exit(exit_code)


if __name__ == "__main__":
    main()

Hmm wait, actually I realize there might be an issue with removeprefix. In Python 3.9+, str.removeprefix(prefix) returns a copy of the string with the prefix removed, or the original string if it doesn't start with the prefix. But the spec says Python 3.10+, so it's fine.

Actually, let me double-check: result.split("\n")[0] gets the first line. If the result is empty or has no newlines, it still returns a single element. So split("\n")[0] is always valid.

But what if the result doesn't start with exit=? That would be a bug in my code. But since my code always produces exit=N as the first line, it's fine. The try/except handles the parsing error gracefully.

Let me also think about: should _truncate handle the case where result is empty? If stdout and stderr are both empty, and exit_code is 0, the result is:

"exit=0\n--- stdout ---\n--- stderr ---\n"

This is 33 bytes, well under 50K. So no truncation needed.

OK, I'm confident in the implementation. Let me write it.

Wait, one more thing: the spec for _truncate says "Slice the tail". I initially interpreted this as "remove from the tail". But let me re-read:

"Slice the tail; do not split mid-byte across a multibyte sequence"

Actually, "slice" can mean "cut off" (remove) or "take a slice of" (keep). In the context of truncation, "slice the tail" likely means "slice off the tail" (remove it). So we keep the head/beginning and remove enough from the end to fit.

This is what my implementation does. Good.

OK but actually, "slice the tail" might mean "keep the tail" (the last N bytes). But that doesn't make sense with truncation... unless you're saying to keep the most recent output. But the spec says "Output truncated at 50,000 bytes total" - this is a hard cap on total output size. "Slice the tail" means "cut off the tail" (truncate).

I'll go with my interpretation: keep the beginning, remove from the end.

OK, one last consideration. Let me think about what shutil.which returns. On macOS, it returns the path to the executable if found, None otherwise. shutil.which("podman") returns something like /opt/homebrew/bin/podman if installed.

OK I think I'm ready. Let me write the file and then run the smoke tests.

Actually, I realize I should also check: does podman/docker exist on this system? Let me check before writing the code.

Actually no, the code should handle the case where neither is found. The smoke checks would show the RuntimeError in that case, which is expected behavior. Let me just write the code and try.

Let me write the file now.

Tool call: bash

Tool call: read

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Docker is available at /usr/local/bin/docker. Good. Let me now write the implementation.

I notice the bench/tasks/sandbox/SPEC.md is slightly different from the root SPEC.md. The root SPEC.md has the detailed format rules. Let me re-read the root SPEC.md to make sure I capture everything.

Actually, I already read the root SPEC.md. The root version has more detailed output format rules. Let me re-read those:

From root SPEC.md:

Return format

exit=<n>
--- stdout ---
<stdout bytes, decoded>
--- stderr ---
<stderr bytes, decoded>

Format rules (normative):

Each header (exit=<n>, --- stdout ---, --- stderr ---) is on its own line, terminated by \n.
The stdout body, if non-empty, comes immediately after the --- stdout ---\n line and ends with exactly one \n before the --- stderr --- header.
The stderr body, if non-empty, comes immediately after the --- stderr ---\n line. It may or may not end with a trailing newline (preserve whatever the underlying stream produced).
If a body is empty, the next header (or end of string) follows directly after the previous header line — no blank line is inserted.

OK wait, there's a significant difference between the root SPEC.md and bench/tasks/sandbox/SPEC.md regarding truncation:

Root SPEC.md (which I should follow since PROMPT.md says to read SPEC.md at the worktree root):

The formatted return string is truncated to 50,000 bytes total after construction

bench version:

Combined output (stdout + stderr) is truncated to 50,000 bytes total before being returned.

These are saying the same thing - truncation is applied to the final formatted string.

Now the root also has the detailed rules about how to handle stdout/stderr body formatting. Let me incorporate all of this.

One thing I notice from the bench version: it says "Combined output (stdout + stderr)" which in context means the full formatted output string.

OK, I'm writing the implementation now. Let me write the file.

Actually, I need to re-read the truncation spec more carefully from the root:

"Output truncated at 50,000 bytes total after construction (i.e. apply truncation to the final string with the headers in place, not to stdout/stderr separately and not via a proportional split). Slice the tail; do not split mid-byte across a multibyte sequence (decode first, then truncate by characters that re-encode within the cap). Truncation is silent (no error), but a clear marker like ... [truncated] may be appended."

OK so:

Build the final string first
Check if it's > 50K bytes
"Slice the tail" - I believe this means slice (cut) the tail off. Keep the head.
Use character-level truncation (after decoding to avoid splitting multi-byte chars)
May add marker

My implementation is correct. Let me write it now.

Wait, one more thing: I just realized I need to handle one edge case with the text=False approach. When I use subprocess.run(argv, capture_output=True, timeout=timeout), the output is bytes. I need to decode it. Simple enough.

But there's a subtlety with TimeoutExpired: the stdout and stderr attributes might be None if no output was captured. I handle this with (e.stdout or b""). Good.

Let me write the file now.

Tool call: write

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Now let me run the smoke checks.

Now run smoke checks:

Tool call: bash

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Hmm, exit=2 with error "sh: 0: Illegal option --". This means the echo hi is being passed wrong. Let me think about this.

The argv constructed is:

['docker', 'run', '--rm', '--pull=missing', '--name', 'sandbox-...', '--network=none', '--memory=2g', '--pids-limit=512', '--cpus=2.0', '--cap-drop=ALL', '--security-opt=no-new-privileges', 'debian:stable-slim', 'sh', '-c', 'echo hi']

Wait, but -- is being consumed by argparse's -- separator, not passed through. But the user's shell also processes --. When the user runs python sandbox.py -- echo hi, the shell passes -- as an argument. argparse handles the -- and everything after it goes to REMAINDER.

So args.command = ['echo', 'hi'], which becomes command = "echo hi". Then sandbox_run("echo hi", ...) builds the argv correctly: sh -c "echo hi".

But the error says "Illegal option --". That's weird. Let me check...

Oh wait, I think the issue is with podman or docker and how they handle the command. Actually, let me look at the error: "sh: 0: Illegal option --"

That's from the sh inside the container, not from our code. Let me think about why sh -c "echo hi" would fail...

Actually, maybe the image isn't downloaded yet? The error might be --pull=missing failing? Or maybe sh in the image is busybox and -c takes the next argument differently?

Wait, "sh: 0: Illegal option --" means sh is receiving - as an argument somehow. Let me check if there's something wrong with how we build the argv.

Actually, I think the issue might be something else. Let me look at the actual Docker invocation. Maybe docker handles -- between its own flags and the command?

Wait, I think the issue might be that --name can't be used, or --pull=missing, or something else. Let me look at the error more carefully.

Actually, the output is "sh: 0: Illegal option --". This is from the shell inside the container. It means that sh received some option it doesn't understand.

Let me think... sh -c "echo hi" should work. Unless -c "echo hi" is being passed incorrectly.

Wait, I wonder if docker's argument parsing is different. Let me check:

docker run --rm --pull=missing --name sandbox-xxx --network=none --memory=2g --pids-limit=512 --cpus=2.0 --cap-drop=ALL --security-opt=no-new-privileges debian:stable-slim sh -c "echo hi"

When we pass sh -c "echo hi" as separate argv items, docker should run the container with sh -c echo hi. Inside the container, sh runs with -c and echo hi as arguments. echo hi is the command string for sh -c. So sh -c echo hi runs echo hi in sh.

Hmm, but the error says "Illegal option --". Let me try running a simpler one without the -- separator to see if the issue is with argparse.

Wait, maybe the problem is with --pull=missing. Let me check if docker supports it.

Actually, let me just run the command directly and see what happens:

Tool call: bash

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

The direct invocation works fine - exit 0, stdout "hi\n", no error. So the issue is with my argparse parsing.

Let me check what args.command is when running python sandbox.py -- echo hi.

Actually wait, let me check the argparse. The issue might be that argparse.REMAINDER is not behaving as expected. Let me debug.

Actually, let me think about this. The command is python sandbox.py -- echo hi. sys.argv would be ['sandbox.py', '--', 'echo', 'hi'].

My argparse setup has:

parser.add_argument("command", nargs=argparse.REMAINDER)

With --, argparse should stop processing and put everything after into REMAINDER. So args.command should be ['echo', 'hi'].

Wait, but actually, there might be an issue with how Python's argparse handles --. Let me check: does argparse treat -- as end-of-options marker by default?

From the argparse docs: "If the argument string '--' is encountered, it is treated as the end of option arguments, and everything after it is treated as positional arguments."

And for REMAINDER: "All the remaining command-line arguments are gathered into a list."

So -- should be handled correctly. But let me check if there's an interaction with --pull=missing in the Docker invocation. The command sandbox.py -- echo hi - maybe argparse is confused by something?

Wait, I think the problem might be simpler. Let me re-read the error output:

exit=2
--- stdout ---
--- stderr ---
sh: 0: Illegal option --

The stderr says "sh: 0: Illegal option --". This is the shell's error. The "0:" means something weird. And exit code is 2.

Hmm, could it be that the --pull=missing flag is being treated by the shell as an option? No, that can't be - --pull=missing is a Docker flag, not a shell argument.

Wait, I think the issue might be that docker on this system doesn't support --pull=missing and it's being passed to the container as the command somehow. Let me test this:

Actually, the debug output above shows that the EXACT same argv works. So the issue must be in how my code is constructing the argv. Maybe there's something different.

Let me add some debug output to my script and re-run.

Actually, let me just print the argv from inside sandbox_run to debug.

Tool call: bash

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Ah-ha! The -- is being captured in args.command! The issue is that argparse.REMAINDER captures -- as part of the remainder rather than stopping at it. This is a known behavior of argparse.REMAINDER. So I need to handle this differently.

Wait, but the debug shows args.command: ['--', 'echo', 'hi']. When joined with space: "-- echo hi". Then inside the container, sh -c "-- echo hi". The sh shell would interpret -- echo hi as... hmm, what would sh -c "-- echo hi" do?

With sh -c "-- echo hi", the -- would be an option to sh. sh -- is used to signal end of options. But then echo hi would be... hmm.

Actually, sh -c takes only ONE argument (the command string). So sh -c "-- echo hi" passes -- echo hi as the command string to sh. Inside the shell, -- is not a command (unless it is), and echo hi would be additional arguments to -- as a command.

Actually, sh -c "string" runs string as a shell command. The string -- echo hi would cause the shell to try to run -- as a command with arguments echo hi. -- is not a valid command, so it fails.

The fix: I need to strip -- from the beginning of args.command if present.

Or, I should use a different approach for the command parsing. Instead of argparse.REMAINDER, I could:

Use nargs="*" with -- handling
Or manually handle the argument parsing

Actually, let me look at the argparse source to understand how -- should interact with REMAINDER.

From Python docs:

argparse.REMAINDER. All the remaining command-line arguments are gathered into a list. This is commonly useful for command-line utilities that dispatch to other command-line utilities

And:

If the argument string '--' is encountered, it is treated as the end of option arguments

But these two interact in a non-obvious way. With REMAINDER, -- is PART of the remainder (it's captured). This is because REMAINDER is designed for "dispatch to another program" where you want ALL remaining args, including --.

So the fix is simple: after parsing, filter out the -- marker:

command_parts = [a for a in args.command if a != "--"]

Or just strip leading --:

if args.command and args.command[0] == "--":
    args.command = args.command[1:]

Let me fix this.

The -- separator is being captured by argparse.REMAINDER. Need to strip it:

Tool call: edit

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Now let me run the smoke check again.

Tool call: bash

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Smoke check 1 passes. The output matches the expected format:

exit=0
--- stdout ---
hi
--- stderr ---

And exit code is 0.

Now let me run smoke check 2: timeout.

Tool call: bash

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Both smoke checks pass:

python sandbox.py -- echo hi → exit=0, correct format (exit=0, stdout "hi", stderr empty)
python sandbox.py --timeout 2 -- sleep 30 → exit=124, no traceback

Let me verify the output once more by printing the file as requested in the prompt.

Wait, the prompt says:

Print the final sandbox.py contents to confirm.

Let me print it.

Both smoke checks pass. Exit=0 for echo hi, exit=124 for --timeout 2 -- sleep 30 (no traceback). Done. Implementation in sandbox.py.

Tool call: read

Assistant (opencode-go/deepseek-v4-flash)

Thinking:

Done. The implementation is in sandbox.py, both smoke checks pass. Let me summarize what was done.

Done. Implementation in sandbox.py. Both smoke checks pass:

python sandbox.py -- echo hi → exit=0, output matches format exactly
python sandbox.py --timeout 2 -- sleep 30 → exit=124, no traceback