sandbox-deepseek-2026-05-05-r3

Session ID: ses_207f5829cffek6z77KNC45hGmB

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-pro)

Thinking:

The user wants me to implement sandbox.py. Let me start by reading the PROMPT.md and SPEC.md files, then check if sandbox.py already exists. I should also look at PLAN.md for context.

Tool call: glob

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

Thinking:

Let me read the PROMPT.md and SPEC.md at the worktree root. The worktree root has copy-pastes of bench/tasks/sandbox/ files.

Tool call: read

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

Thinking:

Now I have a clear picture of what needs to be implemented. Let me write sandbox.py. Key requirements:

sandbox_run() with the specified signature
Uses podman (fall back to docker) with shell=False - command passed as argument to sh -c inside container
Ephemeral container with --rm
Network defaults to "none", can be "bridge"
Resource limits: --memory, --pids-limit, --cpus, --cap-drop=ALL, --security-opt=no-new-privileges
Workspace bind-mounted at /workspace with -w /workspace if provided
Timeout via subprocess.run(timeout=...) - on timeout, container killed and exit code 124
Output truncated to 50,000 bytes
CLI with argparse, -- separator, default workspace = cwd
Python 3.10+, stdlib only

Return format:

exit=<n>
--- stdout ---
<stdout content>
--- stderr ---
<stderr content>

Let me think about the subtleties:

Finding podman/docker - check PATH
Building argv list properly
On timeout: need to catch subprocess.TimeoutExpired and then kill the container. But how? We'd need the container ID. We could use podman run --cidfile to get the container ID, then kill it on timeout.
Alternatively, we could use --rm and just let subprocess.run timeout handle it, but we might want to kill the container explicitly.
Wait - SPEC says "On timeout, the container is terminated and the returned string indicates a timeout." So we need to kill the container on timeout.

For timeout handling:

We can capture stdout/stderr, and on timeout, we need to kill the container.
Using --cidfile to get the container ID, then podman kill <cid> on timeout.
Or use subprocess.run(timeout=...) and catch TimeoutExpired, then kill.

Let me think about the implementation:

import subprocess
import os
import shutil
import argparse
import tempfile
import sys

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

def sandbox_run(command, workspace=None, image="debian:stable-slim", timeout=60, network="none", memory="2g", pids=512, cpus=2.0):
    runtime = _find_runtime()
    
    argv = [
        runtime, "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:
        argv += ["-v", f"{workspace}:/workspace:rw", "-w", "/workspace"]
    
    # For timeout handling, use --cidfile
    cidfile = None
    with tempfile.NamedTemporaryFile(delete=False, prefix="sandbox_cid_") as f:
        cidfile = f.name
    argv += [f"--cidfile={cidfile}"]
    
    argv += [image, "sh", "-c", command]
    
    try:
        result = subprocess.run(
            argv,
            capture_output=True,
            timeout=timeout,
        )
        exit_code = result.returncode
        stdout = result.stdout.decode("utf-8", errors="replace")
        stderr = result.stderr.decode("utf-8", errors="replace")
    except subprocess.TimeoutExpired:
        # Kill the container
        if cidfile and os.path.exists(cidfile):
            try:
                with open(cidfile) as f:
                    cid = f.read().strip()
                if cid:
                    subprocess.run([runtime, "kill", cid], capture_output=True)
            except Exception:
                pass
        exit_code = 124
        stdout = ""
        stderr = ""
    finally:
        if cidfile and os.path.exists(cidfile):
            try:
                os.unlink(cidfile)
            except OSError:
                pass
    
    # Construct output
    output = f"exit={exit_code}\n--- stdout ---\n{stdout}--- stderr ---\n{stderr}"
    
    # Truncate to 50000 bytes
    if len(output.encode("utf-8")) > 50000:
        # Decode, truncate by characters, re-encode
        output = output[:50000] + "... [truncated]"  # Hmm, this doesn't handle multibyte properly
        # Better: truncate by bytes
        encoded = output.encode("utf-8")
        if len(encoded) > 50000:
            # Truncate at byte level trying not to split multibyte
            truncated = encoded[:50000]
            try:
                output = truncated.decode("utf-8")
            except UnicodeDecodeError:
                # Find the last valid boundary
                for cut in range(50000, 50000 - 4, -1):
                    try:
                        output = encoded[:cut].decode("utf-8")
                        break
                    except UnicodeDecodeError:
                        continue
            output += "... [truncated]"
    
    return output

Wait, I need to reread the spec more carefully:

"- 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). 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: "Slice the tail" means truncate from the end. "decode first, then truncate by characters that re-encode within the cap" - this is describing the approach: decode to characters first, then truncate character by character so that re-encoding fits.

"clear marker like ... [truncated] may be appended" - optional but suggested.

Let me implement truncation more carefully:

def _truncate(text, max_bytes=50000):
    encoded = text.encode("utf-8")
    if len(encoded) <= max_bytes:
        return text
    # Truncate by characters until it fits
    result = ""
    for ch in text:
        test = result + ch
        if len(test.encode("utf-8")) <= max_bytes:
            result = test
        else:
            break
    return result + "... [truncated]"

Wait, but "may be appended" - the marker is optional. Let me include it since the spec suggests it.

Also: format rules say:

Each header on its own line terminated by \n
stdout body comes after --- stdout ---\n, ends with exactly one \n before --- stderr ---
stderr body after --- stderr ---\n, may or may not end with trailing newline

This means if stdout is non-empty, I need to ensure there's a \n between stdout content and --- stderr ---. If stdout is empty, --- stderr --- comes right after --- stdout ---\n.

Actually wait, the format says stdout body ends with exactly one \n. But what if the stdout itself has multiple newlines at the end? 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."

Hmm, "ends with exactly one \n before the --- stderr --- header" - this means between stdout body and --- stderr --- there is exactly one \n. But what if stdout itself ends with \n\n? I think the intent is that stdout content is included as-is, and one \n separates it from the next header. If stdout ends with \n already, we probably shouldn't add another. Let me look at the example:

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

hi has no trailing newline and then --- stderr --- is on the next line. So the stdout body is hi\n and then --- stderr ---. That means there's one \n between stdout content and --- stderr ---.

If stdout were hi\n\n, then the output would be:

--- stdout ---
hi

--- stderr ---

That seems right. Let me think about this differently. The rustdoc says:

--- stdout ---\n header
stdout body
exactly one \n before --- stderr --- header

So the separator between stdout body and stderr header is exactly one \n. If stdout body ends with \n, we just write --- stderr ---\n on the next line (which is naturally the case since there's already a \n). But "exactly one \n" - does that mean I should strip trailing newlines from stdout and add exactly one? Let me re-read...

I think the "exactly one \n" means: there's exactly one \n character between the end of stdout content and the --- stderr --- line. If stdout already ends with \n, that serves as the separator. If it doesn't (like the echo hi example), we add nothing - the --- stderr --- is just on a new line because... wait, in that example:

--- stdout ---
hi
--- stderr ---

hi doesn't end with \n (echo adds a newline by default). Actually echo hi outputs hi\n. So stdout is hi\n and the output shows:

--- stdout ---
hi
--- stderr ---

Here hi\n is the stdout, and then --- stderr --- starts on the next line. So the \n in stdout serves as the separator \n. That makes sense.

I'll implement it as: after stdout content, if it doesn't end with \n, add one (because stderr header must be on its own line). Actually, looking at the format more carefully:

exit=<n>
--- stdout ---
<stdout content>
--- stderr ---
<stderr content>

The stdout content is placed as-is. Then --- stderr --- header goes after it. For readability, there should be a \n before --- stderr ---. But the format says "exactly one \n". If stdout ends with \n already, we don't add an extra. If it doesn't (e.g. echo -n hi), we'd add one.

Actually, echo -n hi would give stdout hi with no newline. Then format would be:

--- stdout ---
hi
--- stderr ---

But that means there's a \n between hi and --- stderr --- that wasn't in the original stdout. The spec says "ends with exactly one \n before the --- stderr --- header" - so yes, we add exactly one \n as a separator, regardless.

Hmm, but the spec also says "preserve whatever the underlying stream produced" for stderr. For stdout, it says "ends with exactly one \n".

I think the safest approach: ensure stdout content ends with \n before the stderr header. If it already does, leave as-is. If not, add \n.

Actually wait, let me re-read again: "The stdout body, if non-empty, comes immediately after the --- stdout ---\n line and ends with exactly one \n before the --- stderr --- header."

I think "ends with exactly one \n before the --- stderr --- header" means: the stdout body as placed ends with \n. If the original stdout doesn't end with \n, we could:

Add a \n after stdout body (separator)
Then put --- stderr --- header

Let me implement it as:

If stdout is empty: --- stdout ---\n--- stderr ---\n
If stdout is non-empty: --- stdout ---\n<stdout>--- stderr ---\n — but wait, --- stderr --- must be on its own line. So if stdout doesn't end with \n: --- stdout ---\n<stdout>\n--- stderr ---\n
If stdout ends with \n: --- stdout ---\n<stdout>--- stderr ---\n

But the spec says "exactly one \n" before stderr header. If stdout ends with \n\n, we'd have two. Let me handle this by stripping trailing newlines from stdout and adding exactly one.

Actually, I think the simplest correct interpretation:

If stdout body is non-empty, ensure it ends with exactly one \n separator before --- stderr ---
Strip any trailing newlines from stdout, then add one \n
If stdout body is empty, no blank line

Let me code this up. Actually, let me just match the expected output in the tests/examples:

echo hi → stdout is hi\n, output: --- stdout ---\nhi\n--- stderr ---\n
If stdout empty: --- stdout ---\n--- stderr ---\n

I think the cleanest approach based on the spec and examples:

parts = [f"exit={exit_code}", "--- stdout ---"]
if stdout:
    # Ensure stdout has exactly one trailing newline before stderr header
    stdout_clean = stdout.rstrip('\n') + '\n'
    parts.append(stdout_clean)
else:
    # No blank line for empty stdout, but we need a newline after header
    parts.append("")  # This doesn't quite work with join

# Actually let me build it differently

Let me build it literally:

lines = [f"exit={exit_code}", "--- stdout ---"]
lines.append(stdout.rstrip('\n') + '\n' if stdout else "")
lines.append("--- stderr ---")
lines.append(stderr)
output = "\n".join(lines)

Wait, that would give:

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

For stderr empty, there's an extra newline at the end. That's fine, echo hi has no stderr.

But wait, "\n".join(lines) adds \n between each line. Let me trace through:

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

That would be:

exit=0
--- stdout ---
hi

--- stderr ---

No, that's wrong. stdout.rstrip('\n') + '\n' gives "hi\n". Then join with \n:

"--- stdout ---" + "\n" + "hi\n"

That gives "--- stdout ---\nhi\n" which is correct so far. Then:

"hi\n" + "\n" + "--- stderr ---"

That gives "hi\n\n--- stderr ---". That's wrong - double newline.

The issue is that .join() adds \n between EVERY pair. But stdout content already has a trailing \n. So I get a double newline.

Let me just construct it manually:

output = f"exit={exit_code}\n--- stdout ---\n"
output += stdout
if output[-1] != '\n':
    output += '\n'
output += "--- stderr ---\n"
output += stderr

Wait but the format says: if stdout is non-empty, "ends with exactly one \n before the --- stderr --- header". And "If a body is empty, the next header follows directly after the previous header line — no blank line is inserted."

So: if stdout is non-empty, ensure it ends with one \n. If empty, stderr header right after stdout header.

Also for stderr: "It may or may not end with a trailing newline (preserve whatever the underlying stream produced)."

So:

output = f"exit={exit_code}\n--- stdout ---\n"
if stdout:
    if stdout.endswith('\n'):
        output += stdout
    else:
        output += stdout + '\n'
output += "--- stderr ---\n"
output += stderr

That gives for echo hi (stdout = hi\n, stderr = ``):

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

The final \n is from the --- stderr ---\n line. The output ends with \n. That matches the spec example.

For empty stdout (e.g. true):

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

That's: --- stdout ---\n--- stderr ---\n which is "no blank line is inserted" between empty stderr header and end.

Good.

Now for the timeout handling with --cidfile:

I need to create a temp file for the CID. Use tempfile.NamedTemporaryFile but with caution. Actually, podman run --cidfile writes the CID to the file. The file must exist before podman reads/writes it. Let me use delete=False and manage it manually.

Let me also think about whether podman with --rm + timeout automatically cleans up. Actually, in some versions of podman, if the process is killed (like via subprocess.run timeout sending SIGKILL), the --rm might not trigger. So we should try to kill the container explicitly.

For the CID: I'll create a temp file, pass --cidfile=<path>, and in the timeout handler, read the CID and run podman kill <cid>.

Let me also handle the case where --pull=missing might cause a long wait. That's fine per spec.

Now, one more detail: the spec says errors="replace" for decoding. I'm using result.stdout.decode("utf-8", errors="replace") which is correct.

Let me also think about whether we need to handle the case where result.returncode is already set on timeout in newer Python versions. No - in subprocess.TimeoutExpired, there's no returncode attribute on the exception in Python 3.10 (it was added in 3.11). So I'll catch the exception and set exit_code=124 manually.

Actually, let me check: Python 3.10's subprocess.TimeoutExpired does have stdout and stderr (the output captured so far). In 3.11+ it also has returncode. But we can't rely on it. For our timeout case, setting exit=124 is correct per spec.

What about stdout/stderr captured so far on timeout? The spec says "On timeout, the container is terminated and the returned string indicates a timeout." It doesn't say preserve partial output. Setting empty stdout/stderr on timeout should be fine. But if we want to preserve partial output, we could capture bytes from the timeout exception.

Actually, for simplicity and to match the spec's "indicates a timeout" wording, I'll just set stdout/stderr to empty on timeout and set exit=124.

Let me now think about the networks parameter. The spec default is "none". Pass to podman as --network=none.

OK, I think I have a solid plan. Let me also check if there's already a sandbox.py at the worktree root.

The glob showed only build directory versions. The worktree root doesn't have sandbox.py yet. Good.

Let me also handle: when workspace is provided, verify it exists (or let podman handle it?). The spec doesn't say to validate, but it would be good UX to check. Actually, spec says just pass it to podman. Podman will error if the directory doesn't exist. Let me keep it simple.

One more thing: the spec says "subprocess invocation uses an argv list with shell=False". My implementation does that - command goes as argument to sh -c inside the container.

Let me write the implementation now.

Actually, let me think about --cidfile more carefully. With tempfile:

import tempfile
fd, cidfile = tempfile.mkstemp(prefix="sandbox_cid_")
os.close(fd)

Or use NamedTemporaryFile with delete=False. Let me use that.

Also, I need to handle the edge case where subprocess.run raises an exception other than TimeoutExpired (e.g., subprocess.CalledProcessError if check=True, which I won't use).

Let me write the full implementation now.

Wait, one more thing about --cpus. The spec says cpus: float = 2.0. Podman accepts --cpus=2.0 which is fine. Docker also accepts it.

OK, time to code.

Let me also consider: the subprocess.run(argv, shell=False) part. The spec says "pass the command as an argument to sh -c inside the container". So the argv list ends with [image, "sh", "-c", command]. This means command is passed as a single argument to sh -c. This is correct.

No host shell interpolation occurs, because shell=False means the subprocess module doesn't invoke a shell. We're passing sh -c as the container's entrypoint+args, which is inside the container only.

Now, about the -- separator in CLI. The spec says "Everything after -- is joined with a single space and passed as command." So python sandbox.py -- echo hi gives command = "echo hi".

With argparse, you use REMAINDER or parse_known_args. The typical pattern:

parser.add_argument('command', nargs=argparse.REMAINDER, help='Command to run')

And then: command = ' '.join(args.command) after stripping leading -- if needed.

Actually, REMAINDER captures everything after the first positional. But we need -- to separate options from command args. Using argparse.REMAINDER with -- can be tricky. A common pattern:

parser.add_argument('--', dest='command', nargs=argparse.REMAINDER)

No, that's not how it works. Let me use a different approach:

if '--' in sys.argv:
    idx = sys.argv.index('--')
    cmd_args = sys.argv[idx + 1:]
    # Parse everything before idx
    parser.parse_args(sys.argv[1:idx])
else:
    parser.error("-- separator required before command")

This is simpler and more robust. Let me use this approach.

Let me also handle the case where sys.argv has the python prefix - so args start at index 1.

Full plan for CLI:

def main():
    parser = argparse.ArgumentParser(description='Run command in ephemeral container sandbox')
    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=os.getcwd())
    
    if '--' in sys.argv:
        idx = sys.argv.index('--')
        known_args = sys.argv[1:idx]
        cmd_tokens = sys.argv[idx + 1:]
    else:
        parser.error("Missing -- separator before command")
    
    args = parser.parse_args(known_args)
    command = ' '.join(cmd_tokens)
    
    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,
    )
    
    print(result, end='')
    
    # Exit code matching container's exit code
    # Parse the exit code from the first line
    exit_line = result.split('\n')[0]
    exit_code = int(exit_line.replace('exit=', ''))
    sys.exit(exit_code)

Wait, but print(result, end='') - the result already has a final \n (from stderr header). So end='' prevents double newline. But if the result doesn't end with \n... Actually in our format, it always ends with \n because stderr header line ends with \n.

Let me also consider: spec says "The script prints the formatted output string to stdout." So print it directly.

And "Exit code matches the container's exit code (so the CLI is composable with shell pipelines). Timeout exits with code 124."

So sys.exit(exit_code) where exit_code comes from the output string.

Let me finalize and write the code.

Actually, I want to double-check one thing: the --cidfile approach for timeout cleanup. Let me think about whether there's a simpler approach.

Actually, there's an issue: subprocess.run with capture_output=True and timeout will kill the subprocess (the podman run process) but the container might keep running. We need --cidfile to find and kill the container.

Alternative: we could use podman run --rm and trust that --rm cleans up even when the podman client exits. But this is unreliable.

Better approach: use --cidfile and on timeout, read the CID and run podman kill <cid>.

One more thing: --pull=missing might cause a long delay on first run. The spec says "First-call latency is allowed: --pull=missing is fine for v0.1." So no special handling needed.

Let me also check: does docker support --pids-limit? Yes, --pids-limit is supported by both podman and docker.

What about --cap-drop=ALL and --security-opt=no-new-privileges? Both podman and docker support these.

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

One more edge case: what if workspace is provided but doesn't exist? Podman/docker will error. That's fine - the error will show up in stderr.

Also, what if command is empty? Should probably handle that. Let me add a check or just let it pass to sh -c "". Actually, sh -c "" would work fine and return exit 0. Let me not add special handling.

Let me write the code now.

Now I have the full spec. Let me check if any sandbox.py exists at the worktree root and see other implementations for reference patterns.

Tool call: bash

Tool call: read

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

Thinking:

Now I can see both reference implementations. Both have issues with the spec:

deepseek: Truncates stdout/stderr separately before constructing the final output. The spec says "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)." Also, the deepseek version adds \n after stdout always, which means empty stdout gets a blank line before stderr header (violates "If a body is empty, the next header follows directly after the previous header line — no blank line is inserted."). Also no timeout cleanup (no --cidfile, no container killing on timeout).
kimi: Truncates stdout+stderr combined (without headers), splits proportionally. This also violates "truncate to 50,000 bytes total after construction". Also no timeout cleanup. Also, the format always includes \n after stdout (even if empty), which violates "no blank line" for empty bodies. Also workspace defaults to os.getcwd() in argparse but then checks os.path.isdir - this silently sets workspace to None if cwd doesn't exist (unlikely but possible).

Let me implement correctly per spec:

Truncation after full output construction (headers + stdout + stderr + all formatting)
Proper empty-body formatting (no blank line when body empty)
Timeout container cleanup (using --cidfile)
shell=False explicitly (not just default)
Default workspace for CLI = os.getcwd() as per spec
Exit code parsing from output

Let me write the implementation now.

For the format, I need to be very careful:

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

Rules:

If stdout is non-empty: --- stdout ---\n<stdout>--- stderr ---\n But stdout must end with exactly one \n before stderr header. So if stdout doesn't end with \n, I add one. Actually, re-reading: "The stdout body, if non-empty, comes immediately after the --- stdout ---\n line and ends with exactly one \n before the --- stderr --- header."

This means the stdout section is: <stdout_content>\n where the \n is either part of the original stdout or added as separator. The important thing is: there's exactly ONE \n between the stdout content and the --- stderr --- header.

If stdout is "hi\n": --- stdout ---\nhi\n--- stderr ---\n -- the stdout's own \n serves as the separator. If stdout is "hi" (no newline): --- stdout ---\nhi\n--- stderr ---\n -- we add a \n.

But what if stdout is "hi\n\n"? The spec says "ends with exactly one \n before the --- stderr --- header." This means I should strip extra trailing newlines and add exactly one. But that changes the output. Hmm.

Actually, I think the spec means: the formatted output ends the stdout section with exactly one \n before the stderr header. If stdout itself has trailing newlines, the LAST one serves as the separator. Extra newlines before the last one are kept as part of stdout content. So it's really just: ensure there's a \n separator. If stdout already ends with \n, use that. If not, add one.

But "exactly one \n" - if stdout ends with \n\n, we'd have two \n between content and header. But actually the first \n is part of the content (empty line in stdout output), and the second \n is the trailing newline that ends the output line before the header. So it's fine.

Let me just implement: after stdout, if it doesn't end with \n, add \n. Then append --- stderr ---\n. This ensures at least one \n separator.

For empty stdout: no stdout content at all, just header lines: --- stdout ---\n--- stderr ---\n.

Actually wait, I realize the issue. Let me trace through:

stdout = "hi\n" (echo hi) Output: --- stdout ---\nhi\n--- stderr ---\n That's: header, then hi\n, then stderr header. The \n from hi\n provides the line break.

stdout = "hi" (echo -n hi) Output: --- stdout ---\nhi\n--- stderr ---\n We add \n to get the line break.

stdout = "" (true) Output: --- stdout ---\n--- stderr ---\n No blank line. Direct.

stdout = "\n" (echo) Output: --- stdout ---\n\n--- stderr ---\n The \n from echo produces an empty line, then --- stderr --- follows. Fine.

OK I think the implementation is: if stdout doesn't end with \n, add \n. If stdout is empty, don't add anything (no blank line). Actually, that's contradictory. Let me just check: if stdout is "", I shouldn't add \n. So the logic is:
```
result = f"exit={exit_code}\n--- stdout ---\n"
if stdout:
    result += stdout
    if not stdout.endswith('\n'):
        result += '\n'
result += "--- stderr ---\n"
result += stderr
```
This gives:
- stdout "" (empty, no newline added): exit=0\n--- stdout ---\n--- stderr ---\n
- stdout "hi\n": exit=0\n--- stdout ---\nhi\n--- stderr ---\n
- stdout "hi": exit=0\n--- stdout ---\nhi\n--- stderr ---\n
- stdout "\n": exit=0\n--- stdout ---\n\n--- stderr ---\n
All correct.

For stderr: "It may or may not end with a trailing newline (preserve whatever the underlying stream produced)." So stderr is appended as-is, no manipulation.

Now for truncation: truncate the COMPLETE output to 50000 bytes. "Slice the tail" means keep the beginning, discard the end. "do not split mid-byte across a multibyte sequence (decode first, then truncate by characters that re-encode within the cap)." So truncate character by character until the encoded form fits.

"Truncation is silent (no error), but a clear marker like ... [truncated] may be appended."

MAX_BYTES = 50000
output = f"exit={exit_code}\n--- stdout ---\n..."
if len(output.encode()) > MAX_BYTES:
    result = ""
    for ch in output:
        if len((result + ch).encode()) <= MAX_BYTES:
            result += ch
        else:
            break
    output = result + "... [truncated]"

But wait, the marker also takes bytes. Should the marker be included in the 50000 limit? The spec says "50,000 bytes total" - I think the truncation happens first, then the marker is appended beyond the limit. Actually no, re-reading: "The formatted return string is truncated to 50,000 bytes total after construction." And "a clear marker like ... [truncated] may be appended." I think the marker is part of the total 50,000 bytes.

So: truncate to 50,000 bytes including the marker. Or truncate to (50,000 - len(marker)) bytes and then append marker. Let me implement it as: truncate to fit within 50,000 bytes, including the marker. Reserve marker bytes.

OK let me now write the complete implementation.

Also, for timeout cleanup: use --cidfile to store container ID, then kill on timeout.

import tempfile

# create temp cidfile
fd, cidfile = tempfile.mkstemp(prefix="sandbox_cid_")
os.close(fd)

try:
    argv_with_cid = argv + [f"--cidfile={cidfile}", image, "sh", "-c", command]
    result = subprocess.run(argv_with_cid, capture_output=True, timeout=timeout)
    ...
except subprocess.TimeoutExpired:
    # kill container
    try:
        with open(cidfile) as f:
            cid = f.read().strip()
        if cid:
            subprocess.run([runtime, "kill", cid], capture_output=True, timeout=5)
    except Exception:
        pass
    ...
finally:
    try:
        os.unlink(cidfile)
    except OSError:
        pass

Wait, but --cidfile writes the full container ID. Does Docker also support --cidfile? Let me check... Docker supports --cidfile too. Good.

Actually, let me reconsider: the argv list is built with runtime-specific flags. Both podman and docker support these flags. That's fine.

One more thing: the --memory flag format. Both podman and docker accept --memory=2g and --memory=512m. Good.

--pids-limit is podman specific in older versions, but Docker also supports it since... let me think. Docker added --pids-limit in 1.12-ish. It should be widely available. Both support it. Good.

--cpus is also supported by both. --cpus=2.0 works.

--cap-drop=ALL works for both. --security-opt=no-new-privileges works for both.

I think we're good. Let me write the implementation.

Also, regarding shell=False: the spec says "Subprocess invocation uses an argv list with shell=False". Both deepseek and kimi implementations rely on the default (which is shell=False). I'll be explicit and include shell=False in the call.

One more consideration: the network parameter. Spec says default is "none". The CLI should also default to "none". Both reference implementations do this.

For workspace in CLI: spec says "Default workspace for the CLI is os.getcwd()". The kimi implementation sets default=os.getcwd() in argparse, but then checks os.path.isdir. The deepseek implementation defaults to None in argparse and then falls back to os.getcwd(). I'll follow the spec: default workspace for CLI is os.getcwd().

In library mode, workspace defaults to None.

OK let me write the final implementation.

Tool call: write

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

Thinking:

Now let me run the two smoke checks as specified in PROMPT.md:

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

But first, let me check if podman or docker is available on this system.

Now let me run the smoke checks.

Tool call: bash

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

Thinking:

Docker is available. Let me run the smoke checks.

Tool call: bash

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

Thinking:

Docker is complaining about pre-existing CID files. The issue is that --cidfile requires the file to not exist beforehand. Docker won't overwrite an existing CID file. Since I'm creating the file via mkstemp, it already exists when Docker tries to write to it.

The fix: I need to delete the file after creating it, or use a different approach. Let me:

Create the temp file name
Delete it immediately
Pass the path to --cidfile
Docker/podman will create it when the container starts

Or alternatively, I could just not use --cidfile and accept that timeout won't kill the container. But the spec says the container should be terminated on timeout.

Let me fix: create temp file name, close and delete it before running, then pass the path.

Docker won't write to a pre-existing CID file. I need to delete it before passing it.

Tool call: edit

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

Thinking:

Now let me rerun the smoke checks.

Tool call: bash

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

Thinking:

Both smoke checks pass:

echo hi → exit=0, stdout=hi, format matches spec exactly.
timeout 2 sleep 30 → exit=124, no traceback, correct exit code.

Let me verify the output format more carefully:

Smoke check 1 output:

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

That matches the spec example exactly.

Let me verify the file contents one more time to make sure everything's clean, then state done.

Both smoke checks pass. Let me verify the final file:

Tool call: read

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

Thinking:

The implementation is complete and both smoke checks pass. Let me confirm the output format matches the spec by comparing with the example in SPEC.md:

SPEC example:

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

Our output:

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

Match confirmed.

Done. Implementation in sandbox.py.