Can the agent distinguish stdout and stderr?

Start in src/programbench/container.py around line 85, where the container execution helper appears to merge stdout and stderr. Reproduce the behavior with the cmatrix evaluation case in eval/tests/test_cmatrix.py:120 and compare the observed streams with the command’s actual output. Done means the intended stdout/stderr behavior is clear and the default agent observation no longer obscures the distinction, with relevant evaluation behavior checked.

Hi!

I find that some test functions failed against a generated executable because the test expected to have some string in stderr, while the executable gave that string in stdout.

I also noticed that ProgramBench’s container execution helper appears to merge stdout/stderr streams here.

Is stdout/stderr separation intentionally unavailable to agents during exploration? (I think it is still possible for an agent to manually test stream placement with shell redirection, e.g. cmd >/tmp/out 2>/tmp/err, but the default observation format seems to make the distinction easy to miss.)

Thank you!

The commands I ran

uv run --with mini-swe-agent mini-extra programbench \
    --filter "abishekvashok__cmatrix.5c082c6" \
    --output output \
    --model openai/gpt-5.4

uv run programbench eval output

Score log

                 Evaluation Summary                   
 Instance                        Score  Comment     
 abishekvashok__cmatrix.5c082c6     94  507 tests   
 Average                            94  1 instances 

A failed test in eval.json

• task: abishekvashok/cmatrix
• test function name: eval.tests.test_cmatrix.TestColorOptions.test_color_invalid_shows_error_message
• branch: 1b991a57d4e9

self = <test_cmatrix.TestColorOptions object at 0x7732071f2bf0>

    def test_color_invalid_shows_error_message(self):
        """Test invalid color name produces error message."""
        result = run("-C", "purple")
        assert result.returncode == 0  # Exits with 0 but shows error
>       assert b"Invalid color" in result.stderr
E       AssertionError: assert b'Invalid color' in b''
E        +  where b'' = CompletedProcess(args=['./executable', '-C', 'purple'], returncode=0, stdout=b' Invalid color selection\n Valid colors are green, red, blue, white, yellow, cyan, magenta and black.\n', stderr=b'').stderr

eval/tests/test_cmatrix.py:120: AssertionError