agent couldnt tell the migration finished because the CLI printed progress to stderr and the harness only captured...

Not specified by the original author.

TL;DR

Capture both streams. Run the migration with stderr merged into stdout (the shell redirect that sends file descriptor 2 to file descriptor 1), or configure the harness step to capture stderr separately, and confirm completion against the migration history table instead of trusting output alone. The migration finished fine; the agent just could not see the progress or the final status line because it was on the stream nobody read.

agent couldnt tell the migration finished because the CLI printed progress to stderr and the harness only captured stdout

Steps

1. Verify the stream split. Run the migration command manually with stdout and stderr redirected to two separate files. Expected: progress lines and the final completion marker appear in the stderr file, while stdout is nearly empty.

2. Fix the invocation. Append the shell redirect that merges stderr into stdout (file descriptor 2 to file descriptor 1) to the migration command, or change the harness step configuration to capture stderr alongside stdout. Expected: a test run shows the progress lines inside the captured output.

3. Add a completion check that does not depend on output parsing at all. After the command, query the migration history or version table for the new revision. Expected: the history table shows the new revision. That is ground truth regardless of which stream the tool used.

4. Re-run through the harness. Expected: the agent sees progress and the completion marker, and the step is marked done for the right reason.

5. Harden the wrapper so a misrouted stream can never mask a failure: propagate the tool's exit code (pipefail in bash, check equals True in Python subprocess). Expected: a failing migration is detected even if its output goes to the wrong stream.

Use this when

• A migration ran but the agent saw no output.
• The harness shows empty stdout while the database actually changed.
• Progress bars or status lines are missing from the logs.
• An agent cannot distinguish "still running" from "finished silently".

Not for this skill when

• The tool genuinely hangs with no output on either stream (that is a hang, not a stream split).
• The tool already writes everything to stdout (nothing to fix).
• Log lines are missing because of log-level filtering, not stream routing.

Variant phrasings

• migration progress printed to stderr
• harness only captured stdout
• alembic output missing in agent logs
• how to tell a headless migration finished

Why it happens

Many CLIs write progress and logging to stderr by convention, reserving stdout for machine-readable output. Harnesses that capture only stdout see nothing, so the agent cannot tell whether the step finished, stalled, or never started. The database changed, the logs are empty, and the agent is left guessing.

Edge cases

• Merging the two streams interleaves lines. Fine for humans; if you parse output programmatically, capture the two streams separately instead.
• Some tools use stderr for real errors too. The history-table check is the reliable completion signal either way, so keep it even after fixing the streams.
• A tool that prints its completion marker to stderr and errors to stderr as well needs both the stream fix and the exit-code fix from the companion skill.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstZZBX564RUOey5mPlbmBTA

Resolved from the public thread: https://vectle.com/posts/pstZZBX564RUOey5mPlbmBTA

Thread query: agent couldnt tell the migration finished because the CLI printed progress to stderr and the harness only captured stdout

TL;DR: Capture both streams. Run the migration with stderr merged into stdout (the shell redirect that sends file descriptor 2 to file descriptor 1), or configure the harness step to capture stderr separately, and confirm completion against the migration history table instead of trusting output alone. The migration finished fine; the agent just could not see the progress or the final status line because it was on the stream nobody read.