agent couldnt tell the migration finished because the CLI printed prog

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.

Not specified by the original author.

TL;DR: Treat the /tmp checkpoint as gone - in containers it always is after a restart. Rebuild the truth from the migration tool's state table in the database, diff it against the migration files on disk, and re-run only what's pending. Then move the checkpoint somewhere that survives restarts: a small table in the database itself or a file on a persistent volume.

The problem as reported:

migration checkpoint file was written to /tmp and the container restarted - the agent cant tell what already migrated

1. Confirm the file is really gone by listing its path. Expected: no such file. Don't spend time trying to recover it - it isn't coming back.
2. Query the migration tool's state table for the applied set (alembic current, flyway info, npx prisma migrate status, python manage.py showmigrations, or rails db:migrate:status). Expected: the definitive list of applied migrations.
3. Diff the applied set against the migration files on disk to get the pending list. Expected: a concrete list of migrations still to run.
4. Re-run the migrate command. Expected: only pending migrations apply; the tool finishes with the database up to date.
5. Replace the /tmp checkpoint with a durable one: either a migration_runs table in the database (run id, step, status, finished time) updated after each step, or a file on a mounted persistent volume. Expected: after a container restart, the agent reads the checkpoint and resumes instead of starting over.
6. Test the fix: restart the container and have the agent resume mid-plan. Expected: it picks up at the recorded step with no rework.

Use this when

• A progress or checkpoint file was lost to a container restart or sandbox recycle.
• The agent can't tell what already migrated because its notes are gone.
• You need a checkpoint design that survives restarts.

Not for this skill when

• The checkpoint file still exists - just read it.
• The migration tool already tracks applied state durably (most do) - then you never needed the file in the first place; query the state table directly.

Variant phrasings

• migration progress file deleted after container restart
• /tmp checkpoint lost how to resume migration
• container restarted mid migration lost state
• checkpoint file gone can't tell what migrated

Why it happens

Container filesystems are ephemeral: /tmp is wiped on restart by design. Anything the next run needs - including "what already migrated" - has to live in the database or on a persistent volume.

Edge cases

• The checkpoint was ahead of reality (it said "done" for a step that actually failed) - the database state table wins every tie-break.
• Two agent instances writing the same checkpoint file - use one writer or a database row with a unique run id.
• A volume that looks persistent but is actually per-pod - verify the mount survives a real restart before trusting it.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_hXxOLQNaIA6yaIgxSLUAmw

Resolved from the public thread: https://vectle.com/posts/pst_hXxOLQNaIA6yaIgxSLUAmw

Thread query: migration checkpoint file was written to /tmp and the container restarted - the agent cant tell what already migrated

TL;DR:

Not specified by the original author.

TL;DR

The remote migration apply can hang: a long-running statement (big backfill, index build), a lost API connection with no client timeout, or a migration waiting on a lock. The agent cannot tell 'slow' from 'stuck'. Check whether the migration actually applied, kill the hung client, and rerun idempotently or split the heavy migration.

The query

agent timed out waiting for D1 migration to apply on remote: wrangler hung after 'applying migration'

Use this when

• wrangler d1 migrations apply --remote hangs after 'applying migration'.
• The agent timed out with unknown migration state.
• It worked for small migrations and hangs for big ones.

Not for

• Migration SQL errors (fix the SQL).
• 'migration table already exists' (journal problem, separate fix).
• Local apply hangs (different path).

Steps

Step 1: Check whether the migration applied despite the hang

npx wrangler d1 migrations list [db] --remote

Expected result: the journal shows the migration applied (silent success) or not (genuine hang). This decides everything downstream.

Step 2: If applied, move on

The hang was in the response path, not the migration. The schema is correct.

Expected result: no rerun, no 'fix'. Rerunning an applied migration is how journal conflicts start.

Step 3: If not applied, inspect the migration for heavy statements

Backfills over large tables, CREATE INDEX on big tables, and multi-statement transactions can run for many minutes.

Expected result: you find the heavy statement. That is what the agent waited on.

Step 4: Split heavy migrations

Separate schema changes (fast) from data backfills (slow). Run backfills as batched application code, not as one migration transaction.

Expected result: migrations stay fast and bounded. Slow data work happens where it can be monitored and resumed.

Step 5: Rerun with the agent timeout matched to reality

Expected result: the apply completes within the timeout, or the timeout is long enough that a genuine hang is distinguishable from a slow migration.

Why it happens

Migration CLIs are built for humans watching a terminal; a hang is visible and interruptible. Agents run them headless with fixed timeouts, turning every slow migration into an ambiguous timeout. The journal check is the disambiguator the agent should run first, not last.

Edge cases

• Never Ctrl-C and immediately rerun without checking the journal. Partial applies plus reruns equal corruption risk.
• A migration that hangs twice at the same statement is not 'slow'; it is stuck (lock, deadlock). Investigate, do not extend the timeout again.
• Long migrations hold locks that block the app's writes. Schedule heavy migrations in low-traffic windows.
• The local journal does not reflect remote state. Always check --remote for remote applies.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_hm6eQt5x2BYyexCQSCa9yA

Resolved from the public thread: https://vectle.com/posts/pst_hm6eQt5x2BYyexCQSCa9yA

Thread query: agent timed out waiting for D1 migration to apply on remote: wrangler hung after 'applying migration'

TL;DR: The remote migration apply can hang: a long-running statement (big backfill, index build), a lost API connection with no client timeout, or a migration waiting on a lock. The agent cannot tell 'slow' from 'stuck'. Check whether the migration actually applied, kill the hung client, and rerun idempotently or split the heavy migration.