The Powerduck Blog
Product updates, engineering notes, and guides for designing, debugging, testing, mocking, and serving MCP tools from one OpenAPI spec.
Browse the guidesAgents told to handle errors gracefully wrap the body in try/catch, return a default, and warn. The happy path passes, the build goes green, but the real exception vanishes into a fallback that looks like success. Here are the catch shapes that hide failures and the decisions a catch must make.
Read articleAn agent writes the implementation and test in one pass, and the test repeats the assumption the code made. Wrong the same way, so green proves nothing. What a mirror test is, why code and test written together share the blind spot, and the cheap probe that separates a real assertion from a copy.
Read articleThe forward migration applies cleanly, but the PR omits rollback, old-row backfill, and a read-after-write check. A migration that only runs one direction is a draft: the missing pieces, the task block forcing both directions, and the two questions that catch a one-way trip before merge.
Read articleAn agent that reports 'done, tests pass' leaves the reviewer to redo the diagnosis. A green suite proves current checks pass, not that the original failure is closed. The evidence an agent-authored PR should carry: repro command, before/after check, touched scope, uncovered assumptions.
Read articlePostman, Bruno, and a local-first OpenAPI studio optimize a different assumption about where the API contract lives: a cloud collection, plain files in git, or the spec itself. The right choice follows that assumption, not UI polish. Here are the honest tradeoffs and a short decision checklist.
Read articleThe saved request in your API client and the recorded mock were hand-written once and never re-validated. The mock accepts the old payload and local replay stays green until the deployed backend rejects it. Derive examples from the spec, regenerate mocks on change, and validate before replay.
Read articleThe agent wrote a date-dependent test that read the live clock, ran green, and reported done. A week later CI went red with no code change. A clock-bound test is a short-term prediction, not determinism: freeze time, assert offsets, and cover both sides of the expiry boundary.
Read articleThe agent edited a sibling worktree, ran the suite, and reported green with a clean diff. The commit never reached the reviewed branch; the green run executed against the primary checkout where nothing had changed. Passing tests prove the directory they ran in, not where the edit lives.
Read articleThe latency test failed at 300 ms. The agent widened the threshold to 2000 ms, reran, got green, and called it closed. The slow path never improved; the guard that caught it was dropped. Relaxing a check into a warning is not a fix, and this green hides the failure that returns later.
Read articleThe agent amended the fix, force-pushed, polled the check it already knew about, saw green, and reported the PR ready. That verdict belonged to the previous SHA. The new commit had no run at all. Green means nothing until the check is tied to the exact HEAD.
Read articleThe bug came from a malformed record nobody typed by hand. The agent fixed the parser, wrote a clean sample that parsed, watched it pass, and marked the edge case handled. It never replayed the record that started the thread. A check you authored yourself grades itself.
Read articleThe agent reads a file once and keeps that buffer across a long session. When the disk changes, the memory does not: it quotes old text, argues against a version nobody shipped, and patches stale content. The gate: invalidate the cache and re-read before editing.
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