Repro Fidelity

Repro Fidelity

Use when a user asks whether a bug report is real, asks to reproduce a failure, or mentions a runtime/deployment-specific issue such as a particular language version, worker mode, platform, package manager, browser, SDK, or service model.

If the user asks for investigation only, combine this with $thoughts: verify locally, report, and stop.

Contract

• Reproduce before accepting the report's diagnosis.
• Match the reported environment when feasible: runtime version, package version, OS/platform, deployment mode, long-lived process model, flags, and relevant data/state shape.
• Separate "the symptom is real" from "the proposed root cause is real".
• Prefer small, explicit repro harnesses over broad app runs.
• Do not treat sandbox, dependency, or local-service failures as product bugs until rerun with the right permission/runtime model.

Workflow

1. Pin the report:
   • exact symptom, expected/actual result, affected version, runtime, deployment shape, input/data pattern, and proposed root cause
   • current branch/base and whether main or a previous commit should be compared
2. Build the smallest repro:
   • use the repo package manager and local docs
   • use the requested runtime/version; try nix-comma or nix shell when the tool is missing
   • keep scratch harnesses in /tmp; when repo isolation is needed, follow $thoughts and use a jj workspace when jj is available, otherwise a Git worktree
3. Increase fidelity only as needed:
   • CLI/unit repro
   • target runtime/version
   • deployment mode such as worker/server/browser/device
   • long-lived process/state accumulation
   • before/after or A/B against the proposed fix
4. Prove or falsify root cause:
   • collect direct measurements such as logs, counters, memory, stored payloads, traces, generated files, or persisted state
   • if a suggested patch does not change the symptom, say the diagnosis was falsified even if the bug is real
5. Close out:
   • state what reproduced, what did not, exact commands, environment, and remaining fidelity gaps
   • if implementing, add the smallest meaningful regression test once the failing path is known

Output

Lead with the verdict:

• Reproduced / not reproduced / partially reproduced.
• Root cause: confirmed / falsified / still unknown.
• Fidelity: what matched the report and what still differs.
• Proof: key commands and observed output, not a transcript.
• Next action: fix shape, test to add, or evidence needed.