fix(observability): pass err via { error } in initErrorRoutes for Winston dedup

[PierreBrisorgueil]

Summary

• Fixes fix(observability): logger.error(err.stack) bypasses Winston PostHog dedup → 2 $exception events per uncaught error #3656 — logger.error(err.stack) in initErrorRoutes passed a string, so PostHogErrorTransport couldn't read posthogCaptured on the Error → 2 $exception events per uncaught express error (validated prod 2026-05-11: 3 curl POSTs → 6 events)
• One-line fix: logger.error('Unhandled express error', { error: err }) so transport reads info.error → dedup path works
• Adds 1 unit test covering the info.error.posthogCaptured dedup path (the info instanceof Error path was already tested)

Files touched

• lib/services/express.js — 1-line fix in initErrorRoutes
• lib/services/tests/logger.posthog.transport.unit.tests.js — +1 test (8 total, was 7)

Test plan

• npm run lint — clean
• npm run test:unit — 1380 passed (1 new test added)
• npm run test:coverage — thresholds met (pre-existing $sortArray perf test failure is unrelated to this change, fails on master too)

Summary by CodeRabbit

• Bug Fixes

  • Enhanced error logging with structured format for improved diagnostics and issue tracking.

• Tests

  • Added test coverage to verify error transport handling and prevent duplicate error captures.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 0c019508-c17c-4f07-8823-42e710326b73

📥 Commits

Reviewing files that changed from the base of the PR and between a91b07b and 4db4827.

📒 Files selected for processing (2)

• lib/services/express.js
• lib/services/tests/logger.posthog.transport.unit.tests.js

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Walkthrough

Express error middleware now logs errors as structured objects instead of string stacks, enabling the PostHog transport to read and honor deduplication flags, preventing duplicate exception events.

Changes

Error Deduplication in Express Error Middleware

Layer / File(s)	Summary
Express Error Handler Logging lib/services/express.js	initErrorRoutes middleware changes from logger.error(err.stack) to logger.error('Unhandled express error', { error: err }), passing the error object instead of its string stack to preserve metadata.
PostHog Deduplication Validation lib/services/tests/logger.posthog.transport.unit.tests.js	New test asserts PostHogErrorTransport skips captureException when info.error.posthogCaptured is already true, while still invoking the log callback.

Possibly related issues

• fix(observability): logger.error(err.stack) bypasses Winston PostHog dedup → 2 $exception events per uncaught error #3656 — Directly addresses the bug where logger.error(err.stack) bypasses Winston PostHog deduplication by passing the error object instead of its string representation, allowing the transport to detect and honor the posthogCaptured flag.

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: fixing the error logging format in initErrorRoutes to enable Winston deduplication.
Description check	✅ Passed	The description is comprehensive, covering the issue summary, the one-line fix, test coverage, and validation results. All major template sections are adequately addressed.
Linked Issues check	✅ Passed	The code changes fully address #3656: the error is now passed via { error: err } in initErrorRoutes, allowing PostHogErrorTransport to read the posthogCaptured flag for deduplication, and a test verifies the dedup path.
Out of Scope Changes check	✅ Passed	All changes are in scope: the one-line fix in express.js and the new unit test in logger.posthog.transport.unit.tests.js directly address the deduplication issue identified in #3656.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/winston-dedup-string-err

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.20%. Comparing base (a91b07b) to head (4db4827).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3657   +/-   ##
=======================================
  Coverage   89.20%   89.20%           
=======================================
  Files         136      136           
  Lines        4668     4668           
  Branches     1451     1451           
=======================================
  Hits         4164     4164           
  Misses        392      392           
  Partials      112      112           

Flag	Coverage Δ
integration	59.59% <100.00%> (ø)
unit	64.35% <100.00%> (ø)

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[Copilot]

Pull request overview

Fixes PostHog Error Tracking deduplication for uncaught Express errors by ensuring Winston logs carry the original Error object (so the posthogCaptured flag can be read by the PostHog transport), and adds a unit test covering the { error: err } metadata dedup path.

Changes:

• Update Express error route logging to call logger.error(..., { error: err }) instead of logging err.stack as a plain string.
• Add a unit test to ensure PostHogErrorTransport skips capture when info.error.posthogCaptured === true.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/services/express.js	Adjusts uncaught Express error logging to pass the Error object via Winston metadata for PostHog dedup.
lib/services/tests/logger.posthog.transport.unit.tests.js	Adds coverage for the dedup path when the Error is provided in info.error.