fix(core): clear provider response headers on fallback boundaries

[Huang-404-Q]

Description

clearCtxForFallback wipes the context keys resolved for the previous provider before a fallback attempt runs — but it misses BifrostContextKeyProviderResponseHeaders.

Providers set that key from their own HTTP response before the status check, so error paths can forward it. Normally the fallback's own provider overwrites it, but when a fallback attempt fails pre-flight (key selection fails for the fallback provider, a plugin short-circuits it, the queue is retiring), nothing overwrites the key and the primary's headers survive on the context. The transport then forwards them verbatim on the fallback's error response.

Consequence: the client receives a response attributed to provider B (x-bifrost-routing-info-provider: anthropic) carrying provider A's Retry-After: 60 and x-ratelimit-remaining-* — and a compliant client will wait according to a limit belonging to a provider that never served the request, with no way to reconcile the disagreement from the response alone.

This is the same class of staleness clearCtxForFallback already guards against for key pins and the attempt trail; this key just seems to have been missed when it was added.

Fix

One line: clear BifrostContextKeyProviderResponseHeaders in clearCtxForFallback, with a comment explaining why.

Testing

• Regression test added to the existing clearCtxForFallback coverage in core/bifrost_test.go: headers set to simulate the primary (retry-after, x-ratelimit-remaining-tokens) do not survive the call. Verified red-green (fails on current dev with the exact "survived clearCtxForFallback" message).
• Full core suite: only pre-existing failure is TestContextSpanAttributesEmit (fails identically on unmodified dev; unrelated to this change). go vet / gofmt clean.

Fixes #6973

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[coderabbitai]

📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Prevented response headers from a failed provider attempt from carrying over to fallback error responses.
  • Corrected active-request telemetry so requests are counted down exactly once, including streaming and early-rejection scenarios.
  • Prevented active-request gauges from remaining elevated when request timing information is unavailable.

Walkthrough

The change clears provider response headers during fallback context reset. It also makes telemetry active-request gauge decrements idempotent across early returns and repeated stream hooks.

Changes

Fallback context cleanup

Layer / File(s)	Summary
Clear provider response headers core/utils.go, core/bifrost_test.go	clearCtxForFallback clears provider response headers. The test verifies that supplied headers are removed.

Telemetry active-request gauge

Layer / File(s)	Summary
Idempotent gauge decrement plugins/telemetry/main.go	PostLLMHook decrements ActiveRequests at hook entry with an atomic context flag. The pre-dispatch and stream-final decrement paths are removed.
Gauge regression coverage plugins/telemetry/main_test.go	Tests aggregate gauge values and cover repeated stream hooks and missing startTimeKey state.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Severity of issue fixed: Medium

Suggested reviewers: roroghost17, akshaydeo, kohlivrinda

Merge Risk: 🟡 Moderate · up to 53c4c

Fallback response headers are cleared correctly, but telemetry will undercount active streaming requests after their first chunk, making in-flight request monitoring inaccurate until the stream ends. This should be corrected before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The core changes are in scope for [#6973], but the telemetry changes modify ActiveRequests gauge behavior and add unrelated telemetry tests. These changes are not required to prevent provider response…	Remove the unrelated telemetry changes from this pull request or link them to a relevant issue. If both fixes must ship together, document their relationship and scope explicitly in the pull request description.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: clearing provider response headers at fallback boundaries.
Description check	✅ Passed	The description clearly explains the problem, impact, fix, testing, and linked issue. It omits several template sections, including change type, affected areas, breaking changes, security consideratio…
Linked Issues check	✅ Passed	The core changes satisfy issue [#6973] by clearing BifrostContextKeyProviderResponseHeaders in clearCtxForFallback and adding regression coverage for stale provider headers across fallback boundaries.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 4 files.

Full details: Out of Scope Changes check

Explanation

The core changes are in scope for [#6973], but the telemetry changes modify ActiveRequests gauge behavior and add unrelated telemetry tests. These changes are not required to prevent provider response header leakage across fallback boundaries.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@plugins/telemetry/main.go`:
- Around line 1012-1015: Update the ActiveRequests decrement logic in
PostLLMHook so normal streaming requests decrement only on the terminal stream
callback, while preserving the idempotent activeRequestsDecrementedKey guard.
Add equivalent cleanup on early-return paths without a terminal callback, and
add a regression case verifying the gauge stays at 1 after an intermediate chunk
and reaches 0 only after completion.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7af0d6c and 53c4c17.

📒 Files selected for processing (4)

• core/bifrost_test.go
• core/utils.go
• plugins/telemetry/main.go
• plugins/telemetry/main_test.go

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[Huang-404-Q]

The telemetry comment on this PR belongs to a sibling branch — this PR's diff touches only core/utils.go and core/bifrost_test.go (the fallback-boundary header clearing), and plugins/telemetry isn't part of it. The three branches here share a base and the review stack appears to have mixed their diffs.

That telemetry finding itself is real and is already addressed in PR #7018 (commit e02559d: the decrement gates on a non-stream request or the confirmed final chunk, keeping the idempotent latch).