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feat(gateway): emit backend, cost, and fallback-count response headers - #1207

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AmirF194:fix/401-response-routing-cost-headers
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mozilla-ai:mainfrom
AmirF194:fix/401-response-routing-cost-headers

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@AmirF194

@AmirF194 AmirF194 commented Sep 15, 2026 •

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Description

run_standalone_non_stream already resolves the serving provider, the settled cost, and (when a routing policy fired) how many earlier candidates it fell over before this one, but none of it reaches the caller today: only X-Otari-Request-ID and X-Correlation-ID exist. This adds x-otari-backend, x-otari-response-cost, and x-otari-attempted-fallbacks to the standalone-mode non-streaming response, right next to the rate-limit headers that are already set there.

x-otari-backend and x-otari-attempted-fallbacks are always present (0 fallbacks when no policy routed the request, since that's a true, known answer, not an unknown one). x-otari-response-cost is set only when log_usage actually settled a cost.

Scoped down from the full issue on purpose: hybrid/platform-mode headers and the streaming path are a different code path with different settlement timing, and x-otari-attempted-retries has no data behind it yet (_platform.py's 5xx retry predicate doesn't record anything queryable, per your own comment on this issue). All three are left for follow-up PRs rather than guessed at here.

How to test it locally

uv run pytest tests/unit/test_pipeline_settlement.py -v
make lint && make typecheck

Three new tests in test_pipeline_settlement.py, next to the existing test_standalone_non_stream_applies_rate_limit_headers: the three headers on a normal call, the cost header absent when nothing settled, and the fallback count read from RoutingAttribution.position.

PR Type

  • New Feature
  • Bug Fix
  • Refactor
  • Documentation
  • Infrastructure / CI

Relevant issues

Refs #401

Checklist

  • I understand the code I am submitting.
  • I have added or updated tests that cover my change (tests/unit, tests/integration).
  • I ran the Definition of Done checks locally (make lint, make typecheck, make test).
  • Documentation was updated where necessary.
  • If the API contract changed, I regenerated the OpenAPI spec (uv run python scripts/generate_openapi.py).

AI Usage

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Summary

Standalone non-streaming responses now include backend, attempted-fallback, and settled-cost information in response headers. Cost values preserve exact Decimal precision.

Unit tests cover backend identity, unsettled costs, and fallback counts.

@AmirF194
AmirF194 deployed to integration-tests September 15, 2026 21:14 — with GitHub Actions Active
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Walkthrough

The standalone non-streaming route now sets backend, attempted-fallback, and exact settled-cost response headers. Unit tests cover populated headers, omitted cost, and fallback attribution.

Changes

Standalone response metadata

Layer / File(s) Summary
Response header generation
src/gateway/api/routes/_pipeline.py
The route sets x-otari-backend and x-otari-attempted-fallbacks. When a cost settles, it sets x-otari-response-cost from the exact Decimal string.
Header test coverage
tests/unit/test_pipeline_settlement.py
The test helpers accept compiled routing plans. Tests verify backend, cost, omitted cost, and fallback-count headers.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

Suggested reviewers: njbrake

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 44.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (4 passed)
Check name Status Explanation
Title check ✅ Passed The title uses the Conventional Commit feature prefix with a scope, uses imperative mood, and clearly describes the added response headers. At 70 characters, it is at the stated approximate limit.
Description check ✅ Passed The description includes all required sections, explains the user-facing change, documents local tests, identifies the PR type and issue, and records test coverage. The AI tool fields are empty, and t…
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
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Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag Coverage Δ
integration 83.21% <100.00%> (?)
unit 73.36% <100.00%> (?)

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Files with missing lines Coverage Δ
src/gateway/api/routes/_pipeline.py 93.84% <100.00%> (ø)
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@AmirF194
AmirF194 force-pushed the fix/401-response-routing-cost-headers branch from 34612a5 to d9f0f52 Compare September 17, 2026 05:29
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Actionable comments posted: 1

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Inline comments:
In `@src/gateway/api/routes/_pipeline.py`:
- Around line 4795-4796: Update the response-cost header assignment in the
standalone success path to serialize the settled Decimal directly, removing the
as_float conversion while preserving the existing None check and exact decimal
precision.

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AmirF194 force-pushed the fix/401-response-routing-cost-headers branch from c377725 to bb237eb Compare September 17, 2026 18:07
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🧹 Nitpick comments (1)
src/gateway/api/routes/_pipeline.py (1)

4902-4903: 📐 Maintainability & Code Quality | 🔵 Trivial

Run the required full lint gate before merge. The repository requires make lint; Ruff alone is not sufficient.

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In `@src/gateway/api/routes/_pipeline.py` around lines 4902 - 4903, Run the
repository’s complete lint gate with make lint before merging, rather than
relying on Ruff alone.

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AmirF194 force-pushed the fix/401-response-routing-cost-headers branch from 979d13e to ddef4c9 Compare September 22, 2026 15:15
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@AmirF194

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This has been sitting a week with CI green and no review yet. Anything blocking it, or should I look at splitting it differently?

run_standalone_non_stream already resolves the serving provider, the
settled cost, and (when a routing policy fired) how many earlier
candidates it fell over before this one, but none of it reached the
caller: only X-Otari-Request-ID and X-Correlation-ID exist today.

Add x-otari-backend, x-otari-response-cost, and x-otari-attempted-fallbacks
to the standalone non-streaming response, right where rate-limit headers
are already set. Backend and fallback-count are always known (0 fallbacks
when no policy routed the request); the cost header is set only when
log_usage actually settled one.

Scoped to standalone-mode, non-streaming: hybrid/platform-mode headers,
the streaming path, and attempted-retries (no retry counter exists
anywhere in the codebase yet) are left for follow-up PRs.

Refs mozilla-ai#401
str(as_float(actual_cost)) narrows through a float before serializing to
the header, which can round large valid costs. Serialize the settled
Decimal directly.
@AmirF194
AmirF194 force-pushed the fix/401-response-routing-cost-headers branch from ddef4c9 to 71b4eaf Compare September 27, 2026 08:26
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AmirF194 deployed to integration-tests September 27, 2026 08:26 — with GitHub Actions Active
@AmirF194
AmirF194 deployed to integration-tests September 27, 2026 08:26 — with GitHub Actions Active
@AmirF194
AmirF194 deployed to integration-tests September 27, 2026 08:26 — with GitHub Actions Active
@AmirF194
AmirF194 deployed to integration-tests September 27, 2026 08:26 — with GitHub Actions Active
AGENTS.md requires Otari-defined headers to skip the X- prefix per RFC
6648; these are our own new headers with nothing external depending on
the old names yet, so renaming is the whole fix.
@AmirF194
AmirF194 deployed to integration-tests September 27, 2026 10:22 — with GitHub Actions Active
@AmirF194
AmirF194 deployed to integration-tests September 27, 2026 10:22 — with GitHub Actions Active
@AmirF194
AmirF194 deployed to integration-tests September 27, 2026 10:22 — with GitHub Actions Active
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AmirF194 deployed to integration-tests September 27, 2026 10:22 — with GitHub Actions Active

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integration-tests — 7e94bccf Deployed Sep 27, 2026 by AmirF194 via test-integration (2/4) #2885
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