fix(translation): return HTTP 502 for Chat and Anthropic error bodies

[colinmcnamara]

What

Buffered Chat and Anthropic responses now return UpstreamFailure, the error #703 added for Responses status: "failed", when the provider reports a failure inside an HTTP 200:

• a top-level error object with no choices (Chat) or no content (Anthropic)
• a Chat choices[0].error object, which fails the turn even beside partial text

A top-level error beside real output still decodes. Decode path only, no public Rust API change.

Why

OpenRouter, the Chat target in docs/getting_started.md, documents both 200 shapes for non-streaming requests (docs). The Chat stream decoder already rejects an SSE event with a top-level error (openai_chat/stream.rs:59), but the buffered decoders returned these as successful turns.

Through an Anthropic /v1/messages caller backed by a Chat upstream:

Upstream 200 body	main	this PR
{"error":{"code":503,"message":"model overloaded"}}	200, empty text, end_turn, zero usage	502, api_error "model overloaded"
choices[0] with "content":"partial output", "finish_reason":"error", "error":{"code":502,"message":"Provider disconnected mid-stream"}	200, "partial output" as end_turn	502, api_error "Provider disconnected mid-stream"

Notes for reviewers

• error_bodies_return_upstream_failure_with_provider_message fails on main and passes here. It also checks that a top-level error beside real output, and a choice with "error": null, still decode.

[coderabbitai]

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Walkthrough

The OpenAI Chat and Anthropic response decoders now return UpstreamFailure for object-valued error responses without completion output. Translation tests cover error-only responses and responses that include output.

Changes

Provider error response decoding

Layer / File(s)	Summary
Error response handling and coverage crates/switchyard-translation/src/codecs/openai_chat/buffered.rs, crates/switchyard-translation/src/codecs/anthropic/buffered.rs, crates/switchyard-translation/tests/response_translation.rs	Both decoders return UpstreamFailure when an object-valued error accompanies missing, invalid, or empty completion output. Tests verify the provider message is returned for error-only responses and that output still translates when an error field is also present.

Priority: ➖ Normal

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

Merge Risk: ⚪ Minimal · up to 29d7e

The new error handling is mergeable on the evidence reviewed. Choice-level provider errors can still appear as successful responses and warrant a separate fix, but this change does not introduce that behavior.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: return HTTP 502 for Chat and Anthropic error bodies.

• Fix all pre-merge checks with AI

---

_{A rabbit checks the streams at night,
An error comes without a flight.
The decoders pass the failure through,
While tests check output travels too.
The rabbit hops beneath the moon.}

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Actionable comments posted: 1

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Inline comments:
In @crates/switchyard-translation/src/codecs/openai_chat/buffered.rs:
- Line 273: Before decoding a successful response, check the first choice for an
error object or a finish_reason of "error" and handle it as a failure. Preserve
the existing success behavior when a top-level error appears alongside valid
output.

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📥 Commits

Reviewing files that changed from the base of the PR and between 66bac29 and 29d7ea4.

📒 Files selected for processing (3)

• crates/switchyard-translation/src/codecs/anthropic/buffered.rs
• crates/switchyard-translation/src/codecs/openai_chat/buffered.rs
• crates/switchyard-translation/tests/response_translation.rs

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[afourniernv]

@colinmcnamara did you reproduce these responses against OpenRouter directly, and on both the Chat and Anthropic Messages endpoints? I see the OpenRouter docs link, but the native OpenAI and Anthropic docs don't define buffered HTTP 200 error responses, so I want to make sure we're treating this as OpenRouter-compatible behavior rather than part of the base protocols.

[colinmcnamara]

@afourniernv

Not live, no. I reproduced it with a mock upstream serving the two shapes from OpenRouter's docs, through the Chat decoder. For the Anthropic Messages endpoint I only have OpenRouter's documented error envelope, not a capture.

Agreed, neither base protocol defines a 200 with an error body, so this is gateway-compatible behavior. The top-level guards only fire on an error object with no output, which isn't a valid success in either protocol. The choice-level one fires on an error object inside the choice, which OpenAI's schema doesn't have. So neither should change a normal response.

I'll try to capture a live one from OpenRouter, though it only happens when a provider fails after accepting the request, so it isn't deterministic. If you'd rather keep the Anthropic guard out until there's a real capture, I'm happy to drop it.

[colinmcnamara]

Follow-up on reproducing this. I couldn't trigger it on demand: bursts of non-streaming requests at several :free models came back as normal 200s or with proper 429/404 status codes. The 200-with-error case needs a provider to fail after OpenRouter has accepted the request. But the top-level shape shows up in the wild on buffered requests:

• garrytan/gbrain#5473: a direct curl to /api/v1/chat/completions returned HTTP 200 with {"error":{"code":429,...}}
• antondelafuente/automated-researcher#827: HTTP 200 with {"error":{"message":"Overloaded","code":503}} and no choices
• OpenRouter's own AI SDK provider checks for it: "Check if response is an error (HTTP 200 with error payload)" (src/chat/index.ts)

For the choice-level shape I only found secondhand reports (e.g. the hex/claude-council changelog: "an HTTP 200 with the error on the choice"), no raw capture. For /api/v1/messages I found nothing beyond OpenRouter's docs. Happy to drop either guard if you'd rather keep this to what's been observed.