fix(server): preserve safe headers on terminal upstream errors

[cpakkamisaac-sae]

Summary

Preserve retry guidance and request correlation headers when a routed upstream call ends in an HTTP error. Before this change, the response body and status survived, but useful headers such as Retry-After and x-request-id were lost.

The client retains only an allowlist of retry, rate-limit, and request-ID headers from the final failed attempt. The server applies the same allowlist when constructing the downstream error and when rendering it in OpenAI or Anthropic format. Credentials, cookies, and arbitrary upstream headers are not forwarded.

LlmClientError::UpstreamHttp now carries a boxed header map. This changes the public enum variant's construction pattern; in-tree callers have been updated.

Closes #903.

Verification

• Confirmed the new loopback regression test failed on the original behavior because Retry-After was absent.
• Full Rust workspace tests passed.
• Workspace Clippy passed with warnings denied; formatting check passed.
• Python lint and type checks passed.
• Python tests passed: 166 tests and 2 subtests. The separate Docker integration test was not run because the local Docker daemon is unavailable.
• Integration coverage includes OpenAI and Anthropic error formats, 429 and 503 responses, a successful control request, excluded sensitive headers, retry exhaustion, and fallback to another candidate.

Summary by CodeRabbit

• Improved Error Responses
  • Upstream error responses now include safe retry guidance, request IDs, and rate-limit details when available.
  • Sensitive headers such as cookies and authorization information are not forwarded.
  • When a request uses fallback models, error details reflect the final failed attempt; successful fallback responses do not carry headers from earlier errors.

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Walkthrough

Upstream HTTP errors now retain an allowlist of retry, rate-limit, and request-ID headers from the final failed attempt. The server forwards those headers on routed error responses, including format-specific rendering. Tests cover filtering, retries, Anthropic Messages, and candidate fallback.

Changes

Routed upstream error headers

Layer / File(s)	Summary
Capture headers on upstream errors crates/protocol/src/client.rs, crates/libsy-llm-client/src/client.rs	UpstreamHttp now carries a header map. The client captures selected retry, rate-limit, and request-ID headers. Retry-exhaustion tests check the final attempt’s request ID and exclude set-cookie.
Forward selected headers in error responses crates/switchyard-server/src/lib.rs, crates/libsy-llm-client/src/run.rs, crates/libsy-llm-client/tests/observability.rs, crates/libsy/src/algorithms/..., crates/protocol/src/stream.rs, crates/switchyard-nemo-relay-plugin/src/runtime.rs, crates/switchyard-runner/src/failure.rs, crates/switchyard-server/src/sse.rs	The server forwards allowed upstream error headers, including when format-specific rendering replaces a response. Other upstream error construction sites and fixtures supply empty headers.
Verify terminal errors and fallback headers crates/switchyard-server/tests/server.rs	Tests check header filtering for chat and Anthropic errors. Fallback tests check that successful candidates do not inherit prior error headers and terminal failures use the final candidate’s headers.

Priority: ➖ Normal

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

Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to 93807

A custom upstream client can cause per-hop metadata to reach callers. Filter connection-nominated headers before forwarding; the established impact is narrow and does not otherwise block merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 55.56% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 36 functions across 14 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving safe headers on terminal upstream errors.
Linked Issues check	✅ Passed	Issue [#903] coding requirements are implemented. TranslatingLlmClient captures only retry-after, request-ID, x-ratelimit-*, and anthropic-ratelimit-* headers from the failed response. `Upstre…
Out of Scope Changes check	✅ Passed	The changes stay within issue [#903]. The UpstreamHttp field update, caller and fixture updates, header filtering, error rendering, and added tests directly support safe header propagation and regre…

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---

_{I’m a rabbit with a retry note tucked in my sleeve,
I follow the final request ID through the leaves.
Cookies stay behind; safe headers hop along,
Rate limits mark the path when the wait feels long.
The last candidate’s clues reach the burrow at last.}

_{Comment @coderabbitai help to get the list of available commands.}

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @crates/libsy-llm-client/src/client.rs:
- Around line 866-870: Update upstream_error to remove headers nominated by
Connection from the supplied header map before the allowlist loop appends any
headers to the response. Keep the existing allowlist behavior for headers that
remain.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository: NVIDIA-NeMo/Switchyard/.coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 90922191-6295-43c3-9b65-f804b57371d0

📥 Commits

Reviewing files that changed from the base of the PR and between 16cbe59 and 93807da.

📒 Files selected for processing (14)

• crates/libsy-llm-client/src/client.rs
• crates/libsy-llm-client/src/run.rs
• crates/libsy-llm-client/tests/observability.rs
• crates/libsy/src/algorithms/advisor_gate/tests.rs
• crates/libsy/src/algorithms/util/buffered_response.rs
• crates/libsy/src/algorithms/util/llm_judge.rs
• crates/libsy/src/algorithms/util/robustness.rs
• crates/protocol/src/client.rs
• crates/protocol/src/stream.rs
• crates/switchyard-nemo-relay-plugin/src/runtime.rs
• crates/switchyard-runner/src/failure.rs
• crates/switchyard-server/src/lib.rs
• crates/switchyard-server/src/sse.rs
• crates/switchyard-server/tests/server.rs

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