fix(translation): keep tool-result media and stored tool continuations across Responses

[eric-liu-nvidia]

Summary

Two fixes in the OpenAI Responses codec, both in crates/switchyard-translation/src/codecs/responses/buffered.rs.

1. Cross-format tool results keep image and file content

• Anthropic → Responses: a tool_result whose content held image or document blocks was emitted as function_call_output.output = "<text only>"; an image-only result became output: "". The encoder now emits the typed input_text / input_image / input_file array when the result carries non-text blocks. Text-only results still encode as a plain string, so existing traffic is unchanged.
• Responses → Anthropic: a function_call_output whose output was an array of input_text / input_image / input_file parts was serialized into one JSON string. The decoder now runs the array through the existing Responses content decoder, so the Anthropic target receives typed text / image / document blocks. String and object outputs decode as before.

2. Stored Responses tool continuations stay on the selected model

A function_call_output sent with previous_response_id, where the matching function_call lives in provider-held state, was decoded as ordinary Role::User text ("Tool result <id>: ..."). Two consequences:

• With classify_trigger = "user_turn", the affinity/tier logic saw a new user message, released the retained assignment, ran the judge again, and could move the continuation to a different model mid tool loop.
• A Responses upstream received a user message instead of the function_call_output it was waiting for.

The decoder now keeps such an output as a ToolResult when the request carries a non-empty previous_response_id. A stored custom_tool_call_output records its call id in the request extensions (switchyard_codex_custom_call_outputs) so the encoder re-emits it with the same item type instead of function_call_output. An output with no matching call and no previous_response_id is still malformed and still degrades to readable user text, so the existing OpenAI Chat behaviour for that case is unchanged.

Verification

• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace all green.
• Two new integration tests in request_translation.rs, one per fix: tool-result media surviving Anthropic↔Responses translation (mixed, image-only, text-only, and the reverse direction), and stored tool outputs staying tool results through decode and same-format re-encode, including a stored custom_tool_call_output keeping its type.
• End-to-end against a local switchyard-server build with a loopback capture provider:
  • Anthropic→Responses mixed result now yields [input_text, input_image, input_file]; image-only yields [input_image]; Responses→Anthropic yields [text, image, document]. Same-wire controls unchanged.
  • llm_classifier route with classify_trigger = "user_turn": upstream call order is now judge → weak → weak for both the full-history and the previous_response_id continuation (was judge → weak → judge → strong for the stored one), and the upstream receives the original function_call_output with previous_response_id intact.

Notes

• Ordering nuance: a malformed orphan output that arrives while a tool call is pending but before that call's own output now lands after the tool block (via the existing deferred-message path) instead of between the call and its output. Tool-call/result adjacency is preserved.
• Pre-existing and unchanged here: the Anthropic encoder emits a base64 document from input_file.file_data without media_type. Fixed in a follow-up PR.

🤖 Generated with Claude Code

[coderabbitai]

Walkthrough

The Responses codec preserves typed text, image, and file tool-result content across Anthropic and Responses translation. It also preserves stored Responses tool continuations as tool results and falls back to readable user text when stored state is unavailable.

Changes

Tool-result preservation

Layer / File(s)	Summary
Decode stored and structured tool results crates/switchyard-translation/src/codecs/responses/buffered.rs	Decoding tracks previous_response_id, preserves structured output parts, and classifies unmatched outputs based on stored provider state.
Encode structured tool results crates/switchyard-translation/src/codecs/responses/buffered.rs	Encoding propagates diagnostics and policy and emits either text or typed Responses content for tool results.
Cross-format translation validation CHANGELOG.md, crates/switchyard-translation/tests/request_translation.rs	The changelog and tests cover text, image, file, and stored Responses tool-result translation.

Priority: ⬇️ Low

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

Merge Risk: 🟡 Moderate · up to 720e8

Some tool continuations can fail to match stored calls or lose output content, so these codec issues should be fixed before merge.

🚥 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 12 functions across 2 files. (1 skipped: …	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 both primary changes: preserving tool-result media and stored Responses tool continuations.

Full details: Docstring Coverage

Explanation

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 12 functions across 2 files. (1 skipped: 1 unsupported.)

• Fix all pre-merge checks with AI

---

_{I hop through typed results bright,
Text and images stay in sight.
Files cross formats, neat and true,
Stored tool calls return too.
If state is gone, words still shine,
A tidy trail in every line.}

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

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Actionable comments posted: 2

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Inline comments:
In `@crates/switchyard-translation/src/codecs/responses/buffered.rs`:
- Line 577: Update the normalized re-encoding path around the
answers_pending_call and stored_state check to preserve custom_tool_call_output
type through the IR or provider metadata. When encode_responses_special_input
cannot find the matching ID in current-request custom_call_ids, use the
preserved stored type so the output remains custom_tool_call_output rather than
function_call_output.
- Line 1530: Update the text-only predicate in encode_responses_tool_output to
exclude ContentBlock::Unknown, while retaining Text and Refusal handling. Ensure
encode_responses_content then applies the configured LossyConversionPolicy and
renders object-valued Unknown.raw when allowed, including rejecting it under
Reject.

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📒 Files selected for processing (3)

• CHANGELOG.md
• crates/switchyard-translation/src/codecs/responses/buffered.rs
• crates/switchyard-translation/tests/request_translation.rs

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