fix(translation): encode Responses tool turns correctly

[nachiketb-nvidia]

What

Fix OpenAI Responses request encoding for normalized tool turns.

• Encode function_call.arguments as a JSON string, as required by the Responses API.
• Drop Anthropic-signed private thinking when targeting Responses input.
• Preserve unsigned Responses reasoning items during Responses round trips.

Why

Anthropic tool-use input is represented as structured JSON in the neutral request type, while the
Responses API requires the same arguments to be serialized as a JSON string. Anthropic private
thinking also cannot be replayed as a Responses reasoning item.

Keeping this translation behavior in a focused MR avoids coupling provider codec correctness to the
libsy server refactor.

How

The Responses encoder serializes normalized tool-call arguments with the existing json_string
helper. It filters reasoning blocks carrying an Anthropic signature and continues to encode unsigned
reasoning blocks produced by the Responses decoder.

What to review

1. Anthropic tool-use arguments become valid Responses function_call input.
2. Signed Anthropic thinking does not leak into Responses input.
3. Responses reasoning still survives decode and re-encode.

Validation

• cargo fmt --all --check
• cargo test -p switchyard-translation --test request_translation (34 passed)

Summary by CodeRabbit

• Bug Fixes
  • Improved translation of Anthropic requests to Responses format by excluding private signed reasoning content.
  • Preserved tool calls and their outputs when private thinking content is present.
  • Encoded tool-call arguments consistently as JSON strings.
• Tests
  • Added coverage for tool-call argument encoding and private thinking removal during request translation.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: c3c82f44-48c2-49a4-8e12-94379abc77c1

📥 Commits

Reviewing files that changed from the base of the PR and between 89edae4 and bdce6dd.

📒 Files selected for processing (2)

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

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Walkthrough

Changes

Responses translation

Layer / File(s)	Summary
Responses encoding behavior crates/switchyard-translation/src/codecs/responses/buffered.rs	Signed reasoning blocks are filtered out, unsigned reasoning remains supported, and tool-call arguments are encoded as JSON strings.
Translation regression coverage crates/switchyard-translation/tests/request_translation.rs	Tests verify string-encoded tool arguments and removal of private thinking while retaining function-call items.

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

Poem

I’m a rabbit with bytes in my tray,
Hopping signed thoughts away.
Tool arguments curl into strings,
While function calls grow little wings.
Tests cheer: “The payload’s just right!”

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	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 matches the main change by describing the Responses translation fix for tool turns.
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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