feat(runner): add reasoning_format target setting for Chat reasoning field

[deepujain]

What

Fixes #698. Adds an optional reasoning_format ("openai" | "deepseek") target setting for openai_chat clients. After encoding an OpenAI Chat request, the client moves assistant reasoning to the field the selected target expects and removes the other spelling; the two are never emitted together. Unset keeps the current behavior.

The setting is carried privately on ModelConfig via an additive builder. Config validation rejects it on non-Chat clients and includes it in the existing duplicate target-setting check (targets sharing a model id on one client must agree on it). Scope is switchyard-runner and switchyard-llm-client only; no protocol, codec, or libsy changes.

Why

Some OpenAI-compatible models only accept reasoning replayed under the exact field they produced (e.g. Together documents this; #449 measured DeepSeek rejecting reasoning). The earlier approach carried a source-provenance marker on the public protocol IR across four crates and could not handle routing to a target expecting the other field. This rework puts the choice on the target, per review, matching the direction from #460.

Notes for reviewers

Start at crates/switchyard-runner/src/config.rs: TargetConfig::reasoning_format, the non-Chat rejection, and the extended duplicate-settings check. The client applies it in send_encoded via apply_reasoning_format. Request-level conflict tests (both directions, plus unset-keeps-behavior) live in crates/libsy-llm-client/src/client.rs; config validation tests in crates/switchyard-runner/src/config.rs. Local validation: cargo fmt --check, cargo clippy --workspace --all-targets -D warnings (also with the prefill-router feature), and cargo test --workspace all green.

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Walkthrough

The change adds OpenAI Chat reasoning field provenance to reasoning content blocks. OpenAI Chat translation preserves reasoning or reasoning_content during request rebuilding. Anthropic and Responses reasoning blocks use the default field. Tests cover these behaviors.

Changes

Reasoning field protocol and aggregation

Layer / File(s)	Summary
Protocol reasoning metadata crates/protocol/src/llm.rs, crates/protocol/src/stream.rs	Adds OpenAiChatReasoningField and stores it on ContentBlock::Reasoning. Aggregated reasoning blocks initialize the default field.
Translation field selection crates/switchyard-translation/src/codecs/openai_chat/buffered.rs, crates/switchyard-translation/src/codecs/anthropic/buffered.rs, crates/switchyard-translation/src/codecs/responses/buffered.rs	OpenAI Chat decoding records the source field. Encoding uses that field for plaintext and fallback reasoning. Other codecs initialize the default field and update pattern matching.
Fixture updates and request coverage crates/libsy-llm-client/tests/observability.rs, crates/libsy/src/algorithms/*, crates/switchyard-translation/tests/request_translation.rs	Updates reasoning fixtures for the new metadata. Adds coverage for same-field replay and Anthropic conversion to the default reasoning field.

Priority: ➖ Normal

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

Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to 17940

Add the required concise test comments before merging so the reasoning-field preservation invariants remain clear to future maintainers.

🚥 Pre-merge checks | ✅ 2 | ❌ 3

❌ Failed checks (3 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	Issue #698 is partially implemented. ContentBlock::Reasoning records reasoning or reasoning_content, and the OpenAI Chat encoder replays one recorded spelling without emitting both fields. Tests…	Add a target-dialect or target-reasoning-field signal to the translation path. Use that signal when the source and target dialects differ, while retaining source-field replay for the same dialect. Add tests for explicit target-field selecti…
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 24 functions across 11 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Title check	⚠️ Warning	The title is misleading. The changes preserve the original OpenAI Chat reasoning field spelling and do not add a reasoning_format target setting. The target-field choice remains unresolved.	Rename the pull request to describe the implemented behavior, such as feat(protocol): preserve OpenAI Chat reasoning field spelling.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Out of Scope Changes check	✅ Passed	The protocol metadata, codec changes, compatibility initializers, and request-translation tests support Issue #698. The changes preserve reasoning-field provenance and verify replay without duplicatio…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

Full details: Linked Issues check

Explanation

Issue #698 is partially implemented. ContentBlock::Reasoning records reasoning or reasoning_content, and the OpenAI Chat encoder replays one recorded spelling without emitting both fields. Tests cover both same-format spellings and the foreign-format default. The required explicit target-field choice when routing changes the destination dialect is not implemented. The change uses the historical reasoning default because the translation layer has no target-dialect signal.

Resolution

Add a target-dialect or target-reasoning-field signal to the translation path. Use that signal when the source and target dialects differ, while retaining source-field replay for the same dialect. Add tests for explicit target-field selection.

• Fix all pre-merge checks with AI

---

_{A rabbit records each reasoning trail
reasoning and reasoning_content stay on the rail
The stream keeps its field in view
Rebuilt requests use the spelling true
Anthropic starts with the default name
Tests hop along to confirm the same}

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

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Actionable comments posted: 1

---

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Inline comments:
In `@crates/switchyard-translation/tests/request_translation.rs`:
- Around line 4052-4053: Add concise Rust doc comments immediately above the
tests chat_request_rebuild_preserves_reasoning_field_spelling and
chat_request_rebuild_uses_default_reasoning_spelling_for_foreign_reasoning,
documenting the reasoning-field naming behavior each test preserves.

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📥 Commits

Reviewing files that changed from the base of the PR and between bfcd023 and 1794085.

📒 Files selected for processing (11)

• crates/libsy-llm-client/tests/observability.rs
• crates/libsy/src/algorithms/advisor_gate/tests.rs
• crates/libsy/src/algorithms/llm_class.rs
• crates/libsy/src/algorithms/util/affinity.rs
• crates/libsy/src/algorithms/util/llm_judge.rs
• crates/protocol/src/llm.rs
• crates/protocol/src/stream.rs
• crates/switchyard-translation/src/codecs/anthropic/buffered.rs
• crates/switchyard-translation/src/codecs/openai_chat/buffered.rs
• crates/switchyard-translation/src/codecs/responses/buffered.rs
• crates/switchyard-translation/tests/request_translation.rs

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[afourniernv]

@deepujain Thanks for working on this. I spiked the target-configured path and I don’t think this needs to change the public ContentBlock shape or libsy.

The incoming spelling can help with same-format replay, but it can’t tell us which field a different selected target accepts. We need an explicit target setting for that.

There’s a similar path in #666: reasoning_effort comes from target config and is applied to the encoded request in the LLM client. Could we use the same general shape here?

• Add optional reasoning_format = "openai" | "deepseek" to the target config.
• Reject it for non-Chat clients.
• Include it in the existing duplicate target-setting check.
• Have the configured target format win when it conflicts with the incoming spelling.
• Cover that conflict and the config validation in tests.

I got the configured override working in switchyard-runner and switchyard-llm-client without changing protocol or libsy. The target-level setting also follows Graham’s direction in #460.

[deepujain]

Reworked around the target per your spec. Scope is now switchyard-runner (optional reasoning_format on TargetConfig, rejected for non-Chat clients, covered by the duplicate target-setting check) plus switchyard-llm-client (private field on ModelConfig with an additive builder; after encoding, assistant reasoning is moved to the target's field and the other spelling is removed). No protocol, codec, or libsy changes. Request-level conflict tests live in the llm-client crate, config validation tests in the runner.