fix(translation): replay assistant reasoning as reasoning_content too

[ting-hong-shieh]

What

Replays assistant reasoning under both OpenAI-compatible spellings instead of reasoning alone.

encode_openai_message_plaintext_reasoning and the structured fallback in encode_openai_message_structured_reasoning now go through one helper that writes reasoning and reasoning_content with the same text. No public API change; TargetCapabilities is untouched.

Why

#415 stopped request history from dropping reasoning, which was the original report in #449. It replays it as message["reasoning"], while the response encoder in the same file writes reasoning_content, so the two directions disagree on the name.

Reasoning-required upstreams look for reasoning_content specifically and treat it as present-or-absent rather than reading the alias. @DuanZGit measured this against a production DeepSeek thinking endpoint in #449, varying only the extra field on the assistant history message:

extra field on assistant history message	result
(none)	400 reasoning_content in thinking mode must be passed back
reasoning	400, same message
reasoning_content	200
reasoning_content: ""	200
reasoning_details: [...]	400, same message
unknown field	400, same message

So the second turn still fails on those upstreams today even though the reasoning is replayed.

Sending both spellings rather than swapping one for the other keeps providers that read reasoning working exactly as they do now. The rows above also indicate the rejection is a presence check on reasoning_content, not strict unknown-field validation, since an unknown field alone produced the same error.

I considered gating the field on a new TargetCapabilities flag and decided against it: main already sends reasoning unconditionally, so a provider strict enough to reject reasoning_content is already rejecting reasoning. A second spelling of the same text does not add a new class of field, and a capability flag would add public surface for no additional protection.

Closes #449

How tested

The checklist below is Python-oriented and this change is Rust-only, so those items are not applicable. Commands actually run, on Linux with cargo 1.96.1, at the head of this branch:

cargo fmt --all --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace

All three clean.

Added to crates/switchyard-translation/tests/request_translation.rs:

• the shape reported in [bug] reasoning_content stripped from requests: breaks reasoning-required upstreams (DeepSeek thinking) on multi-turn #449 — an assistant turn carrying reasoning and tool_calls, followed by the tool result — replays both spellings with the same text, keeps the tool calls, and keeps the tool result following the call it answers;
• a turn with no reasoning gains neither key.

Also verified while writing it, beyond the new tests:

• the decode side already accepts either spelling via first_nonempty_string(object, &["reasoning_content", "reasoning"]), so a message carrying both decodes to one reasoning block rather than two;
• re-encoding a translated request is byte-stable, so the two keys neither accumulate nor drift across passes.

Live verification of this branch's exact shape. I have no live endpoint, so the tests above are deterministic regression coverage only. @DuanZGit ran the dual-write against their production DeepSeek endpoint (#449):

assistant history extra	result
both reasoning and reasoning_content, same text — what this PR sends	200
reasoning only — what main sends today	400, unchanged
reasoning and reasoning_content: ""	200

Row two is main's current behavior, and row one is this PR's. That measurement is theirs, not mine; my own evidence remains the code path and the tests.

• uv run ruff check . clean — n/a, no Python changed
• uv run mypy switchyard clean — n/a, no Python changed
• uv run pytest tests/ green — n/a, no Python changed
• Manual smoke — n/a, no live endpoint available; see above

Checklist

• One class per file; filename = snake_case of the primary class. — n/a, no Python changed
• New public symbols exported from switchyard/__init__.py.__all__. — n/a, no new public symbols
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed. — no CLI or README surface changed
• Commits signed off (Signed-off-by:) per the DCO.

Notes for reviewers

One existing test changed. responses_reasoning_item_merges_into_next_assistant_message_for_openai_chat, added by #415, asserts an exact message object and fails when a second key appears. I added reasoning_content to its expected value rather than relaxing the assertion, so it still pins the exact wire shape. Calling it out because it is not my test.

Scope limit worth stating. encode_request consults exact_preserved_request first, so when source and target are both OpenAiChat and PreservationPolicy is InMemory or Embed, the original body is replayed and this code never runs. Reaching it requires cross-format translation, PreservationPolicy::Disabled, or a request built directly from the IR. A gateway that terminates the client and re-emits is in scope; a straight same-format proxy is not.

Signed reasoning is still not replayed. The plaintext path matches only signature: None, so Anthropic thinking blocks that carry a signature continue to be excluded, unchanged by this PR. anthropic_thinking_blocks_do_not_leak_into_openai_chat_messages pins that behavior and still passes.

If you would rather not send both, the alternative is to make reasoning_content replace reasoning for targets that declare support, which reintroduces the capability flag. Say the word and I will switch it; I preferred this shape because it needs no new public surface and cannot regress a provider that reads reasoning today.

[coderabbitai]

Walkthrough

The OpenAI Chat request encoder now emits non-empty reasoning under both reasoning and reasoning_content. Tests cover plaintext and structured reasoning, tool-call replay, empty reasoning omission, and Responses-to-Chat translation.

Changes

Reasoning preservation

Layer / File(s)	Summary
Centralize reasoning field emission crates/switchyard-translation/src/codecs/openai_chat/buffered.rs	Plaintext and structured reasoning use a shared setter. Non-empty text is written to both reasoning and reasoning_content; empty text is omitted.
Validate reasoning replay crates/switchyard-translation/tests/request_translation.rs	Tests verify reasoning replay with tool calls, omission when reasoning is absent, and reasoning_content in Responses-to-Chat translation.

Estimated code review effort: 2 (Simple) | ~15 minutes

Merge Risk: 🟡 Moderate · up to 4f6e4

The structured reasoning fallback now emits both reasoning fields, but an existing regression expectation still requires the reasoning field to be absent, so the current head is not merge-ready until that test is updated.

Poem

I’m a rabbit hopping through the code,
Two reasoning fields now share the load.
Tool calls keep their thinking near,
Empty thoughts disappear clear.
Snuffles cheer: the round trip’s bright! 🐇

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% 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
Linked Issues check	✅ Passed	The PR preserves assistant reasoning under both reasoning and reasoning_content, including tool-calling flows, and adds regression tests for issue #449.
Out of Scope Changes check	✅ Passed	The changes are limited to OpenAI Chat reasoning encoding and related translation tests, which directly support issue #449.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: replaying assistant reasoning as reasoning_content during translation.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@crates/switchyard-translation/src/codecs/openai_chat/buffered.rs`:
- Around line 908-923: Update the structured reasoning regression assertions in
the relevant request translation test so the fallback-text fixture expects both
reasoning and reasoning_content to equal "fallback text"; remove the outdated
expectation that reasoning is absent, matching set_openai_reasoning_text
behavior.

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

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[ting-hong-shieh]

Checked this against the branch, and the named test does not fail — openai_chat_reasoning_details_round_trip_in_assistant_history passes as written, before and after this change.

The reason is what the decoder does with the fixture. That fixture's reasoning_details carry readable text, so prepend_openai_reasoning_blocks sets the block's text from reasoning_text_from_details, not from the "fallback text" string in the body. encode_openai_message_structured_reasoning only collects a fallback when the block text differs from the details text, so for that fixture the fallback is empty, set_openai_reasoning_text returns early, and neither key is emitted. The reasoning absence assertion still holds.

The case you are describing is real, though — it is the other test. openai_chat_encrypted_reasoning_details_retain_fallback uses encrypted details with no readable text, so the fallback is emitted, and that is the structured-path case where both keys now appear. Nothing was asserting reasoning_content there, so my new tests covered the plaintext path only.

Added that assertion in 737adc1. Thanks — the finding pointed at a genuine coverage gap even though the failure prediction did not hold.

cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace are clean at that commit.

[nachiketb-nvidia]

can you reply to coderabbit if you're done?
Also, I don't understand the bug here, can you dumb it down a bit for me?

[ting-hong-shieh]

Sure, and sorry — I opened with the field names, which is the middle of the story.

The one-line version: on the second turn of a conversation, we hand the model's own previous thinking back to it under a name it does not recognise, and it refuses the request.

Walking through it with a reasoning model behind an openai_chat backend:

Turn 1. The user asks something. The model answers and also returns its private reasoning. Switchyard passes both back to the client. This part works.

Turn 2. The client sends the conversation history back so the model can continue. Switchyard re-encodes that history, including the assistant's previous turn. Upstreams in thinking mode require the previous reasoning to come back with it — if it is missing, they refuse to continue, because they will not silently re-think a turn they have already paid for.

The refusal looks like this:

400  The `reasoning_content` in the thinking mode must be passed back to the API

The catch. Two spellings exist in the OpenAI-compatible world. Most providers read reasoning. DeepSeek-style thinking models read reasoning_content, and they check for that exact key — they do not fall back to the alias.

Before #415 we sent neither, so the reasoning was simply lost and every second turn failed. #415 fixed the loss and sends reasoning. For providers that read that key, it now works. For the ones in this issue, the reasoning is present on the wire but under the wrong name, so they still refuse.

So the assistant history message we send today looks like this:

{ "role": "assistant", "content": "...", "reasoning": "the model's earlier thinking" }

and this PR makes it look like this:

{ "role": "assistant", "content": "...",
  "reasoning": "the model's earlier thinking",
  "reasoning_content": "the model's earlier thinking" }

Same text under both names, so whichever key a provider reads, it sees the same reasoning.

Why both rather than switching. Swapping reasoning for reasoning_content would fix these upstreams and break the ones #415 was written for. Sending both costs one duplicated string in the request body and cannot regress a provider that reads either.

The reason I am confident about the name and not guessing: @DuanZGit ran it against a live DeepSeek endpoint in #449, changing only that field on the history message:

field sent	result
nothing	400
reasoning	400, same error
reasoning_content	200
reasoning_content: ""	200

Row two is this PR's reason for existing — it is the state of main today.

One honest caveat: I have not run that matrix myself, and I have no live endpoint. My own evidence is the code path and the tests. @DuanZGit offered in #449 to re-run it against this branch, which would be worth taking them up on before merge, since sending both keys is a combination their table did not cover.

[DuanZGit]

Ran the full matrix against our live DeepSeek endpoint on the exact shape this PR sends — the dual-key combination, which the earlier table didn't cover.

Result: this PR is what fixes it. The precise shape it serializes (row 3) is the first one that passes:

#	reasoning on assistant history message	result
1	neither field	400 ❌
2	reasoning only (what main sends today)	400 ❌
3	reasoning + reasoning_content (PR #460)	200 ✅
4	reasoning_content only	200 ✅
5	reasoning + reasoning_content:""	200 ✅
6	reasoning:"" + reasoning_content	200 ✅

In plain terms for the questions raised: the upstream refuses only when the reasoning_content key is absent — row 3 proves that carrying both spellings satisfies it, and row 2 is literally the current state of main (the reasoning is present but under the wrong name), which is why #449 stays open after #415. The dual-write can't regress the providers #415 fixed, because it keeps reasoning exactly where it was and merely adds the spelling DeepSeek reads.

Two notes on trustworthiness: the decode-side claim in the PR and the no reasoning → neither key case are already pinned by the tests, and our live runs above independently confirm the wire-level behavior this PR relies on (presence check on reasoning_content). We ran all six against deepseek-v4-flash; keys stayed server-side.

This was the re-run I offered in #449 — happy this landed before merge. From our side this is ready to go.