Give a routed turn back the reasoning its target produced

[edwin-zvs]

The bug

A session pinned to DeepSeek from the native picker fails mid-task with:

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

Root cause

Measured against the live API, not read from the docs:

assistant turn replayed	result
tool calls + reasoning_content	200
tool calls + empty reasoning_content	200
tool calls, no reasoning_content, tool-call id DeepSeek issued and still resolves	200
tool calls, no reasoning_content	400
plain text, no reasoning_content	200

DeepSeek v4 is thinking-by-default, and it refuses a replayed tool-calling turn whose reasoning it cannot see. Every such turn needs the field — reasoning on the last turn only still 400s.

The router could not supply it. In the chat-completions dialect, reasoning was dropped in both directions: reasoning_content was never decoded from a target's stream, and canonical thinking blocks were discarded when emitting a request. The harness cannot cover for it either — Claude Code speaks Anthropic, which has no field for another vendor's reasoning, so it is gone by the time the conversation is replayed.

That leaves DeepSeek's own recovery from a recently issued tool-call id, which holds early in a conversation and lapses later — exactly the "worked for a few turns, then failed permanently" shape reported.

The fix

The router is the only participant that sees both the target's response and the request that replays it, so it keeps the reasoning:

• Capture — reasoning a target streams is recorded against that turn's tool-call ids, bounded per session.
• Restore — a replayed tool-calling turn gets its own reasoning back in the field the target's dialect defines.
• Harness first — a turn whose reasoning the harness carried itself is left alone.
• Never invented — a turn no longer remembered is sent with an empty reasoning, which the target accepts. Nothing is fabricated, and reasoning is never re-encoded into what the harness reads.

Marked per target and measured rather than assumed (spec 0160's rule); an arm that needs the echo always rebuilds the request body, since byte-forwarding cannot add a field. Recorded as specs/0181-routed-turns-keep-their-reasoning.md.

Verification

The exact body the patched router emits for a Claude-Code-shaped conversation was posted to the live DeepSeek API:

router body WITH restored reasoning (fixed)    200  content='Both words are: alpha beta'
same body WITHOUT it (today's behavior)        400  The `reasoning_content` in the thinking mode must be passed back to the API.

The restored turn carries the reasoning DeepSeek actually produced; a turn outside the memo carries "".

Full workspace suite green (one unrelated CLI timing test flaked under concurrent build load and passes on its own).

Binaries

Only crates/daemon changes → the relevant binary is construct (the daemon lives in it):
/Users/moon/agentd/.claude/worktrees/router-echo-reasoning/target/debug/construct

No user-visible surface changed, so no recording.