02 — Thought Tokens End-to-End (vendor crate work)

Goal: reasoning/thinking content becomes a first-class, streamed, persisted, replayed, and priced citizen of the crate — so OpenHuman deletes ThinkingForwarder and stops re-projecting thinking via its own ProviderDelta::ThinkingDelta bridge (session/tool_progress.rs:226).

All steps are vendor-crate changes (vendor/tinyagents), committed on a submodule feature branch, gitlink-bumped into OpenHuman, PR'd upstream. Precedent: NoProgressTracker (#7), MicrocompactMiddleware (ac73382).

Step 1 — Message representation

Add ContentBlock::Thinking { text: String, signature: Option<String> } and ContentBlock::RedactedThinking { data: String } to harness/message/types.rs:21-30.
Serde: additively tagged; existing transcripts (no thinking blocks) parse unchanged. Round-trip tests in message/test.rs.
AssistantMessage rendering helpers must skip thinking blocks for plain-text extraction (Message::text()-style accessors) but preserve them for provider replay.

Step 2 — Accumulator persistence

StreamAccumulator::finish (harness/model/mod.rs:683-710): emit the accumulated self.reasoning as a leading ContentBlock::Thinking on the final message instead of dropping it. Carry signature fragments (Anthropic signature_delta) through a new accumulator field.
Keep the existing reasoning() side-channel accessor for backward compat.

Step 3 — Delta plumbing

Populate MessageDelta.reasoning from providers:
- OpenAI path: parse delta.reasoning_content (DeepSeek/compat) and o-series reasoning summaries instead of hardcoding String::new() (providers/openai/mod.rs:772); add the wire fields to openai/types.rs.
- Anthropic path (05): thinking_delta / signature_delta events.
Thread reasoning into the middleware-facing ModelDelta (agent_loop/mod.rs:980-984) — closes sdk-gaps §3. Add ToolDelta.tool_name on the first fragment (tool-name-on-start), the other half of that gap.
Emit reasoning in AgentEvent::ModelDelta with parent/root run attribution (pairs with doc 03's sub-agent propagation so SubagentThinkingDelta can become a projection).

Step 4 — Replay correctness (Anthropic contract)

Anthropic requires thinking blocks (with signatures) to be replayed verbatim in the assistant turn preceding tool results. With Step 1 the blocks live in the transcript; the provider translation (05) must:

Serialize Thinking/RedactedThinking blocks back onto the wire for assistant messages in multi-turn tool-use conversations.
Never send thinking blocks to providers that reject them (OpenAI compat path strips them) — provider capability flag supports_thinking on ModelProfile.
Property test: build a 3-turn tool-use conversation with thinking, assert byte-stable signature replay.

Step 5 — Usage & cost

Add completion_tokens_details.reasoning_tokens to the OpenAI wire struct (openai/types.rs:235-256) and map it in convert_usage (openai/mod.rs:708-719).
Anthropic (05): thinking output tokens are billed as output; keep reasoning_tokens as the reported subset where the API exposes it.
CostTotals.reasoning_cost (cost/mod.rs:69) then prices real numbers — feeds OpenHuman's C3 budget-flip criteria (pricing table wired).

Step 6 — OpenHuman follow-through (after gitlink bump)

Delete ThinkingForwarder (old-plan C7 item; sdk-gaps §3 closes).
AgentProgress::ThinkingDelta/SubagentThinkingDelta become projections of crate AgentEvent::ModelDelta.reasoning — shrinks session/tool_progress.rs toward deletion (with doc 03).
Persisted-transcript compat: OpenHuman's multimodal.rs reasoning-block handling shrinks once the crate owns thinking blocks in messages.

Tests (crate)

Provider-level: SSE fixture streams with reasoning deltas → accumulator → message contains Thinking block; usage carries reasoning tokens.
Loop-level: middleware on_model_delta sees reasoning; AgentRun.messages persists it; replay serialization per provider capability.
Extend tests/e2e_reasoning_and_selection.rs beyond hand-fed deltas.

Effort: L (1–2 weeks) crate-side; OpenHuman follow-through S.

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