Brief P3: request prompt-processing progress so prefill counts as liveness

Approved 2026-09-17. Evidence is in docs/decisions.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/decisions.md

@@ -27,6 +27,7 @@ Approved by the owner on 2026-09-17. One commit each.
 |---|---|---|
 | P1 | Inference contract 6: the `tools` array is fixed per epoch. `find_tool` returns schemas as a tool result and the model calls them through a `call_tool(name, arguments)` meta-tool. Never instruct the model to call an undeclared tool. | M0 (h): adding a tool re-read the whole prompt; undeclared calls were coerced into `write_file`. |
 | P2 | Inference contract 2: the session log stores assistant messages exactly as returned, including `reasoning_content`, and replays them unchanged. Remove the "known risk" about dropped thinking blocks. | M0 (e): Ornith's template keeps every think block. |
+| P3 | Inference contract 7: requests set `return_progress: true`; progress events count as liveness. | M0 (i): otherwise the stream is silent during prefill. |
 
 ## Proposed changes to the design brief
 
@@ -34,7 +35,6 @@ From M0 and the kickoff review. None is applied yet. Each lands as its own commi
 
 | # | Change | Evidence |
 |---|---|---|
-| P3 | Inference contract 7: requests set `return_progress: true`; progress events count as liveness. | M0 (i): otherwise the stream is silent during prefill. |
 | P4 | Inference contract 8: the thinking cap uses `reasoning_control` and the control endpoint. | README b10809. Not yet exercised. |
 | P5 | Settle the open question: chat-completions with server-side tool parsing. | M0 (b), (c). |
 | P6 | Code constraints: `Decision` lives in `brokerd`, has a private field and does not implement `Deserialize`. `proto` carries a plain `DecisionRecord` for the audit log. | Rust privacy is per crate, and a deserializable type can be built by anyone. |

docs/design.md

@@ -100,8 +100,9 @@ ordinary outbound networking can reach the whole tailnet and the internet as the
    `brokerd` applies grants to the target tool, not to `call_tool`. Never prompt the model to call
    a tool that is not in the `tools` array: the server's grammar forces such a call into a
    declared tool.
-7. **Liveness, not deadlines.** Streaming always. The timeout is "no bytes for N seconds", never a
-   total-request deadline.
+7. **Liveness, not deadlines.** Streaming always, with `return_progress: true` so that prompt
+   processing produces events. The timeout is "no bytes for N seconds", never a total-request
+   deadline. Progress events count as bytes.
 8. **Runaway control.** Per-turn thinking-token cap, per-turn tool-iteration cap, detection of
    repeated identical tool calls.
 9. **Startup self-test.** On boot `loopd` checks: tool-call round trip parses, turn-2 prompt