A turn ends when the model stops calling tools, and the record holds the whole request

[deepfates]

A turn ends when the model stops calling tools

Imp.Predict.ReActV2 inherited DSPy's shape: a step with no tool calls bought one more request with tool_choice naming submit. Measured on a live Dwell resident (openrouter:thinkingmachines/inkling), that request costs a full extra call per turn and, when the model had already acted with a tool, comes back with an answer summarizing what it did rather than what it said. Anthropic's tool runner, the OpenAI Agents SDK, LangGraph's ReAct and Pydantic AI all end the turn when the model returns text with no tool call.

A prose step now finishes the run with that prose as the output and termination_reason: :answered, in that one request, when the task signature declares exactly one output of type :string. The prose is validated through the same parse a submit's arguments go through. A signature with several outputs, one non-text output, or a step that says nothing at all still takes the forced submit. prose: :forced_submit is the opt-out for a single-output signature.

New option finish_on maps a tool name to fn arguments, result, inputs -> {:finish, outputs} | :continue end — the shape Pydantic AI calls an output tool. {:finish, outputs} validates against the signature exactly as a submit would, and the run finishes with termination_reason: :finished_by_tool and finished_by_tool naming the tool. When one step calls several terminal tools the first in call order finishes the run; the rest still execute and are recorded, and a submit in the same step still wins. Outputs that fail validation become that call's recorded result, the same error a bad submit records, and the loop continues. The functions persist by registry name, like a tool runner and a tool policy.

The record holds the whole request

Imp.LM.request/2 emitted model_request with only the messages, so a recorded request could not be reproduced. Its metadata now carries :options (the request options with the tools removed) and :tools_hash (SHA-256 of the canonical JSON of the definitions, or nil when none). The definitions are emitted once per run per distinct hash, as the new :tools_offered event. Imp.Run.first_seen?/1 is the per-run state behind "once per run"; the Control process holds the set. Imp.Core.request_parts/1 already returns config.options, the full option keyword list including :tools and :tool_choice, so nothing had to move.

A host can say something about a stored turn

Imp.Adapter.Chat.format/3 gains :history_note_renderer, fn signature, turn -> nil | String.t(), consulted for every stored history turn after that turn's own messages; its text becomes one user message right behind them. A turn carrying tool calls routes through render_native_tool_history_turn/3, which consults neither :output_renderer nor :input_section_renderer, so until now a host had no way to tell the model something that became true after such a turn ended.

Gates

mix format, mix compile --warnings-as-errors, mix check (2857 tests, 0 failures; baseline 2844), mix dialyzer.check (one ignore line renumbered, no new findings).