Hypatia May
4.0 KiB
Agent Note: One selection rule keeps subagent output past an empty terminal message
Status: implemented
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Problem
The agent loop appends an empty-content assistant/message when a max-tokens step assembled only tool-call blocks because BlockAssembler.blocks() drops truncated tool calls; the message records usage only. Three consumers selected the child's output independently and treated that usage record as output. The in-process driver's readResult and the continuable Activation's subagent/end capture selected the last assistant/message without filtering, while the SDK backend's observer let any assistant/message take precedence over accumulated text. In a multi-step turn cut off at max-tokens, the final empty message caused the real partial answer to be omitted from SubagentResult.output, the tool result, telemetry, and subagent/end.lastAssistantMessage. The in-process driver also lacked a streamed-text fallback, so a cancelled child whose only text existed in assistant/chunk events reported [].
Decision
dsh-subagent owns one canonical selection rule in src/assistant-output.ts: select the last non-empty assistant message; without one, select the accumulated text-delta stream; ignore empty-content messages. The incremental AssistantOutputFold implements the rule through push(event) for session-event transports, pushText(text) for chunk-only transports, and collect() for selection. finalAssistantOutput(events) applies it to a complete event suffix for the in-process readResult and Activation capture. The SDK backend folds notification events; the ACP backend exposes no complete assistant messages and folds raw chunk text. SubagentResult.output defines the result contract, and subagent/end.lastAssistantMessage uses the same rule. When a child produces neither form of output, the lifecycle field is absent rather than an empty array for both one-shot and continuable runs. A max-tokens or aborted result retains its actual stop reason.
The foreground delegation tool uses the same selection. A non-completed result remains an isError tool result, but its message appends the child's partial text after the stop-reason headline so the parent model receives both the failure and available output.
Verification
The keyless SDK backend test uses FAKE_EMPTY_MESSAGE to emit a usage-only terminal message. The subagent-max-tokens-partial ACP snapshot records a child that streams text and a tool call, ends at a tool-only max-tokens step with an empty usage message in its durable log, and returns the partial text through the parent's errored tool result. Unit coverage checks empty terminal messages, cancellation, message ordering, textless non-empty messages, and exclusion of tool-result content.
Alternatives considered
Fix each consumer in place without a shared helper. Rejected: three independent selections had diverged, while observers of one run must agree on its output.
Stop the loop from appending the empty message. Rejected: the message records usage and preserves the step in the durable log ("model-visible ⟺ logged"); changing session events to address output selection would affect every replay and projection consumer.
Treat empty-content messages as an error. Rejected: the streamed text is the child's real partial answer, and the stop reason already tells the consumer the turn was cut short.
Consequences
Multi-step children cut off at max-tokens report their earlier text; cancelled in-process children retain text streamed before the abort; one-shot and continuable subagent/end events agree with SubagentResult.output. A message whose content is non-empty but textless, such as reasoning-only content, is selected instead of streamed text because the rule tests content length rather than text presence. A non-empty message is also selected instead of text streamed after it: a child cancelled while streaming a later step reports its earlier complete message, while the stop reason records the truncation.