fix(agent): retry empty final turns instead of silently abandoning the task

[snimu]

Providers occasionally end a turn with a normal stop reason and zero usable content: no text, no tool calls, sometimes a thinking-only message (observed and raw-SSE-confirmed for moonshotai/kimi-k3 via Prime Inference: single-chunk stream, no content, usage.output=0; ~1/10 raw probes). The agent loop treated any no-tool-call turn as completion, so the agent silently abandoned its task: print mode printed nothing and exited 0; interactive mode thought for minutes and then did nothing.

The fix

• streamAssistantResponse now retries an empty final turn silently, up to 3 attempts. The retry happens before message_end is emitted, which is the durability edge: discarded attempts are popped from the request context and never reach session persistence or transcript consumers; their paid usage is carried on the surviving message as discardedUsage, so cost surfaces and goal token budgets still account for it.
• Empty means: no tool calls and no non-whitespace text (thinking does not count). error, aborted, and length stops are excluded - they are signals of their own, and an identical resend cannot help.
• Context-overflow shapes pass through untouched (!isContextOverflow(message, contextWindow) guards the retry, reusing the existing detector): a silent-overflow turn is empty by definition, and retrying it would burn full-context requests and then mask the stop reason that auto-compaction recovery keys on.
• After 3 consecutive empty turns, the final message becomes a standard error turn (Model returned an empty response ... 3 times in a row) and flows the existing error path: print mode exits non-zero with the message on stderr; session-level retry/backoff applies as for any other turn error.
• RLM children: a child whose turn ends in an error now delivers RLM child <name> failed: <error> to its parent instead of the misleading "completed without sending a reply" notice. (Found while wiring exhaustion propagation: graceful error turns resolve promptAndWait, so the parent never saw the error text.)

Tests

Eight new tests in packages/agent (silent retry with clean transcript and exactly one message_end; whitespace-only retry; exhaustion after 3 attempts; no-retry guards for real content, tool calls, aborts, empty length stops, and silent-overflow turns - the last two pin the compaction-recovery contract) plus one in agent-session-recursion (parent receives the child failure message, no terminal notice), an interactive-mode pin (a superseding assistant stream replaces a discarded attempt's bubble), and discarded-spend accounting asserts in the loop pin and the scan/resident usage-parity test. All retry-path pins were proven red against the pre-fix code.

Linear: ENG-5795

---

Note

Medium Risk
Changes core turn completion, retry, and token accounting across agent, session, and RLM paths, though behavior is bounded (3 retries) and covered by new tests.

Overview
Fixes a case where a normal-stop assistant message with no visible text and no tool calls (e.g. thinking-only) was treated as success and the agent stopped with no output.

Agent loop: streamAssistantResponse now silently retries up to 3 times before message_end. Discarded attempts are popped from provider context and never become durable transcript turns; after three empties the turn becomes a standard error message. Context-overflow empty shapes are not retried so compaction recovery still sees them. Paid tokens from dropped attempts are stored on the surviving message as optional discardedUsage (context sizing still uses usage only). A later stream failure during retries wraps prior discard spend in EmptyTurnRetryFailure; Agent.handleRunFailure unwraps it and attaches discardedUsage to the synthetic failure message.

Product layers: Coding-agent goal, autonomous, RLM child usage, telemetry, session stats/scan, and interactive streaming now count discardedUsage and fix UX (RLM child failure notice instead of “completed without reply”; remove orphaned empty streaming bubbles when a superseding stream starts).

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Note

Retry empty assistant turns up to 3 attempts instead of silently abandoning the task

• Adds a retry loop in streamAssistantResponse (agent-loop.ts) that detects empty assistant turns (no text, no tool calls, not error/aborted/length-stop) and retries up to three times. Discarded attempts are removed from context and transcripts, their usage is preserved, and a third consecutive empty response becomes an assistant error.
• Adds discardedUsage to AssistantMessage (types.ts) so paid-but-discarded attempts carry their spend onto the surviving or failure message.
• Updates usage accounting across the coding agent to include discarded spend: session statistics, goal token budgets, autonomous mode counters, telemetry, context-tree totals, and session-file scanning.
• Changes RLM child completion so a child ending with an assistant error reports rlm_child_failure to the parent instead of a completed-without-reply notice.
• Cleans up interactive streaming so a superseding message_start removes orphaned thinking-only components but preserves visible partial output.
• Risk: isEmptyAssistantTurn classifies turns with only thinking or whitespace as empty; turns that end with length stop reason or context-overflow messages bypass the retry path. Reviewers should verify that legitimate thinking-only responses with meaningful reasoning are not discarded.

^{Macroscope summarized d852d24.}

Linear: RES-1279 (https://linear.app/prime-intellect/issue/RES-1279)

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