Session silently stops on empty LLM response (finish: unknown, 0 tokens)

[3351163616]

Description

When the model provider returns an empty completion (0 tokens, no text/reasoning/tool output, finish reason maps to unknown), opencode treats it as a normal completed turn:

• the session loop exits silently (packages/opencode/src/session/prompt.ts breaks on any finish that isn't tool-calls)
• no error is surfaced to the user, no retry happens
• the conversation appears to "suddenly stop" mid-task

Observed with the opencode-go gateway (deepseek-v4-flash) returning an empty response: the session went idle with the last assistant message recorded as finish: unknown with 0 tokens.

Repro

• Provider returns an SSE stream with no content/tool deltas and an unknown finish reason.
• After processing, the loop exits and the session goes idle with no error.

Expected

An empty response should be retried (or surfaced as an error), not silently treated as a completed turn.

Related: #40531 (upstream PR with the same fix, currently conflicting with dev).

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Aborted provider stream recorded as clean stop (finish=unknown, zero usage, no text) — subagent returns empty with no error #37852: Aborted provider stream recorded as clean stop (finish=unknown, zero usage, no text) — describes the same silent exit when a provider stream ends with zero content and finish=unknown
• Empty stop completions are recorded as successful responses #37735: Empty stop completions are recorded as successful responses — related: empty assistant turns being treated as successful completions

Please review those issues and consider whether your report adds new information or context. If it does, a cross-reference comment on those issues may be more useful than a new report.

[liudongyan13701205717-source]

Confirmed a closely related variant on the Windows Desktop app (latest, v1.18.16): the conversation stops mid-thinking as if it had completed normally — the thinking output is present, but there is never a final text reply, and the session just goes idle. No error toast, no retry.

Difference from the pure empty-response case: here the reasoning content was streamed, only the final completion text is missing. From the user's perspective it looks like "the answer ended inside the thinking block" — indistinguishable from a successful completion except that the reply never arrives.

This is distinct from the stuck thinking case (#41365, freeze with no output, requires manual abort): here the turn terminates cleanly, it just terminates early.

For the fix in #41466, please consider covering responses where reasoning/tool deltas were received but the final text completion is empty (finish: unknown, 0 text tokens) — not only fully empty streams.