fix(opencode): fail runs when the model stream ends without a finish frame

[zhang17-24]

Summary

An upstream SSE connection cut mid-turn (EOF with no finish frame, no error event) made the AI SDK emit a synthesized finish-step with finishReason "other"/undefined. opencode mapped that to the benign "unknown" finish, persisted the truncated output as a completed turn, and opencode run exited 0 — silently accepting partial generations as success (issue #39968).

Changes

• packages/opencode/src/session/llm/ai-sdk.ts: "other"/undefined finish reasons now map to "error" instead of "unknown", matching the native runtime's semantics.
• packages/opencode/src/session/processor.ts: a step-finish with reason "error" fails the turn, publishing a session error so opencode run exits non-zero.
• packages/opencode/test/cli/run/run-process.test.ts: updated the two tests that pinned the exit-0 behavior — an interrupted stream now preserves partial output but exits non-zero.
• packages/opencode/test/session/llm.test.ts: updated the adapter expectations.

Test plan

• bun test test/cli/run/run-process.test.ts — 13 pass
• bun test test/session/llm.test.ts — 28 pass
• bun run typecheck in packages/opencode — pass
• biome check — clean

Human note

I am a 19-year-old independent full-stack developer. I reviewed the full diff and confirmed the change: a model stream that ends without a finish frame now fails the run instead of exiting 0 with truncated output.

Fixes #39968

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The following comment was made by an LLM, it may be inaccurate:

Potential related PRs found:

1. fix(opencode): make long-lived provider streams robust to silent SSE terminations #39970 - fix(opencode): make long-lived provider streams robust to silent SSE terminations

   • Directly related to the same issue domain (silent SSE terminations causing stream failures)

2. fix: retry truncated provider streams #39473 - fix: retry truncated provider streams

   • Addresses truncated provider streams, which is the root cause scenario in PR fix(opencode): fail runs when the model stream ends without a finish frame #40032

3. fix(session): retry empty stream truncations and discard partial parts #26167 - fix(session): retry empty stream truncations and discard partial parts

   • Historical PR addressing similar stream truncation handling

These PRs appear to be related to the same problem space of handling incomplete/truncated provider streams. PR #39970 in particular seems highly relevant as it also deals with "silent SSE terminations" which is the exact scenario described in #40032 (EOF with no finish frame).

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