fix(opencode): surface empty model responses

[CyberPunk-2022]

Issue for this PR

Closes#37735

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When a provider finishes with stop and usage but emits neither text nor a tool call, OpenCode currently records the assistant turn as successful. This change stores an EmptyResponseError and emits the session error event instead, while retaining the finish reason and usage for diagnosis.

It is deliberately a harness-level safeguard: it does not claim to identify why a provider produced an empty completion. The related #28507 also contains the normal-stop/non-zero-usage variant, alongside its separate empty-loop report.

How did you verify your code works?

• bun test test/session/prompt.test.ts --timeout 30000 in packages/opencode — 11 passed, 1 existing skip
• bun typecheck in packages/opencode — passed
• Regeneration: bun run script/build.ts in packages/sdk/js

The regression simulates a completed stop stream with usage but no text or tool call, and verifies the persisted assistant error is EmptyResponseError.

Screenshots / recordings

N/A (runtime-only change).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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

Based on the search results, I found several related PRs that are worth noting, though they address different aspects of the response handling:

Potentially Related PRs:

1. PR fix(opencode): stop repeated empty tool loops #37110 - fix(opencode): stop repeated empty tool loops

   • Related to empty response handling but focuses on tool loops rather than overall empty responses

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

   • Addresses empty stream scenarios but in the context of retries and truncations

3. PR fix(core): surface refusal category and explanation on content filter #37392 - fix(core): surface refusal category and explanation on content filter

   • Similar pattern of surfacing specific error conditions (content filter refusals) alongside provider responses

4. PR fix(llm): finalize pending response tool calls #37160 - fix(llm): finalize pending response tool calls

   • Related to provider response handling and finalizing incomplete tool calls

These PRs address overlapping concerns around provider response handling and error surfacing, but none appear to be direct duplicates of PR #37732. PR #37732 is specifically about surfacing empty responses as EmptyResponseError while retaining provider metadata, which is a distinct fix from the related PRs.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

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