Missing array bounds checking in LLM provider response handling

[ryan-mt]

Summary

The LLM provider SDK code accesses array elements without checking if the array is non-empty, which can cause runtime crashes when the API returns unexpected responses.

Locations

• packages/opencode/src/provider/sdk/copilot/chat/openai-compatible-chat-language-model.ts:216
• packages/opencode/src/provider/sdk/copilot/chat/openai-compatible-chat-language-model.ts:435
• packages/opencode/src/provider/sdk/copilot/responses/openai-responses-language-model.ts:435

Issue

// Line 216 - doGenerate()
const choice = responseBody.choices[0]  // No check if choices array is empty
const content: Array<LanguageModelV2Content> = []
const text = choice.message.content  // Will throw if choice is undefined

// Line 435 - doStream()
const choice = value.choices[0]  // Same issue in streaming
if (choice?.finish_reason != null) {
  // ...
}

If the API returns an empty choices array (which can happen on errors or edge cases), accessing choices[0] returns undefined, and subsequent property access throws "Cannot read property 'message' of undefined".

Impact

• Severity: Medium
• Type: Runtime Error
• Effect: Unhandled exception when API returns empty choices

Suggested Fix

Add bounds checking:

const choice = responseBody.choices[0]
if (!choice) {
  throw new InvalidResponseDataError({
    data: responseBody,
    message: "Response contained no choices"
  })
}

Or use optional chaining with fallback:

const choice = responseBody.choices?.[0]
if (!choice?.message) {
  // Handle empty response case
}

[mugnimaestra]

Tangentially related: PR #12567 includes a fix in the same file (openai-compatible-chat-language-model.ts) for a different error path — the Copilot API can send multiple reasoning_opaque deltas in a single response (e.g., one with reasoning text, another with tool calls), and the previous code threw InvalidResponseDataError on the second one. The fix now accepts the latest value instead. Not a bounds-check fix, but addresses another crash scenario in this provider.

[github-actions]

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