fix: handle missing opening <think> tag for deepseek-r1-671b

[MaxwellCalkin]

Summary

Fixes #810.

deepseek-r1-671b updated its chat template and now only emits a closing </think> tag at the end of its thinking output, without an opening <think> tag. The existing thinking-tag logic in useChat.tsx handled the reverse case (opening <think> without closing </think>, which occurs during streaming) but did not handle this case. As a result, the model's thinking content leaked into the main answer instead of being rendered in the ThinkBox component.

Changes

In src/lib/hooks/useChat.tsx, the thinking tag detection in the sections useMemo now:

1. Always counts both opening and closing think tags (previously only counted them inside an if (processedText.includes('<think>')) guard)
2. Prepends <think> when </think> is found without a matching opening tag, so the markdown-to-jsx parser can route the content to the ThinkBox component

This ensures models that omit the opening <think> tag (like deepseek-r1-671b) still have their thinking content rendered correctly in the collapsible ThinkBox, while the existing streaming behavior (closing tag added when opening tag is present but closing is missing) is preserved.

Test plan

• Test with deepseek-r1-671b via custom OpenAI — thinking content should appear in the ThinkBox, not in the main answer
• Test with deepseek-r1-0528 (which uses both <think> and </think>) — should continue working as before
• Test with non-thinking models — no change in behavior (no think tags present)
• Test streaming behavior — incomplete <think> without </think> should still get auto-closed

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Summary by cubic

Fixes #810. Ensures models that only emit a closing (like deepseek-r1-671b) render their thinking in the ThinkBox instead of the main answer.

• Bug Fixes
  • In src/lib/hooks/useChat.tsx, always count both <think> and </think> tags during parsing.
  • When </think> appears without a matching <think>, prepend <think> so the markdown parser routes content to the ThinkBox.
  • Preserve existing streaming behavior that auto-closes an open <think> without a closing tag.

^{Written for commit b688355. Summary will update on new commits.}

[cubic-dev-ai]

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