feat: Jan manages Context Allocation dynamically

[louis-jan]

Describe Your Changes

When a response is cut off due to hitting the context size limit (finish_reason: "length") OR request payload exceeds the context size window (context shift disabled),
Jan now automatically increases the context size by 50% (minimum 32768), reloads the model,
and seamlessly continues generation — prepending the partial response as a prefill so the
model resumes exactly where it stopped rather than regenerating from scratch.

• Transport exposes setContinueFromContent to inject a partial assistant prefill before the
  next request
• Stream wrapper injects the partial text as the first delta so the UI shows it immediately
• Partial message is retained as a placeholder while the model reloads
• Message actions (copy, edit, delete, regenerate) are hidden during continuation

demo.mov

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed

[Copilot]

Pull request overview

Adds automatic context window growth + seamless continuation when a generation is truncated (finish_reason "length") or fails due to context-size limits, so users see an uninterrupted assistant message while the model reloads and resumes.

Changes:

• Detects truncated responses and triggers an automatic context-size increase + regeneration, carrying the partial assistant output forward as a “prefill”.
• Extends the custom transport/hook API with setContinueFromContent and injects the partial content into the next UI stream as the first text delta.
• Updates message rendering to support a continuation placeholder state (hide actions, adjust animation, show “Growing the Mind...” shimmer).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
web-app/src/routes/threads/$threadId.tsx	Continuation orchestration: detect truncation/context errors, increase ctx_len, show placeholder + loading UI, hide actions during continuation
web-app/src/lib/custom-chat-transport.ts	Implements continuation “prefill” and stream wrapper that prepends partial text as the first text-delta
web-app/src/hooks/use-chat.ts	Exposes setContinueFromContent to callers via the useChat wrapper
web-app/src/containers/RenderMarkdown.tsx	Adds isAnimating control for streaming markdown animation
web-app/src/containers/MessageItem.tsx	Plumbs isAnimating/hideActions to message rendering and action visibility

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[github-actions]

Barecheck - Code coverage report

Total: 22.19%

Your code coverage diff: -0.07% ▾

Uncovered files and lines

File	Lines
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web-app/src/containers/RenderMarkdown.tsx	37-38, 75-77, 133-135, 137
web-app/src/hooks/use-chat.ts	1, 5, 15-17, 30-43, 46-47, 49-51, 54-62, 64-68, 71, 74-78, 81-82, 84-89, 91, 93-97, 99-105, 107-111, 114-118, 121-122, 125-127, 129-131, 134-149, 151-156
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