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Michael Suchacz
2ad5af5b54
Closes https://linear.app/codercom/issue/CODAGT-268 ## Problem The chat UI collapses large pastes (>=10 lines or >=1000 chars) into a synthetic `pasted-text-*.txt` attachment. A chat created with only such an attachment had no title input anywhere: the create path derived `titleSource` only from text and file-reference parts (so the chat was named "New Chat"), async auto-titling extracted text the same way and silently skipped generation, and the manual propose/regenerate paths returned an empty title for the same reason. The regular prompt path already inlines these files for the model; only the title paths were blind. ## Fix Add a single title-input derivation in `chatprompt` and use it everywhere: - `chatprompt.TitleText` joins text and file-reference parts (unchanged formatting), and falls back to synthetic pasted-text attachment content (truncated to a 16 KiB title budget) when they yield nothing. - `chatprompt.SyntheticPasteFileIDs` identifies paste attachments; `chatprompt.FallbackTitle` consolidates the previously duplicated `chatTitleFromMessage` / `fallbackChatTitle`. - Chat creation captures paste blob references while validating file parts (the file row was already loaded there) and derives `titleSource` via `TitleText`. Only the create path derives titles; message send and edit reuse the same validation without copying any blob data. - `GenerateChatTitleAsync` and the manual propose/regenerate paths resolve paste content via `titlePasteText`, which only queries when a visible user message has no other title text, so chats with typed text never incur a file fetch. - Title-path paste fetches are bounded: a new `GetChatFileDataPrefixesByIDs` query returns only a `substr` prefix (`chatprompt.TitlePasteBytePrefix`, 64 KiB = 4 bytes x the 16 Ki-rune title budget) so full blobs (up to 10 MiB each) never leave the database for titling, and `chatprompt.TitlePasteText` applies the same bound to the create path which already holds the loaded row. Deliberate side effect: because generation-time extraction now matches create-time `titleSource` exactly, file-reference-only chats also become eligible for AI titles. They were previously skipped by the same derivation mismatch. Non-goals: no frontend changes (attachment chip UX stays as is), and non-synthetic user-uploaded `.txt` files still yield "New Chat". ## Testing - Unit tests for `TitleText`, `TitlePasteText`, `SyntheticPasteFileIDs`, `FallbackTitle`, `titleInput`, `titlePasteText`, and paste-aware `extractManualTitleTurns`. - Real-database test for `GetChatFileDataPrefixesByIDs` (prefix shorter and longer than stored data) plus dbauthz coverage for the new query. - Integration tests: paste-only create gets a fallback title from the paste content, async title generation fires with the paste content as input, and `RegenerateChatTitle` works on a paste-only chat. > This PR was written by [Mux](https://mux.coder.com) on Mike's behalf.