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.
Fixes https://github.com/coder/coder/issues/23910
Adds periodic cleanup of chats and chat files to the dbpurge background
goroutine, with a configurable retention period exposed in the Agent
settings UI.
> 🤖 Written by a Coder Agent. Reviewed by a human.
Needed by #23833
Adds a `chat_file_links` association table to track which files are
associated with each chat.
- `AppendChatFileIDs` query links a file to a chat with deduplication
- `GetChatFileMetadataByIDs` query returns lightweight file metadata by
IDs
- Tool-created files (e.g. `propose_plan`) are linked to the chat after
insert
- User-uploaded files are linked to the chat when the referencing
message is sent
- Single-chat GET endpoint hydrates `files: ChatFileMetadata[]` on the
response
> 🤖 Created by Coder Agents and massaged into shape by a human.
This change adds support for image attachments to chat via add button
and clipboard paste. Files are stored in a new `chat_files` table and
referenced by ID in message content. File data is resolved from storage
at LLM dispatch time, keeping the message content column small.
Upload validates MIME types via content type or content sniffing against
an allowlist (png, jpeg, gif, webp). The retrieval endpoint serves files
with immutable caching headers. On the frontend, uploads start eagerly
on attach with a background fetch to pre-warm the browser HTTP cache so
the timeline renders instantly after send.
Michael Suchacz