Agents can already see workspace files and take screenshots, but users could not download those artifacts from chat. This PR adds durable chat attachments to chatd. `attach_file`, explicit `computer` screenshot actions (not the automatic post-action screenshots), and `propose_plan` now fetch bytes over the agent connection, store them in `chat_files`, link them to the chat, and carry attachment metadata in tool responses so `buildAssistantPartsForPersist` can materialize ordinary `type:"file"` assistant parts that the chat file APIs serve.
The same storage helpers are reused for other artifact-producing paths. `wait_agent` recordings and thumbnails are stored as chat files and linked back to the parent chat, with best-effort relinking so parent chats retain those artifacts without leaving orphaned rows when chat-file caps reject links. `storeChatAttachment` wraps insert + link in one transaction, files are capped at 10 MB each and 20 per chat, and serving defaults to `Content-Disposition: attachment` with an explicit inline-safe allowlist.
This PR also consolidates chat-file media policy in `coderd/chatfiles`. Uploads and tool-generated attachments share byte-based MIME detection, SVG blocking, inline-safety rules, and compatible `text/plain` refinement for JSON, CSV, and Markdown. Prompt construction still only inlines synthetic pasted text for model consumption; assistant-created attachments are persisted for the user and intentionally not replayed into later LLM turns.
UI follow-up lives in #24281.
Relates to CODAGT-91
> This PR was authored by Mux on behalf of Mike.
## Summary
- add persistent plan mode for chats and the chat-specific plan file
flow
- add structured planning tools such as `ask_user_question` and
`propose_plan`
- keep `write_file` and `edit_files` constrained to the chat-specific
plan file during plan turns
- allow shell exploration in plan mode, including subagents, via
`execute` and `process_output`
- block implementation-oriented, provider-native, MCP, dynamic, and
computer-use tools during plan turns
- update the chat UI, tests, and docs for the new planning flow
> This PR was authored by Mux on behalf of Mike.
Chats sharing one workspace (e.g. sibling subagents) all wrote to
`/home/coder/PLAN.md`, causing plan file collisions. This change derives
a unique plan path per chat from the workspace home directory and chat
ID.
## Changes
* `write_file`, `edit_files`, and `propose_plan` reject any `plan.md`
variant (case-insensitive) at the workspace home root, with a clear
error pointing to the chat-specific path.
* Root chats receive a `<plan-file-path>` block inlined in the main
system prompt with the concrete path.
* Prompt and tool descriptions no longer hardcode `/home/coder/PLAN.md`.
* Plan path handling is POSIX-only (forward-slash), relying on the
contract that workspace agent paths are normalized before reaching
chatd.
* Updated `ProposePlanTool.stories.tsx` to use per-chat path examples.
* Full test coverage for plan path detection, legacy-path rejection in
all three tools, inline prompt rendering, and fallback behavior.
Adds a `propose_plan` tool that presents a workspace markdown file as a
dedicated plan card in the agent UI.
The workflow is: the agent uses `write_file`/`edit_files` to build a
plan file (e.g. `/home/coder/PLAN.md`), then calls `propose_plan(path)`
to present it. The backend reads the file via `ReadFile` and the
frontend renders it as an expanded markdown preview card.
**Backend** (`coderd/x/chatd/chattool/proposeplan.go`): new tool
registered as root-chat-only. Validates `.md` suffix, requires an
absolute path, reads raw file content from the workspace agent. Includes
1 MiB size cap.
**Frontend** (`site/src/components/ai-elements/tool/`): dedicated
`ProposePlanTool` component with `ToolCollapsible` + `ScrollArea` +
`Response` markdown renderer, expanded by default. Custom icon
(`ClipboardListIcon`) and filename-based label.
**System prompt** (`coderd/x/chatd/prompt.go`): added `<planning>`
section guiding the agent to research → write plan file → iterate → call
`propose_plan`.
Ethan