The Ctrl+D diff drawer in `coder exp agents` only rendered PR-backed
diffs returned by `/api/experimental/chats/{id}/diff`. Local working
tree changes in a chat's workspace returned an empty diff, so the
drawer showed "No diff contents" with no file summary.
Centralise diff loading behind a single `fetchChatDiffContents` helper
that first hits `/diff`, then falls back to the chat git watcher
WebSocket (`/stream/git`) when the remote diff is empty. Aggregate the
agent's `WorkspaceAgentRepoChanges` into a `ChatDiffContents` value so
the drawer can derive the file summary and styled body from the local
unified diff. Missing workspaces, missing agents, and watcher timeouts
are treated as graceful fallbacks that render the empty-diff
placeholder instead of a hard error.
> Mux is opening this PR on Mike's behalf.
> This PR was authored by Mux on behalf of Mike.
Adds `coder exp agents`, an interactive terminal UI for managing Coder
AI agent chats. Built with bubbletea/lipgloss/glamour, the TUI provides
parity with the web dashboard for chat management, model selection, and
real-time tool execution visibility.
## What it does
- **Chat list view**: tree-based navigation with nested subagent
expansion, search filtering, windowed scrolling, and pagination.
- **Active chat view**: viewport-based transcript with markdown
rendering, WebSocket streaming, and a text input composer for sending
messages.
- **Model picker overlay**: cached model catalog with fuzzy selection.
- **Diff drawer overlay**: git changes inspection with unified diff
rendering.
- **Tool call rendering**: humanized argument summaries, consecutive
duplicate collapsing, and status indicators.
## Key implementation details
- Session lifecycle uses a monotonic `chatGeneration` counter so async
responses from stale sessions are dropped on chat switch.
- Draft mode guards prevent duplicate chat creation on double-Enter.
- Error and loading states render inline without collapsing the TUI
chrome.
- Glamour renderer access is mutex-protected (not thread-safe).
- Intentional WebSocket close is distinguished from dropped connections
to prevent spurious reconnects.
## Testing
~220 unit tests covering rendering, state transitions, keyboard
dispatch, and edge cases. 4-scenario PTY-based E2E suite covers boot,
navigation, search, and direct chat open.
14 new files, ~7,400 lines added.
Michael Suchacz