CAO MCP Apps

Operator + developer playbook for CAO's host-rendered fleet UI. Reference docs: docs/mcp-apps.md; example: examples/mcp-apps/.

Authoritative spec & sources of truth:MCP Apps Overview · Build an MCP App · capability negotiation · client matrix · stable spec 2026-01-26/apps.mdx (SEP-1865, Status: Stable) · SDK @modelcontextprotocol/ext-apps v1.7.4 (API ref · repo) · provenance PR #1865.

Turn it on

The surface is default-off. Enable and run:

export CAO_MCP_APPS_ENABLED=true
uv run cao-server        # :9889 (REST + SSE /events)
uv run cao-mcp-server    # registers tools/resources via the mcp_apps plugin

It is packaged as the built-in mcp_apps plugin (cao.plugins entry-point). The plugin's on_mcp_server hook registers the ui://cao/* resources, the five app tools, the topology widget, and advertises the io.modelcontextprotocol/ui capability — best-effort and default-off, so nothing changes when the flag is unset.

What the operator gets

• ui://cao/dashboard — fleet overview + the mutation entry point.
• ui://cao/agent — one terminal's status, output tail, inbox, sub-agents.
• ui://cao/event-stream — live governance ticker (app-only).
• cao://widget/topology + /widgets/topology/ — build-free live event view.

All mutations flow through submit_command(kind, payload) — kinds: send_message, assign, create_session (standard); interrupt, pause, resume (lifecycle); shutdown_session (destructive). For full payload schemas and scope requirements per kind, see references/submit-command-kinds.md.

Full capability scope (what the views use)

Beyond tools/call, the views exercise the spec's bidirectional channel:

• Host-delegated open-link (ui/open-link) — the dashboard shows "Open full Web UI ↗" → http://127.0.0.1:9889 only when the host advertises hostCapabilities.openLinks (gate on app.canOpenLinks(); the sandbox forbids window.open).
• Display modes (ui/request-display-mode) — views declare availableDisplayModes: ["inline","fullscreen"] at ui/initialize.
• Streamed tool input (ui/notifications/tool-input / -partial) — render before the result lands.
• Model-context notes (ui/update-model-context) — body-free gesture summaries keep the agent aware without leaking message contents.

preferredFrameSize and requiredScopes are CAO additions, not spec _meta.ui fields (the spec sizes via containerDimensions + ui/notifications/size-changed); CAO requests no elevated permissions.

See assets/mcp-apps-example.md for a worked MCP Apps integration example.

Gotchas

• Host doesn't offer the views → confirm CAO_MCP_APPS_ENABLED=true and that initialize advertises io.modelcontextprotocol/ui (the host must speak SEP-1865). Non-SEP-1865 hosts still get text-only tool results.
• Views are blank / fail to load → the React bundles aren't built. Run cd cao_mcp_apps && npm ci && npm run build:all. The topology widget needs no build and is the quickest smoke test (curl /widgets/topology/topology.html).
• Mutations rejected with 403 → the auth layer is enabled and the token lacks cao:write/cao:admin (cao:admin for delete_session). Unset AUTH0_DOMAIN/CAO_AUTH_JWKS_URI to disable enforcement.
• Events don't stream → check GET /events (SSE) directly; the bus is drop-on-slow, so a stalled consumer silently loses events — re-hydrate via cao_fetch_history.

Extending the surface

• Agents emitting UI intents into this surface? Load the agui-author skill — it teaches how to call emit_ui with the six allow-listed components. Your emit_ui intents feed the L2 constructs that these views render.
• Building or migrating an MCP App? Load the mcp-apps-builder skill first. It equips the official ext-apps Agent Skills (create-mcp-app, add-app-to-server, migrate-oai-app, convert-web-app) and the build guide. Use add-app-to-server when adding a new ui://cao/<name> view.
• New command kind → add it to submit_command's classifier + router in mcp_server/app_tools.py (map to a real Backplane HTTP endpoint; never bypass the HTTP-only boundary) and to the scope pre-check.
• New view → add a ui://cao/<name> resource in ext_apps/apps.py + an entry point under cao_mcp_apps/, build it, and tag the rendering tool with ui_meta(...). For the full step-by-step view creation procedure, see references/extending-views.md.
• New host-delegated action → add a thin method on the McpApp bridge (cao_mcp_apps/src/shared/mcpApp.ts) that issues the spec ui/* request (e.g. openLink → ui/open-link, requestDisplayMode → ui/request-display-mode); gate UI on the matching hostCapabilities flag and cover it with a mockHost test.
• Keep the boundary → mcp_server/* must reach state only over HTTP; the AST guard test (test/test_http_only_boundary.py) enforces it.
• Keep bundles JIT-free → no eval/new Function (host CSP forbids it); the CI scan fails the build otherwise.

Recording & Verification

After building or modifying views, regenerate the demo media:

cd cao_mcp_apps && npm run build:all && npm run demo

This runs scripts/record-demo.mjs which:

1. Boots the E2E harness server (serves built bundles in a real MCP-host iframe)
2. Drives Chromium through: dashboard → agent detail → unified → event-stream
3. Records video (docs/media/mcp-apps-demo.webm)
4. Captures screenshots (docs/media/mcp-apps-{dashboard,agent,unified,event-stream}.png)
5. Generates an optimized GIF (docs/media/mcp-apps-demo.gif) when ffmpeg is available

The GIF is referenced in README.md and docs/mcp-apps.md — always regenerate after view changes so docs stay current.

Env overrides: CHROMIUM_BIN (path to Chrome), FFMPEG_BIN (for GIF), DEMO_PORT.

For a worked example of the full MCP Apps surface in action, see assets/mcp-apps-example.md.