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cao-mcp-apps

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Enable, operate, and extend CAO's MCP Apps surface — the sandboxed host-rendered fleet UI (SEP-1865) with the ui://cao/* views, the topology widget, the submit_command mutation choke point, SEP-2133 capability advertisement, and the default-off OAuth scope layer. Use whenever the user wants to turn on the MCP Apps UI, observe/steer a CAO fleet from inside an MCP App host (Claude / Claude Desktop, ChatGPT, VS Code GitHub Copilot, Microsoft 365 Copilot, Goose, Postman, MCPJam, Archestra.AI), debug why the views don't render, build the frontend bundles, or extend the surface (new view, tool, or command kind).

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How to use this skill

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Source SKILL.md: https://github.com/awslabs/cli-agent-orchestrator/blob/HEAD/skills/cao-mcp-apps/SKILL.md

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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).

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.

Troubleshooting

  • 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

  • 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(...).
  • 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.