Canvas Extension API

Canvas Extensions API

Build and validate OpenHands Apps with the Canvas Extensions API, targeting the currently implemented manifest schema 1 and host API 1 routed-page ABI.

Use the naming layers consistently:

• OpenHands Apps - customer-facing category for interfaces people discover, install, enable, and open.
• Apps for Agent Canvas - product phrasing that clarifies where apps run.
• Canvas Extensions API - developer platform used to build apps.
• Canvas extension package - technical artifact containing canvas-extension.json and its entrypoint.
• OpenHands Extensions - ecosystem and repository umbrella for apps, skills, plugins, automations, and integrations.

Use current product labels in user-facing instructions: Apps for Agent Canvas, Add app, Installed apps, App source, Enable trusted app, and Build an app. Keep technical identifiers such as canvas-extension.json, /api/canvas-extensions/*, host.extension, and /extensions/* unchanged unless the target implementation changes them.

Treat apps as trusted, same-realm browser code owned by the active Agent Server. Keep the categories distinct: apps extend what people can do in Agent Canvas, plugins extend agent runtime capabilities, and skills provide agent instructions and knowledge.

Prefer the proven App authoring loop

Treat an App as one authenticated browser dependency graph that Canvas imports from a Blob URL. Do not build a hosted SPA: the production result must be exactly one self-contained browser ESM entrypoint that exports activate(host). Always ship this UI package even when it uses an optional package-owned managed backend or a manual/deployment-owned Sidecar. Make the App page the guided UI for service setup after the App is opened. Never treat a service as another browser chunk or an automatic extension install hook.

For a new App, create an independent package in the target repository and implement its own UI, tests, and build tooling. Follow the Vite library-build reference in references/packaging-recipes.md; do not copy a shared starter or introduce a repository-wide runtime/workspace unless the target repository explicitly requires it.

Keep all source, scripts, dependencies, tests, and checked-in extension.js inside the App package. Implement one declared page with explicit root, nested, and unknown-route behavior. Inject scoped CSS per mount and clean it up with framework roots, requests, listeners, timers, Workers, and object URLs. Build validation must verify dist/extension.js before synchronizing it to the App root. Do not synchronize an unverified artifact. Use CHROME_PATH when the Chrome executable is elsewhere.

For an existing App, run the reusable static gate before and after its own checks:

node /path/to/canvas-extension-api/scripts/validate-extension.mjs /path/to/app
node /path/to/canvas-extension-api/scripts/validate-extension.mjs /path/to/app --dist --marker <required-feature-marker>

Treat the static validator as a gate, not a replacement for the browser Blob smoke test. Read references/packaging-recipes.md before adding CSS, raw assets, dynamic modules, Workers, or WASM. Read references/backend-safety.md before any Agent Server integration or persistence. Read references/sidecar-pattern.md before designing a separately installed service or its onboarding. Read references/acceptance-checklist.md before reporting local Canvas compatibility.

Establish the target

Start by locating the target directory instead of assuming the current workspace. Inspect repository instructions, existing package management, build tooling, tests, and git status before editing.

Make an early architecture decision: browser-only App, Agent Server-integrated App, package-owned managed-backend App, manual/deployment-owned Sidecar App, or explicitly deployment-specific App. Keep the portable host API 1 surface separate from optional Agent Server/host capabilities and deployment-owned behavior.

Use the package-owned managed-backend path only when the target stack includes the lifecycle contract from software-agent-sdk PR #5270 at ac7b322ddf7d2e0e90a3643c5e26d848236af9a3, the bridge contract from PR #5272 at 6d9c82973e765cce4843d6a2fb52f634efa64293, and the host version named in references/v1-contract.md. Until a release containing both SDK PRs lands, those exact revisions are the minimum Agent Server implementation. Feature-detect canvas_app_backend_bridge_v1 and app_backend_ingress_url through GET /server_info; absence is an actionable unsupported state. Do not convert manual Sidecars into managed backends unless the App owns checksum-pinned artifacts for every supported platform.

Clarify only choices that materially affect implementation:

• app purpose and page behavior;
• target repository and subdirectory;
• dependency-free JavaScript versus a bundled TypeScript/framework project;
• Agent Server endpoints or host metadata required;
• whether to build only, install locally, or prepare publication instructions;
• for a Sidecar: execution location, owner, connection route, secrets and data, lifecycle actions, and portability across Agent Canvas deployments.

Default to one app with one routed page when requirements are otherwise clear. Keep the first implementation small and dependency-free unless the requested UI clearly benefits from a framework or the repository already has a bundler.

When creating multiple Apps in one repository, give every App an independent package root containing its own canvas-extension.json, entrypoint, version, tests, README, and build tooling. Do not introduce a monorepo, shared runtime, or common build foundation unless the target repository explicitly requires one. Keep app manifest names globally distinct within the Agent Server installation.

Treat installation as one app per request. The current Customize -> Apps flow accepts one source, optional ref, and optional repo_path; it does not recursively discover or bulk-install every manifest in a repository. For a remote repository, add each app separately using the same source/ref and its own repo_path. For a backend-local source, select each app package directory as the source path.

Read references/v1-contract.md before implementing unfamiliar Canvas Extensions API behavior. Read references/connections.md before connecting to Agent Server, the Automation service, or WebSocket endpoints. Read references/sidecar-pattern.md before proposing a Sidecar bridge, onboarding, or operator action. Read references/testing-and-installation.md before installing or testing inside Agent Canvas, especially for a multi-app repository.

Inspect current upstream behavior

Treat the v1 contract as young and subject to change. Before substantial work, compare the installed or target OpenHands version with the current upstream sources when access is available:

• specs/canvas-extensions.md
• src/types/canvas-extension.ts
• src/components/features/canvas-extensions/canvas-extensions-runtime.tsx
• src/routes/canvas-extension-page.tsx
• docs/CANVAS_EXTENSIONS_TESTING.md

Do not invent planned surfaces such as conversation tabs, slots, themes, or visualizer replacement. Implement only contributions supported by the target version. Routed pages are the only implemented contribution in host API 1.

Create the app package

Place canvas-extension.json at the app package root. Point entrypoint to one self-contained browser ESM file inside that root.

Use this minimal manifest shape:

{
  "schema_version": 1,
  "name": "example-dashboard",
  "display_name": "Example dashboard",
  "version": "0.1.0",
  "description": "A backend-specific project dashboard.",
  "entrypoint": "extension.js",
  "contributes": {
    "pages": [
      {
        "id": "dashboard",
        "title": "Dashboard",
        "path": "/dashboard",
        "nav_label": "Dashboard"
      }
    ]
  }
}

For an optional package-owned managed backend, add this top-level sibling of contributes:

"backend": {
  "schema_version": 1,
  "artifacts": {
    "linux-amd64": {
      "path": "backend/linux-amd64.tar.gz",
      "sha256": "<64 lowercase hex characters>"
    },
    "linux-arm64": {
      "path": "backend/linux-arm64.tar.gz",
      "sha256": "<64 lowercase hex characters>"
    }
  },
  "argv": [
    "{artifact_dir}/bin/server",
    "--host=127.0.0.1",
    "--port={port}",
    "--data-dir={data_dir}"
  ],
  "health": { "path": "/health" }
}

This declaration is optional. Omit it for browser-only Apps and manual/deployment-owned Sidecars. It never authorizes automatic preparation or startup. The server supports Linux amd64 and arm64 initially, verifies package-contained .tar.gz artifacts by SHA-256, and executes the structured argv without a shell. argv[0] must remain inside {artifact_dir}; allowed placeholders are {artifact_dir}, {data_dir}, and {port}.

Follow these invariants:

• Use lowercase letters, digits, and hyphens for app names and page IDs.
• Use absolute kebab-case page paths with a leading slash.
• Keep every manifest path within the app package root.
• Give each page ID and path a unique value.
• Register only page IDs declared in the manifest.
• Keep the entrypoint free of unresolved bare imports and external chunks.
• Bundle dependencies, CSS, and small assets into the entrypoint when using build tooling.

Add a concise README.md when the app is intended for reuse or publication. Document purpose, build command if any, output entrypoint, installation coordinate, and verification steps. Avoid adding explanatory change-log documents.

Implement activation and pages

Export an activate function from the ESM entrypoint:

export function activate(host) {
  if (host.apiVersion !== "1") {
    throw new Error(`This app requires host API 1.`);
  }

  return host.registerPage("dashboard", ({ container, path, navigate }) => {
    const root = document.createElement("section");
    root.textContent = path ? `Nested route: ${path}` : "Dashboard";
    container.append(root);

    return () => root.remove();
  });
}

Use the host contract deliberately:

• Read immutable metadata from host.extension and host.backend.
• Call host.registerPage(id, mount) during activation.
• Use the mount callback's path as the route remainder below the declared page path.
• Treat every nested-route navigation as a fresh mount: Canvas disposes the previous mount, clears the container, and invokes the mount callback with the new remainder. DOM and framework state do not persist across route changes; keep intentional cross-route state in the URL, backend, or activation-scoped state and clean it up on deactivation.
• Use navigate(absoluteCanvasPath) for Canvas-aware routing.
• Use host.agentServer.request({ path, method, body, headers }) for authenticated calls to the owning Agent Server.
• Pass only root-relative request paths beginning with one /; never pass absolute URLs or // paths.

Prefer @openhands/typescript-client for bundled TypeScript integrations whenever it covers the required Agent Server API and the necessary connection inputs are available. Prefer its typed clients, models, compatibility checks, and WebSocket lifecycle over hand-written transport code. Bundle the client into the self-contained app entrypoint; never leave a bare package import or external runtime chunk.

Respect the host API 1 connection boundary:

• Treat host.agentServer.request as the portable Agent Server HTTP path; Canvas selects the active backend and supplies authentication.
• For a managed-backend view, require the feature-detected host helper documented in references/v1-contract.md. The helper owns session creation, validated iframe isolation, retry/new-tab behavior, and revocation; the App never reads the app-scoped cookie or constructs the ingress URL. The ingress must be a separate browser origin from Canvas, and the host must preserve the server-returned sandbox including allow-same-origin.
• Installing, enabling, activating, or mounting an App never prepares or starts a managed backend. Show the resolved revision and declaration, obtain explicit lifecycle consent, and call prepare then start with that exact revision. An update invalidates approval.
• Do not derive an Agent Server URL from window.location or extract session keys from Canvas internals.
• Do not send Automation service requests through host.agentServer.request unless the backend explicitly documents an Agent Server proxy for them. Automation is a separate service with deployment-provided base URL and authentication.
• Do not open a direct Agent Server WebSocket from a portable v1 app. The host currently exposes neither the owning server origin nor a WebSocket/auth capability. Poll through host.agentServer.request, add a backend-owned bridge, or feature-detect a future host subscription API.
• Use the TypeScript client's Agent Server WebSocket support in trusted standalone or future host-enabled contexts, and stop it during disposal. Do not treat the Agent Server client as an Automation service client.

Return cleanup from every layer that creates effects:

• Return the unregister function or an activation disposer from activate.
• Return a mount disposer for DOM nodes, timers, listeners, observers, subscriptions, and outstanding state.
• Make async work disposal-aware so late responses cannot mutate an unmounted page.
• Remove injected styles when the page unmounts.

Assume activation, mounting, and disposal may happen repeatedly during hot enable/disable, updates, backend switches, reconnects, and route changes.