What causes "No handler implemented for this canvas action" in github/copilot-sdk?

Override on_action in your CanvasProvider subclass and implement the declared actions

github/copilot-sdk · error · CanvasError

canvas_action_no_handler

canvas_action_no_handler

Error message

No handler implemented for this canvas action

What it means

CanvasProvider.on_action is the hook for handling non-lifecycle canvas actions declared by a canvas. The base-class default implementation raises CanvasError.no_handler() (code 'canvas_action_no_handler'), signalling that the provider subclass did not override on_action for an action it declared. If your canvas declares actions, you must implement this method.

Solutions

Override on_action in your CanvasProvider subclass and implement the declared actions
Route on ctx.request/ctx.action to the correct handler and return a JSON-serializable result
Remove any declared actions your provider does not implement
Add a guard/deployment check that every declared canvas action has a matching on_action branch

Example fix

// before
class MyProvider(CanvasProvider):
    async def on_open(self, ctx): ...
    # on_action not overridden

// after
class MyProvider(CanvasProvider):
    async def on_action(self, ctx):
        if ctx.action == "submit":
            return {"ok": True}
        raise CanvasError.no_handler()

Defensive patterns

Strategy: try-catch

Validate before calling

declared = {a["name"] for a in canvas_manifest.get("actions", [])}
if declared and not isinstance(provider.on_action, type(CanvasProvider.on_action)):
    raise RuntimeError("canvas declares actions but provider does not override on_action")

Type guard

def implements_on_action(provider) -> bool:
    return type(provider).on_action is not CanvasProvider.on_action

Try / catch

try:
    result = await provider.on_action(ctx)
except CanvasError as e:
    if e.code == "canvas_action_no_handler":
        result = {"error": f"action {ctx.action} not supported"}

Prevention

Always override on_action when your canvas declares any action
Keep the canvas manifest and provider action handlers in sync (test one against the other)
Deploy provider and canvas definition changes together
Return an explicit 'unsupported action' result for unrecognized actions instead of relying on the base stub

When it happens

Trigger: A user or agent invokes an action on a canvas whose provider subclass does not override on_action — the declared action reaches the base-class stub, which always raises.

Common situations: Developers define canvas actions in their canvas manifest but only override lifecycle hooks (on_open/on_close); a new action is added to the canvas while the provider code is not deployed/updated; the provider is wired to a canvas whose action set changed.

Related errors

AI-assisted analysis of github/copilot-sdk@cd8cf15dc3 (2026-09-09). Data as JSON: /api/errors/c7e986177d9a7844. Report an issue: GitHub.

Appendix: source

Thrown at python/copilot/canvas.py:192

        **Experimental.** This type is part of an experimental wire-protocol
        surface and may change or be removed in future SDK or CLI releases.
    """

    @abstractmethod
    async def on_open(self, ctx: CanvasProviderOpenRequest) -> CanvasProviderOpenResult:
        """Open a new canvas instance.

        May raise :class:`CanvasError` to surface a structured failure to
        the host.
        """

    async def on_close(self, ctx: CanvasProviderCloseRequest) -> None:
        """Canvas was closed by the user or agent. Default: no-op."""

    async def on_action(self, ctx: CanvasProviderInvokeActionRequest) -> Any:
        """Handle a non-lifecycle action declared by the canvas."""
        raise CanvasError.no_handler()

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