What causes "Invalid component type '${type}'." in dotnet/aspnetcore?

Ensure the prerendering layer emits exactly 'server', 'webassembly', or 'auto'.

dotnet/aspnetcore · error · Error

Invalid component type

Error message

Invalid component type '${type}'.

What it means

Thrown by parseCommentPayload after JSON.parse of a component comment if the resulting object's 'type' field is not one of 'server', 'webassembly', or 'auto'. These three are the only supported component marker types; any other value (or missing type) is rejected before the payload is returned.

Solutions

Ensure the prerendering layer emits exactly 'server', 'webassembly', or 'auto'.
Update the client-side Blazor runtime to match the server's version so all marker types are recognized.
Remove any custom code that synthesizes Blazor component markers.
Inspect the comment payload in the served HTML and correct the type field.

Defensive patterns

Strategy: validation

Validate before calling

const VALID = new Set(['server', 'webassembly', 'auto']);
function assertMarkerType(payload: any) {
  if (!VALID.has(payload?.type)) {
    throw new Error(`Unsupported marker type '${payload?.type}'.`);
  }
}

Type guard

function isMarkerType(v: unknown): v is 'server' | 'webassembly' | 'auto' {
  return v === 'server' || v === 'webassembly' || v === 'auto';
}

Prevention

Only emit official marker types from the prerendering pipeline.
Align server and client runtime versions.
Don't synthesize Blazor component markers in custom code.

When it happens

Trigger: A Blazor component comment with type set to an unrecognized string (e.g. 'client', 'razor'); the type field missing so it is undefined; a hand-crafted or corrupted marker.

Common situations: Custom tooling that emits component markers with a non-standard type; corruption of the JSON payload; a future/newer Blazor version emitting a type this older runtime doesn't know.

Related errors

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/9c8518fc1c5f609a. Report an issue: GitHub.

Appendix: source

Thrown at src/Components/Web.JS/src/Services/ComponentDescriptorDiscovery.ts:173

          case 'server':
            return createServerComponentComment(componentComment, candidateStart as Comment, candidateEnd);
          case 'auto':
            return createAutoComponentComment(componentComment, candidateStart as Comment, candidateEnd);
        }
      } catch (error) {
        throw new Error(`Found malformed component comment at ${candidateStart.textContent}`);
      }
    } else {
      return;
    }
  }
}

function parseCommentPayload(json: string): ServerComponentMarker | WebAssemblyComponentMarker | AutoComponentMarker {
  const payload = JSON.parse(json);
  const { type } = payload;
  if (type !== 'server' && type !== 'webassembly' && type !== 'auto') {
    throw new Error(`Invalid component type '${type}'.`);
  }

  return payload;
}

function assertNotDirectlyOnDocument(marker: Node) {
  if (marker.parentNode instanceof Document) {
    throw new Error('Root components cannot be marked as interactive. The <html> element must be rendered statically so that scripts are not evaluated multiple times.');
  }
}

function getComponentEndComment(payload: ComponentMarker, start: Comment, iterator: ComponentCommentIterator): Comment | undefined {
  const { prerenderId } = payload;
  if (!prerenderId) {
    return undefined;
  }

  while (iterator.next() && iterator.currentElement) {

View on GitHub (pinned to 3600ca084e)