dotnet/aspnetcore · error · Error

Invalid end of component comment

Error message

Invalid end of component comment: '${json}'

What it means

Thrown by validateEndComponentPayload when the parsed end-of-component comment JSON has more or fewer than exactly one key. The end marker must contain only the single 'prerenderId' property; any extra or missing keys mean the payload is malformed. This is the first structural check on the closing marker before prerenderId is validated.

Solutions

1. Inspect the served HTML: the end comment must be a JSON object with exactly one key, prerenderId.
2. Remove HTML post-processing that alters Blazor comment JSON.
3. Ensure the prerendering pipeline emits the correct end marker shape.
4. Align server and client Blazor runtime versions.

function assertEndMarkerShape(payload: any) {
  const keys = Object.keys(payload ?? {});
  if (keys.length !== 1 || keys[0] !== 'prerenderId') {
    throw new Error(`Invalid end marker; expected exactly { prerenderId }, got ${JSON.stringify(payload)}`);
  }
}

function isEndMarker(p: any): p is { prerenderId: string } {
  return p !== null && typeof p === 'object' && Object.keys(p).length === 1 && typeof p.prerenderId === 'string';
}

When it happens

Trigger: An end component comment whose JSON has zero keys (empty object) or two+ keys; a transform that added/removed fields; a start marker accidentally used as an end marker.

Common situations: HTML minifier that strips or rewrites the end comment; a copy-paste error placing a start-comment payload where the end comment should be; version skew producing a different end-marker schema; corruption.

Related errors

• assembly must be defined when using a descriptor.
• Could not find an end component comment for
• descriptor must be defined when using a descriptor.
• Error parsing the sequence
• Found malformed component comment at

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/92b6a75691b64edd. Report an issue: GitHub.

Appendix: source

Thrown at src/Components/Web.JS/src/Services/ComponentDescriptorDiscovery.ts:288

    throw new Error('assembly must be defined when using a descriptor.');
  }

  if (!typeName) {
    throw new Error('typeName must be defined when using a descriptor.');
  }

  // Parameter definitions and values come Base64 encoded from the server, since they contain random data and can make the
  // comment invalid. We could unencode them in .NET Code, but that would be slower to do and we can leverage the fact that
  // JS provides a native function that will be much faster and that we are doing this work while we are fetching
  // blazor.boot.json
  payload.parameterDefinitions = payload.parameterDefinitions && atob(payload.parameterDefinitions);
  payload.parameterValues = payload.parameterValues && atob(payload.parameterValues);
}

function validateEndComponentPayload(json: string, prerenderId: string): void {
  const payload = JSON.parse(json) as ComponentEndMarker;
  if (Object.keys(payload).length !== 1) {
    throw new Error(`Invalid end of component comment: '${json}'`);
  }
  const prerenderEndId = payload.prerenderId;
  if (!prerenderEndId) {
    throw new Error(`End of component comment must have a value for the prerendered property: '${json}'`);
  }
  if (prerenderEndId !== prerenderId) {
    throw new Error(`End of component comment prerendered property must match the start comment prerender id: '${prerenderId}', '${prerenderEndId}'`);
  }
}

class ComponentCommentIterator {
  private childNodes: NodeListOf<ChildNode>;

  private currentIndex: number;

  private length: number;

  public currentElement: ChildNode | undefined;

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