What causes "No post-processor found for registry ${registry.name}" in mastra-ai/mastra?

Provide a `postProcessServers` function on the registry definition that maps your payload to the expected server format

mastra-ai/mastra · error · Error

No post-processor found for registry ${registry.name}

Error message

No post-processor found for registry ${registry.name}

What it means

Each MCP registry can declare a custom `postProcessServers` function that converts the raw JSON payload into the standard server list format. If a registry has neither a custom post-processor nor a built-in one matching its format, the library cannot interpret the fetched data and throws.

Source

Thrown at packages/mcp-registry-registry/src/registry/fetch-servers.ts:37

    console.info(`Fetching servers from ${registry.name} at ${registry.servers_url}`);

    // Fetch the servers from the registry's servers_url
    const response = await fetch(registry.servers_url);

    if (!response.ok) {
      throw new Error(`Failed to fetch servers from ${registry.servers_url}: ${response.statusText}`);
    }

    const data = (await response.json()) as unknown;

    // If the registry has a custom post-processing function, use it
    if (registry.postProcessServers) {
      console.info(`Using custom post-processor for ${registry.name}`);
      return registry.postProcessServers(data);
    }

    throw new Error(`No post-processor found for registry ${registry.name}`);
  } catch (error) {
    console.error('Error fetching servers:', error);
    throw error;
  }
}

/**
 * Filters server entries based on provided criteria
 */
export function filterServers(
  servers: ServerEntry[],
  filters: {
    tag?: string;
    search?: string;
  },
): ServerEntry[] {
  let filteredServers = [...servers];

View on GitHub (pinned to 75dd419e61)

Solutions

Provide a `postProcessServers` function on the registry definition that maps your payload to the expected server format
Verify the registry response shape matches the format expected by built-in parsers
Log/inspect the fetched JSON (`data`) to write the correct post-processor
Fall back to a registry with a known/supported format

Example fix

// before
const registry = { name: 'custom', servers_url: 'https://internal/api/servers' };
// after
const registry = {
  name: 'custom',
  servers_url: 'https://internal/api/servers',
  postProcessServers: (data) => data.servers.map(s => ({ id: s.name, url: s.endpoint }))
};

Defensive patterns

Strategy: validation

Validate before calling

if (!registry.postProcessServers && !isKnownRegistryFormat(registry)) {
  throw new Error(`Registry "${registry.name}" needs a postProcessServers function`);
}

Try / catch

try {
  const servers = await registryManager.fetchServersFromRegistry({ registryId });
} catch (err) {
  if (err instanceof Error && err.message.startsWith('No post-processor found')) {
    // attach a postProcessServers to the registry definition and retry
  } else throw err;
}

Prevention

Always provide postProcessServers for custom/self-hosted registries
Validate the registry JSON response shape against the expected schema before mapping
Pin registry API versions; watch for upstream response-format changes
Test post-processors with a recorded sample payload

When it happens

Trigger: fetchServersFromRegistry fetches data successfully (response.ok, JSON parsed), the registry has no `postProcessServers`, and no default parser matches the registry — thrown at the end of the try block.

Common situations: Using a custom/self-hosted registry whose response shape doesn't match a known format; forgetting to attach postProcessServers when defining a custom registry; registry API changed its response schema so the default parser path no longer applies.

Related errors

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/403a3555a0478e98. Report an issue: GitHub.