mastra-ai/mastra · error · HTTPException

agent thread subscriptions are not supported by this Mastra

Error message

agent thread subscriptions are not supported by this Mastra core version

What it means

The thread subscription handler requires the agent to implement subscribeToThread for realtime updates. If the method is absent, the installed core version doesn't support thread subscriptions and the server returns HTTP 501. This is the same version-capability pattern as signals and aborts.

Source

Thrown at packages/server/src/server/handlers/agents.ts:2204

export const SUBSCRIBE_AGENT_THREAD_ROUTE = createRoute({
  method: 'POST',
  path: '/agents/:agentId/threads/subscribe',
  responseType: 'stream' as const,
  streamFormat: 'sse' as const,
  sseFlushOnConnect: true,
  pathParamSchema: agentIdPathParams,
  bodySchema: subscribeAgentThreadBodySchema,
  responseSchema: streamResponseSchema,
  summary: 'Subscribe to agent thread runs',
  description: 'Subscribes to future and active stream runs for a memory thread',
  tags: ['Agents', 'Streaming'],
  requiresAuth: true,
  requiresPermission: 'agents:execute',
  handler: async ({ mastra, agentId, resourceId, threadId, abortSignal, requestContext: serverRequestContext }) => {
    try {
      const agent = await getAgentFromSystem({ mastra, agentId, requestContext: serverRequestContext });
      if (typeof (agent as { subscribeToThread?: unknown }).subscribeToThread !== 'function') {
        throw new HTTPException(501, {
          message: 'agent thread subscriptions are not supported by this Mastra core version',
        });
      }

      const effectiveResourceId = getEffectiveResourceId(serverRequestContext, resourceId);
      const effectiveThreadId = getEffectiveThreadId(serverRequestContext, threadId);

      if (!effectiveThreadId) {
        throw new HTTPException(400, { message: 'threadId is required' });
      }

      if (effectiveResourceId) {
        const memory = await agent.getMemory({ requestContext: serverRequestContext });
        if (memory) {
          const thread = await memory.getThreadById({ threadId: effectiveThreadId });
          await validateThreadOwnership(thread, effectiveResourceId);
        }
      }

View on GitHub (pinned to 75dd419e61)

Solutions

Upgrade @mastra/core to a version with subscribeToThread support
Delete the lockfile entry / run pnpm update so core actually resolves to the new version
Verify the running server process was restarted on the upgraded build

Example fix

// before (lockfile pin)
"@mastra/core": "0.10.0" // pinned
// after
pnpm update @mastra/core --latest && pnpm build

Defensive patterns

Strategy: type-guard

Validate before calling

function agentSupportsSubscribe(agent: unknown): boolean {
  return typeof (agent as { subscribeToThread?: unknown })?.subscribeToThread === 'function';
}

Type guard

function supportsSubscribe(a: unknown): a is { subscribeToThread: Function } {
  return !!a && typeof (a as { subscribeToThread?: unknown }).subscribeToThread === 'function';
}

Try / catch

try {
  await subscribeToThread(agentId, threadId);
} catch (e) {
  if (e?.status === 501) fallbackToPolling();
  else throw e;
}

Prevention

Confirm @mastra/core supports subscribeToThread before building realtime UI
Align all mastra package versions; watch for stale lockfile pins
Implement a polling fallback for deployments on older core

When it happens

Trigger: Opening a thread subscription (SSE/WS subscribe route) against an agent lacking subscribeToThread — pre-subscription @mastra/core versions.

Common situations: New playground/server UI subscribed to threads while core is older; partial monorepo upgrade; pinned core version in lockfile resisting the bump.

Related errors

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/09b7ee04928cf80f. Report an issue: GitHub.