mastra-ai/mastra · error · MastraError

PROCESSOR_MISSING_MESSAGE_LIST

PROCESSOR_MISSING_MESSAGE_LIST

Error message

Processor ${processor.id} requires messageList or messages for processInput phase

What it means

A processor implementing processInput needs a MessageList to operate on. The evented workflow builds passThrough data and requires passThrough.messageList; if it is absent (and messages were not converted into one), it throws this structured MastraError (id PROCESSOR_MISSING_MESSAGE_LIST, USER category, MASTRA_WORKFLOW domain).

Source

Thrown at packages/core/src/workflows/evented/workflow.ts:1171

          return result;
        } catch (error) {
          // TripWire errors should end span but bubble up to halt the workflow
          if (error instanceof TripWire) {
            processorSpan?.end({ output: { tripwire: error.message } });
          } else {
            processorSpan?.error({ error: error as Error, endSpan: true });
          }
          throw error;
        }
      };

      // Execute the phase with span lifecycle management
      return executePhaseWithSpan(async () => {
        switch (phase) {
          case 'input': {
            if (processor.processInput) {
              if (!passThrough.messageList) {
                throw new MastraError({
                  category: ErrorCategory.USER,
                  domain: ErrorDomain.MASTRA_WORKFLOW,
                  id: 'PROCESSOR_MISSING_MESSAGE_LIST',
                  text: `Processor ${processor.id} requires messageList or messages for processInput phase`,
                });
              }

              // Create source checker before processing to preserve message sources
              const idsBeforeProcessing = (messages as MastraDBMessage[]).map(m => m.id);
              const check = passThrough.messageList.makeMessageSourceChecker();

              const result = await processor.processInput({
                ...baseContext,
                messages: messages as MastraDBMessage[],
                messageList: passThrough.messageList,
                systemMessages: (systemMessages ?? []) as CoreMessage[],
              });

View on GitHub (pinned to 75dd419e61)

Solutions

1. Ensure the step's inputData includes messages (or messageList) — feed the processor step from agent/loop output or construct a MessageList and pass it.
2. If composing manually, build a MessageList from your messages before invoking the processor step.
3. Only attach processors that implement processInput to flows that supply message data; otherwise use a plain step.

Example fix

// before
await processorStep.execute({ inputData: {} });

// after
await processorStep.execute({
  inputData: { messages: [{ role: 'user', content: 'hello' }] },
});

function assertMessageListInput(inputData: { messageList?: unknown; messages?: unknown }, processorId: string) {
  if (!inputData?.messageList && !inputData?.messages) {
    throw new Error(`Step input for processor ${processorId} must include messages or messageList`);
  }
}

const hasMessages = (d: any): d is { messageList: MessageList } =>
  d?.messageList instanceof MessageList || Array.isArray(d?.messages);

try {
  await processorStep.execute({ inputData });
} catch (e) {
  if (e instanceof MastraError && e.id === 'PROCESSOR_MISSING_MESSAGE_LIST') {
    // re-run with a properly built MessageList
  }
  throw e;
}

When it happens

Trigger: Running a workflow step created via createStep(processor) where processor.processInput exists but the input payload to the step contains neither messageList nor messages — e.g. inputData lacking the messages field the adapter expects.

Common situations: Wiring a processor step into a custom workflow and passing plain objects; forgetting that processor steps expect message-list-shaped input (inputData.messages); skipping the agent loop that normally builds the MessageList.

Related errors

• PROCESSOR_RETURNED_EXTERNAL_MESSAGE_LIST
• PROCESSOR_RETURNED_EXTERNAL_MESSAGE_LIST
• Input data not found
• Forecast data not found
• Repository URL or path is required

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/93639d7c77c85387. Report an issue: GitHub.