fix(core): fail fast when token limiter removes all input (#16063)

Author: mbenhamd

.changeset/two-nights-decide.md

@@ -0,0 +1,5 @@
+---
+'@mastra/core': patch
+---
+
+Fixed TokenLimiterProcessor failing silently when no input messages fit the token budget.

packages/core/src/processors/processors/token-limiter.test.ts

@@ -3,6 +3,7 @@ import { describe, it, expect, vi, beforeEach } from 'vitest';
 
 import type { MastraDBMessage } from '../../agent/message-list';
 import { MessageList } from '../../agent/message-list';
+import { TripWire } from '../../agent/trip-wire';
 import type { IMastraLogger } from '../../logger';
 import { ProcessorRunner } from '../../processors/runner';
 import type { ChunkType } from '../../stream';
@@ -870,6 +871,54 @@ describe('TokenLimiterProcessor', () => {
       ).rejects.toThrow('System messages alone exceed token limit');
     });
 
+    it('should throw TripWire when no messages fit within the remaining token budget', async () => {
+      const processor = new TokenLimiterProcessor({ limit: 25 });
+      const messageList = new MessageList();
+
+      messageList.add(
+        {
+          id: 'user-1',
+          role: 'user',
+          content: { format: 2, content: 'Hello', parts: [{ type: 'text', text: 'Hello' }] },
+          createdAt: new Date('2023-01-01T00:00:00Z'),
+        },
+        'input',
+      );
+
+      try {
+        await processor.processInputStep({
+          messageList,
+          stepNumber: 1,
+          model: createMockModel(),
+          steps: [],
+          systemMessages: [],
+          state: {},
+          retryCount: 0,
+          abort: (() => {
+            throw new Error('aborted');
+          }) as any,
+        });
+        expect.fail('Expected TokenLimiterProcessor to throw a TripWire');
+      } catch (error) {
+        expect(error).toBeInstanceOf(TripWire);
+        expect(error).toHaveProperty(
+          'message',
+          'TokenLimiterProcessor: No messages fit within the remaining token budget. Cannot send LLM a request with no messages.',
+        );
+        expect((error as TripWire).options).toEqual({
+          retry: false,
+          metadata: {
+            systemTokens: 0,
+            limit: 25,
+            remainingBudget: 1,
+            messageCount: 1,
+          },
+        });
+      }
+
+      expect(messageList.get.all.db()).toHaveLength(1);
+    });
+
     it('should handle tool call messages in token counting', async () => {
       const processor = new TokenLimiterProcessor({ limit: 100 });
 

packages/core/src/processors/processors/token-limiter.ts

@@ -36,7 +36,14 @@ export interface TokenLimiterOptions {
  * - Input processor: Filters historical messages to fit within context window, prioritizing recent messages
  * - Output processor: Limits generated response tokens via streaming (processOutputStream) or non-streaming (processOutputResult)
  */
-export class TokenLimiterProcessor implements Processor<'token-limiter', { systemTokens: number; limit: number }> {
+type TokenLimiterTripWireMetadata = {
+  systemTokens: number;
+  limit: number;
+  remainingBudget?: number;
+  messageCount?: number;
+};
+
+export class TokenLimiterProcessor implements Processor<'token-limiter', TokenLimiterTripWireMetadata> {
   public readonly id = 'token-limiter';
   public readonly name = 'Token Limiter';
   private encoderPromise: Promise<Tiktoken> | undefined;
@@ -152,6 +159,16 @@ export class TokenLimiterProcessor implements Processor<'token-limiter', { syste
       }
     }
 
+    if (messagesToKeep.length === 0) {
+      throw new TripWire(
+        'TokenLimiterProcessor: No messages fit within the remaining token budget. Cannot send LLM a request with no messages.',
+        {
+          retry: false,
+          metadata: { systemTokens, limit, remainingBudget, messageCount: messages.length },
+        },
+      );
+    }
+
     // Remove messages that don't fit within the token budget
     const keepIds = new Set(messagesToKeep.map(m => m.id));
     const idsToRemove = messages.filter(m => !keepIds.has(m.id)).map(m => m.id);