fix: throw MaxTokensError when contentBlockStop carries truncated tool_use JSON

strands-ts/src/models/__tests__/model.test.ts

@@ -323,7 +323,7 @@ describe('Model', () => {
         )
       })
 
-      it('preserves SyntaxError instead of overwriting with MaxTokensError when tool input JSON is malformed', async () => {
+      it('throws MaxTokensError when stopReason is maxTokens and contentBlockStop carries truncated tool_use JSON', async () => {
         const provider = new TestModelProvider(async function* () {
           yield { type: 'modelMessageStartEvent', role: 'assistant' }
           yield {
@@ -332,14 +332,36 @@ describe('Model', () => {
           }
           yield {
             type: 'modelContentBlockDeltaEvent',
-            delta: { type: 'toolUseInputDelta', input: '{invalid json' },
+            delta: { type: 'toolUseInputDelta', input: '{"location"' },
           }
           yield { type: 'modelContentBlockStopEvent' }
           yield { type: 'modelMessageStopEvent', stopReason: 'maxTokens' }
         })
 
         const messages = [new Message({ role: 'user', content: [new TextBlock('Hi')] })]
 
+        await expect(async () => await collectGenerator(provider.streamAggregated(messages))).rejects.toThrow(
+          MaxTokensError
+        )
+      })
+
+      it('re-throws SyntaxError when tool_use JSON is malformed and stop reason is not maxTokens', async () => {
+        const provider = new TestModelProvider(async function* () {
+          yield { type: 'modelMessageStartEvent', role: 'assistant' }
+          yield {
+            type: 'modelContentBlockStartEvent',
+            start: { type: 'toolUseStart', toolUseId: 'tool1', name: 'get_weather' },
+          }
+          yield {
+            type: 'modelContentBlockDeltaEvent',
+            delta: { type: 'toolUseInputDelta', input: '{invalid json' },
+          }
+          yield { type: 'modelContentBlockStopEvent' }
+          yield { type: 'modelMessageStopEvent', stopReason: 'endTurn' }
+        })
+
+        const messages = [new Message({ role: 'user', content: [new TextBlock('Hi')] })]
+
         try {
           await collectGenerator(provider.streamAggregated(messages))
           expect.fail('Expected error to be thrown')

strands-ts/src/models/model.ts

@@ -349,6 +349,7 @@ export abstract class Model<T extends BaseModelConfig = BaseModelConfig> {
       let finalStopReason: StopReason | null = null
       let metadata: ModelMetadataEvent | undefined = undefined
       let redactionMessage: string | undefined = undefined
+      let pendingParseError: SyntaxError | null = null
 
       for await (const event_data of this.stream(messages, options)) {
         const event = this._convert_to_class_event(event_data)
@@ -426,7 +427,7 @@ export abstract class Model<T extends BaseModelConfig = BaseModelConfig> {
             } catch (e: unknown) {
               if (e instanceof SyntaxError) {
                 logger.error('unable to parse JSON string', e)
-                throw e
+                pendingParseError = e
               }
             }
             break
@@ -474,6 +475,18 @@ export abstract class Model<T extends BaseModelConfig = BaseModelConfig> {
         throw new ModelError('Stream ended without completing a message')
       }
 
+      // If tool_use JSON failed to parse and the stream stopped due to maxTokens, the JSON was
+      // truncated by the token limit — surface MaxTokensError rather than a bare SyntaxError.
+      if (pendingParseError) {
+        if (finalStopReason === 'maxTokens') {
+          throw new MaxTokensError(
+            'Model reached maximum token limit (tool_use JSON was truncated). This is an unrecoverable state that requires intervention.',
+            stoppedMessage
+          )
+        }
+        throw pendingParseError
+      }
+
       // Attach metadata after redaction so it applies to the final message.
       const messageMetadata: MessageMetadata = {
         ...(metadata?.usage !== undefined && { usage: metadata.usage }),