fix: preserve anthropic assistant ids in tool-first streams

packages/typescript/ai-anthropic/src/adapters/text.ts

@@ -695,6 +695,7 @@ export class AnthropicTextAdapter<
                   toolCallId: existing.id,
                   toolCallName: existing.name,
                   toolName: existing.name,
+                  parentMessageId: messageId,
                   model,
                   timestamp,
                   index: currentToolIndex,
@@ -725,6 +726,7 @@ export class AnthropicTextAdapter<
                   toolCallId: existing.id,
                   toolCallName: existing.name,
                   toolName: existing.name,
+                  parentMessageId: messageId,
                   model,
                   timestamp,
                   index: currentToolIndex,

packages/typescript/ai-anthropic/tests/anthropic-adapter.test.ts

@@ -686,4 +686,71 @@ describe('Anthropic stream processing', () => {
       type: 'RUN_FINISHED',
     })
   })
+
+  it('emits parentMessageId on tool-first tool call chunks', async () => {
+    const mockStream = (async function* () {
+      yield {
+        type: 'content_block_start',
+        index: 0,
+        content_block: {
+          type: 'tool_use',
+          id: 'toolu_weather',
+          name: 'lookup_weather',
+          input: {},
+        },
+      }
+      yield {
+        type: 'content_block_delta',
+        index: 0,
+        delta: {
+          type: 'input_json_delta',
+          partial_json: '{"location":"Berlin"}',
+        },
+      }
+      yield { type: 'content_block_stop', index: 0 }
+      yield {
+        type: 'content_block_start',
+        index: 1,
+        content_block: { type: 'text', text: '' },
+      }
+      yield {
+        type: 'content_block_delta',
+        index: 1,
+        delta: { type: 'text_delta', text: 'It is sunny.' },
+      }
+      yield { type: 'content_block_stop', index: 1 }
+      yield {
+        type: 'message_delta',
+        delta: { stop_reason: 'end_turn' },
+        usage: { output_tokens: 7 },
+      }
+      yield { type: 'message_stop' }
+    })()
+
+    mocks.betaMessagesCreate.mockResolvedValueOnce(mockStream)
+
+    const adapter = createAdapter('claude-3-7-sonnet-20250219')
+
+    const chunks: StreamChunk[] = []
+    for await (const chunk of chat({
+      adapter,
+      messages: [{ role: 'user', content: 'What is the weather in Berlin?' }],
+      tools: [weatherTool],
+    })) {
+      chunks.push(chunk)
+    }
+
+    const textStart = chunks.find(
+      (chunk): chunk is Extract<StreamChunk, { type: 'TEXT_MESSAGE_START' }> =>
+        chunk.type === 'TEXT_MESSAGE_START',
+    )
+    const toolStart = chunks.find(
+      (chunk): chunk is Extract<StreamChunk, { type: 'TOOL_CALL_START' }> =>
+        chunk.type === 'TOOL_CALL_START',
+    )
+
+    expect(textStart).toBeDefined()
+    expect(toolStart).toBeDefined()
+    expect(toolStart?.parentMessageId).toBe(textStart?.messageId)
+  })
 })

packages/typescript/ai/tests/stream-processor.test.ts

@@ -2485,6 +2485,53 @@ describe('StreamProcessor', () => {
         expect(toolCallPart.state).toBe('input-complete')
       }
     })
+
+    it('should preserve the server message id in tool-first flows when parentMessageId is provided', () => {
+      const processor = new StreamProcessor()
+
+      processor.processChunk(
+        chunk('TOOL_CALL_START', {
+          toolCallId: 'tc-1',
+          toolCallName: 'lookupWeather',
+          toolName: 'lookupWeather',
+          parentMessageId: 'anthropic-msg-1',
+        }),
+      )
+
+      let messages = processor.getMessages()
+      expect(messages).toHaveLength(1)
+      expect(messages[0]?.id).toBe('anthropic-msg-1')
+
+      processor.processChunk(ev.toolArgs('tc-1', '{"location":"Berlin"}'))
+      processor.processChunk(ev.toolEnd('tc-1', 'lookupWeather'))
+
+      processor.processChunk(
+        chunk('TEXT_MESSAGE_START', {
+          messageId: 'anthropic-msg-1',
+          role: 'assistant' as const,
+        }),
+      )
+      processor.processChunk(ev.textContent('It is sunny.', 'anthropic-msg-1'))
+      processor.processChunk(ev.textEnd('anthropic-msg-1'))
+      processor.finalizeStream()
+
+      messages = processor.getMessages()
+      expect(messages).toHaveLength(1)
+      expect(messages[0]?.id).toBe('anthropic-msg-1')
+      expect(messages[0]?.parts).toEqual([
+        {
+          type: 'tool-call',
+          id: 'tc-1',
+          name: 'lookupWeather',
+          arguments: '{"location":"Berlin"}',
+          state: 'input-complete',
+        },
+        {
+          type: 'text',
+          content: 'It is sunny.',
+        },
+      ])
+    })
   })
 
   describe('double onStreamEnd guard', () => {