[v6.0] fix(ai): omit empty assistant message for data-only blocks in convertToModelMessages#16793
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…ToModelMessages (#15040) ## Background `convertToModelMessages` was emitting `{ role: 'assistant', content: [] }` when a block contained only unknown data parts (i.e. a data part appears before `step-start` with no `convertDataPart` provided, or `convertDataPart` returns `null`). LLMs reject empty assistant messages, causing downstream failures. ## Summary - Guard `content.length > 0` before pushing the assistant message in `processBlock()` in `convert-to-model-messages.ts` - Add regression test for the data-part-before-step-start scenario ## Manual Verification Verified that this can happen through the public UI stream helpers with `streamText`, not only from a hand-written message fixture. The real route pattern from `content/docs/04-ai-sdk-ui/20-streaming-data.mdx` writes a persistent data part before merging the model stream: ```ts writer.write({ type: 'data-weather', id: 'weather-1', data: { city: 'San Francisco', status: 'loading' }, }); const result = streamText({ model, prompt: 'What is the weather in San Francisco?', }); writer.merge(result.toUIMessageStream()); ``` That ordering is valid because `createUIMessageStream` enqueues `writer.write()` immediately, while `writer.merge()` forwards chunks from the merged stream later. The merged `streamText` UI stream is what emits `start-step`. I reproduced the route shape end-to-end with `streamText` and `MockLanguageModelV4`: ```ts function createWeatherModel() { return new MockLanguageModelV4({ doStream: async () => ({ stream: convertArrayToReadableStream([ { type: 'stream-start', warnings: [] }, { type: 'response-metadata', id: 'response-1', modelId: 'mock-model-id', timestamp: new Date(0), }, { type: 'text-start', id: 'text-1' }, { type: 'text-delta', id: 'text-1', delta: 'It is sunny.' }, { type: 'text-end', id: 'text-1' }, { type: 'finish', finishReason: { unified: 'stop', raw: 'stop' }, usage: { inputTokens: { total: 3, noCache: 3, cacheRead: undefined, cacheWrite: undefined, }, outputTokens: { total: 4, text: 4, reasoning: undefined, }, }, }, ]), }), }); } ``` The test route writes the data part first, then merges `streamText(...).toUIMessageStream()`: ```ts const chunks = await convertReadableStreamToArray( createUIMessageStream<WeatherUIMessage>({ generateId: () => 'msg-123', onEnd({ responseMessage }) { persistedMessage = responseMessage; }, execute({ writer }) { writer.write({ type: 'data-weather', id: 'weather-1', data: { city: 'San Francisco', status: 'loading' }, }); const result = streamText({ model: createWeatherModel(), prompt: 'What is the weather in San Francisco?', }); writer.merge(result.toUIMessageStream()); }, }), ); ``` The recorded UI chunk order was: ```ts [ 'data-weather', 'start', 'start-step', 'text-start', 'text-delta', 'text-end', 'finish-step', 'finish', ] ``` The persisted assistant message had `data-weather` before `step-start`: ```ts [ { type: 'data-weather', id: 'weather-1', data: { city: 'San Francisco', status: 'loading' }, }, { type: 'step-start' }, { type: 'text', text: 'It is sunny.', state: 'done', providerMetadata: undefined, }, ] ``` Against the test-only commit (`6fa891d8b6`), the repro failed because `convertToModelMessages([persistedMessage])` returned an extra empty assistant message: ```ts [ { role: 'assistant', content: [] }, { role: 'assistant', content: [{ type: 'text', text: 'It is sunny.' }] }, ] ``` Against the fixed commit (`872f91b540`), the same repro passed and returned only the valid assistant message: ```ts [ { role: 'assistant', content: [{ type: 'text', text: 'It is sunny.' }] }, ] ``` Focused test on the fixed head: ```sh vitest --config packages/ai/vitest.node.config.js --run packages/ai/src/ui/convert-to-model-messages.test.ts ``` Result: 64 tests passed, no type errors. ## Checklist - [x] All commits are signed - [x] Tests have been added / updated - [ ] Documentation has been added / updated - [x] A _patch_ changeset for relevant packages has been added - [x] I have reviewed this pull request ## Related Issues Fixes #13513 --------- Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com> Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com> (cherry picked from commit d598481)
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Backport of #15040 to
release-v6.0(clean cherry-pick). Data-only blocks previously produced empty assistant messages.Part of the v6.0 backport tracking issue #16767.