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fix(ai): normalize AG-UI snapshot messages to UIMessage[] in MESSAGES_SNAPSHOT handler #698

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fix(ai): normalize AG-UI snapshot messages to UIMessage[] in MESSAGES_SNAPSHOT handler #698

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bc8e675
fix(ai): normalize AG-UI snapshot messages to UIMessage[] in MESSAGES…
amitsaroj Jun 3, 2026
b091adc
fix(ai): preserve original msg.id in MESSAGES_SNAPSHOT normalization
amitsaroj Jun 3, 2026
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14 changes: 11 additions & 3 deletions packages/ai/src/activities/chat/stream/processor.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,7 @@
* @see docs/chat-architecture.md — Canonical reference for AG-UI chunk ordering,
* adapter contract, single-shot flows, and expected UIMessage output.
*/
import { generateMessageId, uiMessageToModelMessages } from '../messages.js'
import { generateMessageId, modelMessageToUIMessage, uiMessageToModelMessages } from '../messages.js'
import { normalizeToolResult } from '../../../utilities/tool-result'
import { defaultJSONParser } from './json-parser'
import {
Expand Down Expand Up @@ -870,8 +870,16 @@ export class StreamProcessor {
chunk: Extract<StreamChunk, { type: 'MESSAGES_SNAPSHOT' }>,
): void {
this.resetStreamState()
// AG-UI Message[] is compatible with UIMessage[] at runtime
this.messages = [...chunk.messages] as Array<UIMessage>
// Normalize AG-UI messages to UIMessage[] to ensure each message has
// a `parts` array. AG-UI snapshot messages carry `content` but no
// `parts`, so casting them directly as UIMessage[] is unsafe and causes
// "Cannot read properties of undefined (reading 'find')" when any code
// later accesses message.parts (e.g. onToolCallStateChange devtools handler).
this.messages = chunk.messages.map((msg) =>
'parts' in msg
? (msg as UIMessage)
: modelMessageToUIMessage(msg as any, (msg as { id?: string }).id ?? generateMessageId()),
)
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this.emitMessagesChange()
}

Expand Down