fix(ui): clear completed assistant stream state

apps/ui/src/lib/chat-utils.ts

@@ -5,7 +5,7 @@ import {
   COMPOSER_TEXTAREA_MAX_RATIO,
   COMPOSER_TEXTAREA_MIN_RATIO,
   COMPOSER_TEXTAREA_SAFE_MIN_HEIGHT
-} from "./constants"
+} from "./constants.ts"
 
 export type MessageStreamState = "streaming" | "error"
 
@@ -92,14 +92,18 @@ export function mergeSessionHistory(current: UiChatMessage[], nextMessages: UiCh
       return message
     }
 
+    if (local.streamState === "streaming" && local.content.length > message.content.length) {
       return {
         ...message,
-      content: local.content.length > message.content.length ? local.content : message.content,
+        content: local.content,
         createdAt: message.createdAt || local.createdAt,
         streamState: local.streamState ?? message.streamState,
         statusLabel: local.statusLabel ?? message.statusLabel,
         statusDetail: local.statusDetail ?? message.statusDetail
       }
+    }
+
+    return message
   })
   const nextIds = new Set(nextMessages.map((message) => message.id))
   const localOnlyMessages = current.filter((message) => !nextIds.has(message.id))

apps/ui/test/chatUtils.test.ts

+import test from "node:test"
+import assert from "node:assert/strict"
+
+import type { UiChatMessage } from "../src/lib/chat-utils.ts"
+import { mergeSessionHistory } from "../src/lib/chat-utils.ts"
+
+test("mergeSessionHistory clears local assistant transient state when server has final content", () => {
+  const current: UiChatMessage[] = [{
+    id: "assistant-1",
+    role: "assistant",
+    content: "Final answer",
+    createdAt: "2026-05-18T08:00:00.000Z",
+    streamState: "streaming",
+    statusLabel: "Thinking",
+    statusDetail: "Using tools"
+  }]
+  const nextMessages: UiChatMessage[] = [{
+    id: "assistant-1",
+    role: "assistant",
+    content: "Final answer",
+    createdAt: "2026-05-18T08:00:01.000Z"
+  }]
+
+  const [message] = mergeSessionHistory(current, nextMessages)
+
+  assert.equal(message.content, "Final answer")
+  assert.equal(message.streamState, undefined)
+  assert.equal(message.statusLabel, undefined)
+  assert.equal(message.statusDetail, undefined)
+})
+
+test("mergeSessionHistory preserves active local streaming state when server content is stale", () => {
+  const current: UiChatMessage[] = [{
+    id: "assistant-1",
+    role: "assistant",
+    content: "Streaming answer with more text",
+    createdAt: "2026-05-18T08:00:00.000Z",
+    streamState: "streaming",
+    statusLabel: "Thinking",
+    statusDetail: "Drafting"
+  }]
+  const nextMessages: UiChatMessage[] = [{
+    id: "assistant-1",
+    role: "assistant",
+    content: "Streaming answer",
+    createdAt: "2026-05-18T08:00:01.000Z"
+  }]
+
+  const [message] = mergeSessionHistory(current, nextMessages)
+
+  assert.equal(message.content, "Streaming answer with more text")
+  assert.equal(message.streamState, "streaming")
+  assert.equal(message.statusLabel, "Thinking")
+  assert.equal(message.statusDetail, "Drafting")
+})

apps/ui/tsconfig.json

@@ -3,6 +3,7 @@
   "compilerOptions": {
     "module": "ESNext",
     "moduleResolution": "Bundler",
+    "allowImportingTsExtensions": true,
     "jsx": "react-jsx",
     "lib": ["ES2022", "DOM", "DOM.Iterable"],
     "types": ["vite/client"]