fix(ui): tolerate absent chat-event messages in text extractors (#111918)

ui/src/lib/chat/message-extract.test.ts

@@ -1,7 +1,12 @@
 // @vitest-environment node
 // Control UI tests cover message extract behavior.
 import { describe, expect, it } from "vitest";
-import { extractText, extractTextCached, extractThinkingCached } from "./message-extract.ts";
+import {
+  extractRawText,
+  extractText,
+  extractTextCached,
+  extractThinkingCached,
+} from "./message-extract.ts";
 
 describe("extractTextCached", () => {
   it("matches extractText output", () => {
@@ -137,3 +142,16 @@ describe("extractThinkingCached", () => {
     expect(extractThinkingCached(message)).toBe("Plan A");
   });
 });
+
+describe("nullish messages", () => {
+  // Chat events can arrive without a message (tool-only or heartbeat finals);
+  // every unknown-typed extractor must read that as "no text", not throw.
+  it("returns null instead of throwing for absent messages", () => {
+    for (const message of [undefined, null]) {
+      expect(extractText(message)).toBeNull();
+      expect(extractTextCached(message)).toBeNull();
+      expect(extractRawText(message)).toBeNull();
+      expect(extractThinkingCached(message)).toBeNull();
+    }
+  });
+});

ui/src/lib/chat/message-extract.ts

@@ -29,6 +29,11 @@ function processMessageText(text: string, role: string): string {
 }
 
 export function extractText(message: unknown): string | null {
+  // Chat events may carry no message at all (tool-only or heartbeat finals);
+  // a nullish message means "no text", never a crash.
+  if (message == null) {
+    return null;
+  }
   const m = message as Record<string, unknown>;
   const role = typeof m.role === "string" ? m.role : "";
   const raw =
@@ -53,6 +58,9 @@ export function extractTextCached(message: unknown): string | null {
 }
 
 function extractThinking(message: unknown): string | null {
+  if (message == null) {
+    return null;
+  }
   const m = message as Record<string, unknown>;
   const content = m.content;
   const parts: string[] = [];
@@ -84,6 +92,9 @@ export function extractThinkingCached(message: unknown): string | null {
 }
 
 export function extractRawText(message: unknown): string | null {
+  if (message == null) {
+    return null;
+  }
   const m = message as Record<string, unknown>;
   const role = normalizeLowercaseStringOrEmpty(m.role);
   const content = m.content;