fix(frontend): render full content for multi-part AI messages (#3649)

* fix(frontend): render full content for multi-part AI messages

    Gemini streams the first content block as a {type:text} object carrying
    the thinking signature, then emits continuation deltas as bare strings.
    The content extractors only kept {type:text} objects and dropped the
    string parts, truncating each AI message to its first block.

    Handle string elements in extractContentFromMessage, extractTextFromMessage,
    and textOfMessage so the full message renders.

    Fixes #1000

* test(frontend): cover multi-part AI message content extraction

Add a regression test for the #1000 truncation: an AI message whose
content is [{type:text, signature}, "bare string"] (Gemini's shape after
LangChain merge_content). Fails on main, passes with the extractor fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(frontend): cover textOfMessage multi-part content extraction

Add tests for the bare-string continuation case and the null-when-empty
contract, and document why textOfMessage joins flat ("") while the body
extractors join with "\n". Addresses review feedback on #3649.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/core/messages/utils.ts

@@ -259,7 +259,13 @@ export function extractTextFromMessage(message: Message) {
   }
   if (Array.isArray(message.content)) {
     return message.content
-      .map((content) => (content.type === "text" ? content.text : ""))
+      .map((content) =>
+        typeof content === "string"
+          ? content
+          : content.type === "text"
+            ? content.text
+            : "",
+      )
       .join("\n")
       .trim();
   }
@@ -323,6 +329,9 @@ export function extractContentFromMessage(message: Message) {
   if (Array.isArray(message.content)) {
     return message.content
       .map((content) => {
+        if (typeof content === "string") {
+          return content;
+        }
         switch (content.type) {
           case "text":
             return content.text;

frontend/src/core/threads/utils.ts

@@ -43,11 +43,14 @@ export function textOfMessage(message: Message) {
   if (typeof message.content === "string") {
     return message.content;
   } else if (Array.isArray(message.content)) {
-    for (const part of message.content) {
-      if (part.type === "text") {
-        return part.text;
-      }
-    }
+    // Flat join ("") for single-line consumers (input box, titles); the rendered
+    // body uses extractContentFromMessage, which joins multi-part content with "\n".
+    const text = message.content
+      .map((part) =>
+        typeof part === "string" ? part : part.type === "text" ? part.text : "",
+      )
+      .join("");
+    return text.length > 0 ? text : null;
   }
   return null;
 }

frontend/tests/unit/core/messages/utils.test.ts

@@ -3,6 +3,7 @@ import { describe, expect, test } from "vitest";
 
 import {
   extractContentFromMessage,
+  extractTextFromMessage,
   extractReasoningContentFromMessage,
   getAssistantTurnCopyData,
   getAssistantTurnUsageMessages,
@@ -508,3 +509,35 @@ test("keeps streaming assistant hidden when a hidden control message follows it"
     ),
   ).toBe(true);
 });
+
+describe("multi-part content with bare-string continuations", () => {
+  // Gemini streams the first content block as a {type:"text"} object carrying
+  // the thinking signature, then emits continuation deltas as plain strings.
+  // LangChain's Python merge_content preserves these as bare-string elements,
+  // so the finalized message content is [{type:"text", ...}, "...rest..."].
+  const geminiMessage = {
+    id: "ai-1",
+    type: "ai",
+    content: [
+      {
+        type: "text",
+        text: "First block carrying the signature.",
+        extras: { signature: "abc123" },
+        index: 0,
+      },
+      "Continuation streamed as a bare string.",
+    ],
+  } as unknown as Message;
+
+  test("extractContentFromMessage includes the bare-string parts", () => {
+    expect(extractContentFromMessage(geminiMessage)).toBe(
+      "First block carrying the signature.\nContinuation streamed as a bare string.",
+    );
+  });
+
+  test("extractTextFromMessage includes the bare-string parts", () => {
+    expect(extractTextFromMessage(geminiMessage)).toBe(
+      "First block carrying the signature.\nContinuation streamed as a bare string.",
+    );
+  });
+});

frontend/tests/unit/core/threads/utils.test.ts

@@ -1,6 +1,11 @@
+import type { Message } from "@langchain/langgraph-sdk";
 import { expect, test } from "vitest";
 
-import { channelSourceOfThread, pathOfThread } from "@/core/threads/utils";
+import {
+  channelSourceOfThread,
+  pathOfThread,
+  textOfMessage,
+} from "@/core/threads/utils";
 
 test("uses standard chat route when thread has no agent context", () => {
   expect(pathOfThread("thread-123")).toBe("/workspace/chats/thread-123");
@@ -81,3 +86,35 @@ test("ignores threads without valid IM channel source metadata", () => {
     }),
   ).toBeNull();
 });
+
+test("textOfMessage concatenates object and bare-string content parts", () => {
+  // Gemini's finalized shape: first signed {type:text} block + bare-string
+  // continuation. textOfMessage joins flat ("") for single-line consumers.
+  const message = {
+    id: "ai-1",
+    type: "ai",
+    content: [
+      {
+        type: "text",
+        text: "First block.",
+        extras: { signature: "abc123" },
+        index: 0,
+      },
+      " Continuation as a bare string.",
+    ],
+  } as unknown as Message;
+
+  expect(textOfMessage(message)).toBe(
+    "First block. Continuation as a bare string.",
+  );
+});
+
+test("textOfMessage returns null when array content has no text", () => {
+  const message = {
+    id: "ai-1",
+    type: "ai",
+    content: [{ type: "image_url", image_url: "https://example.com/x.png" }],
+  } as unknown as Message;
+
+  expect(textOfMessage(message)).toBeNull();
+});