diff --git a/src/gateway/server-methods/chat.test.ts b/src/gateway/server-methods/chat.test.ts
new file mode 100644
index 000000000..f39710c38
--- /dev/null
+++ b/src/gateway/server-methods/chat.test.ts
@@ -0,0 +1,106 @@
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+import { _testExports } from "./chat.js";
+
+const { findTranscriptLeafId, appendAssistantTranscriptMessage } = _testExports;
+
+describe("appendAssistantTranscriptMessage parentId chain", () => {
+  let tempDir: string;
+
+  beforeEach(() => {
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "chat-test-"));
+  });
+
+  afterEach(() => {
+    fs.rmSync(tempDir, { recursive: true, force: true });
+  });
+
+  it("sets parentId to previous message id when appending second message", () => {
+    const transcriptPath = path.join(tempDir, "transcript.jsonl");
+    const sessionId = "test-session";
+
+    // Append first message
+    const result1 = appendAssistantTranscriptMessage({
+      message: "First message",
+      sessionId,
+      storePath: undefined,
+      sessionFile: transcriptPath,
+      createIfMissing: true,
+    });
+    expect(result1.ok).toBe(true);
+    if (!result1.ok) throw new Error("First append failed");
+
+    const firstMessageId = result1.messageId;
+
+    // Append second message
+    const result2 = appendAssistantTranscriptMessage({
+      message: "Second message",
+      sessionId,
+      storePath: undefined,
+      sessionFile: transcriptPath,
+    });
+    expect(result2.ok).toBe(true);
+
+    // Read and parse the transcript
+    const lines = fs
+      .readFileSync(transcriptPath, "utf-8")
+      .trim()
+      .split("\n")
+      .filter((l) => l.trim());
+
+    // Should have: header, first message, second message
+    expect(lines.length).toBe(3);
+
+    const secondMessageEntry = JSON.parse(lines[2]);
+    expect(secondMessageEntry.type).toBe("message");
+
+    // THE KEY ASSERTION: second message's parentId should be first message's id
+    // This fails on upstream main (parentId would be undefined/null)
+    // This passes on fix branch (parentId equals firstMessageId)
+    expect(secondMessageEntry.parentId).toBe(firstMessageId);
+  });
+});
+
+describe("findTranscriptLeafId", () => {
+  let tempDir: string;
+
+  beforeEach(() => {
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "chat-test-"));
+  });
+
+  afterEach(() => {
+    fs.rmSync(tempDir, { recursive: true, force: true });
+  });
+
+  it("returns null for non-existent file", () => {
+    const result = findTranscriptLeafId(path.join(tempDir, "missing.jsonl"));
+    expect(result).toBeNull();
+  });
+
+  it("returns null for file with only header", () => {
+    const transcriptPath = path.join(tempDir, "header-only.jsonl");
+    fs.writeFileSync(
+      transcriptPath,
+      JSON.stringify({ type: "session", id: "test", version: 1 }) + "\n",
+    );
+    const result = findTranscriptLeafId(transcriptPath);
+    expect(result).toBeNull();
+  });
+
+  it("returns last message id", () => {
+    const transcriptPath = path.join(tempDir, "with-messages.jsonl");
+    const lines = [
+      JSON.stringify({ type: "session", id: "test", version: 1 }),
+      JSON.stringify({ type: "message", id: "msg-001", message: {} }),
+      JSON.stringify({ type: "message", id: "msg-002", message: {} }),
+    ];
+    fs.writeFileSync(transcriptPath, lines.join("\n") + "\n");
+
+    const result = findTranscriptLeafId(transcriptPath);
+    expect(result).toBe("msg-002");
+  });
+});
diff --git a/src/gateway/server-methods/chat.ts b/src/gateway/server-methods/chat.ts
index 9010a6f21..c53e737ff 100644
--- a/src/gateway/server-methods/chat.ts
+++ b/src/gateway/server-methods/chat.ts
@@ -82,6 +82,36 @@ function ensureTranscriptFile(params: { transcriptPath: string; sessionId: strin
   }
 }
 
+/**
+ * Find the leaf (last message) ID from a transcript file.
+ * Returns null if file doesn't exist, is empty, or has no messages (header only).
+ */
+function findTranscriptLeafId(transcriptPath: string): string | null {
+  if (!fs.existsSync(transcriptPath)) return null;
+
+  try {
+    const content = fs.readFileSync(transcriptPath, "utf-8");
+    const lines = content.split(/\r?\n/).filter((line) => line.trim());
+    if (lines.length === 0) return null;
+
+    // Walk backwards to find the last message entry with an id
+    for (let i = lines.length - 1; i >= 0; i--) {
+      try {
+        const parsed = JSON.parse(lines[i]);
+        // Skip non-message entries (like session headers)
+        if (parsed?.type === "message" && typeof parsed?.id === "string") {
+          return parsed.id;
+        }
+      } catch {
+        // Skip malformed lines
+      }
+    }
+    return null;
+  } catch {
+    return null;
+  }
+}
+
 function appendAssistantTranscriptMessage(params: {
   message: string;
   label?: string;
@@ -112,6 +142,9 @@ function appendAssistantTranscriptMessage(params: {
     }
   }
 
+  // Find the current leaf ID before we append
+  const parentId = findTranscriptLeafId(transcriptPath);
+
   const now = Date.now();
   const messageId = randomUUID().slice(0, 8);
   const labelPrefix = params.label ? `[${params.label}]\n\n` : "";
@@ -125,6 +158,7 @@ function appendAssistantTranscriptMessage(params: {
   const transcriptEntry = {
     type: "message",
     id: messageId,
+    parentId,
     timestamp: new Date(now).toISOString(),
     message: messageBody,
   };
@@ -679,3 +713,9 @@ export const chatHandlers: GatewayRequestHandlers = {
     respond(true, { ok: true, messageId });
   },
 };
+
+// Exported for testing only
+export const _testExports = {
+  findTranscriptLeafId,
+  appendAssistantTranscriptMessage,
+};