diff --git a/src/acp-agent.ts b/src/acp-agent.ts
index f107af74..1c8e8bb7 100644
--- a/src/acp-agent.ts
+++ b/src/acp-agent.ts
@@ -656,6 +656,39 @@ export function isLocalCommandMetadata(content: unknown): boolean {
   return stripLocalCommandMetadata(content) === null;
 }
 
+/**
+ * True for the synthetic assistant message the CLI injects into the transcript
+ * when a turn fails authentication (e.g. "Not logged in · Please run /login",
+ * "Session expired. Please run /login to sign in again."). The `/login`
+ * instruction is Claude Code TUI-specific and meaningless to ACP clients
+ * (issue #863). The live prompt loop suppresses the text and fails the turn
+ * with `authRequired` so the client can run its own auth flow; replay must
+ * skip it too — both for parity with what the client saw live and because the
+ * message stays in the transcript forever, so it would resurface on every
+ * session/load even after the user has logged back in.
+ *
+ * Takes the API message (`message.message`), which replay only knows as
+ * `unknown`. The persisted record's structured `error: "authentication_failed"`
+ * marker is stripped by `getSessionMessages`, so the synthetic model + text is
+ * all both paths have to match on.
+ */
+export function isSyntheticLoginMessage(apiMessage: unknown): boolean {
+  if (!apiMessage || typeof apiMessage !== "object") {
+    return false;
+  }
+  const { model, content } = apiMessage as { model?: unknown; content?: unknown };
+  if (model !== "<synthetic>" || !Array.isArray(content) || content.length !== 1) {
+    return false;
+  }
+  const block = content[0] as { type?: unknown; text?: unknown } | null;
+  return (
+    !!block &&
+    block.type === "text" &&
+    typeof block.text === "string" &&
+    block.text.includes("Please run /login")
+  );
+}
+
 const PERMISSION_MODE_ALIASES: Record<string, PermissionMode> = {
   auto: "auto",
   default: "default",
@@ -2570,14 +2603,7 @@ export class ClaudeAcpAgent {
               break;
             }
 
-            if (
-              message.type === "assistant" &&
-              message.message.model === "<synthetic>" &&
-              Array.isArray(message.message.content) &&
-              message.message.content.length === 1 &&
-              message.message.content[0].type === "text" &&
-              message.message.content[0].text.includes("Please run /login")
-            ) {
+            if (message.type === "assistant" && isSyntheticLoginMessage(message.message)) {
               failActive(RequestError.authRequired());
               break;
             }
@@ -3192,6 +3218,13 @@ export class ClaudeAcpAgent {
         replaySession.messageIdToUuid.set(replayMessageId, message.uuid);
       }
 
+      // The live prompt loop converts the synthetic "Please run /login"
+      // assistant message into an authRequired error instead of showing its
+      // TUI-specific text; skip it on replay too (issue #863).
+      if (message.type === "assistant" && isSyntheticLoginMessage(message.message)) {
+        continue;
+      }
+
       // @ts-expect-error - untyped in SDK but we handle all of these
       let content: unknown = message.message.content;
       // @ts-expect-error - untyped in SDK but we handle all of these
diff --git a/src/tests/acp-agent.test.ts b/src/tests/acp-agent.test.ts
index f6de2292..45bff93f 100644
--- a/src/tests/acp-agent.test.ts
+++ b/src/tests/acp-agent.test.ts
@@ -30,6 +30,7 @@ import {
   toAcpNotifications,
   promptToClaude,
   isLocalCommandMetadata,
+  isSyntheticLoginMessage,
   stripLocalCommandMetadata,
   ClaudeAcpAgent,
   claudeCliPath,
@@ -59,6 +60,10 @@ vi.mock("@anthropic-ai/claude-agent-sdk", async (importOriginal) => {
     ...actual,
     deleteSession: vi.fn(),
     getSessionInfo: vi.fn(),
+    // Delegates to the real implementation so integration tests that read
+    // actual transcripts keep working; unit tests override per-call with
+    // `mockResolvedValueOnce`.
+    getSessionMessages: vi.fn(actual.getSessionMessages),
   };
 });
 import type {
@@ -1470,6 +1475,94 @@ describe("isLocalCommandMetadata", () => {
   });
 });
 
+describe("synthetic login message (issue #863)", () => {
+  // The exact shape the CLI persists (and streams) when a turn fails auth:
+  // model "<synthetic>", a single text block, structured `error` stripped by
+  // getSessionMessages.
+  const syntheticLoginApiMessage = {
+    id: "0a1dfa6b-1c2f-4aa2-8bff-ae1690acd6e1",
+    model: "<synthetic>",
+    role: "assistant",
+    type: "message",
+    stop_reason: "stop_sequence",
+    content: [{ type: "text", text: "Not logged in · Please run /login" }],
+  };
+
+  it("isSyntheticLoginMessage matches the CLI auth-error message", () => {
+    expect(isSyntheticLoginMessage(syntheticLoginApiMessage)).toBe(true);
+    expect(
+      isSyntheticLoginMessage({
+        ...syntheticLoginApiMessage,
+        content: [{ type: "text", text: "Session expired. Please run /login to sign in again." }],
+      }),
+    ).toBe(true);
+  });
+
+  it("isSyntheticLoginMessage does not match real assistant messages", () => {
+    // Real model output that merely mentions the phrase.
+    expect(
+      isSyntheticLoginMessage({
+        ...syntheticLoginApiMessage,
+        model: "claude-sonnet-5",
+      }),
+    ).toBe(false);
+    // Other synthetic error texts (e.g. API errors) still replay as-is.
+    expect(
+      isSyntheticLoginMessage({
+        ...syntheticLoginApiMessage,
+        content: [{ type: "text", text: "API Error: 500 Internal Server Error" }],
+      }),
+    ).toBe(false);
+    expect(isSyntheticLoginMessage(undefined)).toBe(false);
+    expect(isSyntheticLoginMessage("Not logged in · Please run /login")).toBe(false);
+  });
+
+  it("loadSession replay skips the synthetic login message but keeps the rest", async () => {
+    const updates: SessionNotification[] = [];
+    const client = {
+      sessionUpdate: async (u: SessionNotification) => {
+        updates.push(u);
+      },
+    } as unknown as AcpClient;
+    const agent = new ClaudeAcpAgent(client, { log: () => {}, error: () => {} });
+
+    vi.mocked(getSessionMessages).mockResolvedValueOnce([
+      {
+        type: "user",
+        uuid: "u1",
+        session_id: "s1",
+        parent_tool_use_id: null,
+        parent_agent_id: null,
+        message: { role: "user", content: [{ type: "text", text: "hi, say one word" }] },
+      },
+      {
+        type: "assistant",
+        uuid: "a1",
+        session_id: "s1",
+        parent_tool_use_id: null,
+        parent_agent_id: null,
+        message: syntheticLoginApiMessage,
+      },
+    ] as Awaited<ReturnType<typeof getSessionMessages>>);
+
+    await (
+      agent as unknown as { replaySessionHistory(sessionId: string): Promise<void> }
+    ).replaySessionHistory("s1");
+
+    // The user's prompt still replays…
+    expect(
+      updates.some(
+        (u) =>
+          u.update.sessionUpdate === "user_message_chunk" &&
+          u.update.content.type === "text" &&
+          u.update.content.text.includes("hi, say one word"),
+      ),
+    ).toBe(true);
+    // …but the TUI-specific "/login" instruction never reaches the client.
+    expect(JSON.stringify(updates)).not.toContain("/login");
+  });
+});
+
 describe("escape markdown", () => {
   it("should escape markdown characters", () => {
     let text = "Hello *world*!";