diff --git a/frontend/src/features/agents/CustomAgentChatView.test.tsx b/frontend/src/features/agents/CustomAgentChatView.test.tsx
index 77e42f8..6ee04ff 100644
--- a/frontend/src/features/agents/CustomAgentChatView.test.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.test.tsx
@@ -151,6 +151,250 @@ describe("CustomAgentChatView", () => {
     expect(screen.queryByText(/structured session gone/)).toBeNull();
   });
 
+  it("recovers and retries the prompt when send reports NOT_FOUND", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(async () => ({
+        sessionId: "chat-session-2",
+        assignedConversationId: "conversation-2",
+      })),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi
+        .fn()
+        .mockRejectedValueOnce({
+          code: "NOT_FOUND",
+          message: "not found: structured session stale-session",
+        })
+        .mockImplementationOnce(async (_sessionId: string, _prompt: string, onChunk) => {
+          onChunk({ kind: "final", content: "Recovered reply" });
+        }),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    const onSessionId = vi.fn();
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="stale-session"
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.reattachAgentChat).toHaveBeenCalledWith(
+        "stale-session",
+        expect.any(Function),
+      ),
+    );
+
+    fireEvent.change(screen.getByLabelText(/message CLI custom/), {
+      target: { value: "please recover" },
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Envoyer" }));
+
+    await waitFor(() => expect(agent.sendAgentChat).toHaveBeenCalledTimes(2));
+    expect(agent.sendAgentChat).toHaveBeenNthCalledWith(
+      1,
+      "stale-session",
+      "please recover",
+      expect.any(Function),
+    );
+    expect(agent.sendAgentChat).toHaveBeenNthCalledWith(
+      2,
+      "chat-session-2",
+      "please recover",
+      expect.any(Function),
+    );
+    expect(onSessionId).toHaveBeenCalledWith(null);
+    expect(onSessionId).toHaveBeenCalledWith("chat-session-2");
+    expect(await screen.findByText("Recovered reply")).toBeTruthy();
+    expect(screen.queryByText(/not found: structured session/)).toBeNull();
+  });
+
+  it("treats NOT_FOUND during cancel as an already-gone session", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi.fn(() => new Promise<void>(() => {})),
+      cancelAgentChat: vi.fn().mockRejectedValueOnce({
+        code: "NOT_FOUND",
+        message: "not found: structured session gone-session",
+      }),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    const onSessionId = vi.fn();
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="gone-session"
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.reattachAgentChat).toHaveBeenCalledWith(
+        "gone-session",
+        expect.any(Function),
+      ),
+    );
+
+    fireEvent.change(screen.getByLabelText(/message CLI custom/), {
+      target: { value: "long turn" },
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Envoyer" }));
+    await waitFor(() => expect(agent.sendAgentChat).toHaveBeenCalled());
+
+    fireEvent.click(await screen.findByRole("button", { name: "Cancel" }));
+
+    expect(await screen.findByText("Session déjà fermée.")).toBeTruthy();
+    expect(onSessionId).toHaveBeenCalledWith(null);
+    expect(agent.closeAgentChat).not.toHaveBeenCalled();
+    expect(screen.queryByText(/not found: structured session/)).toBeNull();
+  });
+
+  it("does not swallow post-launch reattach errors", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(async () => ({
+        sessionId: "new-session",
+        assignedConversationId: "conversation-2",
+      })),
+      reattachAgentChat: vi.fn().mockRejectedValueOnce({
+        code: "PROCESS",
+        message: "post-launch attach failed",
+      }),
+      sendAgentChat: vi.fn(() => new Promise<void>(() => {})),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId={null}
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    expect((await screen.findByRole("alert")).textContent).toContain(
+      "post-launch attach failed",
+    );
+  });
+
+  it("recovers when post-launch reattach reports NOT_FOUND", async () => {
+    const agent = {
+      launchAgentChat: vi
+        .fn()
+        .mockResolvedValueOnce({
+          sessionId: "new-session-1",
+          assignedConversationId: "conversation-2",
+        })
+        .mockResolvedValueOnce({
+          sessionId: "new-session-2",
+          assignedConversationId: "conversation-2",
+        }),
+      reattachAgentChat: vi
+        .fn()
+        .mockRejectedValueOnce({
+          code: "NOT_FOUND",
+          message: "not found: structured session new-session-1",
+        })
+        .mockImplementationOnce(async (sessionId: string) => ({
+          sessionId,
+          scrollback: [],
+        })),
+      sendAgentChat: vi.fn(() => new Promise<void>(() => {})),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    const onSessionId = vi.fn();
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId={null}
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() => expect(agent.launchAgentChat).toHaveBeenCalledTimes(2));
+    expect(agent.reattachAgentChat).toHaveBeenNthCalledWith(
+      1,
+      "new-session-1",
+      expect.any(Function),
+    );
+    expect(agent.reattachAgentChat).toHaveBeenNthCalledWith(
+      2,
+      "new-session-2",
+      expect.any(Function),
+    );
+    expect(onSessionId).toHaveBeenCalledWith(null);
+    expect(onSessionId).toHaveBeenCalledWith("new-session-2");
+    expect(screen.queryByText(/not found: structured session/)).toBeNull();
+  });
+
   it("surfaces non-NOT_FOUND reattach errors instead of launching a new session", async () => {
     const agent = {
       launchAgentChat: vi.fn(),
diff --git a/frontend/src/features/agents/CustomAgentChatView.tsx b/frontend/src/features/agents/CustomAgentChatView.tsx
index ad8e60e..bb65809 100644
--- a/frontend/src/features/agents/CustomAgentChatView.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.tsx
@@ -6,7 +6,7 @@
  * deliberately does not try to parse PTY bytes.
  */
 
-import { useEffect, useMemo, useRef, useState } from "react";
+import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 
 import type { AgentProfile, GatewayError, ReplyChunk } from "@/domain";
 import { useGateways } from "@/app/di";
@@ -40,6 +40,10 @@ function describe(e: unknown): string {
   return String(e);
 }
 
+function isNotFound(e: unknown): boolean {
+  return Boolean(e && typeof e === "object" && (e as GatewayError).code === "NOT_FOUND");
+}
+
 function unknownChunkLabel(chunk: unknown): string {
   try {
     return JSON.stringify(chunk);
@@ -122,6 +126,7 @@ export function CustomAgentChatView({
   const scrollRef = useRef<HTMLDivElement | null>(null);
   const sessionRef = useRef<string | null>(sessionId);
   sessionRef.current = currentSession;
+  const mountedRef = useRef(false);
   const onSessionIdRef = useRef(onSessionId);
   onSessionIdRef.current = onSessionId;
   const onConversationIdRef = useRef(onConversationId);
@@ -136,6 +141,78 @@ export function CustomAgentChatView({
       agent.closeAgentChat,
   );
 
+  useEffect(() => {
+    mountedRef.current = true;
+    return () => {
+      mountedRef.current = false;
+    };
+  }, []);
+
+  const receive = useCallback((chunk: ReplyChunk) => {
+    setTurns((prev) => foldChunk(prev, chunk));
+    if (chunk.kind === "final" || chunk.kind === "error") setBusy(false);
+  }, []);
+
+  const reattachStructuredSession = useCallback(
+    async (sid: string, options: { applyScrollback?: boolean } = {}) => {
+      if (!agent.reattachAgentChat) throw new Error("Structured reattach unavailable");
+      const reattached = await agent.reattachAgentChat(sid, receive);
+      if (!mountedRef.current) return reattached.sessionId;
+      setCurrentSession(reattached.sessionId);
+      if (options.applyScrollback) {
+        setTurns(reattached.scrollback.reduce(foldChunk, [] as ChatTurn[]));
+      }
+      return reattached.sessionId;
+    },
+    [agent, receive],
+  );
+
+  const recoverStructuredSession = useCallback(
+    async (options: { applyScrollback?: boolean; retryAttachNotFound?: boolean } = {}) => {
+      if (!agent.launchAgentChat) throw new Error("Structured launch unavailable");
+      let retryAttachNotFound = Boolean(options.retryAttachNotFound);
+      for (;;) {
+        const launched = await agent.launchAgentChat(projectId, agentId, {
+          cwd,
+          rows: 24,
+          cols: 80,
+          conversationId: conversationId ?? undefined,
+          nodeId,
+        });
+        if (!mountedRef.current) return launched.sessionId;
+        setCurrentSession(launched.sessionId);
+        onSessionIdRef.current(launched.sessionId);
+        if (launched.assignedConversationId) {
+          onConversationIdRef.current(launched.assignedConversationId);
+        }
+        try {
+          return await reattachStructuredSession(launched.sessionId, {
+            applyScrollback: options.applyScrollback,
+          });
+        } catch (e) {
+          if (isNotFound(e) && retryAttachNotFound) {
+            retryAttachNotFound = false;
+            if (mountedRef.current) {
+              setCurrentSession(null);
+              onSessionIdRef.current(null);
+            }
+            continue;
+          }
+          throw e;
+        }
+      }
+    },
+    [
+      agent,
+      projectId,
+      agentId,
+      cwd,
+      nodeId,
+      conversationId,
+      reattachStructuredSession,
+    ],
+  );
+
   useEffect(() => {
     const el = scrollRef.current;
     if (el) el.scrollTop = el.scrollHeight;
@@ -144,10 +221,6 @@ export function CustomAgentChatView({
   useEffect(() => {
     if (!supported) return;
     let cancelled = false;
-    const receive = (chunk: ReplyChunk) => {
-      setTurns((prev) => foldChunk(prev, chunk));
-      if (chunk.kind === "final" || chunk.kind === "error") setBusy(false);
-    };
 
     async function openOrAttach() {
       setOpening(true);
@@ -155,33 +228,25 @@ export function CustomAgentChatView({
       try {
         if (sessionId) {
           try {
-            const reattached = await agent.reattachAgentChat!(sessionId, receive);
+            await reattachStructuredSession(sessionId, { applyScrollback: true });
             if (cancelled) return;
-            setCurrentSession(reattached.sessionId);
-            setTurns(reattached.scrollback.reduce(foldChunk, [] as ChatTurn[]));
             return;
           } catch (e) {
             // The backend contract for `reattach_agent_chat` (NOT_FOUND) is that
             // the session is gone (closed/never live) and the caller falls back
             // to a fresh launch — any other error still surfaces to the user.
-            if ((e as GatewayError)?.code !== "NOT_FOUND") throw e;
+            if (!isNotFound(e)) throw e;
+            if (!cancelled) {
+              setCurrentSession(null);
+              onSessionIdRef.current(null);
+            }
           }
         }
-        const launched = await agent.launchAgentChat!(projectId, agentId, {
-          cwd,
-          rows: 24,
-          cols: 80,
-          conversationId: conversationId ?? undefined,
-          nodeId,
+        await recoverStructuredSession({
+          applyScrollback: false,
+          retryAttachNotFound: true,
         });
         if (cancelled) return;
-        setCurrentSession(launched.sessionId);
-        onSessionIdRef.current(launched.sessionId);
-        if (launched.assignedConversationId) {
-          onConversationIdRef.current(launched.assignedConversationId);
-        }
-        // Attach the view so any in-flight chunks can be replayed after launch.
-        await agent.reattachAgentChat!(launched.sessionId, receive).catch(() => {});
       } catch (e) {
         if (!cancelled) setError(describe(e));
       } finally {
@@ -195,23 +260,18 @@ export function CustomAgentChatView({
     };
   }, [
     supported,
-    agent,
-    projectId,
-    agentId,
-    cwd,
-    nodeId,
     sessionId,
-    conversationId,
+    reattachStructuredSession,
+    recoverStructuredSession,
   ]);
 
   const canSend = useMemo(
     () =>
       supported &&
-      Boolean(currentSession) &&
       Boolean(draft.trim()) &&
       !busy &&
       !opening,
-    [supported, currentSession, draft, busy, opening],
+    [supported, draft, busy, opening],
   );
 
   async function pickAttachment() {
@@ -221,7 +281,7 @@ export function CustomAgentChatView({
 
   async function send() {
     const text = draft.trim();
-    if (!canSend || !currentSession || !agent.sendAgentChat) return;
+    if (!canSend || !agent.sendAgentChat) return;
     const prompt = attachment ? `${text}\n\n[Fichier joint: ${attachment}]` : text;
     setDraft("");
     setAttachment(null);
@@ -229,11 +289,31 @@ export function CustomAgentChatView({
     setError(null);
     setTurns((prev) => [...prev, { role: "user", text, attachment: attachment ?? undefined }]);
     try {
-      await agent.sendAgentChat(currentSession, prompt, (chunk) => {
-        setTurns((prev) => foldChunk(prev, chunk));
-        if (chunk.kind === "final" || chunk.kind === "error") setBusy(false);
-      });
+      const sid =
+        currentSession ??
+        (await recoverStructuredSession({
+          applyScrollback: false,
+          retryAttachNotFound: true,
+        }));
+      await agent.sendAgentChat(sid, prompt, receive);
     } catch (e) {
+      if (isNotFound(e)) {
+        try {
+          setCurrentSession(null);
+          onSessionIdRef.current(null);
+          const recoveredSession = await recoverStructuredSession({
+            applyScrollback: false,
+            retryAttachNotFound: true,
+          });
+          await agent.sendAgentChat(recoveredSession, prompt, receive);
+          return;
+        } catch (recoveryError) {
+          setBusy(false);
+          setError(describe(recoveryError));
+          setTurns((prev) => [...prev, { role: "error", text: describe(recoveryError) }]);
+          return;
+        }
+      }
       setBusy(false);
       setError(describe(e));
       setTurns((prev) => [...prev, { role: "error", text: describe(e) }]);
@@ -250,6 +330,12 @@ export function CustomAgentChatView({
       await agent.cancelAgentChat(sid);
       setTurns((prev) => [...prev, { role: "tool", label: "Tour interrompu." }]);
     } catch (e) {
+      if (isNotFound(e)) {
+        setCurrentSession(null);
+        onSessionIdRef.current(null);
+        setTurns((prev) => [...prev, { role: "tool", label: "Session déjà fermée." }]);
+        return;
+      }
       setError(describe(e));
     }
   }