diff --git a/frontend/src/features/agents/CustomAgentChatView.test.tsx b/frontend/src/features/agents/CustomAgentChatView.test.tsx
index f9d45687..18e15003 100644
--- a/frontend/src/features/agents/CustomAgentChatView.test.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.test.tsx
@@ -522,6 +522,85 @@ describe("CustomAgentChatView", () => {
     expect(screen.getByLabelText(/message CLI custom/).hasAttribute("disabled")).toBe(true);
   });
 
+  it("restores visible conversation from conversationId cache when layout returns before sessionId", async () => {
+    let scrollback: ReplyChunk[] = [
+      { kind: "userPrompt" as const, text: "Initial prompt" },
+      { kind: "final" as const, content: "Initial reply" },
+    ];
+    const secondReattach = deferred<{ sessionId: string; scrollback: ReplyChunk[] }>();
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi
+        .fn()
+        .mockImplementationOnce(async (sessionId: string) => ({
+          sessionId,
+          scrollback,
+        }))
+        .mockImplementationOnce(() => secondReattach.promise),
+      detachAgentChat: vi.fn(async () => {}),
+      sendAgentChat: vi.fn(async () => {}),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    const props = {
+      projectId: "project-1",
+      agentId: "agent-1",
+      agentName: "Worker",
+      profile,
+      cwd: "/repo",
+      nodeId: "node-1",
+      sessionId: "chat-session-1" as string | null,
+      conversationId: "conversation-1",
+      onSessionId: vi.fn(),
+      onConversationId: vi.fn(),
+    };
+
+    const first = render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView {...props} />
+      </DIProvider>,
+    );
+
+    expect(await screen.findByText("Initial reply")).toBeTruthy();
+    first.unmount();
+    await waitFor(() => expect(agent.detachAgentChat).toHaveBeenCalledWith("chat-session-1"));
+
+    scrollback = [
+      { kind: "userPrompt" as const, text: "Initial prompt" },
+      { kind: "final" as const, content: "Initial reply" },
+      { kind: "userPrompt" as const, text: "Question during graph view" },
+      { kind: "final" as const, content: "Answer restored from replay" },
+    ];
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView {...props} sessionId={null} />
+      </DIProvider>,
+    );
+
+    expect(screen.getByText("Initial reply")).toBeTruthy();
+    expect(agent.launchAgentChat).not.toHaveBeenCalled();
+    expect(screen.getByRole("button", { name: "Envoyer" }).hasAttribute("disabled")).toBe(true);
+
+    act(() => {
+      secondReattach.resolve({ sessionId: "chat-session-1", scrollback });
+    });
+
+    expect(await screen.findByText("Answer restored from replay")).toBeTruthy();
+    expect(screen.getByText("Question during graph view")).toBeTruthy();
+    expect(agent.launchAgentChat).not.toHaveBeenCalled();
+  });
+
   it("drops a stale no-response chunk when replay contains a later real answer (#190)", async () => {
     const agent = {
       launchAgentChat: vi.fn(),
diff --git a/frontend/src/features/agents/CustomAgentChatView.tsx b/frontend/src/features/agents/CustomAgentChatView.tsx
index 113f9ada..6439ac46 100644
--- a/frontend/src/features/agents/CustomAgentChatView.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.tsx
@@ -67,15 +67,18 @@ interface CachedChatView {
   turns: ChatTurn[];
   progressIndex: Map<string, ProgressIndexEntry>;
   chunkCount: number;
+  sessionId?: string;
 }
 
 const chatSessionViewCache = new Map<string, CachedChatView>();
+const chatConversationViewCache = new Map<string, CachedChatView>();
 const EMPTY_MODEL_RESPONSE_MESSAGE = "Le modèle n'a renvoyé aucune réponse.";
 const COMPOSER_RESIZE_STEP = 12;
 const FALLBACK_COMPOSER_HEIGHT = 64;
 
 export function __clearCustomAgentChatViewCacheForTests() {
   chatSessionViewCache.clear();
+  chatConversationViewCache.clear();
 }
 
 interface AttachmentDraft {
@@ -310,6 +313,14 @@ function emptyChatViewState(): ChatViewState {
   return { turns: [], progressIndex: new Map() };
 }
 
+function conversationCacheKey(
+  projectId: string,
+  agentId: string,
+  conversationId: string | null,
+): string | null {
+  return conversationId ? `${projectId}::${agentId}::${conversationId}` : null;
+}
+
 function appendProgress(state: ChatViewState, progress: ReplyProgress): ChatViewState {
   const key = progressUpsertKey(progress);
   const existingIndex = state.turns.findIndex(
@@ -711,10 +722,27 @@ export function CustomAgentChatView({
 }: CustomAgentChatViewProps) {
   const { agent, profile: profileGateway, system } = useGateways();
   const pluginRuntime = usePluginRuntime();
-  const [turns, setTurns] = useState<ChatTurn[]>(
-    () => (sessionId ? (chatSessionViewCache.get(sessionId)?.turns ?? []) : []),
+  const initialConversationCacheKey = conversationCacheKey(
+    projectId,
+    agentId,
+    conversationId,
+  );
+  const [turns, setTurns] = useState<ChatTurn[]>(
+    () =>
+      sessionId
+        ? (chatSessionViewCache.get(sessionId)?.turns ?? [])
+        : initialConversationCacheKey
+          ? (chatConversationViewCache.get(initialConversationCacheKey)?.turns ?? [])
+          : [],
+  );
+  const [progressIndex, setProgressIndex] = useState<Map<string, ProgressIndexEntry>>(
+    () =>
+      sessionId
+        ? (chatSessionViewCache.get(sessionId)?.progressIndex ?? new Map())
+        : initialConversationCacheKey
+          ? (chatConversationViewCache.get(initialConversationCacheKey)?.progressIndex ?? new Map())
+          : new Map(),
   );
-  const [progressIndex, setProgressIndex] = useState<Map<string, ProgressIndexEntry>>(new Map());
   const [currentSession, setCurrentSession] = useState(sessionId);
   const [externalSessionId, setExternalSessionId] = useState(sessionId);
   const [draft, setDraft] = useState("");
@@ -724,7 +752,7 @@ export function CustomAgentChatView({
   const [slashActiveIndex, setSlashActiveIndex] = useState(0);
   const [profileDialog, setProfileDialog] =
     useState<ProfileCommandDialogState | null>(null);
-  const [opening, setOpening] = useState(Boolean(sessionId));
+  const [opening, setOpening] = useState(Boolean(sessionId || conversationId));
   const [busy, setBusy] = useState(false);
   const [stopping, setStopping] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -849,6 +877,10 @@ export function CustomAgentChatView({
     () => (layoutId ? { projectId, layoutId, nodeId } : null),
     [layoutId, nodeId, projectId],
   );
+  const currentConversationCacheKey = useMemo(
+    () => conversationCacheKey(projectId, agentId, conversationId),
+    [agentId, conversationId, projectId],
+  );
   const displayTurns = turns;
 
   const clearPersistedDraft = useCallback(async () => {
@@ -918,13 +950,27 @@ export function CustomAgentChatView({
   );
 
   useEffect(() => {
-    if (!currentSession) return;
-    chatSessionViewCache.set(currentSession, {
-      turns,
-      progressIndex,
-      chunkCount: scrollbackChunkCountRef.current,
-    });
-  }, [currentSession, progressIndex, turns]);
+    if (currentSession) {
+      chatSessionViewCache.set(currentSession, {
+        turns,
+        progressIndex,
+        chunkCount: scrollbackChunkCountRef.current,
+        sessionId: currentSession,
+      });
+    }
+    if (currentConversationCacheKey) {
+      const previous = chatConversationViewCache.get(currentConversationCacheKey);
+      const cachedSessionId = currentSession ?? previous?.sessionId;
+      if (cachedSessionId || turns.length > 0 || progressIndex.size > 0) {
+        chatConversationViewCache.set(currentConversationCacheKey, {
+          turns,
+          progressIndex,
+          chunkCount: scrollbackChunkCountRef.current,
+          ...(cachedSessionId ? { sessionId: cachedSessionId } : {}),
+        });
+      }
+    }
+  }, [currentConversationCacheKey, currentSession, progressIndex, turns]);
 
   useEffect(() => {
     draftSaveSeqRef.current += 1;
@@ -1216,6 +1262,15 @@ export function CustomAgentChatView({
       setOpening(true);
       setError(null);
       try {
+        const localSession = sessionRef.current;
+        if (!externalSessionId && localSession) {
+          console.debug("[ticket149] openOrAttach:local-session-present:skip", {
+            timestamp: new Date().toISOString(),
+            execution,
+            currentSession: localSession,
+          });
+          return;
+        }
         if (externalSessionId) {
           const cached = chatSessionViewCache.get(externalSessionId);
           if (cached) {
@@ -1250,9 +1305,32 @@ export function CustomAgentChatView({
               error: instrumentationError(e),
             });
           }
+        } else if (currentConversationCacheKey) {
+          const cached = chatConversationViewCache.get(currentConversationCacheKey);
+          if (cached) {
+            scrollbackChunkCountRef.current = cached.chunkCount;
+            applyChatViewState({
+              turns: cached.turns,
+              progressIndex: cached.progressIndex ?? new Map(),
+            });
+            if (cached.sessionId) {
+              const reattachedSessionId = await reattachStructuredSession(cached.sessionId, {
+                applyScrollback: true,
+              });
+              if (cancelled) return;
+              publishSessionId(reattachedSessionId);
+              console.debug("[ticket149] openOrAttach:reattach-conversation-cache:success", {
+                timestamp: new Date().toISOString(),
+                execution,
+                sessionId: reattachedSessionId,
+              });
+              return;
+            }
+            return;
+          }
         }
         await recoverStructuredSession({
-          applyScrollback: false,
+          applyScrollback: Boolean(conversationId),
           retryAttachNotFound: true,
         });
         if (cancelled) return;
@@ -1285,6 +1363,8 @@ export function CustomAgentChatView({
     recoverStructuredSession,
     publishSessionId,
     applyChatViewState,
+    currentConversationCacheKey,
+    conversationId,
   ]);
 
   const canSend = useMemo(
diff --git a/frontend/src/features/layout/LayoutGrid.chat.test.tsx b/frontend/src/features/layout/LayoutGrid.chat.test.tsx
index b814b476..eeb2d032 100644
--- a/frontend/src/features/layout/LayoutGrid.chat.test.tsx
+++ b/frontend/src/features/layout/LayoutGrid.chat.test.tsx
@@ -266,6 +266,71 @@ describe("LayoutGrid custom agent CLI (#147)", () => {
     expect(screen.queryByText(/déjà actif dans une autre cellule/)).toBeNull();
   });
 
+  it("restores a custom CLI transcript when returning with a conversation id but no session (#217)", async () => {
+    const layout = new MockLayoutGateway();
+    const agent = new MockAgentGateway();
+    const profileGateway = new MockProfileGateway();
+    const terminal = new MockTerminalGateway();
+    const system = new MockSystemGateway();
+    await profileGateway.configureProfiles([structuredProfile]);
+    const created = await agent.createAgent("p1", {
+      name: "Worker",
+      profileId: structuredProfile.id,
+    });
+    const tree = await layout.loadLayout("p1");
+    const leafId = leaves(tree)[0].id;
+    await layout.mutateLayout("p1", {
+      type: "setCellAgent",
+      target: leafId,
+      agent: created.id,
+    });
+    await layout.mutateLayout("p1", {
+      type: "setCellConversation",
+      target: leafId,
+      conversationId: "conversation-known",
+    });
+    window.localStorage.setItem(`idea.agent-cell-mode.p1.${leafId}`, "custom");
+    const launchChat = vi.spyOn(agent, "launchAgentChat").mockResolvedValue({
+      sessionId: "chat-resumed",
+      cellKind: "chat" as const,
+    });
+    const reattachChat = vi.spyOn(agent, "reattachAgentChat").mockResolvedValue({
+      sessionId: "chat-resumed",
+      scrollback: [
+        { kind: "userPrompt", text: "known prompt" },
+        { kind: "final", content: "known answer" },
+      ],
+    });
+
+    renderGrid({
+      layout,
+      agent,
+      profile: profileGateway,
+      terminal,
+      system,
+    } as unknown as Gateways);
+
+    expect(await screen.findByText("known answer")).toBeTruthy();
+    expect(screen.getByText("known prompt")).toBeTruthy();
+    expect(launchChat).toHaveBeenCalledWith(
+      "p1",
+      created.id,
+      expect.objectContaining({
+        nodeId: leafId,
+        conversationId: "conversation-known",
+      }),
+    );
+    expect(reattachChat).toHaveBeenCalledWith("chat-resumed", expect.any(Function));
+    await waitFor(async () => {
+      const updated = await layout.loadLayout("p1");
+      const leaf = leaves(updated)[0];
+      expect(leaf.conversationId).toBe("conversation-known");
+      expect(leaf.session).toBe("chat-resumed");
+    });
+    expect(screen.queryByText("Envoyez un message pour démarrer la conversation structurée."))
+      .toBeNull();
+  });
+
   it("relaunches a stopped custom CLI agent from scratch (#197)", async () => {
     const layout = new MockLayoutGateway();
     const agent = new MockAgentGateway();