diff --git a/frontend/src/features/agents/CustomAgentChatView.test.tsx b/frontend/src/features/agents/CustomAgentChatView.test.tsx
index 57a3d2bf..a80d5f7f 100644
--- a/frontend/src/features/agents/CustomAgentChatView.test.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.test.tsx
@@ -1,4 +1,6 @@
 import { act, fireEvent, render, screen, waitFor, within } from "@testing-library/react";
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
 import { useState } from "react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
@@ -25,6 +27,11 @@ import {
   customAgentChatQueueCount,
 } from "./CustomAgentChatView";
 
+const themeCss = readFileSync(
+  resolve(process.cwd(), "src/shared/styles/theme.css"),
+  "utf8",
+);
+
 const profile: AgentProfile = {
   id: "structured",
   name: "Structured Codex",
@@ -199,6 +206,63 @@ describe("CustomAgentChatView", () => {
     __resetProjectCliInteractionActivityForTests();
   });
 
+  it.each([
+    { width: 600, mode: "single-row" },
+    { width: 360, mode: "second-row" },
+    { width: 240, mode: "full-width" },
+  ])("keeps its conversation access clear of cell controls at $width px ($mode)", ({ width, mode }) => {
+    const agent = {
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({ sessionId, scrollback: [] })),
+      sendAgentChat: vi.fn(),
+      cancelAgentChat: vi.fn(),
+      closeAgentChat: vi.fn(),
+    };
+
+    render(
+      <div data-testid="representative-cell" style={{ width, height: 640 }}>
+        <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="chat-1"
+            conversationId="conversation-1"
+            onSessionId={vi.fn()}
+            onConversationId={vi.fn()}
+          />
+        </DIProvider>
+      </div>,
+    );
+
+    const header = screen.getByTestId("custom-agent-chat-header");
+    const identity = screen.getByTestId("custom-agent-chat-identity");
+    const conversations = screen.getByTestId("custom-agent-chat-conversations");
+    const button = screen.getByRole("button", { name: "Conversations" });
+
+    expect(screen.getByTestId("representative-cell").style.width).toBe(`${width}px`);
+    expect(header.className).toContain("custom-agent-chat-header");
+    expect(header.contains(identity)).toBe(true);
+    expect(header.contains(conversations)).toBe(true);
+    expect(conversations.contains(button)).toBe(true);
+    expect(screen.getByText("CLI personnalisée · Structured Codex")).toBeTruthy();
+    expect(themeCss).toContain("padding: 2.25rem 0.75rem 0.5rem");
+
+    if (mode === "single-row") {
+      expect(themeCss).toContain(".custom-agent-chat-conversations-menu {\n    right: 0;");
+    } else {
+      expect(themeCss).toContain("@container (max-width: 419.99px)");
+      expect(themeCss).toContain("flex-direction: column");
+      expect(themeCss).toContain("left: 0");
+    }
+    if (mode === "full-width") {
+      expect(themeCss).toContain("@container (max-width: 279.99px)");
+      expect(themeCss).toContain(".custom-agent-chat-conversations-button,\n    .custom-agent-chat-conversations-menu");
+    }
+  });
+
   it("renders the project icon as a non-interactive custom-CLI watermark and updates it immediately", async () => {
     const agent = {
       reattachAgentChat: vi.fn(async (sessionId: string) => ({ sessionId, scrollback: [] })),
diff --git a/frontend/src/features/agents/CustomAgentChatView.tsx b/frontend/src/features/agents/CustomAgentChatView.tsx
index 29f29b62..469b9912 100644
--- a/frontend/src/features/agents/CustomAgentChatView.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.tsx
@@ -2966,18 +2966,26 @@ export function CustomAgentChatView({
       <div
         role="toolbar"
         aria-label={t("custom.actions")}
-        className="relative z-20 flex shrink-0 items-center gap-2 border-b border-border px-3 py-2"
+        data-testid="custom-agent-chat-header"
+        className="custom-agent-chat-header relative z-20 shrink-0 border-b border-border"
       >
-        <div className="min-w-0 flex-1 overflow-hidden">
+        <div
+          data-testid="custom-agent-chat-identity"
+          className="custom-agent-chat-identity min-w-0 overflow-hidden"
+        >
           <div className="truncate text-sm font-medium">{agentName}</div>
           <div className="truncate text-xs text-muted">
             {tr("CLI personnalisée", "Custom CLI")} · {profile.name}
           </div>
         </div>
-        <div className="relative shrink-0">
+        <div
+          data-testid="custom-agent-chat-conversations"
+          className="custom-agent-chat-conversations relative shrink-0"
+        >
           <Button
             size="sm"
             variant="ghost"
+            className="custom-agent-chat-conversations-button"
             aria-expanded={conversationMenuOpen}
             aria-haspopup="menu"
             onClick={() => setConversationMenuOpen((open) => !open)}
@@ -2988,7 +2996,8 @@ export function CustomAgentChatView({
             <div
               role="menu"
               aria-label={tr("Conversations", "Conversations")}
-              className="absolute right-0 top-full z-50 mt-1 w-[min(22rem,calc(100vw-1rem))] overflow-hidden rounded-md border border-border bg-raised shadow-xl"
+              data-testid="custom-agent-chat-conversations-menu"
+              className="custom-agent-chat-conversations-menu absolute top-full z-50 mt-1 overflow-hidden rounded-md border border-border bg-raised shadow-xl"
             >
               {effectiveBusy && (
                 <p className="border-b border-border bg-warning/10 px-3 py-2 text-xs text-warning">
diff --git a/frontend/src/features/layout/LayoutGrid.cellControlsLayering.test.tsx b/frontend/src/features/layout/LayoutGrid.cellControlsLayering.test.tsx
index ef4e3ff5..2358375a 100644
--- a/frontend/src/features/layout/LayoutGrid.cellControlsLayering.test.tsx
+++ b/frontend/src/features/layout/LayoutGrid.cellControlsLayering.test.tsx
@@ -415,9 +415,12 @@ describe("LayoutGrid — ticket #48 cell control layering", () => {
     const customToolbar = screen.getByRole("toolbar", {
       name: "custom agent chat actions",
     });
+    const conversations = screen.getByRole("button", { name: "Conversations" });
     const mutate = vi.spyOn(setup.layout, "mutateLayout");
 
     expect(customToolbar.className).toContain("z-20");
+    expect(customToolbar.className).toContain("custom-agent-chat-header");
+    expect(customToolbar.contains(conversations)).toBe(true);
     expect(screen.getByLabelText(`Sélecteur d’agent ${setup.bId}`)).toBeTruthy();
     expect(screen.getByRole("group", {
       name: `Mode CLI de l’agent ${setup.bId}`,
diff --git a/frontend/src/shared/styles/theme.css b/frontend/src/shared/styles/theme.css
index 446309ab..45886150 100644
--- a/frontend/src/shared/styles/theme.css
+++ b/frontend/src/shared/styles/theme.css
@@ -107,6 +107,55 @@
     container-type: size;
   }
 
+  /* The LayoutGrid cell controls occupy the top strip as an absolute sibling.
+     Keep the custom-chat header below that strip, inside the chat's local
+     stacking context, so its conversation affordance can never sit underneath
+     the agent selector/split/close controls. */
+  .custom-agent-chat-header {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    padding: 2.25rem 0.75rem 0.5rem;
+  }
+
+  .custom-agent-chat-identity {
+    flex: 1 1 auto;
+  }
+
+  .custom-agent-chat-conversations-menu {
+    right: 0;
+    width: 22rem;
+    max-width: calc(100cqw - 1.5rem);
+  }
+
+  /* Medium split cells put the action on its own left-aligned row while
+     keeping the menu constrained to the cell rather than the viewport. */
+  @container (max-width: 419.99px) {
+    .custom-agent-chat-header {
+      flex-direction: column;
+      align-items: flex-start;
+    }
+
+    .custom-agent-chat-identity {
+      width: 100%;
+    }
+
+    .custom-agent-chat-conversations-menu {
+      right: auto;
+      left: 0;
+    }
+  }
+
+  /* Very narrow cells retain a usable, non-overlapping full-width target. */
+  @container (max-width: 279.99px) {
+    .custom-agent-chat-conversations,
+    .custom-agent-chat-conversations-button,
+    .custom-agent-chat-conversations-menu {
+      width: 100%;
+      max-width: 100%;
+    }
+  }
+
   .project-cell-illustration-area {
     container-type: size;
   }