diff --git a/packages/app/e2e/regression/composer-command-draft.spec.ts b/packages/app/e2e/regression/composer-command-draft.spec.ts
index 757a0f40c76..4ef2f32a5aa 100644
--- a/packages/app/e2e/regression/composer-command-draft.spec.ts
+++ b/packages/app/e2e/regression/composer-command-draft.spec.ts
@@ -8,7 +8,8 @@ const projectID = "proj_composer_editing"
 const sessionID = "ses_composer_editing"
 const server = `http://${process.env.PLAYWRIGHT_SERVER_HOST ?? "127.0.0.1"}:${process.env.PLAYWRIGHT_SERVER_PORT ?? "4096"}`
 
-test("preserves the draft when a populated command menu triggers a built-in", async ({ page }) => {
+test("keeps a narrow session composer contained when invoking a built-in", async ({ page }) => {
+  await page.setViewportSize({ width: 800, height: 600 })
   await mockOpenCodeServer(page, {
     directory,
     project: {
@@ -40,6 +41,8 @@ test("preserves the draft when a populated command menu triggers a built-in", as
     .poll(() => input.evaluate((element) => getComputedStyle(element, "::before").content))
     .toBe(`"${String.fromCodePoint(0x200b)}"`)
   await expectAppVisible(composer)
+  await expect(page.locator('[data-slot="session-chat-panel"]')).toHaveCSS("min-width", "0px")
+  await expect.poll(() => page.evaluate(() => document.documentElement.scrollWidth <= innerWidth)).toBe(true)
 
   await input.fill("keep me")
   await composer.getByRole("button", { name: "Add images and files" }).click()
diff --git a/packages/app/src/session/screen.tsx b/packages/app/src/session/screen.tsx
index 222ed54168f..7bec153f9d2 100644
--- a/packages/app/src/session/screen.tsx
+++ b/packages/app/src/session/screen.tsx
@@ -276,7 +276,8 @@ export function SessionScreen(props: { session: SessionModel }) {
         <div ref={screen.panel.ref} class="relative flex-1 min-h-0 flex flex-col md:flex-row gap-2">
           <div
             classList={{
-              "@container relative z-10 shrink-0 flex flex-col min-h-0 h-full flex-1 md:flex-none transition-[width]": true,
+              "@container relative z-10 min-w-0 shrink-0 flex flex-col min-h-0 h-full flex-1 md:flex-none transition-[width]":
+                true,
               "duration-[240ms] ease-[cubic-bezier(0.22,1,0.36,1)] will-change-[width] motion-reduce:transition-none":
                 !screen.size.active() && sidePresence.animate(),
               "transition-none": screen.size.active() || !sidePresence.animate(),