diff --git a/packages/app/e2e/regression/session-prompt-spacing.spec.ts b/packages/app/e2e/regression/session-prompt-spacing.spec.ts
new file mode 100644
index 00000000000..6719a317fc4
--- /dev/null
+++ b/packages/app/e2e/regression/session-prompt-spacing.spec.ts
@@ -0,0 +1,74 @@
+import { expect, test } from "@playwright/test"
+import { sessionID, setupTimeline, userMessage } from "../performance/timeline-stability/fixture"
+
+test("keeps a submitted prompt in place while its optimistic rows are measured", async ({ page }) => {
+  await setupTimeline(page, { messages: [userMessage()], seedHistory: true })
+  const release = Promise.withResolvers<void>()
+  await page.route(`**/api/session/${sessionID}/prompt`, async (route) => {
+    if (route.request().method() !== "POST") return route.fallback()
+    await release.promise
+    return route.fallback()
+  })
+
+  const editor = page.locator('[data-component="composer"]').getByRole("textbox")
+  await expect(editor).toBeEditable()
+  await editor.fill("Observe optimistic prompt spacing.")
+  await expect
+    .poll(() =>
+      page.locator("[data-timeline-virtual-content]").evaluate((element) => {
+        const root = element.parentElement!
+        return root.scrollHeight - root.clientHeight - root.scrollTop
+      }),
+    )
+    .toBe(0)
+
+  const observation = await page.evaluateHandle(() => {
+    const frames: { prompt?: number; working: boolean }[] = []
+    let frame = 0
+    const sample = () => {
+      const prompt = [...document.querySelectorAll<HTMLElement>('[data-timeline-row="UserMessage"]')].find((row) =>
+        row.textContent?.includes("Observe optimistic prompt spacing."),
+      )
+      frames.push({
+        ...(prompt ? { prompt: prompt.getBoundingClientRect().y } : {}),
+        working: !!document.querySelector('[data-component="session-working"]'),
+      })
+      frame = requestAnimationFrame(sample)
+    }
+    frame = requestAnimationFrame(sample)
+    return {
+      stop: () => {
+        cancelAnimationFrame(frame)
+        return frames
+      },
+    }
+  })
+  const requested = page.waitForRequest(
+    (request) => request.method() === "POST" && new URL(request.url()).pathname === `/api/session/${sessionID}/prompt`,
+  )
+  try {
+    await editor.press("Enter")
+    await requested
+    const prompt = page
+      .locator('[data-timeline-row="UserMessage"]')
+      .filter({ hasText: "Observe optimistic prompt spacing." })
+    await expect(prompt).toBeInViewport()
+    await expect(page.locator('[data-component="session-working"]')).toBeVisible()
+    await expect
+      .poll(() =>
+        page.locator("[data-timeline-virtual-content]").evaluate((element) => {
+          const root = element.parentElement!
+          return root.scrollHeight - root.clientHeight - root.scrollTop
+        }),
+      )
+      .toBe(0)
+    const frames = await observation.evaluate((value) => value.stop())
+    expect(frames.some((frame) => frame.working && frame.prompt === undefined)).toBe(false)
+    const positions = frames.flatMap((frame) => (frame.prompt === undefined ? [] : [frame.prompt]))
+    expect(positions.length).toBeGreaterThan(0)
+    expect(new Set(positions).size).toBe(1)
+  } finally {
+    release.resolve()
+    await observation.dispose()
+  }
+})
diff --git a/packages/app/src/composer/submit.ts b/packages/app/src/composer/submit.ts
index b1c7e473519..2fe2c063595 100644
--- a/packages/app/src/composer/submit.ts
+++ b/packages/app/src/composer/submit.ts
@@ -88,8 +88,12 @@ export function createComposerSubmit(input: ComposerSubmitInput) {
       if (value.mode === "normal" && !command) {
         session.handoff?.set(handoffMessage(value))
         const optimisticBusy = !input.adapter.working()
-        if (optimisticBusy) session.data.session.setStatus(session.id, "running")
-        const sending = sendPrompt(session, value, input.adapter.controls().model.selection.trackSessionCommit).then(
+        if (optimisticBusy && input.adapter.kind === "new-session")
+          session.data.session.setStatus(session.id, "running")
+        const sending = sendPrompt(session, value, input.adapter.controls().model.selection.trackSessionCommit, () => {
+          if (optimisticBusy && input.adapter.kind === "active-session")
+            session.data.session.setStatus(session.id, "running")
+        }).then(
           () => ({ ok: true as const }),
           (error) => ({ ok: false as const, error }),
         )
@@ -359,7 +363,8 @@ async function applySelection(
 async function sendPrompt(
   session: ComposerSession,
   value: ComposerSubmission,
-  track?: ModelSelection["trackSessionCommit"],
+  track: ModelSelection["trackSessionCommit"] | undefined,
+  onAdmit: () => void,
 ) {
   const request = await buildSubmissionRequest(session, value)
   // Switching agent or model reconfigures the session immediately, and with it
@@ -389,7 +394,9 @@ async function sendPrompt(
       },
     },
   }
-  await session.data.session.prompt(admission).catch(() => session.data.session.prompt(admission))
+  const sending = session.data.session.prompt(admission).catch(() => session.data.session.prompt(admission))
+  onAdmit()
+  await sending
 }
 
 async function buildSubmissionRequest(session: ComposerSession, value: ComposerSubmission) {
diff --git a/packages/app/src/session/timeline/virtualizer.tsx b/packages/app/src/session/timeline/virtualizer.tsx
index e924d346892..c09dc0c10e0 100644
--- a/packages/app/src/session/timeline/virtualizer.tsx
+++ b/packages/app/src/session/timeline/virtualizer.tsx
@@ -515,6 +515,19 @@ export function createTimelineVirtualizer(input: Input) {
           <div
             ref={(value) => {
               element = value
+              if (row()._tag !== "UserMessage" || !addedKeys.has(rowProps.rowKey) || !input.pinned() || coldPending)
+                return
+              // The optimistic row can paint before ResizeObserver corrects the tail estimates.
+              // Measure the mounted tail and pin it in this render's microtask instead.
+              queueMicrotask(() => {
+                if (!input.pinned() || !virtualContent?.isConnected) return
+                virtualizer.elementsCache.forEach((item) => {
+                  if (item.isConnected) virtualizer.resizeItem(virtualizer.indexFromElement(item), item.offsetHeight)
+                })
+                virtualizer.resizeItem(item().index, element.offsetHeight)
+                virtualContent.style.height = `${virtualizer.getTotalSize()}px`
+                virtualizer.scrollToEnd()
+              })
             }}
             data-index={item().index}
             style={{ "min-height": ready() ? undefined : `${initialItem.size}px` }}