From 5ed03ad1377b549df83aa52dc11bdb07593a5327 Mon Sep 17 00:00:00 2001
From: Kit Langton <kit.langton@gmail.com>
Date: Sun, 25 Oct 2026 08:32:58 -0700
Subject: [PATCH] feat(session-ui): quiet automatic compaction divider (#58996)

---
 .../e2e/regression/timeline-stream.spec.ts    |  29 ++
 packages/app/e2e/utils/timeline.ts            |   1 +
 .../src/shell/routes/session-ui-provider.tsx  |  23 +-
 .../session-compaction.spec.ts                | 215 ++++++++++++++
 .../src/components/session-compaction.css     | 262 ++++++++++++++++++
 packages/session-ui/src/context/data.tsx      |   5 +
 .../src/message/message-content.tsx           | 164 +++++++++--
 .../src/storybook/current-session-fixtures.ts |  14 +
 .../src/storybook/current-session-story.tsx   |  10 +-
 packages/session-ui/src/styles/index.css      |   1 +
 .../src/timeline/timeline-row.stories.tsx     | 108 +++++++-
 packages/ui/src/i18n/en.ts                    |   1 +
 12 files changed, 792 insertions(+), 41 deletions(-)
 create mode 100644 packages/session-ui/component-tests/session-compaction.spec.ts
 create mode 100644 packages/session-ui/src/components/session-compaction.css

diff --git a/packages/app/e2e/regression/timeline-stream.spec.ts b/packages/app/e2e/regression/timeline-stream.spec.ts
index a927f8dfc3f..39f474bb912 100644
--- a/packages/app/e2e/regression/timeline-stream.spec.ts
+++ b/packages/app/e2e/regression/timeline-stream.spec.ts
@@ -1462,6 +1462,35 @@ test.describe("compaction", () => {
     await expect(page.locator('[data-component="session-working"]')).toBeVisible()
   })
 
+  test("shows an automatic compaction as one quiet divider with background compaction on", async ({ page }) => {
+    const config = page.waitForResponse((response) => new URL(response.url()).pathname === "/api/config")
+
+    const timeline = await setupTimeline(page, {
+      sessionMessages: [user, completed],
+      sessionStatus: { [sessionID]: { type: "busy" } },
+      configEntries: [{ type: "document", info: { experimental: { background_compaction: true } } }],
+    })
+
+    await config
+
+    await timeline.send(compactionStarted({ sessionID, reason: "auto" }))
+    await timeline.send(compactionDelta({ sessionID, text: "Partial automatic summary." }))
+    const compaction = page.locator('[data-component="session-compaction-message"]')
+    const divider = compaction.locator('[data-slot="session-compaction-divider"]')
+    await expect(divider).toHaveAttribute("data-state", "running")
+    await expect(compaction).not.toContainText("Partial automatic summary.")
+    await expect(compaction).not.toContainText("Session compaction started")
+
+    await timeline.send(
+      compactionEnded({ sessionID, reason: "auto", text: "## Checkpoint\n\nAutomatic details." }),
+    )
+    await expect(divider).toHaveAttribute("data-state", "done")
+    await expect(divider).toHaveAccessibleName("Compacted")
+    await expect(compaction).not.toContainText("Automatic details.")
+    await divider.click()
+    await expect(compaction).toContainText("Automatic details.")
+  })
+
   const outcomes = {
     failed: {
       reason: "auto",
diff --git a/packages/app/e2e/utils/timeline.ts b/packages/app/e2e/utils/timeline.ts
index 4dae3612585..b774c3bac0a 100644
--- a/packages/app/e2e/utils/timeline.ts
+++ b/packages/app/e2e/utils/timeline.ts
@@ -109,6 +109,7 @@ type TimelineServerInput = Partial<
   Pick<
     MockServerConfig,
     | "inbox"
+    | "configEntries"
     | "forms"
     | "permissions"
     | "sessionPermissions"
diff --git a/packages/app/src/shell/routes/session-ui-provider.tsx b/packages/app/src/shell/routes/session-ui-provider.tsx
index e17c307e261..f2e8420282c 100644
--- a/packages/app/src/shell/routes/session-ui-provider.tsx
+++ b/packages/app/src/shell/routes/session-ui-provider.tsx
@@ -1,7 +1,8 @@
 import { DataProvider } from "@opencode/session-ui/context"
 import { MarkdownProvider, type ReadMarkdownImage } from "@opencode/session-ui/context/markdown"
 import { useNavigate, useParams } from "@solidjs/router"
-import { createMemo, type ParentProps } from "solid-js"
+import { useQuery } from "@tanstack/solid-query"
+import { createMemo, onCleanup, type ParentProps } from "solid-js"
 import { useProviders } from "@/providers/catalog/providers"
 import { LocalProvider } from "@/providers/models/selection"
 import type { ServerConnection } from "@/runtime/server/registry"
@@ -66,6 +67,25 @@ export function SessionUIProvider(
 
   const providers = useProviders(directory)
 
+  const config = useQuery(() => ({
+    queryKey: [serverSDK.scope, "session-ui-config", directory()],
+    enabled: serverSDK.connection.status() === "connected",
+    queryFn: () => serverSDK.api.config.get({ location: { directory: directory() } }),
+  }))
+
+  onCleanup(
+    serverSDK.event.on("config.updated", (event) => {
+      if (event.location && event.location.directory !== directory()) return
+      void config.refetch()
+    }),
+  )
+
+  // Matches the server: the highest-priority document that sets `experimental` decides.
+  const backgroundCompaction = () =>
+    config.data
+      ?.flatMap((entry) => (entry.type === "document" && entry.info.experimental ? [entry.info.experimental] : []))
+      .at(-1)?.background_compaction === true
+
   // Turn diffs live in the tab's in-memory state, so they outlast timeline remounts and go when the tab closes.
   const turnDiffs = () => {
     const id = params.id
@@ -112,6 +132,7 @@ export function SessionUIProvider(
       }}
       onNavigateToSession={navigateToSession}
       onSessionHref={href}
+      backgroundCompaction={backgroundCompaction()}
     >
       <MarkdownProvider readImage={readImage()} openSession={openReferencedSession}>
         <LocalProvider>{props.children}</LocalProvider>
diff --git a/packages/session-ui/component-tests/session-compaction.spec.ts b/packages/session-ui/component-tests/session-compaction.spec.ts
new file mode 100644
index 00000000000..03ad1fd8cae
--- /dev/null
+++ b/packages/session-ui/component-tests/session-compaction.spec.ts
@@ -0,0 +1,215 @@
+import { expect, story } from "../../storybook/playwright/story"
+
+const rows = "current-session-timeline-rows"
+
+story("shows a running automatic compaction as one Compacting divider", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-in-progress`)
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+  const divider = message.locator('[data-slot="session-compaction-divider"]')
+
+  await expect(message).toHaveAttribute("data-quiet", "")
+  // Already running when the history loads, so it shows at once rather than waiting out the grace.
+  await expect(message).not.toHaveAttribute("data-enter")
+  await expect(divider).toHaveAttribute("data-state", "running")
+  await expect(divider).toBeDisabled()
+  await expect(divider).toHaveAttribute("aria-busy", "true")
+  await expect(divider).toHaveAccessibleName("Compacting")
+  await expect(divider).not.toHaveAttribute("aria-expanded")
+  await expect(message.locator('[data-slot="session-compaction-running"]')).toHaveCSS("opacity", "1")
+  await expect(message).not.toContainText("Session compaction started")
+  await expect(message).not.toContainText("Restore compaction output")
+  await expect(message.locator('[data-component="text-part"]')).toHaveCount(0)
+})
+
+story("opens a finished automatic compaction's summary from its divider", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-and-continuation`)
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+  const divider = message.locator('[data-slot="session-compaction-divider"]')
+  const summary = message.locator('[data-slot="session-compaction-summary"]')
+
+  // Loaded with the history, so it is already there rather than fading in.
+  await expect(message).not.toHaveAttribute("data-enter")
+  await expect(divider).toHaveAttribute("data-state", "done")
+  await expect(message.locator('[data-slot="session-compaction-done"]')).toHaveCSS("opacity", "1")
+  await expect(divider).toHaveAccessibleName("Compacted")
+  await expect(divider).toHaveAttribute("aria-expanded", "false")
+  await expect(divider).not.toHaveAttribute("aria-controls")
+  await expect(summary).toHaveCount(0)
+
+  await divider.click()
+  await expect(divider).toHaveAttribute("aria-expanded", "true")
+  await expect(summary).toContainText("The Session timeline now consumes current nested assistant content.")
+  await expect(summary).toContainText("11.9K in · 508 out")
+  await expect(divider).toHaveAttribute("aria-controls", (await summary.getAttribute("id")) ?? "")
+
+  await divider.click()
+  await expect(divider).toHaveAttribute("aria-expanded", "false")
+  await expect(divider).not.toHaveAttribute("aria-controls")
+  await expect(summary).toHaveCount(0)
+})
+
+story("fades in an instant automatic compaction as Compacted", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-replay`, { args: { run: "instant" } })
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+  const divider = message.locator('[data-slot="session-compaction-divider"]')
+
+  await expect(message).toHaveAttribute("data-enter", "now")
+  await expect(divider).toHaveAttribute("data-state", "done")
+  await expect(divider).toHaveCSS("animation-name", "session-compaction-enter")
+  await expect(divider).toHaveAccessibleName("Compacted")
+  await expect(message.locator('[data-slot="session-compaction-light"]').first()).toBeHidden()
+})
+
+story("fades in a compaction that ends inside the grace as Compacted only", async ({ mount, page }) => {
+  await mount(`${rows}--compaction-replay`, { args: { run: "quick" } })
+
+  // Sample the frame after it finishes: Compacting must already be gone rather than crossfading under Compacted.
+  const ended = await page.evaluate(
+    () =>
+      new Promise<{ enter: string | null; animation: string; running: string; done: string }>((resolve) => {
+        const sample = () => {
+          const message = document.querySelector('[data-component="session-compaction-message"]')
+          const divider = message?.querySelector('[data-slot="session-compaction-divider"]')
+
+          if (!message || !divider || divider.getAttribute("data-state") !== "done")
+            return requestAnimationFrame(sample)
+
+          requestAnimationFrame(() =>
+            resolve({
+              enter: message.getAttribute("data-enter"),
+              animation: getComputedStyle(divider).animationName,
+              running: getComputedStyle(message.querySelector('[data-slot="session-compaction-running"]')!).opacity,
+              done: getComputedStyle(message.querySelector('[data-slot="session-compaction-done"]')!).opacity,
+            }),
+          )
+        }
+
+        sample()
+      }),
+  )
+
+  expect(ended).toEqual({ enter: "now", animation: "session-compaction-enter", running: "0", done: "1" })
+})
+
+story("warms the label only once the gathering light reaches it", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-in-progress`)
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+
+  // Hold the divider's animations at a point in their 3.2 s cycle and read the light and the label's glint there.
+  const at = (progress: number) =>
+    message.evaluate((root, progress) => {
+      root.ownerDocument
+        .getAnimations()
+        .filter(
+          (animation) => animation instanceof CSSAnimation && animation.animationName.startsWith("session-compaction-"),
+        )
+        .forEach((animation) => {
+          animation.pause()
+          animation.currentTime = progress * 3_200
+        })
+
+      const light = root.querySelector('[data-slot="session-compaction-light"]')
+      const glint = root.querySelector('[data-slot="session-compaction-glint"]')
+
+      if (!light || !glint) throw new Error("Expected the light and the glint")
+
+      return [getComputedStyle(light, "::after").opacity, getComputedStyle(glint).opacity].map(Number)
+    }, progress)
+
+  const [travelling, arriving, landed, cooled] = await Promise.all(
+    [0.3, 0.48, 0.57, 0.9].map((progress) => at(progress)),
+  )
+
+  expect(travelling).toEqual([1, 0])
+  expect(arriving).toEqual([1, 0])
+  expect(landed[0]).toBeLessThan(1)
+  expect(landed[1]).toBeCloseTo(0.75, 2)
+  expect(cooled).toEqual([0, 0])
+})
+
+story("keeps the full record for a running automatic compaction with background compaction off", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-in-progress`, { args: { backgroundCompaction: false } })
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+
+  await expect(message).not.toHaveAttribute("data-quiet")
+  await expect(message.locator('[data-slot="session-compaction-divider"]')).toHaveCount(0)
+  await expect(message).toContainText("Session compaction started")
+  await expect(message.getByRole("status")).toBeVisible()
+})
+
+story("keeps the full record for a finished automatic compaction with background compaction off", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-and-continuation`, { args: { backgroundCompaction: false } })
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+
+  await expect(message).not.toHaveAttribute("data-quiet")
+  await expect(message.locator('[data-slot="session-compaction-divider"]')).toHaveCount(0)
+  await expect(message).toContainText("Session compaction started")
+  await expect(message).toContainText("The Session timeline now consumes current nested assistant content.")
+  await expect(message).toContainText("Session compacted · 11.9K in · 508 out")
+})
+
+story("keeps the full record for a manual compaction", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-manual`)
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+
+  await expect(message).not.toHaveAttribute("data-quiet")
+  await expect(message).toContainText("Session compaction started")
+  await expect(message).toContainText("The Session timeline now consumes current nested assistant content.")
+  await expect(message).toContainText("Session compacted · 11.9K in · 508 out")
+})
+
+story("keeps the full record for a failed automatic compaction", async ({ mount }) => {
+  const timeline = await mount(`${rows}--compaction-failed`)
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+
+  await expect(message).not.toHaveAttribute("data-quiet")
+  await expect(message).toContainText("Session compaction started")
+  await expect(message).toContainText("Session compaction failed")
+  await expect(message.locator('[data-component="card"]')).toContainText(
+    "The provider rejected the compaction request.",
+  )
+})
+
+story("keeps the divider's box while an automatic compaction runs and finishes", async ({ mount, page }) => {
+  const timeline = await mount(`${rows}--conversation`, { args: { scenario: "compaction" } })
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+  const label = message.locator('[data-slot="session-compaction-label"]')
+  const stem = label.locator("span").first()
+
+  // The story's turn above gains its footer when the Session goes idle, so measure the divider against itself.
+  const box = async () => {
+    const [outer, text, start] = await Promise.all([message.boundingBox(), label.boundingBox(), stem.boundingBox()])
+
+    if (!outer || !text || !start) throw new Error("Expected the compaction divider to be laid out")
+
+    return {
+      width: outer.width,
+      height: outer.height,
+      label: [text.x - outer.x, text.y - outer.y, text.width],
+      stem: [start.x - outer.x, start.width],
+    }
+  }
+
+  await expect(message.locator('[data-slot="session-compaction-divider"]')).toHaveAttribute("data-state", "running")
+  const running = await box()
+
+  await page.getByRole("button", { name: "Stream summary" }).click()
+  await expect(message).not.toContainText("Streamed implementation details.")
+  expect(await box()).toEqual(running)
+
+  await page.getByRole("button", { name: "Complete summary" }).click()
+  await expect(message.locator('[data-slot="session-compaction-divider"]')).toHaveAttribute("data-state", "done")
+  expect(await box()).toEqual(running)
+})
+
+story("holds the lines still under reduced motion", async ({ mount, page }) => {
+  await page.emulateMedia({ reducedMotion: "reduce" })
+  const timeline = await mount(`${rows}--compaction-in-progress`)
+  const message = timeline.locator('[data-component="session-compaction-message"]')
+  const line = message.locator('[data-slot="session-compaction-line"]').first()
+
+  await expect(message.locator('[data-slot="session-compaction-light"]').first()).toBeHidden()
+  await expect(message.locator('[data-slot="session-compaction-glint"]')).toBeHidden()
+  await expect.poll(() => line.evaluate((element) => getComputedStyle(element, "::before").opacity)).toBe("1")
+  await expect(message.locator('[data-slot="session-compaction-running"]')).toHaveCSS("opacity", "1")
+})
diff --git a/packages/session-ui/src/components/session-compaction.css b/packages/session-ui/src/components/session-compaction.css
new file mode 100644
index 00000000000..d93b531b335
--- /dev/null
+++ b/packages/session-ui/src/components/session-compaction.css
@@ -0,0 +1,262 @@
+/* The quiet automatic compaction divider. Every state keeps the same box: only opacity, transforms, a light blur and
+   the light's position move, so the timeline never shifts. */
+[data-component="session-compaction-message"][data-quiet] {
+  --session-compaction-delay: 0ms;
+  --session-compaction-cycle: 3.2s;
+
+  &[data-enter="grace"] {
+    --session-compaction-delay: 400ms;
+  }
+
+  &[data-enter] [data-slot="session-compaction-divider"] {
+    animation: session-compaction-fade 320ms ease-out var(--session-compaction-delay) both;
+  }
+
+  /* Its own name restarts the fade when a compaction ends inside the grace, and nothing inside transitions, so only
+     Compacted appears. */
+  &[data-enter="now"] {
+    [data-slot="session-compaction-divider"] {
+      animation-name: session-compaction-enter;
+    }
+
+    [data-slot="session-compaction-line"]::before,
+    [data-slot="session-compaction-light"],
+    [data-slot="session-compaction-running"],
+    [data-slot="session-compaction-glint-stem"],
+    [data-slot="session-compaction-glint-ending"] {
+      transition: none;
+    }
+
+    [data-slot="session-compaction-done"] {
+      transition: color 150ms ease-out;
+    }
+  }
+
+  [data-slot="session-compaction-divider"] {
+    padding: 0;
+    border: 0;
+    border-radius: 6px;
+    background: none;
+    font: inherit;
+    cursor: pointer;
+
+    &:disabled {
+      cursor: default;
+    }
+
+    &:focus-visible {
+      outline: 1px solid var(--v2-text-text-accent);
+      outline-offset: 2px;
+    }
+  }
+
+  [data-slot="session-compaction-line"] {
+    position: relative;
+    overflow: hidden;
+
+    &[data-side="end"] {
+      transform: scaleX(-1);
+    }
+
+    &::before {
+      content: "";
+      position: absolute;
+      inset: 0;
+      background: var(--v2-border-border-strong);
+      opacity: 0.6;
+      transition: opacity 360ms ease-out;
+    }
+  }
+
+  /* Each light starts at the outer end and travels to the label. Its brightest point reaches the line's end at 49.5% of
+     the cycle, where it starts to cool as it slides off. */
+  [data-slot="session-compaction-light"] {
+    position: absolute;
+    inset: 0;
+    transition:
+      opacity 360ms ease-out,
+      display 360ms allow-discrete;
+
+    &::after {
+      content: "";
+      position: absolute;
+      inset-block: 0;
+      left: 0;
+      width: 40%;
+      background: linear-gradient(
+        90deg,
+        transparent,
+        color-mix(in srgb, var(--v2-text-text-muted) 55%, transparent) 65%,
+        transparent
+      );
+      animation: session-compaction-gather var(--session-compaction-cycle) linear var(--session-compaction-delay)
+        infinite backwards;
+    }
+  }
+
+  [data-slot="session-compaction-ending"] > span {
+    grid-area: 1 / 1;
+  }
+
+  /* Only the ending changes when the compaction ends: ing rolls up and out, then ed rolls up into its place. The
+     ending's box fits the wider of the two, so the stem and lines hold still. */
+  [data-slot="session-compaction-running"],
+  [data-slot="session-compaction-glint-ending"] {
+    transition:
+      opacity 200ms cubic-bezier(0.4, 0, 1, 1),
+      transform 200ms cubic-bezier(0.4, 0, 1, 1),
+      filter 200ms cubic-bezier(0.4, 0, 1, 1);
+  }
+
+  [data-slot="session-compaction-done"] {
+    opacity: 0;
+    transform: translateY(3px);
+    filter: blur(2px);
+    transition:
+      opacity 280ms cubic-bezier(0.2, 0.8, 0.2, 1) 80ms,
+      transform 280ms cubic-bezier(0.2, 0.8, 0.2, 1) 80ms,
+      filter 280ms cubic-bezier(0.2, 0.8, 0.2, 1) 80ms,
+      color 150ms ease-out;
+  }
+
+  /* The label warms only once each light lands on it, then cools before the next one arrives. */
+  [data-slot="session-compaction-glint"] {
+    position: absolute;
+    inset: 0;
+    display: flex;
+    pointer-events: none;
+    color: var(--v2-text-text-muted);
+    opacity: 0;
+    animation: session-compaction-glint var(--session-compaction-cycle) linear var(--session-compaction-delay) infinite
+      backwards;
+  }
+
+  [data-slot="session-compaction-glint-stem"] {
+    transition: opacity 200ms ease-out;
+  }
+
+  [data-state="done"] {
+    [data-slot="session-compaction-line"]::before {
+      opacity: 1;
+    }
+
+    [data-slot="session-compaction-light"] {
+      display: none;
+      opacity: 0;
+    }
+
+    [data-slot="session-compaction-running"],
+    [data-slot="session-compaction-glint-ending"] {
+      opacity: 0;
+      transform: translateY(-3px);
+      filter: blur(2px);
+    }
+
+    [data-slot="session-compaction-glint"] {
+      animation-play-state: paused;
+    }
+
+    [data-slot="session-compaction-glint-stem"] {
+      opacity: 0;
+    }
+
+    [data-slot="session-compaction-done"] {
+      opacity: 1;
+      transform: none;
+      filter: none;
+    }
+
+    &:not(:disabled):hover [data-slot="session-compaction-done"],
+    &[aria-expanded="true"] [data-slot="session-compaction-done"] {
+      color: var(--v2-text-text-muted);
+    }
+  }
+
+  /* The summary is reference material, so its body sits a step quieter than the conversation around it. */
+  [data-slot="session-compaction-summary"] {
+    padding-bottom: 8px;
+    animation: session-compaction-fade 200ms ease-out both;
+
+    [data-component="markdown"] {
+      color: var(--v2-text-text-muted);
+    }
+  }
+
+  @media (prefers-reduced-motion: reduce) {
+    [data-slot="session-compaction-line"]::before {
+      opacity: 1;
+      transition: none;
+    }
+
+    [data-slot="session-compaction-light"],
+    [data-slot="session-compaction-glint"] {
+      display: none;
+    }
+
+    &[data-enter] [data-slot="session-compaction-divider"] {
+      animation-duration: 200ms;
+    }
+
+    [data-slot="session-compaction-running"],
+    [data-state="done"] [data-slot="session-compaction-running"],
+    [data-slot="session-compaction-done"] {
+      transform: none;
+      filter: none;
+    }
+  }
+}
+
+:dir(rtl) [data-component="session-compaction-message"][data-quiet] [data-slot="session-compaction-line"] {
+  transform: scaleX(-1);
+
+  &[data-side="end"] {
+    transform: none;
+  }
+}
+
+@keyframes session-compaction-fade {
+  from {
+    opacity: 0;
+  }
+}
+
+@keyframes session-compaction-enter {
+  from {
+    opacity: 0;
+  }
+}
+
+/* Travel runs 0–70% on one ease-in-out, so the light never stops or jumps. Opacity has its own keyframes: it fades up
+   early, holds, and cools from the moment the light reaches the label. */
+@keyframes session-compaction-gather {
+  0% {
+    transform: translateX(-100%);
+    opacity: 0;
+    animation-timing-function: cubic-bezier(0.4, 0, 0.6, 1);
+  }
+  14%,
+  49.5% {
+    opacity: 1;
+  }
+  70%,
+  100% {
+    transform: translateX(250%);
+    opacity: 0;
+  }
+}
+
+@keyframes session-compaction-glint {
+  0%,
+  49% {
+    opacity: 0;
+    animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
+  }
+  57% {
+    opacity: 0.75;
+    animation-timing-function: cubic-bezier(0.4, 0, 0.6, 1);
+  }
+  82%,
+  100% {
+    opacity: 0;
+  }
+}
diff --git a/packages/session-ui/src/context/data.tsx b/packages/session-ui/src/context/data.tsx
index ffd6a9041d2..7faad71501a 100644
--- a/packages/session-ui/src/context/data.tsx
+++ b/packages/session-ui/src/context/data.tsx
@@ -62,6 +62,8 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
     turnDiff?: TurnDiffSource
     onNavigateToSession?: NavigateToSessionFn
     onSessionHref?: SessionHrefFn
+    /** Whether `experimental.background_compaction` is on for this location; automatic compactions then show quietly. */
+    backgroundCompaction?: boolean
   }) => {
     return {
       get store() {
@@ -73,6 +75,9 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
       get sessionID() {
         return props.sessionID
       },
+      get backgroundCompaction() {
+        return props.backgroundCompaction === true
+      },
       navigateToSession: props.onNavigateToSession,
       sessionHref: props.onSessionHref,
       shellRunning: props.shellRunning,
diff --git a/packages/session-ui/src/message/message-content.tsx b/packages/session-ui/src/message/message-content.tsx
index 8f0d18c32b2..2098826500d 100644
--- a/packages/session-ui/src/message/message-content.tsx
+++ b/packages/session-ui/src/message/message-content.tsx
@@ -585,6 +585,7 @@ function CompactionDivider(props: { label: string }) {
 
 export function SessionCompactionMessage(props: { message: SessionMessageCompaction; error: string }) {
   const i18n = useI18n()
+  const data = useData()
   const summary = () => (props.message.status === "failed" ? "" : props.message.summary)
 
   const error = () => {
@@ -630,44 +631,147 @@ export function SessionCompactionMessage(props: { message: SessionMessageCompact
   const label = createMemo(() => [i18n.t(outcome()), usage()].filter(Boolean).join(" · "))
 
   return (
-    <div data-component="session-compaction-message">
-      <CompactionDivider label={i18n.t("ui.messagePart.compaction.started")} />
-      <Show when={summary().trim()}>
-        <div data-component="text-part" data-timeline-part-id={props.message.id}>
-          <div data-slot="text-part-body">
-            <PacedMarkdown
-              text={summary()}
-              cacheKey={props.message.id}
-              streaming={props.message.status === "running"}
+    <Show
+      when={!data.backgroundCompaction || props.message.reason === "manual" || props.message.status === "failed"}
+      fallback={
+        <QuietCompaction
+          id={props.message.id}
+          created={props.message.time.created}
+          running={props.message.status === "running"}
+          summary={summary()}
+          usage={usage()}
+        />
+      }
+    >
+      <div data-component="session-compaction-message">
+        <CompactionDivider label={i18n.t("ui.messagePart.compaction.started")} />
+        <Show when={summary().trim()}>
+          <div data-component="text-part" data-timeline-part-id={props.message.id}>
+            <div data-slot="text-part-body">
+              <PacedMarkdown
+                text={summary()}
+                cacheKey={props.message.id}
+                streaming={props.message.status === "running"}
+              />
+            </div>
+          </div>
+        </Show>
+        <Show when={props.message.status === "running"}>
+          <div role="status" class="py-2">
+            <BasicTool
+              icon="archive"
+              trigger={{ title: i18n.t("ui.messagePart.compaction.running") }}
+              status="running"
+              locked
+              hideDetails
             />
           </div>
+        </Show>
+        <Show when={props.message.status !== "running"}>
+          <div class="py-2">
+            <TimelineSeparator label={label()} />
+          </div>
+        </Show>
+        <Show when={error()}>
+          <Card variant="error" class="error-card">
+            {error()}
+          </Card>
+        </Show>
+      </div>
+    </Show>
+  )
+}
+
+/**
+ * An automatic compaction, drawn as one divider that reads Compacting, then Compacted. Its height never changes, and
+ * clicking it opens the summary.
+ */
+function QuietCompaction(props: { id: string; created: number; running: boolean; summary: string; usage: string }) {
+  const i18n = useI18n()
+  const [open, setOpen] = createSignal(false)
+  // A new one waits out the grace (400 ms, matched in session-compaction.css) before showing Compacting; one that
+  // ends inside it, as a precomputed summary does, only fades in as Compacted. One already running or finished when
+  // the history loads shows without an entrance.
+  const mounted = performance.now()
+  const age = Date.now() - props.created
+
+  const [enter, setEnter] = createSignal(
+    props.running ? (age < 400 ? "grace" : undefined) : age < 1_000 ? "now" : undefined,
+  )
+
+  const expandable = () => !props.running && !!props.summary.trim()
+
+  const label = createMemo(() =>
+    splitEnding(i18n.t("ui.messagePart.compaction.running"), i18n.t("ui.messagePart.compaction.done")),
+  )
+
+  createEffect(() => {
+    if (!props.running && enter() === "grace" && performance.now() - mounted < 400) setEnter("now")
+  })
+
+  return (
+    <div data-component="session-compaction-message" data-quiet="" data-enter={enter()}>
+      <div class="py-2">
+        <button
+          type="button"
+          data-slot="session-compaction-divider"
+          data-state={props.running ? "running" : "done"}
+          class="flex h-8 w-full items-center gap-3 text-v2-text-text-faint"
+          disabled={!expandable()}
+          aria-busy={props.running}
+          aria-expanded={expandable() ? open() : undefined}
+          aria-controls={expandable() && open() ? `${props.id}-summary` : undefined}
+          aria-label={props.running ? label().running : label().done}
+          onClick={() => setOpen((value) => !value)}
+        >
+          <span data-slot="session-compaction-line" data-side="start" class="h-px min-w-0 flex-1">
+            <span data-slot="session-compaction-light" />
+          </span>
+          <span
+            data-slot="session-compaction-label"
+            class="relative flex shrink-0 whitespace-nowrap text-[13px] font-[440] leading-text-compact tracking-[-0.04px]"
+          >
+            <span>{label().stem}</span>
+            <span data-slot="session-compaction-ending" class="grid justify-items-start">
+              <span data-slot="session-compaction-running">{label().ending.running}</span>
+              <span data-slot="session-compaction-done">{label().ending.done}</span>
+            </span>
+            <span data-slot="session-compaction-glint" aria-hidden="true">
+              <span data-slot="session-compaction-glint-stem">{label().stem}</span>
+              <span data-slot="session-compaction-glint-ending">{label().ending.running}</span>
+            </span>
+          </span>
+          <span data-slot="session-compaction-line" data-side="end" class="h-px min-w-0 flex-1">
+            <span data-slot="session-compaction-light" />
+          </span>
+        </button>
+      </div>
+      <Show when={expandable() && open()}>
+        <div id={`${props.id}-summary`} data-slot="session-compaction-summary">
+          <div data-component="text-part" data-timeline-part-id={props.id}>
+            <div data-slot="text-part-body">
+              <PacedMarkdown text={props.summary} cacheKey={props.id} streaming={false} />
+            </div>
+          </div>
+          <Show when={props.usage}>
+            <div class="mt-2 text-[13px] leading-text-compact text-v2-text-text-faint tabular-nums">{props.usage}</div>
+          </Show>
         </div>
       </Show>
-      <Show when={props.message.status === "running"}>
-        <div role="status" class="py-2">
-          <BasicTool
-            icon="archive"
-            trigger={{ title: i18n.t("ui.messagePart.compaction.running") }}
-            status="running"
-            locked
-            hideDetails
-          />
-        </div>
-      </Show>
-      <Show when={props.message.status !== "running"}>
-        <div class="py-2">
-          <TimelineSeparator label={label()} />
-        </div>
-      </Show>
-      <Show when={error()}>
-        <Card variant="error" class="error-card">
-          {error()}
-        </Card>
-      </Show>
     </div>
   )
 }
 
+/** Splits Compacting and Compacted at their shared stem, so only the ending changes when the compaction ends. */
+function splitEnding(running: string, done: string) {
+  const chars = [...running]
+  const others = [...done]
+  const at = chars.findIndex((char, index) => char !== others[index])
+  const stem = chars.slice(0, at === -1 ? chars.length : at).join("")
+
+  return { running, done, stem, ending: { running: running.slice(stem.length), done: done.slice(stem.length) } }
+}
+
 export function AssistantTextContent(props: {
   id: string
   text: string
diff --git a/packages/session-ui/src/storybook/current-session-fixtures.ts b/packages/session-ui/src/storybook/current-session-fixtures.ts
index be939cf1c96..f6fe22fa9ae 100644
--- a/packages/session-ui/src/storybook/current-session-fixtures.ts
+++ b/packages/session-ui/src/storybook/current-session-fixtures.ts
@@ -1010,6 +1010,20 @@ export const compactionDocument = document([
   }),
 ] satisfies SessionMessageInfo[])
 
+export const compactionManualDocument = document([
+  ...compactionPrelude,
+  {
+    id: "msg_compaction_manual",
+    type: "compaction",
+    status: "completed",
+    reason: "manual",
+    summary: "The Session timeline now consumes current nested assistant content.",
+    cost: 0.0142,
+    tokens: { input: 3_180, output: 412, reasoning: 96, cache: { read: 8_704, write: 0 } },
+    time: { created: STORY_TIME + 63_000 },
+  },
+] satisfies SessionMessageInfo[])
+
 export const compactionFailedDocument = document([
   ...compactionPrelude,
   {
diff --git a/packages/session-ui/src/storybook/current-session-story.tsx b/packages/session-ui/src/storybook/current-session-story.tsx
index 17357ef3c00..00cc74525a5 100644
--- a/packages/session-ui/src/storybook/current-session-story.tsx
+++ b/packages/session-ui/src/storybook/current-session-story.tsx
@@ -9,11 +9,16 @@ import { Button } from "@opencode/ui/button"
 import { Show, createMemo, createSignal, type JSX } from "solid-js"
 import { CURRENT_SESSION_ID, STORY_TIME } from "./current-session-fixtures"
 
-export function CurrentSessionProviders(props: { document: SessionDocument; children: JSX.Element }) {
+export function CurrentSessionProviders(props: {
+  document: SessionDocument
+  backgroundCompaction?: boolean
+  children: JSX.Element
+}) {
   return (
     <DataProvider
       directory="C:/workspaces/opencode"
       sessionID={props.document.sessionID}
+      backgroundCompaction={props.backgroundCompaction}
       data={{
         agent: [
           { name: "build", color: "blue" },
@@ -78,6 +83,7 @@ export function CurrentSessionTimelineStory(props: {
   shellToolDefaultOpen?: boolean
   editToolDefaultOpen?: boolean
   turnDiffs?: boolean
+  backgroundCompaction?: boolean
 }) {
   // Inline story documents rebuild their messages on every prop read, and the timeline projection keys text by identity.
   const document = createMemo(() => props.document)
@@ -113,7 +119,7 @@ export function CurrentSessionTimelineStory(props: {
         <Show when={revision()} keyed>
           {(revision) => (
             <div data-story-revision={revision}>
-              <CurrentSessionProviders document={document()}>
+              <CurrentSessionProviders document={document()} backgroundCompaction={props.backgroundCompaction}>
                 <SessionTimeline
                   document={document()}
                   presentation={props.presentation}
diff --git a/packages/session-ui/src/styles/index.css b/packages/session-ui/src/styles/index.css
index 6f1a6cae1a9..4c8b1c0a53e 100644
--- a/packages/session-ui/src/styles/index.css
+++ b/packages/session-ui/src/styles/index.css
@@ -7,6 +7,7 @@
 @import "../components/message-nav.css" layer(components);
 @import "../components/question-prompt.css" layer(components);
 @import "../components/session-review.css" layer(components);
+@import "../components/session-compaction.css" layer(components);
 @import "../components/session-turn.css" layer(components);
 @import "../components/shell-submessage.css" layer(components);
 @import "../components/tool-count-label.css" layer(components);
diff --git a/packages/session-ui/src/timeline/timeline-row.stories.tsx b/packages/session-ui/src/timeline/timeline-row.stories.tsx
index efb0ec0a7b6..bc799b39de0 100644
--- a/packages/session-ui/src/timeline/timeline-row.stories.tsx
+++ b/packages/session-ui/src/timeline/timeline-row.stories.tsx
@@ -1,5 +1,5 @@
 import type { SessionMessageAssistant, SessionMessageInfo } from "@opencode/client/promise"
-import { createMemo } from "solid-js"
+import { createMemo, onCleanup, onMount } from "solid-js"
 import { createStore } from "solid-js/store"
 import type { SessionDocument } from "../document"
 import { SessionTimeline } from "./session-timeline"
@@ -14,6 +14,7 @@ import {
   compactionCancelledDocument,
   compactionDocument,
   compactionFailedDocument,
+  compactionManualDocument,
   compactionRunningDocument,
   instructionsUpdatedMultipleDocument,
   instructionsUpdatedSingleDocument,
@@ -207,7 +208,10 @@ function RetryAndRecoverStory() {
             ? {
                 attempt: 2,
                 at: 1_900_000_000_000,
-                error: { type: "ProviderRateLimitError" as const, message: "Rate limit reached. Retrying with backoff." },
+                error: {
+                  type: "ProviderRateLimitError" as const,
+                  message: "Rate limit reached. Retrying with backoff.",
+                },
               }
             : undefined,
           time: { created: STORY_TIME, completed: finished ? STORY_TIME + 300 : undefined },
@@ -231,7 +235,7 @@ function RetryAndRecoverStory() {
           Finish response
         </button>
       </div>
-      <CurrentSessionProviders document={document()}>
+      <CurrentSessionProviders document={document()} backgroundCompaction>
         <SessionTimeline document={document()} />
       </CurrentSessionProviders>
     </section>
@@ -364,7 +368,7 @@ function CompactSessionStory() {
           Cancel next compaction
         </button>
       </div>
-      <CurrentSessionProviders document={document()}>
+      <CurrentSessionProviders document={document()} backgroundCompaction>
         <SessionTimeline document={document()} />
       </CurrentSessionProviders>
     </section>
@@ -373,28 +377,114 @@ function CompactSessionStory() {
 
 const CompactSession = { render: () => <CompactSessionStory /> }
 
+// Automatic compactions only show quietly with experimental.background_compaction on; turn it off to see today's record.
+const backgroundCompaction = {
+  args: { backgroundCompaction: true },
+  argTypes: { backgroundCompaction: { control: "boolean" } },
+}
+
 export const CompactionInProgress = {
-  render: () => (
+  ...backgroundCompaction,
+  render: (args: { backgroundCompaction: boolean }) => (
     <CurrentSessionTimelineStory
       title="Compaction in progress"
-      description="The production divider remains stable while the context summary streams below it."
+      description="With background compaction on, an automatic compaction is one quiet divider whose lines carry light toward Compacting."
       document={compactionRunningDocument}
       width="600px"
+      backgroundCompaction={args.backgroundCompaction}
     />
   ),
 }
 
 export const CompactionAndContinuation = {
-  render: () => (
+  ...backgroundCompaction,
+  render: (args: { backgroundCompaction: boolean }) => (
     <CurrentSessionTimelineStory
       title="Compaction and continuation"
-      description="The context summary separates earlier output from the continued response."
+      description="With background compaction on, a finished automatic compaction reads Compacted; clicking it opens the summary."
       document={compactionDocument}
       width="600px"
+      backgroundCompaction={args.backgroundCompaction}
     />
   ),
 }
 
+export const CompactionManual = {
+  render: () => (
+    <CurrentSessionTimelineStory
+      title="Manual compaction"
+      description="A /compact keeps the full record: started divider, summary and usage."
+      document={compactionManualDocument}
+      width="600px"
+      backgroundCompaction
+    />
+  ),
+}
+
+/** Replays an automatic compaction live: Working, then the divider for `duration` ms, then the continued reply. */
+function CompactionReplayStory(props: { duration: number }) {
+  // The fixture's earlier reply was cut off for the compaction; here it finished, so no Interrupted divider shows first.
+  const prelude = compactionDocument.messages
+    .slice(0, 2)
+    .map((message) => (message.type === "assistant" ? { ...message, error: undefined } : message))
+
+  const [compaction, continuation] = [compactionDocument.messages[2], compactionDocument.messages[3]]
+  const [state, setState] = createStore({ phase: "working", created: 0, run: 0 })
+
+  const start = () => {
+    const run = state.run + 1
+    setState({ phase: "working", created: 0, run })
+    const at = (ms: number, apply: () => void) => setTimeout(() => state.run === run && apply(), ms)
+    at(1_500, () => setState({ phase: props.duration > 0 ? "running" : "completed", created: Date.now() }))
+    at(1_500 + props.duration, () => setState("phase", "completed"))
+    at(2_900 + props.duration, () => setState("phase", "continued"))
+  }
+
+  onMount(start)
+  onCleanup(() => setState("run", -1))
+
+  const document = createMemo(() => {
+    if (state.phase === "working" || compaction?.type !== "compaction" || compaction.status !== "completed")
+      return { ...compactionDocument, messages: prelude, status: { type: "busy" } } satisfies SessionDocument
+
+    const message =
+      state.phase === "running"
+        ? { ...compaction, status: "running" as const, time: { created: state.created } }
+        : { ...compaction, time: { created: state.created } }
+
+    return {
+      ...compactionDocument,
+      messages: [...prelude, message, ...(state.phase === "continued" && continuation ? [continuation] : [])],
+      status: { type: state.phase === "continued" ? "idle" : "busy" },
+    } satisfies SessionDocument
+  })
+
+  return (
+    <section class="mx-auto flex w-full max-w-[640px] flex-col gap-4 p-6">
+      <div class="flex gap-3">
+        <button type="button" onClick={start}>
+          Replay
+        </button>
+      </div>
+      <CurrentSessionProviders document={document()} backgroundCompaction>
+        <SessionTimeline document={document()} />
+      </CurrentSessionProviders>
+    </section>
+  )
+}
+
+// `instant` arrives finished, as when started and ended land in one flush; `quick` ends inside the 400 ms grace.
+const replayDurations = { instant: 0, quick: 150, short: 1_200, long: 20_000 }
+
+export const CompactionReplay = {
+  args: { run: "long" },
+  argTypes: { run: { control: "select", options: Object.keys(replayDurations) } },
+  render: (args: { run: string }) => (
+    // SAFETY: the run control only offers replayDurations keys.
+    <CompactionReplayStory duration={replayDurations[args.run as keyof typeof replayDurations]} />
+  ),
+}
+
 export const CompactionFailed = {
   render: () => (
     <CurrentSessionTimelineStory
@@ -402,6 +492,7 @@ export const CompactionFailed = {
       description="A failed compaction keeps the production divider and shows the provider error."
       document={compactionFailedDocument}
       width="600px"
+      backgroundCompaction
     />
   ),
 }
@@ -413,6 +504,7 @@ export const CompactionCancelled = {
       description="An interrupted compaction keeps the production divider without an empty error block."
       document={compactionCancelledDocument}
       width="600px"
+      backgroundCompaction
     />
   ),
 }
diff --git a/packages/ui/src/i18n/en.ts b/packages/ui/src/i18n/en.ts
index f4c50837089..1b0d52876d6 100644
--- a/packages/ui/src/i18n/en.ts
+++ b/packages/ui/src/i18n/en.ts
@@ -113,6 +113,7 @@ const source = {
   "ui.messagePart.compaction.started": "Session compaction started",
   "ui.messagePart.compaction.queued": "Session compaction queued",
   "ui.messagePart.compaction.running": "Compacting",
+  "ui.messagePart.compaction.done": "Compacted",
   "ui.messagePart.compaction.failed": "Session compaction failed",
   "ui.messagePart.compaction.cancelled": "Session compaction cancelled",
   "ui.messagePart.compaction.interrupted": "Session compaction interrupted",