diff --git a/ui/src/components/assistant/PromoteTransition.test.tsx b/ui/src/components/assistant/PromoteTransition.test.tsx
new file mode 100644
index 00000000..df6e5cd1
--- /dev/null
+++ b/ui/src/components/assistant/PromoteTransition.test.tsx
@@ -0,0 +1,154 @@
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { PromoteTransition } from "./PromoteTransition";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
+
+describe("<PromoteTransition />", () => {
+  let container: HTMLDivElement;
+  let root: ReturnType<typeof createRoot> | null = null;
+
+  beforeEach(() => {
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = null;
+  });
+
+  afterEach(() => {
+    if (root) {
+      act(() => {
+        root!.unmount();
+      });
+      root = null;
+    }
+    if (container.parentNode) container.remove();
+  });
+
+  function render(node: React.ReactNode) {
+    root = createRoot(container);
+    act(() => {
+      root!.render(node);
+    });
+  }
+
+  it("renders nothing in idle state", () => {
+    render(
+      <PromoteTransition
+        state="idle"
+        panelChildren={<div data-testid="panel">panel</div>}
+      >
+        <div data-testid="ribbon">ribbon</div>
+      </PromoteTransition>,
+    );
+    expect(container.querySelector('[data-testid="promote-transition"]')).toBeNull();
+  });
+
+  it("renders nothing in done state", () => {
+    render(
+      <PromoteTransition
+        state="done"
+        panelChildren={<div data-testid="panel">panel</div>}
+      >
+        <div data-testid="ribbon">ribbon</div>
+      </PromoteTransition>,
+    );
+    expect(container.querySelector('[data-testid="promote-transition"]')).toBeNull();
+  });
+
+  it("renders nothing in error state", () => {
+    render(
+      <PromoteTransition
+        state="error"
+        panelChildren={<div data-testid="panel">panel</div>}
+      >
+        <div data-testid="ribbon">ribbon</div>
+      </PromoteTransition>,
+    );
+    expect(container.querySelector('[data-testid="promote-transition"]')).toBeNull();
+  });
+
+  it("renders the overlay, ribbon, panel and label in prompting state", () => {
+    render(
+      <PromoteTransition
+        state="prompting"
+        panelChildren={<div data-testid="panel">panel</div>}
+      >
+        <div data-testid="ribbon">ribbon</div>
+      </PromoteTransition>,
+    );
+    const overlay = container.querySelector('[data-testid="promote-transition"]') as HTMLElement;
+    expect(overlay).not.toBeNull();
+    expect(overlay.getAttribute("data-state")).toBe("prompting");
+    expect(overlay.getAttribute("aria-live")).toBe("polite");
+    expect(overlay.getAttribute("role")).toBe("region");
+
+    const ribbon = container.querySelector('[data-testid="promote-chat-ribbon"]') as HTMLElement;
+    const panelContainer = container.querySelector('[data-testid="promote-panel-container"]') as HTMLElement;
+    const label = container.querySelector('[data-testid="promote-source-label"]') as HTMLElement;
+
+    expect(ribbon).not.toBeNull();
+    expect(panelContainer).not.toBeNull();
+    expect(label).not.toBeNull();
+
+    // State data attribute is threaded to the animated children so CSS can
+    // select them.
+    expect(ribbon.getAttribute("data-pstate")).toBe("prompting");
+    expect(panelContainer.getAttribute("data-pstate")).toBe("prompting");
+    expect(label.getAttribute("data-pstate")).toBe("prompting");
+
+    // Children render inside the right slots.
+    expect(ribbon.querySelector('[data-testid="ribbon"]')).not.toBeNull();
+    expect(panelContainer.querySelector('[data-testid="panel"]')).not.toBeNull();
+
+    // Source conversation label text.
+    expect(label.textContent?.trim().toLowerCase()).toBe("source conversation");
+  });
+
+  it("updates data-state to creating while the API call is in flight", () => {
+    render(
+      <PromoteTransition
+        state="creating"
+        panelChildren={<div data-testid="panel">panel</div>}
+      >
+        <div data-testid="ribbon">ribbon</div>
+      </PromoteTransition>,
+    );
+    const overlay = container.querySelector('[data-testid="promote-transition"]') as HTMLElement;
+    expect(overlay.getAttribute("data-state")).toBe("creating");
+    const ribbon = container.querySelector('[data-testid="promote-chat-ribbon"]') as HTMLElement;
+    expect(ribbon.getAttribute("data-pstate")).toBe("creating");
+  });
+
+  it("inlines a <style> tag with the keyframe CSS", () => {
+    render(
+      <PromoteTransition
+        state="prompting"
+        panelChildren={<div data-testid="panel">panel</div>}
+      >
+        <div data-testid="ribbon">ribbon</div>
+      </PromoteTransition>,
+    );
+    const style = container.querySelector("style") as HTMLStyleElement;
+    expect(style).not.toBeNull();
+    expect(style.textContent ?? "").toContain("nx-promote-ribbon");
+    expect(style.textContent ?? "").toContain("max-height");
+    expect(style.textContent ?? "").toContain("prefers-reduced-motion");
+  });
+
+  it("has an accessible label on the overlay region", () => {
+    render(
+      <PromoteTransition
+        state="prompting"
+        panelChildren={<div>panel</div>}
+      >
+        <div>ribbon</div>
+      </PromoteTransition>,
+    );
+    const overlay = container.querySelector('[data-testid="promote-transition"]') as HTMLElement;
+    expect(overlay.getAttribute("aria-label")).toBe("Promote to project");
+  });
+});
diff --git a/ui/src/components/assistant/PromoteTransition.tsx b/ui/src/components/assistant/PromoteTransition.tsx
new file mode 100644
index 00000000..7da81132
--- /dev/null
+++ b/ui/src/components/assistant/PromoteTransition.tsx
@@ -0,0 +1,148 @@
+// [nexus] Phase 12 — PromoteTransition.
+//
+// CSS-first animation container that drives the 700ms compress-and-rise
+// moment from spec §5.6. When `state` is `prompting` or `creating`:
+//
+//   0–200ms  : chat ribbon collapses 100% → 30vh (inset shadow fades in)
+//   200–500ms: brainstormer panel slides up from below into bottom 70%
+//   500–700ms: SOURCE CONVERSATION label fades in above the ribbon
+//
+// On `idle` / `done` the overlay unmounts. `prefers-reduced-motion` is
+// honored via inline CSS — transitions are set to `none`.
+//
+// This component is intentionally CSS-only; no motion library dependency.
+import { useEffect, useRef } from "react";
+import { cn } from "@/lib/utils";
+
+export type PromoteTransitionState = "idle" | "prompting" | "creating" | "done" | "error";
+
+export interface PromoteTransitionProps {
+  /** The hook's promote state kind. */
+  state: PromoteTransitionState;
+  /**
+   * The compressed chat ribbon. Caller passes whatever the source view
+   * should look like — usually the current ChatMessageList in a scrollable
+   * container.
+   */
+  children: React.ReactNode;
+  /**
+   * The rising brainstormer panel content. Usually <BrainstormerPanel />.
+   */
+  panelChildren: React.ReactNode;
+  className?: string;
+}
+
+// Inlined styles for the transition so we do NOT need to touch the global
+// stylesheet. A <style> tag is rendered once per overlay mount and scopes
+// via a class name.
+const TRANSITION_STYLE = `
+.nx-promote-ribbon {
+  position: relative;
+  overflow: hidden;
+  max-height: 100%;
+  transition: max-height 200ms cubic-bezier(0.22, 1, 0.36, 1),
+              box-shadow 200ms ease-out;
+  box-shadow: none;
+}
+.nx-promote-ribbon[data-pstate="prompting"],
+.nx-promote-ribbon[data-pstate="creating"] {
+  max-height: 30vh;
+  box-shadow: inset 0 -16px 24px -12px rgba(0, 0, 0, 0.55);
+}
+.nx-promote-panel {
+  transform: translateY(100%);
+  opacity: 0;
+  transition: transform 300ms cubic-bezier(0.22, 1, 0.36, 1),
+              opacity 300ms ease-out;
+  transition-delay: 0ms;
+}
+.nx-promote-panel[data-pstate="prompting"],
+.nx-promote-panel[data-pstate="creating"] {
+  transform: translateY(0);
+  opacity: 1;
+  transition-delay: 200ms;
+}
+.nx-promote-label {
+  opacity: 0;
+  transition: opacity 200ms ease-out;
+  transition-delay: 0ms;
+}
+.nx-promote-label[data-pstate="prompting"],
+.nx-promote-label[data-pstate="creating"] {
+  opacity: 1;
+  transition-delay: 500ms;
+}
+@media (prefers-reduced-motion: reduce) {
+  .nx-promote-ribbon,
+  .nx-promote-panel,
+  .nx-promote-label {
+    transition: none !important;
+  }
+}
+`;
+
+export function PromoteTransition({
+  state,
+  children,
+  panelChildren,
+  className,
+}: PromoteTransitionProps) {
+  const rootRef = useRef<HTMLDivElement>(null);
+
+  // When the transition first mounts (state flipped to prompting), move
+  // focus to the overlay so screen readers announce the aria-live region.
+  useEffect(() => {
+    if (state === "prompting" || state === "creating") {
+      rootRef.current?.focus();
+    }
+  }, [state]);
+
+  if (state === "idle" || state === "done" || state === "error") {
+    return null;
+  }
+
+  return (
+    <div
+      ref={rootRef}
+      tabIndex={-1}
+      data-testid="promote-transition"
+      data-state={state}
+      aria-live="polite"
+      role="region"
+      aria-label="Promote to project"
+      className={cn(
+        "absolute inset-0 z-40 flex flex-col bg-background",
+        className,
+      )}
+    >
+      <style>{TRANSITION_STYLE}</style>
+
+      {/* SOURCE CONVERSATION label */}
+      <div
+        data-testid="promote-source-label"
+        data-pstate={state}
+        className="nx-promote-label px-6 pt-3 text-[10px] uppercase tracking-[0.14em] text-muted-foreground"
+      >
+        Source Conversation
+      </div>
+
+      {/* Compressed chat ribbon */}
+      <div
+        data-testid="promote-chat-ribbon"
+        data-pstate={state}
+        className="nx-promote-ribbon flex-shrink-0 px-6 py-2"
+      >
+        {children}
+      </div>
+
+      {/* Rising brainstormer panel */}
+      <div
+        data-testid="promote-panel-container"
+        data-pstate={state}
+        className="nx-promote-panel relative flex-1 min-h-0"
+      >
+        {panelChildren}
+      </div>
+    </div>
+  );
+}