diff --git a/src/components/ui/__tests__/sheet-section.test.tsx b/src/components/ui/__tests__/sheet-section.test.tsx
new file mode 100644
index 000000000..b3f719c71
--- /dev/null
+++ b/src/components/ui/__tests__/sheet-section.test.tsx
@@ -0,0 +1,74 @@
+import { describe, expect, it } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { SheetSection, SheetSectionCount } from "../sheet-section";
+
+describe("<SheetSection>", () => {
+  it("renders the title and summary slot on the trigger row", () => {
+    const html = renderToStaticMarkup(
+      <SheetSection title="More tags" summary={<span>summary-badge</span>}>
+        <p>section body</p>
+      </SheetSection>,
+    );
+    expect(html).toContain("More tags");
+    expect(html).toContain("summary-badge");
+  });
+
+  it("renders children inside an open section", () => {
+    // Radix Collapsible omits the content subtree while collapsed, so the
+    // body is asserted on an expanded section.
+    const html = renderToStaticMarkup(
+      <SheetSection title="More tags" defaultOpen>
+        <p>section body</p>
+      </SheetSection>,
+    );
+    expect(html).toContain("section body");
+  });
+
+  it("starts collapsed by default and open when defaultOpen", () => {
+    const closed = renderToStaticMarkup(
+      <SheetSection title="Factors">
+        <p>x</p>
+      </SheetSection>,
+    );
+    const open = renderToStaticMarkup(
+      <SheetSection title="Factors" defaultOpen>
+        <p>x</p>
+      </SheetSection>,
+    );
+    expect(closed).toContain('data-state="closed"');
+    expect(open).toContain('data-state="open"');
+  });
+
+  it("hides the leading icon from assistive tech", () => {
+    const html = renderToStaticMarkup(
+      <SheetSection title="Note" icon={<svg data-testid="icon" />}>
+        <p>x</p>
+      </SheetSection>,
+    );
+    expect(html).toMatch(/aria-hidden="true"[^>]*>[^<]*<svg/);
+  });
+
+  it("respects reduced motion on the content and chevron", () => {
+    const html = renderToStaticMarkup(
+      <SheetSection title="x">
+        <p>y</p>
+      </SheetSection>,
+    );
+    expect(html).toContain("motion-reduce:animate-none");
+    expect(html).toContain("motion-reduce:transition-none");
+  });
+});
+
+describe("<SheetSectionCount>", () => {
+  it("renders the count when positive", () => {
+    const html = renderToStaticMarkup(<SheetSectionCount count={3} />);
+    expect(html).toContain("3");
+    expect(html).toContain('data-slot="sheet-section-count"');
+  });
+
+  it("renders nothing for zero or negative counts", () => {
+    expect(renderToStaticMarkup(<SheetSectionCount count={0} />)).toBe("");
+    expect(renderToStaticMarkup(<SheetSectionCount count={-1} />)).toBe("");
+  });
+});
diff --git a/src/components/ui/sheet-section.tsx b/src/components/ui/sheet-section.tsx
new file mode 100644
index 000000000..1eadf6028
--- /dev/null
+++ b/src/components/ui/sheet-section.tsx
@@ -0,0 +1,107 @@
+"use client";
+
+import * as React from "react";
+import { ChevronDown } from "lucide-react";
+import { Collapsible as CollapsiblePrimitive } from "radix-ui";
+
+import { cn } from "@/lib/utils";
+
+/**
+ * v1.17.0 — `SheetSection` is the collapsible disclosure primitive that
+ * powers the sectioned single-sheet add flows (mood, cycle). Each section
+ * carries a title, an optional leading icon, and a `summary` slot that
+ * communicates what is selected inside it while collapsed (e.g. a count
+ * badge) — so a closed section never hides its contents silently.
+ *
+ * Built on Radix Collapsible. The chevron rotates and the content height
+ * animates open/closed via the `tw-animate-css` `collapsible-down/up`
+ * keyframes; both motions are suppressed under `prefers-reduced-motion`.
+ * Matches the shadcn new-york + zinc token language used across the app.
+ */
+
+interface SheetSectionProps {
+  /** Section heading shown on the always-visible trigger row. */
+  title: string;
+  /**
+   * A node rendered on the trigger row's trailing edge — typically a count
+   * badge that reports what is selected inside while the section is
+   * collapsed. Stays visible whether the section is open or closed.
+   */
+  summary?: React.ReactNode;
+  /** Optional leading icon (a Lucide component or any node). */
+  icon?: React.ReactNode;
+  /** Whether the section starts expanded. Defaults to collapsed. */
+  defaultOpen?: boolean;
+  /** Optional className merged onto the section wrapper. */
+  className?: string;
+  children: React.ReactNode;
+}
+
+export function SheetSection({
+  title,
+  summary,
+  icon,
+  defaultOpen = false,
+  className,
+  children,
+}: SheetSectionProps) {
+  return (
+    <CollapsiblePrimitive.Root
+      defaultOpen={defaultOpen}
+      data-slot="sheet-section"
+      className={cn("border-border/60 border-t pt-3", className)}
+    >
+      <CollapsiblePrimitive.Trigger
+        data-slot="sheet-section-trigger"
+        className="group focus-visible:ring-ring/50 -mx-1 flex w-[calc(100%+0.5rem)] items-center gap-2 rounded-md px-1 py-1.5 text-left transition-colors focus-visible:ring-2 focus-visible:outline-none"
+      >
+        {icon ? (
+          <span
+            className="text-muted-foreground flex h-4 w-4 shrink-0 items-center justify-center [&>svg]:h-4 [&>svg]:w-4"
+            aria-hidden="true"
+          >
+            {icon}
+          </span>
+        ) : null}
+        <span className="text-foreground flex-1 text-sm font-medium">
+          {title}
+        </span>
+        {summary ? (
+          <span
+            data-slot="sheet-section-summary"
+            className="flex items-center"
+          >
+            {summary}
+          </span>
+        ) : null}
+        <ChevronDown
+          aria-hidden="true"
+          className="text-muted-foreground h-4 w-4 shrink-0 transition-transform duration-200 group-data-[state=open]:rotate-180 motion-reduce:transition-none"
+        />
+      </CollapsiblePrimitive.Trigger>
+      <CollapsiblePrimitive.Content
+        data-slot="sheet-section-content"
+        className="overflow-hidden data-[state=closed]:animate-collapsible-up data-[state=open]:animate-collapsible-down motion-reduce:animate-none"
+      >
+        <div className="space-y-3 pt-3 pb-1">{children}</div>
+      </CollapsiblePrimitive.Content>
+    </CollapsiblePrimitive.Root>
+  );
+}
+
+/**
+ * A compact "(N)" count badge for a `SheetSection` summary slot. Renders
+ * nothing when `count <= 0`, so a section with no selections shows a bare
+ * chevron rather than a "(0)".
+ */
+export function SheetSectionCount({ count }: { count: number }) {
+  if (count <= 0) return null;
+  return (
+    <span
+      data-slot="sheet-section-count"
+      className="bg-primary/15 text-primary inline-flex min-w-5 items-center justify-center rounded-full px-1.5 py-0.5 text-xs font-medium tabular-nums"
+    >
+      {count}
+    </span>
+  );
+}