diff --git a/web/src/activity.test.ts b/web/src/activity.test.ts
new file mode 100644
index 0000000..206c97f
--- /dev/null
+++ b/web/src/activity.test.ts
@@ -0,0 +1,101 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  activityColorSlug,
+  activityDurationLabel,
+  activityHeldMs,
+  showsMetaLine,
+} from "./activity.js";
+import type { Session } from "./types.js";
+
+describe("activityHeldMs", () => {
+  it("measures time spent in the current state", () => {
+    const now = Date.now();
+    const since = new Date(now - 90_000).toISOString();
+    expect(activityHeldMs(since, now)).toBe(90_000);
+  });
+
+  it("returns null for payloads that predate #102 (epoch default)", () => {
+    expect(activityHeldMs(undefined, Date.now())).toBeNull();
+    expect(activityHeldMs(new Date(0).toISOString(), Date.now())).toBeNull();
+  });
+
+  it("returns null for unparseable timestamps", () => {
+    expect(activityHeldMs("not-a-time", Date.now())).toBeNull();
+  });
+
+  it("never goes negative", () => {
+    const now = Date.now();
+    expect(activityHeldMs(new Date(now + 5_000).toISOString(), now)).toBe(0);
+  });
+});
+
+describe("activityDurationLabel (#102)", () => {
+  it("floors whole seconds under a minute", () => {
+    expect(activityDurationLabel(30_400)).toBe("30s");
+    expect(activityDurationLabel(59_900)).toBe("59s");
+  });
+
+  it("floors whole minutes under an hour", () => {
+    expect(activityDurationLabel(60_000)).toBe("1m");
+    expect(activityDurationLabel(4 * 60_000 + 59_999)).toBe("4m");
+  });
+
+  it("floors whole hours from an hour up, with no days unit", () => {
+    expect(activityDurationLabel(60 * 60_000)).toBe("1h");
+    expect(activityDurationLabel(2 * 3_600_000 + 59 * 60_000 + 59_999)).toBe("2h");
+    expect(activityDurationLabel(26 * 3_600_000)).toBe("26h");
+  });
+});
+
+describe("activityColorSlug", () => {
+  it("maps each known state to its contract slug", () => {
+    expect(activityColorSlug("Active")).toBe("active");
+    expect(activityColorSlug("Idle")).toBe("idle");
+    expect(activityColorSlug("AwaitingInput")).toBe("awaiting-input");
+    expect(activityColorSlug("Error")).toBe("error");
+  });
+
+  it("treats an unknown variant as idle so rendering never fails", () => {
+    expect(activityColorSlug("Teleported")).toBe("idle");
+  });
+});
+
+function session(overrides: Partial<Session>): Session {
+  return {
+    id: "11111111-2222-3333-4444-555555555555",
+    cwd: "/home/naps/tea/maestro",
+    command: "claude",
+    env: {},
+    metadata: {},
+    status: "Running",
+    created_at: "2026-08-24T10:00:00Z",
+    activity: "Active",
+    ...overrides,
+  };
+}
+
+describe("showsMetaLine", () => {
+  const fresh = new Date(Date.now() - 10_000).toISOString();
+
+  it("stays hidden for a freshly-active session", () => {
+    expect(showsMetaLine(session({ activity_since: fresh }))).toBe(false);
+  });
+
+  it("appears once the state has been held past 30 seconds", () => {
+    const held = new Date(Date.now() - 31_000).toISOString();
+    expect(showsMetaLine(session({ activity_since: held }))).toBe(true);
+  });
+
+  it("appears for blocked work, exited sessions and reported errors", () => {
+    expect(
+      showsMetaLine(session({ activity_since: fresh, blocked_work: { uncommitted_files: 1 } })),
+    ).toBe(true);
+    expect(showsMetaLine(session({ activity_since: fresh, status: "Dead" }))).toBe(true);
+    expect(showsMetaLine(session({ activity_since: fresh, activity: "Error" }))).toBe(true);
+  });
+
+  it("needs no activity timestamp when another trigger holds", () => {
+    expect(showsMetaLine(session({ activity_since: undefined, status: "Dead" }))).toBe(true);
+  });
+});
diff --git a/web/src/activity.ts b/web/src/activity.ts
new file mode 100644
index 0000000..2d3aa83
--- /dev/null
+++ b/web/src/activity.ts
@@ -0,0 +1,56 @@
+// docs/SIDEBAR.md: the session card's meta line — activity colour mapping and
+// the shared duration format. Both clients compute it identically; the TUI
+// mirrors this in crates/maestro-tui.
+
+import type { ActivityState, IsoTimestamp, Session } from "./types.js";
+import { sessionBlockers } from "./types.js";
+
+/** A state held shorter than this keeps the card to one line (contract rule 1). */
+const META_LINE_MIN_HOLD_MS = 30_000;
+
+// Payloads predating #102 default `activity_since` to the Unix epoch; anything
+// implausibly old is treated as "never observed" rather than a huge duration.
+const PLAUSIBLE_EPOCH_MS = Date.UTC(2010, 0, 1);
+
+export function activityHeldMs(since: IsoTimestamp | undefined, now: number): number | null {
+  if (!since) return null;
+  const t = Date.parse(since);
+  if (Number.isNaN(t) || t < PLAUSIBLE_EPOCH_MS) return null;
+  return Math.max(0, now - t);
+}
+
+/** `<60s` → whole seconds, `<60m` → whole minutes, else whole hours. Floored
+ *  everywhere so the counter never shows a duration the state hasn't reached. */
+export function activityDurationLabel(heldMs: number): string {
+  const s = Math.floor(heldMs / 1000);
+  if (s < 60) return `${s}s`;
+  const m = Math.floor(s / 60);
+  if (m < 60) return `${m}m`;
+  return `${Math.floor(m / 60)}h`;
+}
+
+/** CSS hook for the state's colour (`styles.css` maps slugs to tokens). An
+ *  unknown variant degrades to idle so rendering never fails (Precedence). */
+export function activityColorSlug(state: ActivityState | string): string {
+  switch (state) {
+    case "Active":
+      return "active";
+    case "AwaitingInput":
+      return "awaiting-input";
+    case "Error":
+      return "error";
+    default:
+      return "idle";
+  }
+}
+
+/** Line 2 renders only when at least one contract trigger holds. */
+export function showsMetaLine(s: Session, now: number = Date.now()): boolean {
+  const held = activityHeldMs(s.activity_since, now);
+  return (
+    (held !== null && held >= META_LINE_MIN_HOLD_MS) ||
+    sessionBlockers(s) !== null ||
+    s.status === "Dead" ||
+    s.activity === "Error"
+  );
+}