diff --git a/canvas/src/store/__tests__/classNames.test.ts b/canvas/src/store/__tests__/classNames.test.ts
new file mode 100644
index 00000000..7c6d1548
--- /dev/null
+++ b/canvas/src/store/__tests__/classNames.test.ts
@@ -0,0 +1,168 @@
+// @vitest-environment jsdom
+/**
+ * Tests for store/classNames helpers — the centralised string
+ * manipulation for React Flow's space-separated className strings.
+ *
+ * Why this is load-bearing: every spawn / parent-pulse / one-shot
+ * animation flow runs through these helpers. Dedup correctness
+ * matters because React Flow's diffing treats className identity
+ * by string equality, so a stray double-class on every render
+ * thrashes layout repeatedly. Whitespace handling matters because
+ * upstream class strings sometimes arrive with multiple spaces
+ * (legacy concat) — the helpers must collapse them.
+ *
+ * Issue: #1815 follow-up — store/classNames.ts was at 17% coverage.
+ */
+import {
+  describe,
+  it,
+  expect,
+  vi,
+  beforeEach,
+  afterEach,
+} from "vitest";
+import {
+  appendClass,
+  removeClass,
+  scheduleNodeClassRemoval,
+} from "../classNames";
+
+// ── appendClass ──────────────────────────────────────────────────────────────
+
+describe("appendClass", () => {
+  it("returns just `cls` when existing is undefined", () => {
+    expect(appendClass(undefined, "spawn")).toBe("spawn");
+  });
+
+  it("returns just `cls` when existing is empty string", () => {
+    expect(appendClass("", "spawn")).toBe("spawn");
+  });
+
+  it("appends to a single-class existing", () => {
+    expect(appendClass("a", "b")).toBe("a b");
+  });
+
+  it("does NOT duplicate when class already present (the dedup contract)", () => {
+    // The whole reason this lives in classNames.ts: pre-helper, the
+    // call sites inlined `${existing} ${cls}` with no dedup, so a
+    // tick that fired the same class twice produced "a a" and
+    // React Flow treated it as a className change every render
+    // (string equality fails) → constant re-render thrash.
+    expect(appendClass("a b spawn", "spawn")).toBe("a b spawn");
+    expect(appendClass("spawn", "spawn")).toBe("spawn");
+  });
+
+  it("collapses multiple spaces in the input (whitespace normalization)", () => {
+    // Upstream sometimes arrives with double spaces (legacy concat
+    // path). Filter+join normalizes regardless of input shape.
+    expect(appendClass("a   b", "c")).toBe("a b c");
+  });
+
+  it("ignores leading/trailing whitespace in existing", () => {
+    expect(appendClass("  a b  ", "c")).toBe("a b c");
+  });
+});
+
+// ── removeClass ──────────────────────────────────────────────────────────────
+
+describe("removeClass", () => {
+  it("returns empty string when existing is undefined", () => {
+    expect(removeClass(undefined, "spawn")).toBe("");
+  });
+
+  it("returns empty string when existing is empty", () => {
+    expect(removeClass("", "spawn")).toBe("");
+  });
+
+  it("removes the named class", () => {
+    expect(removeClass("a spawn b", "spawn")).toBe("a b");
+  });
+
+  it("removes only exact matches (not substrings)", () => {
+    // "spawn" must NOT match "spawn-fast". String split on
+    // whitespace + exact compare gives this for free.
+    expect(removeClass("spawn spawn-fast", "spawn")).toBe("spawn-fast");
+  });
+
+  it("returns empty string when removing the only class", () => {
+    expect(removeClass("spawn", "spawn")).toBe("");
+  });
+
+  it("is a no-op when class isn't present", () => {
+    expect(removeClass("a b c", "missing")).toBe("a b c");
+  });
+
+  it("collapses multiple spaces and removes empty entries", () => {
+    expect(removeClass("a   spawn   b", "spawn")).toBe("a b");
+  });
+});
+
+// ── scheduleNodeClassRemoval ─────────────────────────────────────────────────
+
+describe("scheduleNodeClassRemoval", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("calls set() with className-removed nodes after delayMs", () => {
+    const get = vi.fn(() => ({
+      nodes: [
+        { id: "ws-a", className: "spawn animate-pulse" },
+        { id: "ws-b", className: "spawn" },
+      ],
+    }));
+    const set = vi.fn();
+
+    scheduleNodeClassRemoval("ws-a", "spawn", 200, get, set);
+
+    // Timer hasn't fired yet — no set call.
+    expect(set).not.toHaveBeenCalled();
+
+    vi.advanceTimersByTime(200);
+
+    expect(set).toHaveBeenCalledTimes(1);
+    const patch = set.mock.calls[0][0] as {
+      nodes: Array<{ id: string; className?: string }>;
+    };
+    // Target node had `spawn` removed, kept `animate-pulse`.
+    const wsA = patch.nodes.find((n) => n.id === "ws-a")!;
+    expect(wsA.className).toBe("animate-pulse");
+    // Other node UNTOUCHED — class still present, NOT pruned by id mismatch.
+    const wsB = patch.nodes.find((n) => n.id === "ws-b")!;
+    expect(wsB.className).toBe("spawn");
+  });
+
+  it("does not fire before the delay elapses", () => {
+    const get = vi.fn(() => ({ nodes: [{ id: "x", className: "spawn" }] }));
+    const set = vi.fn();
+    scheduleNodeClassRemoval("x", "spawn", 500, get, set);
+    vi.advanceTimersByTime(499);
+    expect(set).not.toHaveBeenCalled();
+    vi.advanceTimersByTime(1);
+    expect(set).toHaveBeenCalledTimes(1);
+  });
+
+  it("is a no-op when window is undefined (SSR safety)", () => {
+    // jsdom defines `window` by default; mock it to undefined for
+    // this case so the SSR guard is exercised. Don't `vi.useFakeTimers`
+    // here since we're asserting NO timer was ever scheduled.
+    vi.useRealTimers();
+    const originalWindow = globalThis.window;
+    // @ts-expect-error — deliberately undefining window to simulate SSR.
+    globalThis.window = undefined;
+
+    const get = vi.fn();
+    const set = vi.fn();
+    try {
+      scheduleNodeClassRemoval("x", "spawn", 100, get, set);
+    } finally {
+      globalThis.window = originalWindow;
+    }
+
+    expect(get).not.toHaveBeenCalled();
+    expect(set).not.toHaveBeenCalled();
+  });
+});