diff --git a/replay-ui/src/__tests__/action-format.test.ts b/replay-ui/src/__tests__/action-format.test.ts
new file mode 100644
index 0000000..17b8cae
--- /dev/null
+++ b/replay-ui/src/__tests__/action-format.test.ts
@@ -0,0 +1,84 @@
+import { describe, it, expect } from "bun:test";
+import {
+  formatActionRow,
+  formatElapsed,
+  parseSelector,
+  tagFromSelector,
+} from "../lib/action-format";
+import type { StepSummary } from "../types";
+
+function summary(over: Partial<StepSummary>): StepSummary {
+  return {
+    index: 0,
+    timestamp: "1970-01-01T00:00:00.000Z",
+    has_violations: false,
+    has_exceptions: false,
+    ...over,
+  };
+}
+
+// Bug class: selector parsing mislabels every action row — splitting on the
+// wrong colon, treating a value-with-colon as the kind, or dropping the prefix
+// ellipsis would render the wrong target tag for every step.
+describe("parseSelector", () => {
+  const cases: { input: string; out: ReturnType<typeof parseSelector> }[] = [
+    { input: "id:login", out: { kind: "id", value: "login" } },
+    { input: "text:Sign In", out: { kind: "text", value: "Sign In" } },
+    { input: "textPrefix:Hello", out: { kind: "textPrefix", value: "Hello" } },
+    { input: "id:com.app:id/btn", out: { kind: "id", value: "com.app:id/btn" } },
+    { input: "bogus:x", out: null },
+    { input: ":leading", out: null },
+    { input: "no-colon", out: null },
+  ];
+  for (const { input, out } of cases) {
+    it(`parses ${input}`, () => {
+      expect(parseSelector(input)).toEqual(out);
+    });
+  }
+});
+
+describe("tagFromSelector", () => {
+  it("appends ellipsis only for prefix selectors", () => {
+    expect(tagFromSelector("textPrefix:Hel")).toBe("Hel...");
+    expect(tagFromSelector("text:Hello")).toBe("Hello");
+    expect(tagFromSelector("plain")).toBe("plain");
+  });
+});
+
+describe("formatActionRow", () => {
+  it("observes when no action kind, with and without screen", () => {
+    expect(formatActionRow(summary({}))).toEqual({
+      verb: "Observe",
+      target: "",
+      targetIsTag: false,
+    });
+    expect(formatActionRow(summary({ screen: "Home" }))).toEqual({
+      verb: "Observe",
+      target: "@ Home",
+      targetIsTag: false,
+    });
+  });
+
+  it("treats a selector label as a tag and a coordinate label as literal", () => {
+    expect(
+      formatActionRow(summary({ action_kind: "Tap", action_label: "id:btn" })),
+    ).toEqual({ verb: "Click", target: "btn", targetIsTag: true });
+    expect(
+      formatActionRow(summary({ action_kind: "Tap", action_label: "(10, 20)" })),
+    ).toEqual({ verb: "Click", target: "(10, 20)", targetIsTag: false });
+  });
+
+  it("maps known verbs and falls back to the raw kind", () => {
+    expect(formatActionRow(summary({ action_kind: "InputText", action_label: "hi" })).verb).toBe("Type");
+    expect(formatActionRow(summary({ action_kind: "Swipe" })).verb).toBe("Swipe");
+    expect(formatActionRow(summary({ action_kind: "Custom" })).verb).toBe("Custom");
+  });
+});
+
+describe("formatElapsed", () => {
+  it("formats mm:ss.mmm and clamps negatives to zero", () => {
+    expect(formatElapsed(0)).toBe("00:00.000");
+    expect(formatElapsed(65_432)).toBe("01:05.432");
+    expect(formatElapsed(-50)).toBe("00:00.000");
+  });
+});