diff --git a/__tests__/lib/extract-subagent-ids.test.ts b/__tests__/lib/extract-subagent-ids.test.ts
new file mode 100644
index 0000000..a29709e
--- /dev/null
+++ b/__tests__/lib/extract-subagent-ids.test.ts
@@ -0,0 +1,56 @@
+import { describe, it, expect } from "vitest";
+import { extractSubagentIds } from "@/lib/extract-subagent-ids";
+
+describe("extractSubagentIds", () => {
+  it("returns deduped ids from valid user entries", () => {
+    const fileContent = [
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "abc123" } }),
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "def456" } }),
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "abc123" } }),
+    ].join("\n");
+
+    expect(extractSubagentIds(fileContent)).toEqual(["abc123", "def456"]);
+  });
+
+  it("skips malformed JSON lines silently", () => {
+    const fileContent = [
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "abc123" } }),
+      "{bad json",
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "def456" } }),
+    ].join("\n");
+
+    expect(extractSubagentIds(fileContent)).toEqual(["abc123", "def456"]);
+  });
+
+  it("ignores entries where type is not user", () => {
+    const fileContent = [
+      JSON.stringify({ type: "assistant", toolUseResult: { agentId: "abc123" } }),
+      JSON.stringify({ type: "system", toolUseResult: { agentId: "def456" } }),
+    ].join("\n");
+
+    expect(extractSubagentIds(fileContent)).toEqual([]);
+  });
+
+  it("rejects non-hex agent ids", () => {
+    const fileContent = [
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "abc123" } }),
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "abc123g" } }),
+      JSON.stringify({ type: "user", toolUseResult: { agentId: "ABC123" } }),
+    ].join("\n");
+
+    expect(extractSubagentIds(fileContent)).toEqual(["abc123"]);
+  });
+
+  it("ignores entries missing toolUseResult", () => {
+    const fileContent = [
+      JSON.stringify({ type: "user" }),
+      JSON.stringify({ type: "user", toolUseResult: null }),
+    ].join("\n");
+
+    expect(extractSubagentIds(fileContent)).toEqual([]);
+  });
+
+  it("returns an empty array for empty input", () => {
+    expect(extractSubagentIds("")).toEqual([]);
+  });
+});