diff --git a/src/discord/probe.parse-token.test.ts b/src/discord/probe.parse-token.test.ts
new file mode 100644
index 00000000000..8439c79ac46
--- /dev/null
+++ b/src/discord/probe.parse-token.test.ts
@@ -0,0 +1,43 @@
+import { describe, expect, it } from "vitest";
+import { parseApplicationIdFromToken } from "./probe.js";
+
+describe("parseApplicationIdFromToken", () => {
+  it("extracts application ID from a valid token", () => {
+    // "1234567890" base64-encoded is "MTIzNDU2Nzg5MA=="
+    const token = `${Buffer.from("1234567890").toString("base64")}.timestamp.hmac`;
+    expect(parseApplicationIdFromToken(token)).toBe("1234567890");
+  });
+
+  it("extracts large snowflake IDs without precision loss", () => {
+    // ID that exceeds Number.MAX_SAFE_INTEGER (2^53 - 1 = 9007199254740991)
+    const largeId = "1477179610322964541";
+    const token = `${Buffer.from(largeId).toString("base64")}.GhIiP9.vU1xEpJ6NjFm`;
+    expect(parseApplicationIdFromToken(token)).toBe(largeId);
+  });
+
+  it("handles tokens with Bot prefix", () => {
+    const token = `Bot ${Buffer.from("9876543210").toString("base64")}.ts.hmac`;
+    expect(parseApplicationIdFromToken(token)).toBe("9876543210");
+  });
+
+  it("returns undefined for empty string", () => {
+    expect(parseApplicationIdFromToken("")).toBeUndefined();
+  });
+
+  it("returns undefined for token without dots", () => {
+    expect(parseApplicationIdFromToken("nodots")).toBeUndefined();
+  });
+
+  it("returns undefined when decoded segment is not numeric", () => {
+    const token = `${Buffer.from("not-a-number").toString("base64")}.ts.hmac`;
+    expect(parseApplicationIdFromToken(token)).toBeUndefined();
+  });
+
+  it("returns undefined for whitespace-only input", () => {
+    expect(parseApplicationIdFromToken("   ")).toBeUndefined();
+  });
+
+  it("returns undefined when first segment is empty (starts with dot)", () => {
+    expect(parseApplicationIdFromToken(".ts.hmac")).toBeUndefined();
+  });
+});