diff --git a/packages/agentstack/src/index.test.ts b/packages/agentstack/src/index.test.ts
index 99bc1a7..31b3b6d 100644
--- a/packages/agentstack/src/index.test.ts
+++ b/packages/agentstack/src/index.test.ts
@@ -29,6 +29,7 @@ describe("DID helpers", () => {
   it("rejects non-CoinPay DIDs", () => {
     expect(parseDid("did:web:example.com")).toBeNull();
     expect(parseDid("did:coinpay:bot:abc")).toBeNull();
+    expect(parseDid("did:coinpay:agent:abc:extra")).toBeNull();
   });
 });
 
diff --git a/packages/agentstack/src/index.ts b/packages/agentstack/src/index.ts
index d5ea46c..319ad1f 100644
--- a/packages/agentstack/src/index.ts
+++ b/packages/agentstack/src/index.ts
@@ -25,7 +25,9 @@ export const agentDid = (id: string) => makeDid("agent", id);
 export function parseDid(did: string): { kind: DidKind; id: string } | null {
   const prefix = `${DID_METHOD}:`;
   if (!did.startsWith(prefix)) return null;
-  const [kind, id] = did.slice(prefix.length).split(":");
+  const parts = did.slice(prefix.length).split(":");
+  if (parts.length !== 2) return null;
+  const [kind, id] = parts;
   if ((kind !== "user" && kind !== "agent") || !id) return null;
   return { kind, id };
 }