fix: only register completions handler when server supports completions

src/InMemoryEventStore.test.ts

@@ -157,4 +157,76 @@ describe("InMemoryEventStore", () => {
     );
     expect(emptyResult).toBe("");
   });
+
+  it("keeps deterministic ordering even when events share the same timestamp", async () => {
+    const store = new InMemoryEventStore();
+    const streamId = "deterministic-stream";
+
+    const messages: JSONRPCMessage[] = [
+      { id: 1, jsonrpc: "2.0", method: "step/one" },
+      { id: 2, jsonrpc: "2.0", method: "step/two" },
+      { id: 3, jsonrpc: "2.0", method: "step/three" },
+      { id: 4, jsonrpc: "2.0", method: "step/four" },
+    ];
+
+    const fixedTimestamp = 1_730_000_000_000;
+    const nowSpy = vi.spyOn(Date, "now").mockReturnValue(fixedTimestamp);
+
+    const eventIds: string[] = [];
+    try {
+      for (const message of messages) {
+        eventIds.push(await store.storeEvent(streamId, message));
+      }
+    } finally {
+      nowSpy.mockRestore();
+    }
+
+    // Ensure IDs already arrive sorted since we stored sequentially
+    expect(eventIds).toEqual([...eventIds].sort());
+
+    const parts = eventIds.map((eventId) => eventId.split("_"));
+
+    const timestampParts = parts.map(([, timestamp]) => timestamp);
+    expect(timestampParts).toEqual(
+      Array(messages.length).fill(fixedTimestamp.toString())
+    );
+
+    const counterSuffixes = parts.map(([, , counter]) => counter);
+    expect(counterSuffixes).toEqual(
+      messages.map((_, index) => (index).toString(36).padStart(4, "0"))
+    );
+
+    // Random parts should be 3 base36 characters each (due to substring(2, 5))
+    const randomParts = parts.map(([, , , random]) => random);
+    for (const random of randomParts) {
+      expect(random).toMatch(/^[0-9a-z]{3}$/);
+    }
+
+    // Replay after the first event and ensure the remainder flow in order
+    const replayedMessages: JSONRPCMessage[] = [];
+    const returnedStreamId = await store.replayEventsAfter(eventIds[0], {
+      send: async (_eventId: string, message: JSONRPCMessage) => {
+        replayedMessages.push(message);
+      },
+    });
+
+    expect(returnedStreamId).toBe(streamId);
+    expect(replayedMessages).toEqual(messages.slice(1));
+
+    // Now allow timestamp to advance to ensure counter resets
+    const nextTimestamp = fixedTimestamp + 1;
+    const secondSpy = vi.spyOn(Date, "now").mockReturnValue(nextTimestamp);
+    try {
+      const nextId = await store.storeEvent(streamId, {
+        id: 5,
+        jsonrpc: "2.0",
+        method: "step/five",
+      });
+      const [, , counter, random] = nextId.split("_");
+      expect(counter).toBe("0000");
+      expect(random).toMatch(/^[0-9a-z]{3}$/);
+    } finally {
+      secondSpy.mockRestore();
+    }
+  });
 });

src/InMemoryEventStore.ts

@@ -14,6 +14,8 @@ import type { JSONRPCMessage } from "@modelcontextprotocol/sdk/types.js";
 export class InMemoryEventStore implements EventStore {
   private events: Map<string, { message: JSONRPCMessage; streamId: string }> =
     new Map();
+  private lastTimestamp = 0;
+  private lastTimestampCounter = 0;
 
   /**
    * Replays events that occurred after a specific event ID
@@ -79,10 +81,25 @@ export class InMemoryEventStore implements EventStore {
   }
 
   /**
-   * Generates a unique event ID for a given stream ID
+   * Generates a monotonic unique event ID in 
+   * `${streamId}_${timestamp}_${counter}_${random}` format.
    */
   private generateEventId(streamId: string): string {
-    return `${streamId}_${Date.now()}_${Math.random().toString(36).substring(2, 10)}`;
+
+    const now = Date.now();
+
+    if (now === this.lastTimestamp) {
+      this.lastTimestampCounter++;
+    } else {
+      this.lastTimestampCounter = 0;
+      this.lastTimestamp = now;
+    }
+
+    const timestamp = now.toString();
+    const counter = this.lastTimestampCounter.toString(36).padStart(4, "0");
+    const random = Math.random().toString(36).substring(2, 5);
+
+    return `${streamId}_${timestamp}_${counter}_${random}`;
   }
 
   /**

src/proxyServer.ts

@@ -124,10 +124,12 @@ export const proxyServer = async ({
     });
   }
 
-  server.setRequestHandler(CompleteRequestSchema, async (args) => {
-    return client.complete(
-      args.params,
-      requestTimeout ? { timeout: requestTimeout } : undefined,
-    );
-  });
+  if (serverCapabilities?.completions) {
+    server.setRequestHandler(CompleteRequestSchema, async (args) => {
+      return client.complete(
+        args.params,
+        requestTimeout ? { timeout: requestTimeout } : undefined,
+      );
+    });
+  }
 };