diff --git a/Sources/MCP/Base/Transports/HTTPServer/StatelessHTTPServerTransport.swift b/Sources/MCP/Base/Transports/HTTPServer/StatelessHTTPServerTransport.swift
index abe9576c..736ee5bc 100644
--- a/Sources/MCP/Base/Transports/HTTPServer/StatelessHTTPServerTransport.swift
+++ b/Sources/MCP/Base/Transports/HTTPServer/StatelessHTTPServerTransport.swift
@@ -29,6 +29,13 @@ import Logging
 /// - Session management is handled externally or not needed
 ///
 /// For full streaming and session support, use ``StatefulHTTPServerTransport`` instead.
+///
+/// ## Concurrency
+///
+/// In-flight requests are correlated to their HTTP exchange by JSON-RPC id. Because ids are
+/// client-scoped, a concurrent request that reuses an id already in flight is rejected with
+/// HTTP 409 and a JSON-RPC `-32600` error rather than silently displacing the first request.
+/// Clients must therefore keep ids unique among their own concurrent in-flight requests.
 public actor StatelessHTTPServerTransport: Transport, HTTPContextProviding {
     public nonisolated let logger: Logger
 
@@ -221,6 +228,30 @@ public actor StatelessHTTPServerTransport: Transport, HTTPContextProviding {
         requestID: String,
         request: HTTPRequest
     ) async -> HTTPResponse {
+        // Reject a second request that reuses a JSON-RPC id already in flight.
+        //
+        // JSON-RPC ids are client-scoped, and this transport is explicitly multi-client, so
+        // colliding ids across concurrent POSTs are legal and common (most clients start their
+        // id sequence at 1). Keying per-request state on the raw id means a second request
+        // would silently overwrite the first's response waiter (the first POST hangs forever
+        // and its continuation leaks — issue #254) and its HTTP context (a handler could then
+        // read another client's `Authorization` header — issue #265). Failing the duplicate
+        // fast keeps every in-flight exchange isolated. Transparent support for concurrent
+        // colliding ids arrives with the stateless core (SEP-2575).
+        guard httpRequestContexts[requestID] == nil else {
+            logger.warning(
+                "Rejecting request: a JSON-RPC id is already in flight for another concurrent request",
+                metadata: ["requestID": "\(requestID)"]
+            )
+            return .error(
+                statusCode: 409,
+                .invalidRequest(
+                    "Duplicate in-flight JSON-RPC request id '\(requestID)'; "
+                        + "ids must be unique among concurrent in-flight requests"
+                )
+            )
+        }
+
         httpRequestContexts[requestID] = request
         // Yield the incoming message to the server
         incomingContinuation.yield(body)
diff --git a/Tests/MCPTests/HTTPServerTransportTests.swift b/Tests/MCPTests/HTTPServerTransportTests.swift
index 8e1c89b4..1fcc9c83 100644
--- a/Tests/MCPTests/HTTPServerTransportTests.swift
+++ b/Tests/MCPTests/HTTPServerTransportTests.swift
@@ -1051,6 +1051,104 @@ struct StatelessHTTPServerTransportTests {
         // Should return error (500 or similar) since waiter was cancelled
         #expect(response.statusCode == 500)
     }
+
+    // MARK: - Concurrent colliding JSON-RPC ids (issues #254, #265)
+
+    @Test(
+        "Colliding in-flight JSON-RPC ids: duplicate is rejected; first request's context and response stay intact",
+        .timeLimit(.minutes(1))
+    )
+    func testCollidingInFlightRequestIDsRejected() async throws {
+        let transport = makeStatelessTransport()
+        try await transport.connect()
+
+        // Track which request bodies actually reach the server via receive().
+        // The rejected duplicate must never be dispatched.
+        actor SeenIDs {
+            private(set) var ids: [String] = []
+            func add(_ id: String) { ids.append(id) }
+        }
+        let seen = SeenIDs()
+        let drain = Task {
+            let stream = await transport.receive()
+            for try await data in stream {
+                if case .request(let id, _)? = JSONRPCMessageKind(data: data) {
+                    await seen.add(id)
+                }
+            }
+        }
+
+        // Request A — id "1", Authorization "Bearer A". Parks until we respond.
+        let requestA = HTTPRequest(
+            method: "POST",
+            headers: [
+                "Content-Type": "application/json",
+                "Accept": "application/json",
+                "Authorization": "Bearer A",
+            ],
+            body: makeRequestBody(id: "1", method: "tools/list"),
+            path: "/mcp"
+        )
+        let taskA = Task { await transport.handleRequest(requestA) }
+
+        // Let A register its context + waiter and park.
+        try await Task.sleep(for: .milliseconds(50))
+
+        // The in-flight context for id "1" is A's.
+        let ctxDuringA = await transport.httpRequestContext(for: .string("1"))
+        #expect(ctxDuringA?.header("Authorization") == "Bearer A")
+
+        // Request B — SAME id "1", Authorization "Bearer B". Must be rejected promptly,
+        // NOT overwrite A's waiter/context, and NOT hang.
+        let requestB = HTTPRequest(
+            method: "POST",
+            headers: [
+                "Content-Type": "application/json",
+                "Accept": "application/json",
+                "Authorization": "Bearer B",
+            ],
+            body: makeRequestBody(id: "1", method: "tools/list"),
+            path: "/mcp"
+        )
+        actor ResponseBox {
+            private(set) var response: HTTPResponse?
+            func set(_ value: HTTPResponse) { response = value }
+        }
+        let box = ResponseBox()
+        let taskB = Task { await box.set(await transport.handleRequest(requestB)) }
+
+        // Poll up to ~2s: B must return, not hang.
+        var responseB: HTTPResponse?
+        for _ in 0..<200 {
+            if let r = await box.response {
+                responseB = r
+                break
+            }
+            try await Task.sleep(for: .milliseconds(10))
+        }
+        #expect(responseB != nil, "duplicate-id request must return promptly, not hang")
+        #expect(responseB?.statusCode == 409)
+
+        // A's context is untouched — B never overwrote it (no auth cross-talk, #265).
+        let ctxStillA = await transport.httpRequestContext(for: .string("1"))
+        #expect(ctxStillA?.header("Authorization") == "Bearer A")
+
+        // Respond to A — it completes normally with its own response (#254: waiter not lost).
+        // Capture the body once: JSONSerialization key order isn't stable across calls.
+        let responseBodyA = makeResponseBody(id: "1")
+        try await transport.send(responseBodyA)
+        let resultA = await taskA.value
+        #expect(resultA.statusCode == 200)
+        #expect(resultA.bodyData == responseBodyA)
+
+        // Only A's body ever reached the server; B was rejected pre-dispatch.
+        let seenIDs = await seen.ids
+        #expect(seenIDs == ["1"])
+
+        _ = await taskB.value
+        drain.cancel()
+        await transport.disconnect()
+    }
 }
 
 // MARK: - HTTPContextProviding / Server.currentHandlerContext