@@ -904,6 +904,14 @@ def handle_subscriptions_listen(body)
             return invalid_request_response("Invalid Request: subscriptions/listen requires an id")
           end
 
+          # The id is echoed in every message of the stream, so one that cannot be written back as JSON
+          # (a String holding bytes that are not valid UTF-8 parses, but does not generate) is refused here,
+          # while the refusal can still answer with a null id, rather than surfacing as a failed write after
+          # the stream has been registered.
+          unless json_encodable?(request_id)
+            return invalid_request_response("Invalid Request: subscriptions/listen id must be valid UTF-8")
+          end
+
           begin
             if RequestEnvelope.modern?(params)
               RequestEnvelope.parse!(params, request: params)
@@ -946,7 +954,43 @@ def handle_subscriptions_listen(body)
             return too_many_listen_subscriptions_response(request_id)
           end
 
-          [200, SSE_HEADERS.dup, listen_sse_body(request_id, honored_filter(filter))]
+          honored = honored_filter(filter)
+
+          # The acknowledgement echoes the honored filter, whose resource URIs are client-supplied strings,
+          # so it is encoded before the stream is committed to: a URI that cannot be written as JSON is
+          # refused with a 400 instead of failing the first write after the stream was registered.
+          unless (acknowledgement = listen_acknowledgement_json(request_id, honored))
+            return json_rpc_error_response(
+              status: 400,
+              code: JsonRpcHandler::ErrorCode::INVALID_PARAMS,
+              message: "Invalid params: subscriptions/listen `notifications` must be valid UTF-8",
+              id: request_id,
+            )
+          end
+
+          [200, SSE_HEADERS.dup, listen_sse_body(request_id, honored, acknowledgement)]
+        end
+
+        # The `notifications/subscriptions/acknowledged` frame for a listen stream as JSON, or `nil` when
+        # the request id or the honored filter holds a string JSON cannot encode.
+        def listen_acknowledgement_json(request_id, honored)
+          {
+            jsonrpc: "2.0",
+            method: Methods::NOTIFICATIONS_SUBSCRIPTIONS_ACKNOWLEDGED,
+            params: {
+              notifications: honored,
+              _meta: { RequestEnvelope::SUBSCRIPTION_ID_META_KEY.to_sym => request_id },
+            },
+          }.to_json
+        rescue JSON::GeneratorError
+          nil
+        end
+
+        def json_encodable?(value)
+          value.to_json
+          true
+        rescue JSON::GeneratorError
+          false
         end
 
         def listen_subscriptions_full?
@@ -1005,7 +1049,7 @@ def first
         # The entry is keyed by an identifier minted here, not by the request id: that id is unique only among
         # the requesting client's own in-flight requests, and two clients that pick the same one must each get
         # their stream, stamped with the id they sent.
-        def listen_sse_body(request_id, honored)
+        def listen_sse_body(request_id, honored, acknowledgement)
           ListenStreamBody.new do |stream|
             subscription_key = SecureRandom.uuid
             subscription = nil
@@ -1029,15 +1073,6 @@ def listen_sse_body(request_id, honored)
             if subscription.nil?
               close_stream_safely(stream)
             else
-              acknowledgement = {
-                jsonrpc: "2.0",
-                method: Methods::NOTIFICATIONS_SUBSCRIPTIONS_ACKNOWLEDGED,
-                params: {
-                  notifications: honored,
-                  _meta: { RequestEnvelope::SUBSCRIPTION_ID_META_KEY.to_sym => request_id },
-                },
-              }
-
               begin
                 acknowledged = subscription[:write_mutex].synchronize do
                   next false if subscription[:closed]
 
@@ -6564,6 +6564,80 @@ def string
           refute(events.any? { |event| event["method"] == "notifications/tools/list_changed" })
         end
 
+        test "subscriptions/listen refuses an id that is not valid UTF-8 without registering a stream" do
+          body = modern_listen_body(id: "listen-BAD", params: { notifications: { toolsListChanged: true } }).b.sub("BAD", "\xFF".b)
+
+          response = @transport.handle_request(modern_rack_request(body))
+
+          assert_equal 400, response[0]
+
+          parsed = JSON.parse(response[2][0])
+
+          # The member is present and null, as JSON-RPC 2.0 requires when the id cannot be determined.
+          assert parsed.key?("id")
+          assert_nil parsed["id"]
+          assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, parsed.dig("error", "code")
+          assert_empty @transport.instance_variable_get(:@listen_subscriptions)
+        end
+
+        test "subscriptions/listen refuses a resource URI that is not valid UTF-8 without registering a stream" do
+          server = Server.new(name: "listen_test", capabilities: { resources: { subscribe: true } })
+          transport = StreamableHTTPTransport.new(server, listen_keepalive_interval: nil)
+          body = modern_listen_body(
+            id: "listen-1",
+            params: { notifications: { resourceSubscriptions: ["file:///BAD"] } }
+          ).b.sub("BAD", "\xFF".b)
+
+          response = transport.handle_request(modern_rack_request(body))
+
+          assert_equal 400, response[0]
+
+          parsed = JSON.parse(response[2][0])
+
+          assert_equal "listen-1", parsed["id"]
+          assert_equal(-32602, parsed.dig("error", "code"))
+          assert_empty transport.instance_variable_get(:@listen_subscriptions)
+        ensure
+          transport.close
+        end
+
+        test "subscriptions/listen acknowledges without a resource URI that is not valid UTF-8 when subscribe is not declared" do
+          # Without the `subscribe` capability the URIs are not honored, so the acknowledgement never echoes
+          # the unencodable one and the request is served.
+          body = modern_listen_body(
+            id: "listen-1",
+            params: { notifications: { toolsListChanged: true, resourceSubscriptions: ["file:///BAD"] } },
+          ).b.sub("BAD", "\xFF".b)
+
+          response = @transport.handle_request(modern_rack_request(body))
+
+          assert_equal 200, response[0]
+
+          io = StringIO.new
+          response[2].call(io)
+          acknowledgement = sse_events(io).first
+
+          assert_equal "notifications/subscriptions/acknowledged", acknowledgement["method"]
+          refute acknowledgement.dig("params", "notifications").key?("resourceSubscriptions")
+        end
+
+        test "subscriptions/listen refuses a request past the stream cap before checking its resource URIs" do
+          server = Server.new(name: "listen_test", capabilities: { resources: { subscribe: true } })
+          transport = StreamableHTTPTransport.new(server, max_listen_subscriptions: 1, listen_keepalive_interval: nil)
+          open_listen_stream(id: "listen-1", notifications: { toolsListChanged: true }, transport: transport)
+          body = modern_listen_body(
+            id: "listen-2",
+            params: { notifications: { resourceSubscriptions: ["file:///BAD"] } }
+          ).b.sub("BAD", "\xFF".b)
+
+          response = transport.handle_request(modern_rack_request(body))
+
+          assert_equal 503, response[0]
+          assert_equal "listen-2", JSON.parse(response[2][0])["id"]
+        ensure
+          transport.close
+        end
+
         test "transport close during the acknowledgement write still sends the acknowledgement first" do
           # The stream blocks its first write, the acknowledgement, until released, so the close arrives while
           # that write is in progress and has to queue behind it on the stream's write mutex.