Lowercase response header names in StreamableHTTPTransport

AGENTS.md

@@ -32,6 +32,7 @@ This is the official Ruby SDK for the Model Context Protocol (MCP), implementing
 - Use frozen string literals
 - Follow Ruby community conventions
 - Keep dependencies minimal
+- Use lowercase HTTP response header names (e.g. `mcp-session-id`); the Rack 3 SPEC requires this, and the MCP spec's `Mcp-Session-Id` casing is prose convention only
 
 ## Commit message conventions
 

examples/streamable_http_server.rb

@@ -105,7 +105,7 @@ def call(env)
           @mcp_logger.error("Response error: #{parsed["error"]["message"]}")
         else
           @mcp_logger.info("Response: success (id: #{parsed["id"]})")
-          @sse_logger.info("Session created: #{headers["Mcp-Session-Id"]}") if headers["Mcp-Session-Id"]
+          @sse_logger.info("Session created: #{headers["mcp-session-id"]}") if headers["mcp-session-id"]
         end
       rescue JSON::ParserError
         @mcp_logger.warn("Invalid JSON in response")

lib/mcp/server/transports/streamable_http_transport.rb

@@ -19,9 +19,9 @@ class StreamableHTTPTransport < Transport
         class InvalidJsonError < StandardError; end
 
         SSE_HEADERS = {
-          "Content-Type" => "text/event-stream",
-          "Cache-Control" => "no-cache",
-          "Connection" => "keep-alive",
+          "content-type" => "text/event-stream",
+          "cache-control" => "no-cache",
+          "connection" => "keep-alive",
         }.freeze
 
         # Secure defaults for stateful mode. Without a finite idle timeout, sessions live until an explicit client DELETE,
@@ -566,7 +566,7 @@ def handle_get(request)
         end
 
         def handle_delete(request)
-          success_response = [200, { "Content-Type" => "application/json" }, [{ success: true }.to_json]]
+          success_response = [200, { "content-type" => "application/json" }, [{ success: true }.to_json]]
 
           if @stateless
             protocol_version_error = validate_protocol_version_header(request)
@@ -762,7 +762,7 @@ def validate_protocol_version_header(request)
 
         def json_rpc_error_response(status:, code:, message:)
           body = { jsonrpc: "2.0", id: nil, error: { code: code, message: message } }
-          [status, { "Content-Type" => "application/json" }, [body.to_json]]
+          [status, { "content-type" => "application/json" }, [body.to_json]]
         end
 
         def notification?(body)
@@ -870,10 +870,10 @@ def handle_initialization(request, body_string, body)
           end
 
           headers = {
-            "Content-Type" => "application/json",
+            "content-type" => "application/json",
           }
 
-          headers["Mcp-Session-Id"] = session_id if session_id
+          headers["mcp-session-id"] = session_id if session_id
 
           [200, headers, [response]]
         end
@@ -915,7 +915,7 @@ def handle_regular_request(body_string, session_id, related_request_id: nil)
             # which would produce an empty body the client cannot parse as JSON.
             return handle_accepted if response.nil?
 
-            [200, { "Content-Type" => "application/json" }, [response]]
+            [200, { "content-type" => "application/json" }, [response]]
           end
         end
 
@@ -1148,7 +1148,7 @@ def invalid_request_response(message, request_id: nil)
               message: message,
             },
           }
-          [400, { "Content-Type" => "application/json" }, [body.to_json]]
+          [400, { "content-type" => "application/json" }, [body.to_json]]
         end
 
         def session_already_connected_response

test/mcp/server/transports/streamable_http_notification_integration_test.rb

@@ -28,7 +28,7 @@ class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
             { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
           )
           init_response = @transport.handle_request(init_request)
-          session_id = init_response[1]["Mcp-Session-Id"]
+          session_id = init_response[1]["mcp-session-id"]
 
           # Connect with SSE
           io = StringIO.new
@@ -77,7 +77,7 @@ class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
             { jsonrpc: "2.0", method: "initialize", id: "123", params: initialize_params }.to_json,
           )
           init_response1 = @transport.handle_request(init_request1)
-          session_id1 = init_response1[1]["Mcp-Session-Id"]
+          session_id1 = init_response1[1]["mcp-session-id"]
 
           init_request2 = create_rack_request(
             "POST",
@@ -86,7 +86,7 @@ class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
             { jsonrpc: "2.0", method: "initialize", id: "456", params: initialize_params }.to_json,
           )
           init_response2 = @transport.handle_request(init_request2)
-          session_id2 = init_response2[1]["Mcp-Session-Id"]
+          session_id2 = init_response2[1]["mcp-session-id"]
 
           # Connect both sessions with SSE
           io1 = StringIO.new
@@ -132,7 +132,7 @@ class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
             { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
           )
           init_response = @transport.handle_request(init_request)
-          session_id = init_response[1]["Mcp-Session-Id"]
+          session_id = init_response[1]["mcp-session-id"]
 
           # Connect with SSE
           io = StringIO.new
@@ -168,7 +168,7 @@ class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
             { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
           )
           init_response = @transport.handle_request(init_request)
-          session_id = init_response[1]["Mcp-Session-Id"]
+          session_id = init_response[1]["mcp-session-id"]
 
           # Connect with SSE
           io = StringIO.new
@@ -212,7 +212,7 @@ class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
             { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
           )
           init_response = @transport.handle_request(init_request)
-          session_id = init_response[1]["Mcp-Session-Id"]
+          session_id = init_response[1]["mcp-session-id"]
 
           # Connect with SSE
           io = StringIO.new