diff --git a/docs/mcpgodebug.md b/docs/mcpgodebug.md
index 696aa9f92..01e96285c 100644
--- a/docs/mcpgodebug.md
+++ b/docs/mcpgodebug.md
@@ -42,6 +42,13 @@ Options listed below were added and will be removed in the 1.11.0 version of the
   behavior was changed to reject such messages as invalid requests, as the
   specification lists `params` as required for all three.
 
+- `nopreinitinvalidrequest` added. If set to `1`, a legacy server session that
+  has not been initialized refuses a request other than `initialize` or `ping`
+  with JSON-RPC error code `0`, restoring the previous behavior. The default
+  behavior was changed to refuse it with `-32600` (Invalid Request): `0`
+  tells a client nothing and is indistinguishable from an unset code. See
+  issue #1367.
+
 ### 1.8.0
 
 Options listed below were added and will be removed in the 1.9.0 version of the SDK.
diff --git a/internal/docs/mcpgodebug.src.md b/internal/docs/mcpgodebug.src.md
index b1b5016b5..0a75d81de 100644
--- a/internal/docs/mcpgodebug.src.md
+++ b/internal/docs/mcpgodebug.src.md
@@ -41,6 +41,13 @@ Options listed below were added and will be removed in the 1.11.0 version of the
   behavior was changed to reject such messages as invalid requests, as the
   specification lists `params` as required for all three.
 
+- `nopreinitinvalidrequest` added. If set to `1`, a legacy server session that
+  has not been initialized refuses a request other than `initialize` or `ping`
+  with JSON-RPC error code `0`, restoring the previous behavior. The default
+  behavior was changed to refuse it with `-32600` (Invalid Request): `0`
+  tells a client nothing and is indistinguishable from an unset code. See
+  issue #1367.
+
 ### 1.8.0
 
 Options listed below were added and will be removed in the 1.9.0 version of the SDK.
diff --git a/mcp/server.go b/mcp/server.go
index 3efa3b834..a6bbdea72 100644
--- a/mcp/server.go
+++ b/mcp/server.go
@@ -2010,6 +2010,14 @@ func (ss *ServerSession) receivingMethodHandler() MethodHandler {
 // getConn implements [session.getConn].
 func (ss *ServerSession) getConn() *jsonrpc2.Connection { return ss.conn }
 
+// nopreinitinvalidrequest is a compatibility parameter that restores the
+// previous behavior of a legacy server session that has not been initialized,
+// where a request other than initialize or ping was refused with JSON-RPC
+// error code 0 instead of -32600 (Invalid Request). See the documentation for
+// the mcpgodebug package for instructions how to enable it. The option will be
+// removed in a future version of the SDK.
+var nopreinitinvalidrequest = mcpgodebug.Value("nopreinitinvalidrequest")
+
 // handle invokes the method described by the given JSON RPC request.
 func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any, error) {
 	ss.mu.Lock()
@@ -2076,7 +2084,11 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	if !initialized && !validatedMeta.usesNewProtocol && req.IsCall() &&
 		req.Method != methodInitialize && req.Method != methodPing {
 		ss.server.opts.Logger.Error("method invalid during initialization", "method", req.Method)
-		return nil, fmt.Errorf("method %q is invalid during session initialization", req.Method)
+		err := fmt.Errorf("method %q is invalid during session initialization", req.Method)
+		if nopreinitinvalidrequest == "1" {
+			return nil, err
+		}
+		return nil, &jsonrpc.Error{Code: jsonrpc.CodeInvalidRequest, Message: err.Error()}
 	}
 
 	// modelcontextprotocol/go-sdk#26: handle calls asynchronously, and
diff --git a/mcp/server_test.go b/mcp/server_test.go
index 314afefa1..c3bfcee8f 100644
--- a/mcp/server_test.go
+++ b/mcp/server_test.go
@@ -2445,6 +2445,26 @@ func TestServerHandle_LegacyCallBeforeInitialize(t *testing.T) {
 			if want := fmt.Sprintf("method %q is invalid during session initialization", tc.method); err.Error() != want {
 				t.Errorf("handle(%q) error = %q, want %q", tc.method, err.Error(), want)
 			}
+			var rpcErr *jsonrpc.Error
+			if !errors.As(err, &rpcErr) || rpcErr.Code != jsonrpc.CodeInvalidRequest {
+				t.Errorf("handle(%q) error = %#v, want a JSON-RPC error with code %d", tc.method, err, jsonrpc.CodeInvalidRequest)
+			}
+			// MCPGODEBUG=nopreinitinvalidrequest=1 restores the old code, 0.
+			func() {
+				defer func(old string) { nopreinitinvalidrequest = old }(nopreinitinvalidrequest)
+				nopreinitinvalidrequest = "1"
+				_, err := ss.handle(ctx, &jsonrpc.Request{
+					ID:     jsonrpc2.Int64ID(2),
+					Method: tc.method,
+					Params: json.RawMessage(tc.params),
+				})
+				if err == nil {
+					t.Fatalf("handle(%q), old error code restored, served the call before initialize", tc.method)
+				}
+				if errors.As(err, &rpcErr) {
+					t.Errorf("handle(%q), old error code restored, error = %#v, want a plain error (code 0 on the wire)", tc.method, err)
+				}
+			}()
 			if handlerRan {
 				t.Errorf("handle(%q) ran the server's handler for a call it refused", tc.method)
 			}
diff --git a/mcp/streamable_test.go b/mcp/streamable_test.go
index a481e7b7c..a5d987ab8 100644
--- a/mcp/streamable_test.go
+++ b/mcp/streamable_test.go
@@ -1291,6 +1291,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					messages:       []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Name: "tool"})},
 					wantStatusCode: http.StatusOK,
 					wantMessages: []jsonrpc.Message{resp(2, nil, &jsonrpc.Error{
+						Code:    jsonrpc.CodeInvalidRequest,
 						Message: `method "tools/call" is invalid during session initialization`,
 					})},
 				},
diff --git a/mcp/testdata/conformance/server/lifecycle.txtar b/mcp/testdata/conformance/server/lifecycle.txtar
index 652aa7f32..edaf27f3a 100644
--- a/mcp/testdata/conformance/server/lifecycle.txtar
+++ b/mcp/testdata/conformance/server/lifecycle.txtar
@@ -26,7 +26,7 @@ See also modelcontextprotocol/go-sdk#225.
 	"jsonrpc": "2.0",
 	"id": 2,
 	"error": {
-		"code": 0,
+		"code": -32600,
 		"message": "method \"tools/list\" is invalid during session initialization"
 	}
 }