diff --git a/.golangci.yml b/.golangci.yml
index fc627292..d94bf062 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -132,3 +132,6 @@ issues:
     # We want to show examples with http.Get
     - linters: [noctx]
       path: internal/memhttp/memhttp_test.go
+    # We need to initialize a map of all protocol headers
+    - linters: [gochecknoglobals]
+      path: header.go
diff --git a/connect_ext_test.go b/connect_ext_test.go
index 42294086..81ea030a 100644
--- a/connect_ext_test.go
+++ b/connect_ext_test.go
@@ -2631,6 +2631,70 @@ func TestBlankImportCodeGeneration(t *testing.T) {
 	assert.NotNil(t, desc)
 }
 
+// TestSetProtocolHeaders tests that headers required by the protocols are set
+// overriding user provided headers.
+func TestSetProtocolHeaders(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name              string
+		clientOption      connect.ClientOption
+		expectContentType string
+	}{{
+		name:              "connect",
+		expectContentType: "application/proto",
+	}, {
+		name:              "grpc",
+		clientOption:      connect.WithGRPC(),
+		expectContentType: "application/grpc",
+	}, {
+		name:              "grpcweb",
+		clientOption:      connect.WithGRPCWeb(),
+		expectContentType: "application/grpc-web+proto",
+	}}
+	for _, tt := range tests {
+		testcase := tt
+		t.Run(testcase.name, func(t *testing.T) {
+			t.Parallel()
+			pingServer := &pingServer{}
+			mux := http.NewServeMux()
+			mux.Handle(pingv1connect.NewPingServiceHandler(pingServer))
+			server := memhttptest.NewServer(t, mux)
+
+			clientOpts := []connect.ClientOption{}
+			if testcase.clientOption == nil {
+				// Use a different protocol to test the override.
+				clientOpts = append(clientOpts, connect.WithGRPC())
+			}
+			client := pingv1connect.NewPingServiceClient(server.Client(), server.URL(), clientOpts...)
+
+			pingProxyServer := &pluggablePingServer{
+				ping: func(ctx context.Context, request *connect.Request[pingv1.PingRequest]) (*connect.Response[pingv1.PingResponse], error) {
+					return client.Ping(ctx, request)
+				},
+			}
+			proxyMux := http.NewServeMux()
+			proxyMux.Handle(pingv1connect.NewPingServiceHandler(pingProxyServer))
+			proxyServer := memhttptest.NewServer(t, proxyMux)
+
+			proxyClientOpts := []connect.ClientOption{}
+			if testcase.clientOption != nil {
+				proxyClientOpts = append(proxyClientOpts, testcase.clientOption)
+			}
+			proxyClient := pingv1connect.NewPingServiceClient(proxyServer.Client(), proxyServer.URL(), proxyClientOpts...)
+
+			request := connect.NewRequest(&pingv1.PingRequest{Number: 42})
+			request.Header().Set("X-Test", t.Name())
+			response, err := proxyClient.Ping(context.Background(), request)
+			if !assert.Nil(t, err) {
+				return
+			}
+			// Assert the Content-Type is set for the proxy clients protocol and not the client's.
+			assert.Equal(t, response.Header().Get("Content-Type"), testcase.expectContentType)
+			assert.Equal(t, len(response.Header().Values("Content-Type")), 1)
+		})
+	}
+}
+
 type unflushableWriter struct {
 	w http.ResponseWriter
 }
diff --git a/error_writer.go b/error_writer.go
index 58ce3c42..f05d19ec 100644
--- a/error_writer.go
+++ b/error_writer.go
@@ -128,7 +128,7 @@ func (w *ErrorWriter) Write(response http.ResponseWriter, request *http.Request,
 
 func (w *ErrorWriter) writeConnectUnary(response http.ResponseWriter, err error) error {
 	if connectErr, ok := asError(err); ok && !connectErr.wireErr {
-		mergeMetadataHeaders(response.Header(), connectErr.meta)
+		mergeNonProtocolHeaders(response.Header(), connectErr.meta)
 	}
 	response.WriteHeader(connectCodeToHTTP(CodeOf(err)))
 	data, marshalErr := json.Marshal(newConnectWireError(err))
diff --git a/handler.go b/handler.go
index 1d573291..5eab6c71 100644
--- a/handler.go
+++ b/handler.go
@@ -71,8 +71,8 @@ func NewUnaryHandler[Req, Res any](
 		if err != nil {
 			return err
 		}
-		mergeHeaders(conn.ResponseHeader(), response.Header())
-		mergeHeaders(conn.ResponseTrailer(), response.Trailer())
+		mergeNonProtocolHeaders(conn.ResponseHeader(), response.Header())
+		mergeNonProtocolHeaders(conn.ResponseTrailer(), response.Trailer())
 		return conn.Send(response.Any())
 	}
 
diff --git a/header.go b/header.go
index f3c7cacd..b3f05432 100644
--- a/header.go
+++ b/header.go
@@ -19,6 +19,33 @@ import (
 	"net/http"
 )
 
+var (
+	protocolHeaders = map[string]struct{}{
+		// HTTP headers.
+		headerContentType:     {},
+		headerContentLength:   {},
+		headerContentEncoding: {},
+		headerHost:            {},
+		headerUserAgent:       {},
+		headerTrailer:         {},
+		headerDate:            {},
+		// Connect headers.
+		connectUnaryHeaderAcceptCompression:     {},
+		connectUnaryTrailerPrefix:               {},
+		connectStreamingHeaderCompression:       {},
+		connectStreamingHeaderAcceptCompression: {},
+		connectHeaderTimeout:                    {},
+		connectHeaderProtocolVersion:            {},
+		// gRPC headers.
+		grpcHeaderCompression:       {},
+		grpcHeaderAcceptCompression: {},
+		grpcHeaderTimeout:           {},
+		grpcHeaderStatus:            {},
+		grpcHeaderMessage:           {},
+		grpcHeaderDetails:           {},
+	}
+)
+
 // EncodeBinaryHeader base64-encodes the data. It always emits unpadded values.
 //
 // In the Connect, gRPC, and gRPC-Web protocols, binary headers must have keys
@@ -57,10 +84,9 @@ func mergeHeaders(into, from http.Header) {
 	}
 }
 
-// mergeMetdataHeaders merges the metadata headers from the "from" header into
-// the "into" header. It skips over non metadata headers that should not be
-// propagated from the server to the client.
-func mergeMetadataHeaders(into, from http.Header) {
+// mergeNonProtocolHeaders merges headers excluding protocol headers defined in
+// protocolHeaders.
+func mergeNonProtocolHeaders(into, from http.Header) {
 	for key, vals := range from {
 		if len(vals) == 0 {
 			// For response trailers, net/http will pre-populate entries
@@ -68,30 +94,7 @@ func mergeMetadataHeaders(into, from http.Header) {
 			// are no actual values for those keys, we skip them.
 			continue
 		}
-		switch http.CanonicalHeaderKey(key) {
-		case headerContentType,
-			headerContentLength,
-			headerContentEncoding,
-			headerHost,
-			headerUserAgent,
-			headerTrailer,
-			headerDate:
-			// HTTP headers.
-		case connectUnaryHeaderAcceptCompression,
-			connectUnaryTrailerPrefix,
-			connectStreamingHeaderCompression,
-			connectStreamingHeaderAcceptCompression,
-			connectHeaderTimeout,
-			connectHeaderProtocolVersion:
-			// Connect headers.
-		case grpcHeaderCompression,
-			grpcHeaderAcceptCompression,
-			grpcHeaderTimeout,
-			grpcHeaderStatus,
-			grpcHeaderMessage,
-			grpcHeaderDetails:
-			// gRPC headers.
-		default:
+		if _, isProtocolHeader := protocolHeaders[key]; !isProtocolHeader {
 			into[key] = append(into[key], vals...)
 		}
 	}
diff --git a/protocol_connect.go b/protocol_connect.go
index e3c5e4a5..bf26f1aa 100644
--- a/protocol_connect.go
+++ b/protocol_connect.go
@@ -765,7 +765,7 @@ func (hc *connectUnaryHandlerConn) mergeResponseHeader(err error) {
 	}
 	if err != nil {
 		if connectErr, ok := asError(err); ok && !connectErr.wireErr {
-			mergeMetadataHeaders(header, connectErr.meta)
+			mergeNonProtocolHeaders(header, connectErr.meta)
 		}
 	}
 	for k, v := range hc.responseTrailer {
@@ -850,7 +850,7 @@ func (m *connectStreamingMarshaler) MarshalEndStream(err error, trailer http.Hea
 	if err != nil {
 		end.Error = newConnectWireError(err)
 		if connectErr, ok := asError(err); ok && !connectErr.wireErr {
-			mergeMetadataHeaders(end.Trailer, connectErr.meta)
+			mergeNonProtocolHeaders(end.Trailer, connectErr.meta)
 		}
 	}
 	data, marshalErr := json.Marshal(end)
diff --git a/protocol_grpc.go b/protocol_grpc.go
index 32b116f4..5addf31e 100644
--- a/protocol_grpc.go
+++ b/protocol_grpc.go
@@ -841,7 +841,7 @@ func grpcErrorToTrailer(trailer http.Header, protobuf Codec, err error) {
 		return
 	}
 	if connectErr, ok := asError(err); ok && !connectErr.wireErr {
-		mergeMetadataHeaders(trailer, connectErr.meta)
+		mergeNonProtocolHeaders(trailer, connectErr.meta)
 	}
 	var (
 		status  = grpcStatusFromError(err)