mcp: make request headers available to tools (modelcontextprotocol#333)

Add HTTP request headers to RequestExtra, so tools and other
user-defined handlers can access them.

Fixes modelcontextprotocol#331.

Author: jba

mcp/shared.go

@@ -16,6 +16,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"net/http"
 	"reflect"
 	"slices"
 	"strings"
@@ -400,6 +401,7 @@ type ServerRequest[P Params] struct {
 // the transport layer.
 type RequestExtra struct {
 	TokenInfo *auth.TokenInfo // bearer token info (e.g. from OAuth) if any
+	Header    http.Header     // header from HTTP request, if any
 }
 
 func (*ClientRequest[P]) isRequest() {}

mcp/streamable.go

@@ -580,10 +580,6 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 	// This also requires access to the negotiated version, which would either be
 	// set by the MCP-Protocol-Version header, or would require peeking into the
 	// session.
-	if err != nil {
-		http.Error(w, fmt.Sprintf("malformed payload: %v", err), http.StatusBadRequest)
-		return
-	}
 	incoming, _, err := readBatch(body)
 	if err != nil {
 		http.Error(w, fmt.Sprintf("malformed payload: %v", err), http.StatusBadRequest)
@@ -592,17 +588,20 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 	requests := make(map[jsonrpc.ID]struct{})
 	tokenInfo := auth.TokenInfoFromContext(req.Context())
 	for _, msg := range incoming {
-		if req, ok := msg.(*jsonrpc.Request); ok {
+		if jreq, ok := msg.(*jsonrpc.Request); ok {
 			// Preemptively check that this is a valid request, so that we can fail
 			// the HTTP request. If we didn't do this, a request with a bad method or
 			// missing ID could be silently swallowed.
-			if _, err := checkRequest(req, serverMethodInfos); err != nil {
+			if _, err := checkRequest(jreq, serverMethodInfos); err != nil {
 				http.Error(w, err.Error(), http.StatusBadRequest)
 				return
 			}
-			req.Extra = &RequestExtra{TokenInfo: tokenInfo}
-			if req.IsCall() {
-				requests[req.ID] = struct{}{}
+			jreq.Extra = &RequestExtra{
+				TokenInfo: tokenInfo,
+				Header:    req.Header,
+			}
+			if jreq.IsCall() {
+				requests[jreq.ID] = struct{}{}
 			}
 		}
 	}