mcp: infer elicit mode from ElicitParams (#1078)

Fixes #1072

mcp/protocol.go

@@ -63,6 +63,9 @@ func (m InputRequestMap) MarshalJSON() ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
+		if ep, ok := v.(*ElicitParams); ok {
+			v = ep.inferElicitMode()
+		}
 		converted[k] = &wire{Method: method, Params: v}
 	}
 	return json.Marshal(converted)
@@ -2126,6 +2129,21 @@ func (x *ElicitParams) isNil() bool     { return x == nil }
 func (x *ElicitParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ElicitParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
+// inferElicitMode returns x with Mode populated by inference if it was empty.
+// Mode is inferred as "url" when URL or ElicitationID is set, otherwise "form".
+func (x *ElicitParams) inferElicitMode() *ElicitParams {
+	if x == nil || x.Mode != "" {
+		return x
+	}
+	x2 := *x
+	if x.URL != "" || x.ElicitationID != "" {
+		x2.Mode = "url"
+	} else {
+		x2.Mode = "form"
+	}
+	return &x2
+}
+
 // The client's response to an elicitation/create request from the server.
 type ElicitResult struct {
 	// This property is reserved by the protocol to allow clients and servers to

mcp/server.go

@@ -1639,15 +1639,7 @@ func (ss *ServerSession) Elicit(ctx context.Context, params *ElicitParams) (*Eli
 		return nil, fmt.Errorf("%w: params cannot be nil", jsonrpc2.ErrInvalidParams)
 	}
 
-	if params.Mode == "" {
-		params2 := *params
-		if params.URL != "" || params.ElicitationID != "" {
-			params2.Mode = "url"
-		} else {
-			params2.Mode = "form"
-		}
-		params = &params2
-	}
+	params = params.inferElicitMode()
 
 	if iparams := ss.InitializeParams(); iparams == nil || iparams.Capabilities == nil || iparams.Capabilities.Elicitation == nil {
 		return nil, fmt.Errorf("client does not support elicitation")

mcp/testdata/conformance/server/mrtr.txtar

@@ -52,7 +52,7 @@ confirmThenGreet
 			"who": {
 				"method": "elicitation/create",
 				"params": {
-					"mode": "",
+					"mode": "form",
 					"message": "What is your name?",
 					"requestedSchema": {
 						"type": "object",