mcp: preserve existing Content in SetError

SetError now only populates the Content field when it has not already
been populated (len check handles both nil and empty slice). This allows
callers to provide a user-friendly message while still recording the
underlying error for middleware inspection via GetError.

Fixes #858

Author: ravyg

mcp/mcp_test.go

@@ -2244,4 +2244,45 @@ func TestToolErrorMiddleware(t *testing.T) {
 	}
 }
 
+func TestSetErrorPreservesContent(t *testing.T) {
+	for _, tt := range []struct {
+		name        string
+		content     []Content
+		err         error
+		wantContent string
+	}{
+		{
+			name:        "nil content",
+			err:         errors.New("internal failure"),
+			wantContent: "internal failure",
+		},
+		{
+			name:        "empty slice content",
+			content:     []Content{},
+			err:         errors.New("internal failure"),
+			wantContent: "internal failure",
+		},
+		{
+			name:        "existing content preserved",
+			content:     []Content{&TextContent{Text: "user-friendly msg"}},
+			err:         errors.New("db timeout"),
+			wantContent: "user-friendly msg",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			res := CallToolResult{Content: tt.content}
+			res.SetError(tt.err)
+			if !res.IsError {
+				t.Fatal("want IsError=true")
+			}
+			if got := res.Content[0].(*TextContent).Text; got != tt.wantContent {
+				t.Errorf("Content text = %q, want %q", got, tt.wantContent)
+			}
+			if got := res.GetError(); got != tt.err {
+				t.Errorf("GetError() = %v, want %v", got, tt.err)
+			}
+		})
+	}
+}
+
 var ctrCmpOpts = []cmp.Option{cmp.AllowUnexported(CallToolResult{})}

mcp/protocol.go

@@ -16,6 +16,7 @@ import (
 	"maps"
 
 	internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
+	"github.com/modelcontextprotocol/go-sdk/internal/mcpgodebug"
 )
 
 // Optional annotations for the client. The client can use annotations to inform
@@ -119,10 +120,25 @@ type CallToolResult struct {
 	err error
 }
 
-// SetError sets the error for the tool result and populates the Content field
-// with the error text. It also sets IsError to true.
+// seterroroverwrite is a compatibility parameter that restores the pre-1.6.0
+// behavior of [CallToolResult.SetError], where Content was always overwritten
+// with the error text. See the documentation for the mcpgodebug package for
+// instructions on how to enable it.
+// The option will be removed in the 1.8.0 version of the SDK.
+var seterroroverwrite = mcpgodebug.Value("seterroroverwrite")
+
+// SetError sets the error for the tool result and sets IsError to true.
+// If Content has not already been populated, it is set to the error text.
+// If Content has already been populated, it is left unchanged, allowing callers
+// to provide a user-friendly message while still recording the underlying error
+// for inspection via [GetError] in server middleware.
+//
+// To restore the previous behavior where Content was always overwritten,
+// set MCPGODEBUG=seterroroverwrite=1.
 func (r *CallToolResult) SetError(err error) {
-	r.Content = []Content{&TextContent{Text: err.Error()}}
+	if len(r.Content) == 0 || seterroroverwrite == "1" {
+		r.Content = []Content{&TextContent{Text: err.Error()}}
+	}
 	r.IsError = true
 	r.err = err
 }