mcp: preserve existing Content in SetError (#864)

SetError now only populates the Content field when it has not already
been populated. This allows callers to provide a user-friendly message
while still recording the underlying error for middleware inspection
via GetError.
The len check (rather than nil check) ensures both nil and empty-slice
Content are treated as "not yet populated."
  Fixes #858

mcp/mcp_test.go

@@ -2228,4 +2228,45 @@ 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

@@ -10,6 +10,7 @@ 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
@@ -113,10 +114,25 @@ type CallToolResult struct {
 	err error
 }
 
-// SetError sets the error for the tool result and populates the Content field
+// seterroroverwrite is a compatibility parameter that restores the pre-1.6.0
-// with the error text. It also sets IsError to true.
+// 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
 }