Export GetError and SetError methods (#753)

Exported GetError and SetError. 

Fixes #64

Co-authored-by: Claude <noreply@anthropic.com>

Author: jba

mcp/mcp_test.go

@@ -2124,7 +2124,7 @@ func TestToolErrorMiddleware(t *testing.T) {
 			res, err := h(ctx, method, req)
 			if err == nil {
 				if ctr, ok := res.(*CallToolResult); ok {
-					middleErr = ctr.getError()
+					middleErr = ctr.GetError()
 				}
 			}
 			return res, err
@@ -2161,7 +2161,7 @@ func TestToolErrorMiddleware(t *testing.T) {
 		t.Fatal("want error, got none")
 	}
 	// Clients can't see the error, because it isn't marshaled.
-	if err := res.getError(); err != nil {
+	if err := res.GetError(); err != nil {
 		t.Fatalf("got %v, want nil", err)
 	}
 	if middleErr != errTestFailure {

mcp/protocol.go

@@ -116,17 +116,17 @@ type CallToolResult struct {
 	err error
 }
 
-// TODO(#64): consider exposing setError (and getError), by adding an error
-// field on CallToolResult.
-func (r *CallToolResult) setError(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.
+func (r *CallToolResult) SetError(err error) {
 	r.Content = []Content{&TextContent{Text: err.Error()}}
 	r.IsError = true
 	r.err = err
 }
 
-// getError returns the error set with setError, or nil if none.
+// GetError returns the error set with SetError, or nil if none.
 // This function always returns nil on clients.
-func (r *CallToolResult) getError() error {
+func (r *CallToolResult) GetError() error {
 	return r.err
 }
 

mcp/server.go

@@ -339,7 +339,7 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
 			}
 			// For regular errors, embed them in the tool result as per MCP spec
 			var errRes CallToolResult
-			errRes.setError(err)
+			errRes.SetError(err)
 			return &errRes, nil
 		}