fix(mcp): preserve structured plugin tool errors (#3815)

apps/backend/internal/mcp/server/plugin_result_test.go

@@ -0,0 +1,60 @@
+package mcp
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/kandev/kandev/internal/mcp/plugintools"
+	mcplib "github.com/mark3labs/mcp-go/mcp"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPluginToolPreservesStructuredResultAndErrorStatus(t *testing.T) {
+	for _, isError := range []bool{false, true} {
+		name := "success"
+		if isError {
+			name = "error"
+		}
+		t.Run(name, func(t *testing.T) {
+			structured := map[string]any{"status": "uncertain", "recipient": "U01234567", "duplicate": true}
+			backend := &testBackend{response: map[string]any{
+				"text": "Check delivery before retrying.", "structured_content": structured, "is_error": isError,
+			}}
+			server := New(backend, "session-1", "task-1", 10005, newTestLogger(t), "", false, ModeTask)
+			definition := plugintools.Definition{
+				PluginID: "notifications", LocalName: "notify", ExposedName: plugintools.ExposedName("notifications", "notify"),
+				Description: "Notify a user", InputSchema: []byte(`{"type":"object"}`),
+				Surfaces: []string{plugintools.SurfaceKanban},
+			}
+			require.NoError(t, server.SetPluginTools(plugintools.Snapshot{
+				Generation: "g", Revision: 1, Tools: []plugintools.Definition{definition},
+			}))
+
+			result := callTool(t, server, definition.ExposedName, map[string]any{})
+			require.Equal(t, isError, result.IsError)
+			require.Equal(t, structured, result.StructuredContent)
+			require.Len(t, result.Content, 1)
+			require.Equal(t, "Check delivery before retrying.", result.Content[0].(mcplib.TextContent).Text)
+		})
+	}
+}
+
+func TestPluginToolErrorOmitsNilStructuredContent(t *testing.T) {
+	backend := &testBackend{response: map[string]any{
+		"text": "Check delivery before retrying.", "structured_content": nil, "is_error": true,
+	}}
+	server := New(backend, "session-1", "task-1", 10005, newTestLogger(t), "", false, ModeTask)
+	definition := plugintools.Definition{
+		PluginID: "notifications", LocalName: "notify", ExposedName: plugintools.ExposedName("notifications", "notify"),
+		Description: "Notify a user", InputSchema: []byte(`{"type":"object"}`),
+		Surfaces: []string{plugintools.SurfaceKanban},
+	}
+	require.NoError(t, server.SetPluginTools(plugintools.Snapshot{
+		Generation: "g", Revision: 1, Tools: []plugintools.Definition{definition},
+	}))
+
+	result := callTool(t, server, definition.ExposedName, map[string]any{})
+	serialized, err := json.Marshal(result)
+	require.NoError(t, err)
+	require.NotContains(t, string(serialized), `"structuredContent"`)
+}

apps/backend/internal/mcp/server/server.go

@@ -989,7 +989,11 @@ func (s *Server) registerPluginTools() {
 				return nil, err
 			}
 			if result.IsError {
-				return mcp.NewToolResultError(result.Text), nil
+				response := mcp.NewToolResultError(result.Text)
+				if result.StructuredContent != nil {
+					response.StructuredContent = result.StructuredContent
+				}
+				return response, nil
 			}
 			if result.StructuredContent != nil {
 				return mcp.NewToolResultStructured(result.StructuredContent, result.Text), nil