fix: return non-nil empty completion results

pkg/github/repository_resource_completions.go

@@ -28,7 +28,9 @@ var RepositoryResourceArgumentResolvers = map[string]CompleteHandler{
 func RepositoryResourceCompletionHandler(getClient GetClientFn) func(ctx context.Context, req *mcp.CompleteRequest) (*mcp.CompleteResult, error) {
 	return func(ctx context.Context, req *mcp.CompleteRequest) (*mcp.CompleteResult, error) {
 		if req.Params.Ref.Type != "ref/resource" {
-			return nil, nil // Not a resource completion
+			return &mcp.CompleteResult{
+				Completion: mcp.CompletionResultDetails{Values: []string{}},
+			}, nil
 		}
 
 		argName := req.Params.Argument.Name

pkg/github/repository_resource_completions_test.go

@@ -2,6 +2,7 @@ package github
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"testing"
@@ -20,16 +21,18 @@ func TestRepositoryResourceCompletionHandler(t *testing.T) {
 		wantErr  bool
 	}{
 		{
-			name: "non-resource completion request",
+			name: "prompt completion request",
 			request: &mcp.CompleteRequest{
 				Params: &mcp.CompleteParams{
 					Ref: &mcp.CompleteReference{
-						Type: "something-else",
+						Type: "ref/prompt",
 					},
 				},
 			},
-			expected: nil,
-			wantErr:  false,
+			expected: &mcp.CompleteResult{
+				Completion: mcp.CompletionResultDetails{Values: []string{}},
+			},
+			wantErr: false,
 		},
 		{
 			name: "invalid ref type",
@@ -40,8 +43,10 @@ func TestRepositoryResourceCompletionHandler(t *testing.T) {
 					},
 				},
 			},
-			expected: nil,
-			wantErr:  false,
+			expected: &mcp.CompleteResult{
+				Completion: mcp.CompletionResultDetails{Values: []string{}},
+			},
+			wantErr: false,
 		},
 		{
 			name: "unknown argument",
@@ -65,6 +70,7 @@ func TestRepositoryResourceCompletionHandler(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			getClient := func(_ context.Context) (*github.Client, error) {
+				assert.Equal(t, "ref/resource", tt.request.Params.Ref.Type, "non-resource completions must not fetch a client")
 				return &github.Client{}, nil
 			}
 
@@ -77,6 +83,12 @@ func TestRepositoryResourceCompletionHandler(t *testing.T) {
 				require.NoError(t, err)
 			}
 			assert.Equal(t, tt.expected, result)
+			if !tt.wantErr {
+				require.NotNil(t, result)
+				data, err := json.Marshal(result)
+				require.NoError(t, err)
+				assert.JSONEq(t, `{"completion":{"values":[]}}`, string(data))
+			}
 		})
 	}
 }

pkg/github/server.go

@@ -212,7 +212,9 @@ func CompletionsHandler(getClient GetClientFn) func(ctx context.Context, req *mc
 			}
 			return nil, fmt.Errorf("unsupported resource URI: %s", req.Params.Ref.URI)
 		case "ref/prompt":
-			return nil, nil
+			return &mcp.CompleteResult{
+				Completion: mcp.CompletionResultDetails{Values: []string{}},
+			}, nil
 		default:
 			return nil, fmt.Errorf("unsupported ref type: %s", req.Params.Ref.Type)
 		}

pkg/github/server_test.go

@@ -528,3 +528,58 @@ func TestCompletionsHandler_RejectsMissingRef(t *testing.T) {
 		})
 	}
 }
+
+func TestCompletionsHandler(t *testing.T) {
+	clientErr := errors.New("client error")
+	tests := []struct {
+		name       string
+		ref        *mcp.CompleteReference
+		wantErr    string
+		wantClient bool
+	}{
+		{
+			name: "prompt completion",
+			ref:  &mcp.CompleteReference{Type: "ref/prompt"},
+		},
+		{
+			name:       "repository resource completion",
+			ref:        &mcp.CompleteReference{Type: "ref/resource", URI: "repo://{owner}/{repo}/contents/{path}"},
+			wantErr:    clientErr.Error(),
+			wantClient: true,
+		},
+		{
+			name:    "unsupported resource URI",
+			ref:     &mcp.CompleteReference{Type: "ref/resource", URI: "unsupported://resource"},
+			wantErr: "unsupported resource URI: unsupported://resource",
+		},
+		{
+			name:    "unsupported ref type",
+			ref:     &mcp.CompleteReference{Type: "unsupported"},
+			wantErr: "unsupported ref type: unsupported",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			clientCalled := false
+			handler := CompletionsHandler(func(context.Context) (*gogithub.Client, error) {
+				clientCalled = true
+				return nil, clientErr
+			})
+			result, err := handler(t.Context(), &mcp.CompleteRequest{
+				Params: &mcp.CompleteParams{Ref: tt.ref},
+			})
+			assert.Equal(t, tt.wantClient, clientCalled)
+			if tt.wantErr != "" {
+				require.EqualError(t, err, tt.wantErr)
+				assert.Nil(t, result)
+				return
+			}
+			require.NoError(t, err)
+			require.NotNil(t, result)
+			assert.Equal(t, []string{}, result.Completion.Values)
+			data, err := json.Marshal(result)
+			require.NoError(t, err)
+			assert.JSONEq(t, `{"completion":{"values":[]}}`, string(data))
+		})
+	}
+}