diff --git a/private/buf/buflsp/completion_test.go b/private/buf/buflsp/completion_test.go
new file mode 100644
index 0000000000..6b531e8dec
--- /dev/null
+++ b/private/buf/buflsp/completion_test.go
@@ -0,0 +1,109 @@
+// Copyright 2020-2025 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package buflsp_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.lsp.dev/protocol"
+)
+
+func TestCompletion(t *testing.T) {
+	t.Parallel()
+
+	ctx := t.Context()
+
+	testProtoPath, err := filepath.Abs("testdata/completion/test.proto")
+	require.NoError(t, err)
+
+	clientJSONConn, testURI := setupLSPServer(t, testProtoPath)
+
+	tests := []struct {
+		name                string
+		line                uint32
+		character           uint32
+		expectedContains    []string
+		expectedNotContains []string
+		expectNoCompletions bool
+	}{
+		{
+			name:             "complete_builtin_toplevel",
+			line:             5,
+			character:        1, // After the "m"
+			expectedContains: []string{"message"},
+		},
+		{
+			name:             "complete_message_field_types",
+			line:             9, // Empty line in User message where field would go
+			character:        2, // Indented position where field type would be
+			expectedContains: []string{"string", "int32", "int64", "bool", "bytes", "User", "GetUserRequest", "GetUserResponse"},
+		},
+		{
+			name:             "complete_builtin_service",
+			line:             14,
+			character:        2, // Indented position where "rpc" would be
+			expectedContains: []string{"rpc", "option"},
+		},
+		{
+			name:             "complete_rpc_request_type",
+			line:             13,
+			character:        uint32(len("  rpc GetUser(Get") - 1),
+			expectedContains: []string{"GetUserRequest", "GetUserResponse"},
+		},
+		{
+			name:             "complete_rpc_response_type",
+			line:             13,
+			character:        uint32(len("  rpc GetUser(Get) returns (Get") - 1),
+			expectedContains: []string{"GetUserRequest", "GetUserResponse"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			var completionList *protocol.CompletionList
+			_, completionErr := clientJSONConn.Call(ctx, protocol.MethodTextDocumentCompletion, protocol.CompletionParams{
+				TextDocumentPositionParams: protocol.TextDocumentPositionParams{
+					TextDocument: protocol.TextDocumentIdentifier{
+						URI: testURI,
+					},
+					Position: protocol.Position{
+						Line:      tt.line,
+						Character: tt.character,
+					},
+				},
+			}, &completionList)
+			require.NoError(t, completionErr)
+			if tt.expectNoCompletions {
+				assert.Nil(t, completionList, "expected no completions")
+				return
+			}
+			require.NotNil(t, completionList, "expected completion list to be non-nil")
+			labels := make([]string, 0, len(completionList.Items))
+			for _, item := range completionList.Items {
+				labels = append(labels, item.Label)
+			}
+			for _, expected := range tt.expectedContains {
+				assert.Contains(t, labels, expected, "expected completion list to contain %q", expected)
+			}
+			for _, notExpected := range tt.expectedNotContains {
+				assert.NotContains(t, labels, notExpected, "expected completion list to not contain %q", notExpected)
+			}
+		})
+	}
+}
diff --git a/private/buf/buflsp/testdata/completion/buf.yaml b/private/buf/buflsp/testdata/completion/buf.yaml
new file mode 100644
index 0000000000..f74da98a3c
--- /dev/null
+++ b/private/buf/buflsp/testdata/completion/buf.yaml
@@ -0,0 +1,9 @@
+version: v2
+modules:
+  - path: .
+lint:
+  use:
+    - STANDARD
+breaking:
+  use:
+    - FILE
diff --git a/private/buf/buflsp/testdata/completion/test.proto b/private/buf/buflsp/testdata/completion/test.proto
new file mode 100644
index 0000000000..e2f30fd9ab
--- /dev/null
+++ b/private/buf/buflsp/testdata/completion/test.proto
@@ -0,0 +1,24 @@
+syntax = "proto3";
+
+package example.v1;
+
+// complete_builtin_toplevel
+m {}
+
+message User {
+  string id = 1;
+  // complete_message_field_types
+}
+
+service UserService {
+  rpc GetUser(Get) returns (Get); // complete_rpc_request_type, complete_rpc_response_type
+  // complete_builtin_service
+}
+
+message GetUserRequest {
+  string user_id = 1;
+}
+
+message GetUserResponse {
+  User user = 1;
+}