gopls/internal/mcp: add go_rename tool

This tool uses the LSP server's Rename method to rename Go symbols
(variables, functions, types, etc.) at a specified position. It supports
both preview mode (dryRun: true, default) and actual application
(dryRun: false) of changes across the workspace.

- Add renameTool() and renameHandler() to the MCP server
- Support dryRun parameter: true for preview, false to apply changes
- Add TestMCPRename to test the tool for both dryRun modes
- Update MCP instructions with go_rename usage examples for both modes

Author: arjunmahishi

gopls/internal/cmd/mcp_test.go

@@ -285,6 +285,146 @@ func getRandomPort() int {
 	return listener.Addr().(*net.TCPAddr).Port
 }
 
+func TestMCPRename(t *testing.T) {
+	// Test the go_rename tool via MCP.
+	if !(supportsFsnotify(runtime.GOOS)) {
+		// See golang/go#74580
+		t.Skipf(`skipping on %s; fsnotify is not supported`, runtime.GOOS)
+	}
+	testenv.NeedsExec(t) // stdio transport uses execve(2)
+
+	tests := []struct {
+		name        string
+		dryRunValue interface{} // nil, true, or false
+		wantStrings []string
+	}{
+		{
+			name:        "default_dry_run",
+			dryRunValue: nil, // omit dryRun parameter
+			wantStrings: []string{"a.go", "b.go", "c.go", "NewName", "OldName", "Rename would make", "6 changes", "3 files"},
+		},
+		{
+			name:        "explicit_dry_run",
+			dryRunValue: true,
+			wantStrings: []string{"a.go", "b.go", "c.go", "NewName", "OldName", "Rename would make", "6 changes", "3 files"},
+		},
+		{
+			name:        "apply_changes",
+			dryRunValue: false,
+			wantStrings: []string{"Successfully applied", "6 changes", "3 files"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Test cross-file rename with: function definition (a.go), function calls (b.go, c.go),
+			// and variable assignments (all files) = 6 total references across 3 files
+			tree := writeTree(t, `
+-- go.mod --
+module example.com
+go 1.18
+
+-- a.go --
+package main
+
+func OldName() {
+}
+
+var globalVar = OldName
+
+-- b.go --
+package main
+
+func TestFunc() {
+	OldName()
+	x := OldName
+	_ = x
+}
+
+-- c.go --
+package main
+
+func AnotherFunc() {
+	OldName()
+	result := OldName
+	_ = result
+}
+`)
+
+			goplsCmd := exec.Command(os.Args[0], "mcp")
+			goplsCmd.Env = append(os.Environ(), "ENTRYPOINT=goplsMain")
+			goplsCmd.Dir = tree
+
+			ctx := t.Context()
+			client := mcp.NewClient("client", "v0.0.1", nil)
+			mcpSession, err := client.Connect(ctx, mcp.NewCommandTransport(goplsCmd))
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer func() {
+				if err := mcpSession.Close(); err != nil {
+					t.Errorf("closing MCP connection: %v", err)
+				}
+			}()
+
+			tool := "go_rename"
+			args := map[string]any{
+				"file":    filepath.Join(tree, "a.go"),
+				"line":    2,
+				"column":  5,
+				"newName": "NewName",
+			}
+
+			// Add dryRun parameter if specified
+			if tt.dryRunValue != nil {
+				args["dryRun"] = tt.dryRunValue
+			}
+
+			res, err := mcpSession.CallTool(ctx, &mcp.CallToolParams{Name: tool, Arguments: args})
+			if err != nil {
+				t.Fatal(err)
+			}
+			got := resultText(t, res)
+
+			// Check expected strings in output
+			for _, want := range tt.wantStrings {
+				if !strings.Contains(got, want) {
+					t.Errorf("CallTool(%s, %v) = %v, want containing %q", tool, args, got, want)
+				}
+			}
+
+			// Check file modification based on dryRun value across all files
+			isDryRun := tt.dryRunValue == nil || tt.dryRunValue == true
+
+			for _, filename := range []string{"a.go", "b.go", "c.go"} {
+				content, err := os.ReadFile(filepath.Join(tree, filename))
+				if err != nil {
+					t.Fatal(err)
+				}
+
+				if isDryRun {
+					// Files should NOT be modified in dry run
+					if strings.Contains(string(content), "NewName") {
+						t.Errorf("Expected %s to NOT contain 'NewName' in dry run, but it was modified: %s", filename, string(content))
+					}
+					if !strings.Contains(string(content), "OldName") {
+						t.Errorf("Expected %s to still contain 'OldName' in dry run, got: %s", filename, string(content))
+					}
+					continue
+				}
+
+				// Files should be modified when applying changes - verify cross-file rename
+				if !strings.Contains(string(content), "NewName") {
+					t.Errorf("Expected %s to contain 'NewName' after cross-file rename, got: %s", filename, string(content))
+				}
+				if strings.Contains(string(content), "OldName") {
+					t.Errorf("Expected %s to no longer contain 'OldName' after rename, but found: %s", filename, string(content))
+				}
+			}
+		})
+	}
+}
+
 // supportsFsnotify returns true if fsnotify supports the os.
 func supportsFsnotify(os string) bool {
 	return os == "darwin" || os == "linux" || os == "windows"

gopls/internal/mcp/instructions.md

@@ -38,10 +38,16 @@ The editing workflow is iterative. You should cycle through these steps until th
 
 3. **Make edits**: Make the required edits, including edits to references you identified in the previous step. Don't proceed to the next step until all planned edits are complete.
 
-4. **Check for errors**: After every code modification, you MUST call the `go_diagnostics` tool. Pass the paths of the files you have edited. This tool will report any build or analysis errors.
+4. **Rename symbols**: For renaming symbols across the workspace, use the `go_rename` tool which will automatically find and rename all references. ALWAYS follow this workflow:
+   - First, run with `dryRun: true` (default) to preview changes: `go_rename({"file":"/path/to/server.go","line":10,"column":5,"newName":"NewServerName"})`
+   - Show the user what changes would be made and ask for confirmation
+   - Only if the user approves, apply the changes: `go_rename({"file":"/path/to/server.go","line":10,"column":5,"newName":"NewServerName","dryRun":false})`
+   - Exception: Only skip the preview step if the user explicitly requests to apply changes immediately
+
+5. **Check for errors**: After every code modification, you MUST call the `go_diagnostics` tool. Pass the paths of the files you have edited. This tool will report any build or analysis errors.
    EXAMPLE: `go_diagnostics({"files":["/path/to/server.go"]})`
 
-5. **Fix errors**: If `go_diagnostics` reports any errors, fix them. The tool may provide suggested quick fixes in the form of diffs. You should review these diffs and apply them if they are correct. Once you've applied a fix, re-run `go_diagnostics` to confirm that the issue is resolved. It is OK to ignore 'hint' or 'info' diagnostics if they are not relevant to the current task. Note that Go diagnostic messages may contain a summary of the source code, which may not match its exact text.
+6. **Fix errors**: If `go_diagnostics` reports any errors, fix them. The tool may provide suggested quick fixes in the form of diffs. You should review these diffs and apply them if they are correct. Once you've applied a fix, re-run `go_diagnostics` to confirm that the issue is resolved. It is OK to ignore 'hint' or 'info' diagnostics if they are not relevant to the current task. Note that Go diagnostic messages may contain a summary of the source code, which may not match its exact text.
 
-6. **Run tests**: Once `go_diagnostics` reports no errors (and ONLY once there are no errors), run the tests for the packages you have changed. You can do this with `go test [packagePath...]`. Don't run `go test ./...` unless the user explicitly requests it, as doing so may slow down the iteration loop.
+7. **Run tests**: Once `go_diagnostics` reports no errors (and ONLY once there are no errors), run the tests for the packages you have changed. You can do this with `go test [packagePath...]`. Don't run `go test ./...` unless the user explicitly requests it, as doing so may slow down the iteration loop.
 

gopls/internal/mcp/mcp.go

@@ -75,7 +75,9 @@ func Serve(ctx context.Context, address string, sessions Sessions, isDaemon bool
 }
 
 // StartStdIO starts an MCP server over stdio.
-func StartStdIO(ctx context.Context, session *cache.Session, server protocol.Server, rpcLog io.Writer) error {
+func StartStdIO(
+	ctx context.Context, session *cache.Session, server protocol.Server, rpcLog io.Writer,
+) error {
 	transport := mcp.NewStdioTransport()
 	var t mcp.Transport = transport
 	if rpcLog != nil {
@@ -166,6 +168,7 @@ func newServer(session *cache.Session, lspServer protocol.Server) *mcp.Server {
 		h.symbolReferencesTool(),
 		h.searchTool(),
 		h.fileContextTool(),
+		h.renameTool(),
 	}
 	disabledTools := append(defaultTools,
 		// The fileMetadata tool is redundant with fileContext.
@@ -222,7 +225,9 @@ func (h *handler) snapshot() (*cache.Snapshot, func(), error) {
 //
 // This helps avoid stale packages, but is not a substitute for real file
 // watching, as it misses things like files being added to a package.
-func (h *handler) fileOf(ctx context.Context, file string) (file.Handle, *cache.Snapshot, func(), error) {
+func (h *handler) fileOf(
+	ctx context.Context, file string,
+) (file.Handle, *cache.Snapshot, func(), error) {
 	uri := protocol.URIFromPath(file)
 	fh, snapshot, release, err := h.session.FileOf(ctx, uri)
 	if err != nil {
@@ -260,7 +265,9 @@ func (h *handler) fileOf(ctx context.Context, file string) (file.Handle, *cache.
 //
 // It also doesn't catch package changes that occur due to added files or
 // changes to the go.mod file.
-func checkForFileChanges(ctx context.Context, snapshot *cache.Snapshot, id metadata.PackageID) ([]protocol.FileEvent, error) {
+func checkForFileChanges(
+	ctx context.Context, snapshot *cache.Snapshot, id metadata.PackageID,
+) ([]protocol.FileEvent, error) {
 	var events []protocol.FileEvent
 
 	seen := make(map[metadata.PackageID]struct{})