gopls/internal/mcp: more tuning of tools and prompts

After further testing, the go_file_metadata tool does not seem
particularly useful replace it with a go_file_context tool, which
summarizes APIs used within the file.

For golang/go#73580

Change-Id: I8a41f916b7f2a71dac5b6b800088be595ecd8c21
Reviewed-on: https://go-review.googlesource.com/c/tools/+/687355
Reviewed-by: Madeline Kalil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>

Author: findleyr

gopls/internal/cmd/mcp_test.go

@@ -231,7 +231,7 @@ func MyFun() {}
 	}()
 
 	var (
-		tool = "go_file_metadata"
+		tool = "go_file_context"
 		args = map[string]any{"file": filepath.Join(tree, "a.go")}
 	)
 	res, err := mcpSession.CallTool(ctx, &mcp.CallToolParams{Name: tool, Arguments: args})

gopls/internal/mcp/context.go

@@ -66,7 +66,7 @@ func (h *handler) contextHandler(ctx context.Context, _ *mcp.ServerSession, para
 	{
 		fmt.Fprintf(&result, "%s (current file):\n", pgf.URI.Base())
 		result.WriteString("```go\n")
-		if err := writeFileSummary(ctx, snapshot, pgf.URI, &result, false); err != nil {
+		if err := writeFileSummary(ctx, snapshot, pgf.URI, &result, false, nil); err != nil {
 			return nil, err
 		}
 		result.WriteString("```\n\n")
@@ -81,7 +81,7 @@ func (h *handler) contextHandler(ctx context.Context, _ *mcp.ServerSession, para
 
 			fmt.Fprintf(&result, "%s:\n", file.URI.Base())
 			result.WriteString("```go\n")
-			if err := writeFileSummary(ctx, snapshot, file.URI, &result, false); err != nil {
+			if err := writeFileSummary(ctx, snapshot, file.URI, &result, false, nil); err != nil {
 				return nil, err
 			}
 			result.WriteString("```\n\n")
@@ -153,7 +153,7 @@ func summarizePackage(ctx context.Context, snapshot *cache.Snapshot, md *metadat
 	for _, f := range md.CompiledGoFiles {
 		fmt.Fprintf(&buf, "%s:\n", f.Base())
 		buf.WriteString("```go\n")
-		if err := writeFileSummary(ctx, snapshot, f, &buf, true); err != nil {
+		if err := writeFileSummary(ctx, snapshot, f, &buf, true, nil); err != nil {
 			return "" // ignore error
 		}
 		buf.WriteString("```\n\n")
@@ -163,7 +163,7 @@ func summarizePackage(ctx context.Context, snapshot *cache.Snapshot, md *metadat
 
 // writeFileSummary writes the file summary to the string builder based on
 // the input file URI.
-func writeFileSummary(ctx context.Context, snapshot *cache.Snapshot, f protocol.DocumentURI, out *strings.Builder, onlyExported bool) error {
+func writeFileSummary(ctx context.Context, snapshot *cache.Snapshot, f protocol.DocumentURI, out *strings.Builder, onlyExported bool, declsToSummarize map[string]bool) error {
 	fh, err := snapshot.ReadFile(ctx, f)
 	if err != nil {
 		return err
@@ -173,52 +173,60 @@ func writeFileSummary(ctx context.Context, snapshot *cache.Snapshot, f protocol.
 		return err
 	}
 
-	// Copy everything before the first non-import declaration:
-	// package decl, imports decl(s), and all comments (excluding copyright).
-	{
-		endPos := pgf.File.FileEnd
+	// If we're summarizing specific declarations, we don't need to copy the header.
+	if declsToSummarize == nil {
+		// Copy everything before the first non-import declaration:
+		// package decl, imports decl(s), and all comments (excluding copyright).
+		{
+			endPos := pgf.File.FileEnd
 
-	outerloop:
-		for _, decl := range pgf.File.Decls {
-			switch decl := decl.(type) {
-			case *ast.FuncDecl:
-				if decl.Doc != nil {
-					endPos = decl.Doc.Pos()
-				} else {
-					endPos = decl.Pos()
-				}
-				break outerloop
-			case *ast.GenDecl:
-				if decl.Tok == token.IMPORT {
-					continue
-				}
-				if decl.Doc != nil {
-					endPos = decl.Doc.Pos()
-				} else {
-					endPos = decl.Pos()
+		outerloop:
+			for _, decl := range pgf.File.Decls {
+				switch decl := decl.(type) {
+				case *ast.FuncDecl:
+					if decl.Doc != nil {
+						endPos = decl.Doc.Pos()
+					} else {
+						endPos = decl.Pos()
+					}
+					break outerloop
+				case *ast.GenDecl:
+					if decl.Tok == token.IMPORT {
+						continue
+					}
+					if decl.Doc != nil {
+						endPos = decl.Doc.Pos()
+					} else {
+						endPos = decl.Pos()
+					}
+					break outerloop
 				}
-				break outerloop
 			}
-		}
 
-		startPos := pgf.File.FileStart
-		if copyright := golang.CopyrightComment(pgf.File); copyright != nil {
-			startPos = copyright.End()
-		}
+			startPos := pgf.File.FileStart
+			if copyright := golang.CopyrightComment(pgf.File); copyright != nil {
+				startPos = copyright.End()
+			}
 
-		text, err := pgf.PosText(startPos, endPos)
-		if err != nil {
-			return err
-		}
+			text, err := pgf.PosText(startPos, endPos)
+			if err != nil {
+				return err
+			}
 
-		out.Write(bytes.TrimSpace(text))
-		out.WriteString("\n\n")
+			out.Write(bytes.TrimSpace(text))
+			out.WriteString("\n\n")
+		}
 	}
 
 	// Write func decl and gen decl.
 	for _, decl := range pgf.File.Decls {
 		switch decl := decl.(type) {
 		case *ast.FuncDecl:
+			if declsToSummarize != nil {
+				if _, ok := declsToSummarize[decl.Name.Name]; !ok {
+					continue
+				}
+			}
 			if onlyExported {
 				if !decl.Name.IsExported() {
 					continue
@@ -251,6 +259,28 @@ func writeFileSummary(ctx context.Context, snapshot *cache.Snapshot, f protocol.
 				continue
 			}
 
+			// If we are summarizing specific decls, check if any of them are in this GenDecl.
+			if declsToSummarize != nil {
+				found := false
+				for _, spec := range decl.Specs {
+					switch spec := spec.(type) {
+					case *ast.TypeSpec:
+						if _, ok := declsToSummarize[spec.Name.Name]; ok {
+							found = true
+						}
+					case *ast.ValueSpec:
+						for _, name := range spec.Names {
+							if _, ok := declsToSummarize[name.Name]; ok {
+								found = true
+							}
+						}
+					}
+				}
+				if !found {
+					continue
+				}
+			}
+
 			// Dump the entire GenDecl (exported or unexported)
 			// including doc comment without any filtering to the output.
 			if !onlyExported {
@@ -301,6 +331,11 @@ func writeFileSummary(ctx context.Context, snapshot *cache.Snapshot, f protocol.
 
 				switch spec := spec.(type) {
 				case *ast.TypeSpec:
+					if declsToSummarize != nil {
+						if _, ok := declsToSummarize[spec.Name.Name]; !ok {
+							continue
+						}
+					}
 					// TODO(hxjiang): only keep the exported field of
 					// struct spec and exported method of interface spec.
 					if !spec.Name.IsExported() {
@@ -323,6 +358,17 @@ func writeFileSummary(ctx context.Context, snapshot *cache.Snapshot, f protocol.
 					}
 
 				case *ast.ValueSpec:
+					if declsToSummarize != nil {
+						found := false
+						for _, name := range spec.Names {
+							if _, ok := declsToSummarize[name.Name]; ok {
+								found = true
+							}
+						}
+						if !found {
+							continue
+						}
+					}
 					// TODO(hxjiang): only keep the exported identifier.
 					if !slices.ContainsFunc(spec.Names, (*ast.Ident).IsExported) {
 						continue

gopls/internal/mcp/file_context.go

@@ -0,0 +1,104 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mcp
+
+import (
+	"context"
+	"fmt"
+	"go/ast"
+	"go/types"
+	"strings"
+
+	"golang.org/x/tools/gopls/internal/golang"
+	"golang.org/x/tools/gopls/internal/protocol"
+	"golang.org/x/tools/internal/mcp"
+)
+
+type fileContextParams struct {
+	File string `json:"file"`
+}
+
+func (h *handler) fileContextTool() *mcp.ServerTool {
+	return mcp.NewServerTool(
+		"go_file_context",
+		"Summarizes a file's cross-file dependencies",
+		h.fileContextHandler,
+		mcp.Input(
+			mcp.Property("file", mcp.Description("the absolute path to the file")),
+		),
+	)
+}
+
+func (h *handler) fileContextHandler(ctx context.Context, _ *mcp.ServerSession, params *mcp.CallToolParamsFor[fileContextParams]) (*mcp.CallToolResultFor[any], error) {
+	fh, snapshot, release, err := h.fileOf(ctx, params.Arguments.File)
+	if err != nil {
+		return nil, err
+	}
+	defer release()
+
+	pkg, pgf, err := golang.NarrowestPackageForFile(ctx, snapshot, fh.URI())
+	if err != nil {
+		return nil, err
+	}
+
+	info := pkg.TypesInfo()
+	if info == nil {
+		return nil, fmt.Errorf("no types info for package %q", pkg.Metadata().PkgPath)
+	}
+
+	// Group objects defined in other files by file URI.
+	otherFiles := make(map[protocol.DocumentURI]map[string]bool)
+	addObj := func(obj types.Object) {
+		if obj == nil {
+			return
+		}
+		pos := obj.Pos()
+		if !pos.IsValid() {
+			return
+		}
+		objFile := pkg.FileSet().File(pos)
+		if objFile == nil {
+			return
+		}
+		uri := protocol.URIFromPath(objFile.Name())
+		if uri == fh.URI() {
+			return
+		}
+		if _, ok := otherFiles[uri]; !ok {
+			otherFiles[uri] = make(map[string]bool)
+		}
+		otherFiles[uri][obj.Name()] = true
+	}
+
+	for cur := range pgf.Cursor.Preorder((*ast.Ident)(nil)) {
+		id := cur.Node().(*ast.Ident)
+		addObj(info.Uses[id])
+		addObj(info.Defs[id])
+	}
+
+	var result strings.Builder
+	fmt.Fprintf(&result, "File `%s` is in package %q.\n", params.Arguments.File, pkg.Metadata().PkgPath)
+	fmt.Fprintf(&result, "Below is a summary of the APIs it uses from other files.\n")
+	fmt.Fprintf(&result, "To read the full API of any package, use go_package_api.\n")
+	for uri, decls := range otherFiles {
+		pkgPath := "UNKNOWN"
+		md, err := snapshot.NarrowestMetadataForFile(ctx, uri)
+		if err != nil {
+			if ctx.Err() != nil {
+				return nil, ctx.Err()
+			}
+		} else {
+			pkgPath = string(md.PkgPath)
+		}
+		fmt.Fprintf(&result, "Referenced declarations from %s (package %q):\n", uri.Path(), pkgPath)
+		result.WriteString("```go\n")
+		if err := writeFileSummary(ctx, snapshot, uri, &result, false, decls); err != nil {
+			return nil, err
+		}
+		result.WriteString("```\n\n")
+	}
+
+	return textResult(result.String()), nil
+}

gopls/internal/mcp/instructions.md

@@ -1,51 +1,46 @@
 # The gopls MCP server
 
-These instructions describe how to efficiently work in the Go programming
-language using the gopls MCP server. They are intended to be provided as context
-for an interactive session using the gopls MCP tool: you can load this file
-directly into a session where the gopls MCP server is connected.
+These instructions describe how to efficiently work in the Go programming language using the gopls MCP server. You can load this file directly into a session where the gopls MCP server is connected.
 
 ## Detecting a Go workspace
 
-Use the `go_workspace` tool to learn about the Go workspace. These instructions
-apply whenever that tool indicates that the user is in a Go workspace.
-
-## Go Programming Guidelines
-
-These guidelines MUST be followed whenever working in a Go workspace. There
-are two workflows described below: the 'Read Workflow' must be followed when
-the user asks a question about a Go workspace. The 'Edit Workflow' must be
-followed when the user edits a Go workspace.
-
-You may re-do parts of each workflow as necessary to recover from errors.
-However, you cannot skip any steps.
-
-### Read Workflow
-
-1. **Search the workspace:** When the user asks about a symbol, use
-   `go_search` to search for the symbol in question. If you find no matches,
-   search for a substring of the user's referenced symbol. If `go_search`
-   fails, you may fall back to regular textual search.
-2. **Read files:** Read the relevant file(s). Use the `go_file_metadata` tool
-   to get package information for the file.
-3. **Understand packages:** If the user is asking about the use of one or more Go
-   package, use the `go_package_outline` command to summarize their API.
-
-### Editing Workflow
-
-1. **Read first:** Before making any edits, follow the Read Workflow to
-   understand the user's request.
-2. **Find references:** Before modifying the definition of any symbol, use the
-   `go_symbol_references` tool to find references to that identifier. These
-   references may need to be updated after editing the symbol. Read files
-   containing references to evaluate if any further edits are required.
-3. **Make edits:** Make the primary edit, as well as any edits to references.
-4. **Run diagnostics:** Every time, after making edits to one or more files,
-   you must call the `go_diagnostics` tool, passing the paths to the edited
-   files, to verify that the build is not broken. Apply edits to fix any
-   relevant diagnostics, and re-run the `go_diagnostics` tool to verify the
-   fixes. It is OK to ignore 'hint' or 'info' diagnostics if they are not
-   relevant.
-5. **Run tests** run `go test` for any packages that were edited. Invoke `go
-   test` with the package paths returned from `go_file_metadata`. Fix any test
-   failures.
+At the start of every session, you MUST use the `go_workspace` tool to learn about the Go workspace. The rest of these instructions apply whenever that tool indicates that the user is in a Go workspace.
+
+## Go programming workflows
+
+These guidelines MUST be followed whenever working in a Go workspace. There are two workflows described below: the 'Read Workflow' must be followed when the user asks a question about a Go workspace. The 'Edit Workflow' must be followed when the user edits a Go workspace.
+
+You may re-do parts of each workflow as necessary to recover from errors. However, you must not skip any steps.
+
+### Read workflow
+
+The goal of the read workflow is to understand the codebase.
+
+1. **Understand the workspace layout**: Start by using `go_workspace` to understand the overall structure of the workspace, such as whether it's a module, a workspace, or a GOPATH project.
+
+2. **Find relevant symbols**: If you're looking for a specific type, function, or variable, use `go_search`. This is a fuzzy search that will help you locate symbols even if you don't know the exact name or location.
+   EXAMPLE: search for the 'Server' type: `go_search({"query":"server"})`
+
+3. **Understand a file and its intra-package dependencies**: When you have a file path and want to understand its contents and how it connects to other files *in the same package*, use `go_file_context`. This tool will show you a summary of the declarations from other files in the same package that are used by the current file. `go_file_context` MUST be used immediately after reading any Go file for the first time, and MAY be re-used if dependencies have changed.
+   EXAMPLE: to understand `server.go`'s dependencies on other files in its package: `go_file_context({"file":"/path/to/server.go"})`
+
+4. **Understand a package's public API**: When you need to understand what a package provides to external code (i.e., its public API), use `go_package_api`. This is especially useful for understanding third-party dependencies or other packages in the same monorepo.
+   EXAMPLE: to see the API of the `storage` package: `go_package_api({"packagePaths":["example.com/internal/storage"]})`
+
+### Editing workflow
+
+The editing workflow is iterative. You should cycle through these steps until the task is complete.
+
+1. **Read first**: Before making any edits, follow the Read Workflow to understand the user's request and the relevant code.
+
+2. **Find references**: Before modifying the definition of any exported symbol, use the `go_symbol_references` tool to find all references to that identifier. This is critical for understanding the impact of your change. Read the files containing references to evaluate if any further edits are required.
+   EXAMPLE: `go_symbol_references({"file":"/path/to/server.go","symbol":"Server.Run"})`
+
+3. **Make edits**: Make the primary edit, as well as any edits to references you identified in the previous step.
+
+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.
+   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.
+
+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.