diff --git a/cmd/fiximports/main.go b/cmd/fiximports/main.go
new file mode 100644
index 0000000000..5a912e89c0
--- /dev/null
+++ b/cmd/fiximports/main.go
@@ -0,0 +1,387 @@
+// Copyright 2015 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.
+
+// The fiximports command fixes import declarations to use the canonical
+// import path for packages that have an "import comment" as defined by
+// https://golang.org/s/go14customimport.
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// flags
+var (
+	dryrun     = flag.Bool("n", false, "dry run: show changes, but don't apply them")
+	badDomains = flag.String("baddomains", "code.google.com",
+		"a comma-separated list of domains from which packages should not be imported")
+)
+
+// seams for testing
+var (
+	stderr    io.Writer = os.Stderr
+	writeFile           = ioutil.WriteFile
+)
+
+const usage = `fiximports: rewrite import paths to use canonical package names.
+
+Usage: fiximports [-n] package...
+
+The package... arguments specify a list of packages
+in the style of the go tool; see "go help packages".
+Hint: use "all" or "..." to match the entire workspace.
+
+Flags:
+  -n:	       dry run: show changes, but don't apply them
+  -baddomains  a comma-separated list of domains from which packages
+               should not be imported
+`
+
+func main() {
+	flag.Parse()
+
+	if len(flag.Args()) == 0 {
+		fmt.Fprintf(stderr, usage)
+		os.Exit(1)
+	}
+	if !fiximports(flag.Args()...) {
+		os.Exit(1)
+	}
+}
+
+// fiximports fixes imports in the specified packages.
+// Invariant: a false result implies an error was already printed.
+func fiximports(packages ...string) bool {
+	// importedBy is the transpose of the package import graph.
+	importedBy := make(map[string]map[*build.Package]bool)
+
+	// addEdge adds an edge to the import graph.
+	addEdge := func(from *build.Package, to string) {
+		if to == "C" || to == "unsafe" {
+			return // fake
+		}
+		pkgs := importedBy[to]
+		if pkgs == nil {
+			pkgs = make(map[*build.Package]bool)
+			importedBy[to] = pkgs
+		}
+		pkgs[from] = true
+	}
+
+	// List metadata for all packages in the workspace.
+	pkgs, err := list("...")
+	if err != nil {
+		fmt.Fprintf(stderr, "importfix: %v\n", err)
+		return false
+	}
+
+	// noncanonical maps each non-canonical package path to
+	// its canonical name.
+	// A present nil value indicates that the canonical package
+	// is unknown: hosted on a bad domain with no redirect.
+	noncanonical := make(map[string]*build.Package)
+	domains := strings.Split(*badDomains, ",")
+
+	// Find non-canonical packages and populate importedBy graph.
+	for _, p := range pkgs {
+		if p.Error != nil {
+			msg := p.Error.Err
+			if strings.Contains(msg, "code in directory") &&
+				strings.Contains(msg, "expects import") {
+				// don't show the very errors we're trying to fix
+			} else {
+				fmt.Fprintln(stderr, msg)
+			}
+		}
+
+		for _, imp := range p.Imports {
+			addEdge(&p.Package, imp)
+		}
+		for _, imp := range p.TestImports {
+			addEdge(&p.Package, imp)
+		}
+		for _, imp := range p.XTestImports {
+			addEdge(&p.Package, imp)
+		}
+
+		if p.ImportComment != "" {
+			if p.ImportComment != p.ImportPath {
+				noncanonical[p.ImportPath] = &p.Package
+			}
+		} else {
+			for _, domain := range domains {
+				slash := strings.Index(p.ImportPath, "/")
+				if slash < 0 {
+					continue // no slash: standard package
+				}
+				if p.ImportPath[:slash] == domain {
+					// Package comes from bad domain and has no import comment.
+					// Report an error each time this package is imported.
+					noncanonical[p.ImportPath] = nil
+
+					// TODO(adonovan): should we make an HTTP request to
+					// see if there's an HTTP redirect, a "go-import" meta tag,
+					// or an import comment in the the latest revision?
+					// It would duplicate a lot of logic from "go get".
+				}
+				break
+			}
+		}
+	}
+
+	// Find all clients (direct importers) of noncanonical packages.
+	// These are the packages that need fixing up.
+	clients := make(map[*build.Package]bool)
+	for path := range noncanonical {
+		for client := range importedBy[path] {
+			clients[client] = true
+		}
+	}
+
+	// Restrict rewrites to the set of packages specified by the user.
+	if len(packages) == 1 && (packages[0] == "all" || packages[0] == "...") {
+		// no restriction
+	} else {
+		pkgs, err := list(packages...)
+		if err != nil {
+			fmt.Fprintf(stderr, "importfix: %v\n", err)
+			return false
+		}
+		seen := make(map[string]bool)
+		for _, p := range pkgs {
+			seen[p.ImportPath] = true
+		}
+		for client := range clients {
+			if !seen[client.ImportPath] {
+				delete(clients, client)
+			}
+		}
+	}
+
+	// Rewrite selected client packages.
+	ok := true
+	for client := range clients {
+		if !rewritePackage(client, noncanonical) {
+			ok = false
+
+			// There were errors.
+			// Show direct and indirect imports of client.
+			seen := make(map[string]bool)
+			var direct, indirect []string
+			for p := range importedBy[client.ImportPath] {
+				direct = append(direct, p.ImportPath)
+				seen[p.ImportPath] = true
+			}
+
+			var visit func(path string)
+			visit = func(path string) {
+				for q := range importedBy[path] {
+					qpath := q.ImportPath
+					if !seen[qpath] {
+						seen[qpath] = true
+						indirect = append(indirect, qpath)
+						visit(qpath)
+					}
+				}
+			}
+
+			if direct != nil {
+				fmt.Fprintf(stderr, "\timported directly by:\n")
+				sort.Strings(direct)
+				for _, path := range direct {
+					fmt.Fprintf(stderr, "\t\t%s\n", path)
+					visit(path)
+				}
+
+				if indirect != nil {
+					fmt.Fprintf(stderr, "\timported indirectly by:\n")
+					sort.Strings(indirect)
+					for _, path := range indirect {
+						fmt.Fprintf(stderr, "\t\t%s\n", path)
+					}
+				}
+			}
+		}
+	}
+
+	return ok
+}
+
+// Invariant: false result => error already printed.
+func rewritePackage(client *build.Package, noncanonical map[string]*build.Package) bool {
+	ok := true
+
+	used := make(map[string]bool)
+	var filenames []string
+	filenames = append(filenames, client.GoFiles...)
+	filenames = append(filenames, client.TestGoFiles...)
+	filenames = append(filenames, client.XTestGoFiles...)
+	var first bool
+	for _, filename := range filenames {
+		if !first {
+			first = true
+			fmt.Fprintf(stderr, "%s\n", client.ImportPath)
+		}
+		err := rewriteFile(filepath.Join(client.Dir, filename), noncanonical, used)
+		if err != nil {
+			fmt.Fprintf(stderr, "\tERROR: %v\n", err)
+			ok = false
+		}
+	}
+
+	// Show which imports were renamed in this package.
+	var keys []string
+	for key := range used {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
+		if p := noncanonical[key]; p != nil {
+			fmt.Fprintf(stderr, "\tfixed: %s -> %s\n", key, p.ImportComment)
+		} else {
+			fmt.Fprintf(stderr, "\tERROR: %s has no import comment\n", key)
+			ok = false
+		}
+	}
+
+	return ok
+}
+
+// rewrite reads, modifies, and writes filename, replacing all imports
+// of packages P in noncanonical by noncanonical[P].
+// It records in used which noncanonical packages were imported.
+// used[P]=="" indicates that P was imported but its canonical path is unknown.
+func rewriteFile(filename string, noncanonical map[string]*build.Package, used map[string]bool) error {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
+	if err != nil {
+		return err
+	}
+	var changed bool
+	for _, imp := range f.Imports {
+		impPath, err := strconv.Unquote(imp.Path.Value)
+		if err != nil {
+			log.Printf("%s: bad import spec %q: %v",
+				fset.Position(imp.Pos()), imp.Path.Value, err)
+			continue
+		}
+		p, ok := noncanonical[impPath]
+		if !ok {
+			continue // import path is canonical
+		}
+
+		used[impPath] = true
+
+		if p == nil {
+			// The canonical path is unknown.
+			// Show the offending import.
+			// TODO(adonovan): should we show the actual source text?
+			fmt.Fprintf(stderr, "\t%s:%d: import %q\n",
+				shortPath(filename),
+				fset.Position(imp.Pos()).Line, impPath)
+			continue
+		}
+
+		changed = true
+
+		imp.Path.Value = strconv.Quote(p.ImportComment)
+
+		// Add a renaming import if necessary.
+		//
+		// This is a guess at best.  We can't see whether a 'go
+		// get' of the canonical import path would have the same
+		// name or not.  Assume it's the last segment.
+		//
+		// TODO(adonovan): should we make an HTTP request?
+		newBase := path.Base(p.ImportComment)
+		if imp.Name == nil && newBase != p.Name {
+			imp.Name = &ast.Ident{Name: p.Name}
+		}
+	}
+
+	if changed && !*dryrun {
+		var buf bytes.Buffer
+		if err := format.Node(&buf, fset, f); err != nil {
+			return fmt.Errorf("%s: couldn't format file: %v", filename, err)
+		}
+		return writeFile(filename, buf.Bytes(), 0644)
+	}
+
+	return nil
+}
+
+// listPackage is a copy of cmd/go/list.Package.
+// It has more fields than build.Package and we need some of them.
+type listPackage struct {
+	build.Package
+	Error *packageError // error loading package
+}
+
+// A packageError describes an error loading information about a package.
+type packageError struct {
+	ImportStack []string // shortest path from package named on command line to this one
+	Pos         string   // position of error
+	Err         string   // the error itself
+}
+
+// list runs 'go list' with the specified arguments and returns the
+// metadata for matching packages.
+func list(args ...string) ([]*listPackage, error) {
+	cmd := exec.Command("go", append([]string{"list", "-e", "-json"}, args...)...)
+	cmd.Stdout = new(bytes.Buffer)
+	cmd.Stderr = stderr
+	if err := cmd.Run(); err != nil {
+		return nil, err
+	}
+
+	dec := json.NewDecoder(cmd.Stdout.(io.Reader))
+	var pkgs []*listPackage
+	for {
+		var p listPackage
+		if err := dec.Decode(&p); err == io.EOF {
+			break
+		} else if err != nil {
+			return nil, err
+		}
+		pkgs = append(pkgs, &p)
+	}
+	return pkgs, nil
+}
+
+var cwd string
+
+func init() {
+	var err error
+	cwd, err = os.Getwd()
+	if err != nil {
+		log.Fatalf("os.Getwd: %v", err)
+	}
+}
+
+// shortPath returns an absolute or relative name for path, whatever is shorter.
+// Plundered from $GOROOT/src/cmd/go/build.go.
+func shortPath(path string) string {
+	if rel, err := filepath.Rel(cwd, path); err == nil && len(rel) < len(path) {
+		return rel
+	}
+	return path
+}
diff --git a/cmd/fiximports/main_test.go b/cmd/fiximports/main_test.go
new file mode 100644
index 0000000000..80adbc437f
--- /dev/null
+++ b/cmd/fiximports/main_test.go
@@ -0,0 +1,157 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bytes"
+	"os"
+	"strings"
+	"testing"
+)
+
+// TODO(adonovan):
+// - test introduction of renaming imports.
+// - test induced failures of rewriteFile.
+
+// Guide to the test packages:
+//
+// new.com/one		-- canonical name for old.com/one
+// old.com/one		-- non-canonical; has import comment "new.com/one"
+// old.com/bad		-- has a parse error
+// fruit.io/orange	\
+// fruit.io/banana	 } orange -> pear -> banana -> titanic.biz/bar
+// fruit.io/pear	/
+// titanic.biz/bar	-- domain is sinking; package has jumped ship to new.com/bar
+// titanic.biz/foo	-- domain is sinking but package has no import comment yet
+
+func TestFixImports(t *testing.T) {
+	gopath := cwd + "/testdata"
+	if err := os.Setenv("GOPATH", gopath); err != nil {
+		t.Fatalf("os.Setenv: %v", err)
+	}
+	defer func() {
+		stderr = os.Stderr
+		*badDomains = "code.google.com"
+	}()
+
+	for i, test := range []struct {
+		packages    []string // packages to rewrite, "go list" syntax
+		badDomains  string   // -baddomains flag
+		wantOK      bool
+		wantStderr  string
+		wantRewrite map[string]string
+	}{
+		// #0. No errors.
+		{
+			packages:   []string{"all"},
+			badDomains: "code.google.com",
+			wantOK:     true,
+			wantStderr: `
+testdata/src/old.com/bad/bad.go:2:43: expected 'package', found 'EOF'
+fruit.io/banana
+	fixed: old.com/one -> new.com/one
+	fixed: titanic.biz/bar -> new.com/bar
+`,
+			wantRewrite: map[string]string{
+				"$GOPATH/src/fruit.io/banana/banana.go": `package banana
+
+import (
+	_ "new.com/bar"
+	_ "new.com/one"
+	_ "titanic.biz/foo"
+)`,
+			},
+		},
+		// #1. No packages needed rewriting.
+		{
+			packages:   []string{"titanic.biz/...", "old.com/...", "new.com/..."},
+			badDomains: "code.google.com",
+			wantOK:     true,
+			wantStderr: `
+testdata/src/old.com/bad/bad.go:2:43: expected 'package', found 'EOF'
+`,
+		},
+		// #2. Some packages without import comments matched bad domains.
+		{
+			packages:   []string{"all"},
+			badDomains: "titanic.biz",
+			wantOK:     false,
+			wantStderr: `
+testdata/src/old.com/bad/bad.go:2:43: expected 'package', found 'EOF'
+fruit.io/banana
+	testdata/src/fruit.io/banana/banana.go:6: import "titanic.biz/foo"
+	fixed: old.com/one -> new.com/one
+	fixed: titanic.biz/bar -> new.com/bar
+	ERROR: titanic.biz/foo has no import comment
+	imported directly by:
+		fruit.io/pear
+	imported indirectly by:
+		fruit.io/orange
+`,
+			wantRewrite: map[string]string{
+				"$GOPATH/src/fruit.io/banana/banana.go": `package banana
+
+import (
+	_ "new.com/bar"
+	_ "new.com/one"
+	_ "titanic.biz/foo"
+)`,
+			},
+		},
+	} {
+		*badDomains = test.badDomains
+
+		stderr = new(bytes.Buffer)
+		gotRewrite := make(map[string]string)
+		writeFile = func(filename string, content []byte, mode os.FileMode) error {
+			filename = strings.Replace(filename, gopath, "$GOPATH", 1)
+			gotRewrite[filename] = string(bytes.TrimSpace(content))
+			return nil
+		}
+
+		// Check status code.
+		if fiximports(test.packages...) != test.wantOK {
+			t.Errorf("#%d. fiximports() = %t", i, !test.wantOK)
+		}
+
+		// Compare stderr output.
+		if stderr.(*bytes.Buffer).String() != test.wantStderr {
+			t.Errorf("#%d. stderr: got <<%s>>, want <<%s>>",
+				i, stderr, test.wantStderr)
+		}
+
+		// Compare rewrites.
+		for k, v := range gotRewrite {
+			if test.wantRewrite[k] != v {
+				t.Errorf("#%d. rewrite[%s] = <<%s>>, want <<%s>>",
+					i, k, v, test.wantRewrite[k])
+			}
+			delete(test.wantRewrite, k)
+		}
+		for k, v := range test.wantRewrite {
+			t.Errorf("#%d. rewrite[%s] missing, want <<%s>>", i, k, v)
+		}
+	}
+}
+
+// TestDryRun tests that the -n flag suppresses calls to writeFile.
+func TestDryRun(t *testing.T) {
+	gopath := cwd + "/testdata"
+	if err := os.Setenv("GOPATH", gopath); err != nil {
+		t.Fatalf("os.Setenv: %v", err)
+	}
+
+	*dryrun = true
+	defer func() { *dryrun = false }() // restore
+	stderr = new(bytes.Buffer)
+	writeFile = func(filename string, content []byte, mode os.FileMode) error {
+		t.Fatalf("writeFile(%s) called in dryrun mode", filename)
+		return nil
+	}
+
+	if !fiximports("all") {
+		t.Fatalf("fiximports failed: %s", stderr)
+	}
+}
diff --git a/cmd/fiximports/testdata/src/fruit.io/banana/banana.go b/cmd/fiximports/testdata/src/fruit.io/banana/banana.go
new file mode 100644
index 0000000000..04e0242eda
--- /dev/null
+++ b/cmd/fiximports/testdata/src/fruit.io/banana/banana.go
@@ -0,0 +1,7 @@
+package banana
+
+import (
+	_ "old.com/one"
+	_ "titanic.biz/bar"
+	_ "titanic.biz/foo"
+)
diff --git a/cmd/fiximports/testdata/src/fruit.io/orange/orange.go b/cmd/fiximports/testdata/src/fruit.io/orange/orange.go
new file mode 100644
index 0000000000..ae65daa6b3
--- /dev/null
+++ b/cmd/fiximports/testdata/src/fruit.io/orange/orange.go
@@ -0,0 +1,3 @@
+package orange
+
+import _ "fruit.io/pear"
diff --git a/cmd/fiximports/testdata/src/fruit.io/pear/pear.go b/cmd/fiximports/testdata/src/fruit.io/pear/pear.go
new file mode 100644
index 0000000000..de92df05d4
--- /dev/null
+++ b/cmd/fiximports/testdata/src/fruit.io/pear/pear.go
@@ -0,0 +1,3 @@
+package pear
+
+import _ "fruit.io/banana"
diff --git a/cmd/fiximports/testdata/src/new.com/one/one.go b/cmd/fiximports/testdata/src/new.com/one/one.go
new file mode 100644
index 0000000000..a8c5e83e93
--- /dev/null
+++ b/cmd/fiximports/testdata/src/new.com/one/one.go
@@ -0,0 +1 @@
+package one // import "new.com/one"
diff --git a/cmd/fiximports/testdata/src/old.com/bad/bad.go b/cmd/fiximports/testdata/src/old.com/bad/bad.go
new file mode 100644
index 0000000000..a1a3d1ad5f
--- /dev/null
+++ b/cmd/fiximports/testdata/src/old.com/bad/bad.go
@@ -0,0 +1,2 @@
+// This ill-formed Go source file is here to ensure the tool is robust
+// against bad packages in the workspace.
diff --git a/cmd/fiximports/testdata/src/old.com/one/one.go b/cmd/fiximports/testdata/src/old.com/one/one.go
new file mode 100644
index 0000000000..a8c5e83e93
--- /dev/null
+++ b/cmd/fiximports/testdata/src/old.com/one/one.go
@@ -0,0 +1 @@
+package one // import "new.com/one"
diff --git a/cmd/fiximports/testdata/src/titanic.biz/bar/bar.go b/cmd/fiximports/testdata/src/titanic.biz/bar/bar.go
new file mode 100644
index 0000000000..cc720bc76a
--- /dev/null
+++ b/cmd/fiximports/testdata/src/titanic.biz/bar/bar.go
@@ -0,0 +1,2 @@
+// This package is moving to new.com too.
+package bar // import "new.com/bar"
diff --git a/cmd/fiximports/testdata/src/titanic.biz/foo/foo.go b/cmd/fiximports/testdata/src/titanic.biz/foo/foo.go
new file mode 100644
index 0000000000..145c31b8d2
--- /dev/null
+++ b/cmd/fiximports/testdata/src/titanic.biz/foo/foo.go
@@ -0,0 +1,2 @@
+// This package hasn't jumped ship yet.
+package foo