diff --git a/tools/fix-tests/fix-tests.go b/tools/fix-tests/fix-tests.go
new file mode 100644
index 0000000..6b8b860
--- /dev/null
+++ b/tools/fix-tests/fix-tests.go
@@ -0,0 +1,277 @@
+// Copyright 2021 The Tint Authors.
+//
+// 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.
+
+// fix-tests is a tool to update tests with new expected output.
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+func main() {
+	if err := run(); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+}
+
+func showUsage() {
+	fmt.Println(`
+fix-tests is a tool to update tests with new expected output.
+
+Usage:
+  fix-tests <executable>
+
+  executable         - the path to the test executable to run.`)
+	os.Exit(1)
+}
+
+func run() error {
+	flag.Parse()
+	args := flag.Args()
+	if len(args) < 1 {
+		showUsage()
+	}
+
+	exe := args[0]          // The path to the test executable
+	wd := filepath.Dir(exe) // The directory holding the test exe
+
+	// Create a temporary directory to hold the 'test-results.json' file
+	tmpDir, err := ioutil.TempDir("", "fix-tests")
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(tmpDir, 0666); err != nil {
+		return fmt.Errorf("Failed to create temporary directory: %w", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Full path to the 'test-results.json' in the temporary directory
+	testResultsPath := filepath.Join(tmpDir, "test-results.json")
+
+	// Run the tests
+	switch err := exec.Command(exe, "--gtest_output=json:"+testResultsPath).Run().(type) {
+	default:
+		return err
+	case nil:
+		fmt.Println("All tests passed")
+	case *exec.ExitError:
+	}
+
+	// Read the 'test-results.json' file
+	testResultsFile, err := os.Open(testResultsPath)
+	if err != nil {
+		return err
+	}
+
+	var testResults Results
+	if err := json.NewDecoder(testResultsFile).Decode(&testResults); err != nil {
+		return err
+	}
+
+	// For each failing test...
+	var errs []error
+	numFixes := 0
+	for _, group := range testResults.Groups {
+		for _, suite := range group.Testsuites {
+			for _, failure := range suite.Failures {
+				// .. attempt to fix the problem
+				test := group.Name + "." + suite.Name
+				if err := processFailure(test, wd, failure.Failure); err != nil {
+					errs = append(errs, fmt.Errorf("%v: %w", test, err))
+				} else {
+					numFixes++
+				}
+			}
+		}
+	}
+
+	if numFixes > 0 {
+		fmt.Printf("%v tests fixed\n", numFixes)
+	}
+	if n := len(errs); n > 0 {
+		fmt.Printf("%v tests could not be fixed:\n", n)
+		for _, err := range errs {
+			fmt.Println(err)
+		}
+	}
+	return nil
+}
+
+var (
+	// Regular expression to match a test declaration
+	reTests = regexp.MustCompile(`TEST(?:_[FP])?\((\w+),[ \n]*(\w+)\)`)
+	// Regular expression to match a EXPECT_EQ failure for strings
+	reExpectEq = regexp.MustCompile(`^([./\\a-z_-]*):(\d+).*\nExpected equality of these values:\n(?:.|\n)*?(?:Which is: |  )"((?:.|\n)*?)[^\\]"\n(?:.|\n)*?(?:Which is: |  )"((?:.|\n)*?)[^\\]"`)
+)
+
+func processFailure(test, wd, failure string) error {
+	// Start by un-escaping newlines in the failure message
+	failure = strings.ReplaceAll(failure, "\\n", "\n")
+
+	// Look for a EXPECT_EQ failure pattern
+	var file, a, b string
+	if parts := reExpectEq.FindStringSubmatch(failure); len(parts) == 5 {
+		file, a, b = parts[1], parts[3], parts[4]
+	} else {
+		return fmt.Errorf("Cannot fix this type of failure")
+	}
+
+	// Now un-escape any quotes (the regex is sensitive to these)
+	a = strings.ReplaceAll(a, `\"`, `"`)
+	b = strings.ReplaceAll(b, `\"`, `"`)
+
+	// Get the path to the source file containing the test failure
+	sourcePath := filepath.Join(wd, file)
+
+	// Parse the source file, split into tests
+	sourceFile, err := parseSourceFile(sourcePath)
+	if err != nil {
+		return fmt.Errorf("Couldn't parse tests from file '%v': %w", file, err)
+	}
+
+	// Find the test
+	testIdx, ok := sourceFile.tests[test]
+	if !ok {
+		return fmt.Errorf("Test '%v' not found in '%v'", test, file)
+	}
+
+	// Grab the source for the particular test
+	testSource := sourceFile.parts[testIdx]
+
+	// We don't know if a or b is the expected, so just try flipping the string
+	// to the other form.
+	switch {
+	case strings.Contains(testSource, a):
+		testSource = strings.Replace(testSource, a, b, -1)
+	case strings.Contains(testSource, b):
+		testSource = strings.Replace(testSource, b, a, -1)
+	default:
+		// Try escaping for R"(...)" strings
+		a = strings.ReplaceAll(a, "\n", `\n`)
+		b = strings.ReplaceAll(b, "\n", `\n`)
+		a = strings.ReplaceAll(a, "\"", `\"`)
+		b = strings.ReplaceAll(b, "\"", `\"`)
+		switch {
+		case strings.Contains(testSource, a):
+			testSource = strings.Replace(testSource, a, b, -1)
+		case strings.Contains(testSource, b):
+			testSource = strings.Replace(testSource, b, a, -1)
+		default:
+			return fmt.Errorf("Could not fix test '%v' in '%v'", test, file)
+		}
+	}
+
+	// Replace the part of the source file
+	sourceFile.parts[testIdx] = testSource
+
+	// Write out the source file
+	return writeSourceFile(sourcePath, sourceFile)
+}
+
+// parseSourceFile() reads the file at path, splitting the content into chunks
+// for each TEST.
+func parseSourceFile(path string) (sourceFile, error) {
+	fileBytes, err := ioutil.ReadFile(path)
+	if err != nil {
+		return sourceFile{}, err
+	}
+	fileContent := string(fileBytes)
+
+	out := sourceFile{
+		tests: map[string]int{},
+	}
+
+	pos := 0
+	for _, span := range reTests.FindAllStringIndex(fileContent, -1) {
+		out.parts = append(out.parts, fileContent[pos:span[0]])
+		pos = span[0]
+
+		match := reTests.FindStringSubmatch(fileContent[span[0]:span[1]])
+		group := match[1]
+		suite := match[2]
+		out.tests[group+"."+suite] = len(out.parts)
+	}
+	out.parts = append(out.parts, fileContent[pos:])
+
+	return out, nil
+}
+
+// writeSourceFile() joins the chunks of the file, and writes the content out to
+// path.
+func writeSourceFile(path string, file sourceFile) error {
+	body := strings.Join(file.parts, "")
+	return ioutil.WriteFile(path, []byte(body), 0666)
+}
+
+type sourceFile struct {
+	parts []string
+	tests map[string]int // "X.Y" -> part index
+}
+
+// Results is the root JSON structure of the JSON --gtest_output file .
+type Results struct {
+	Tests     int              `json:"tests"`
+	Failures  int              `json:"failures"`
+	Disabled  int              `json:"disabled"`
+	Errors    int              `json:"errors"`
+	Timestamp string           `json:"timestamp"`
+	Time      string           `json:"time"`
+	Name      string           `json:"name"`
+	Groups    []TestsuiteGroup `json:"testsuites"`
+}
+
+// TestsuiteGroup is a group of test suites in the JSON --gtest_output file .
+type TestsuiteGroup struct {
+	Name       string      `json:"name"`
+	Tests      int         `json:"tests"`
+	Failures   int         `json:"failures"`
+	Disabled   int         `json:"disabled"`
+	Errors     int         `json:"errors"`
+	Timestamp  string      `json:"timestamp"`
+	Time       string      `json:"time"`
+	Testsuites []Testsuite `json:"testsuite"`
+}
+
+// Testsuite is a suite of tests in the JSON --gtest_output file.
+type Testsuite struct {
+	Name       string    `json:"name"`
+	ValueParam string    `json:"value_param,omitempty"`
+	Status     Status    `json:"status"`
+	Result     Result    `json:"result"`
+	Timestamp  string    `json:"timestamp"`
+	Time       string    `json:"time"`
+	Classname  string    `json:"classname"`
+	Failures   []Failure `json:"failures,omitempty"`
+}
+
+// Failure is a reported test failure in the JSON --gtest_output file.
+type Failure struct {
+	Failure string `json:"failure"`
+	Type    string `json:"type"`
+}
+
+// Status is a status code in the JSON --gtest_output file.
+type Status string
+
+// Result is a result code in the JSON --gtest_output file.
+type Result string