diff --git a/.github/workflows/go_test.yml b/.github/workflows/go_test.yml
index d55a45d..55d2aa3 100644
--- a/.github/workflows/go_test.yml
+++ b/.github/workflows/go_test.yml
@@ -29,6 +29,8 @@
         run: |
           go install golang.org/x/tools/gopls@latest
           go install gotest.tools/gotestsum@latest
+          go install mvdan.cc/gofumpt@latest
+          go install golang.org/x/tools/cmd/goimports@latest
 
       - name: Go generate
         run: |
diff --git a/claudetool/codereview_test.go b/claudetool/codereview_test.go
new file mode 100644
index 0000000..71934a2
--- /dev/null
+++ b/claudetool/codereview_test.go
@@ -0,0 +1,290 @@
+package claudetool
+
+import (
+	"bytes"
+	"cmp"
+	"context"
+	"flag"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+
+	"golang.org/x/tools/txtar"
+)
+
+// updateTests is set to true when the -update flag is used.
+// This will update the expected test results instead of failing tests.
+var updateTests = flag.Bool("update", false, "update expected test results instead of failing tests")
+
+// TestCodereviewDifferential runs all the end-to-end tests for the codereview and differential packages.
+// Each test is defined in a .txtar file in the testdata directory.
+func TestCodereviewDifferential(t *testing.T) {
+	entries, err := os.ReadDir("testdata")
+	if err != nil {
+		t.Fatalf("failed to read testdata directory: %v", err)
+	}
+	for _, entry := range entries {
+		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".txtar") {
+			continue
+		}
+		testPath := filepath.Join("testdata", entry.Name())
+		testName := strings.TrimSuffix(entry.Name(), ".txtar")
+		t.Run(testName, func(t *testing.T) {
+			t.Parallel()
+			runE2ETest(t, testPath, *updateTests)
+		})
+	}
+}
+
+// runE2ETest executes a single end-to-end test from a .txtar file.
+func runE2ETest(t *testing.T, testPath string, update bool) {
+	orig, err := os.ReadFile(testPath)
+	if err != nil {
+		t.Fatalf("failed to read test file %s: %v", testPath, err)
+	}
+	archive, err := txtar.ParseFile(testPath)
+	if err != nil {
+		t.Fatalf("failed to parse txtar file %s: %v", testPath, err)
+	}
+
+	tmpDir := t.TempDir()
+	// resolve temp dir path so that we can canonicalize/normalize it later
+	tmpDir = resolveRealPath(tmpDir)
+
+	if err := initGoModule(tmpDir); err != nil {
+		t.Fatalf("failed to initialize Go module: %v", err)
+	}
+	if err := initGitRepo(tmpDir); err != nil {
+		t.Fatalf("failed to initialize git repository: %v", err)
+	}
+	if err := processTestFiles(t, archive, tmpDir, update); err != nil {
+		t.Fatalf("error processing test files: %v", err)
+	}
+
+	// If we're updating, write back the modified archive to the file
+	if update {
+		updatedContent := txtar.Format(archive)
+		// only write back changes, avoids git status churn
+		if !bytes.Equal(orig, updatedContent) {
+			if err := os.WriteFile(testPath, updatedContent, 0o644); err != nil {
+				t.Errorf("Failed to update test file %s: %v", testPath, err)
+			}
+		}
+	}
+}
+
+func gitCommitEnv() []string {
+	return append(os.Environ(),
+		"GIT_AUTHOR_NAME=Test Author",
+		"GIT_AUTHOR_EMAIL=test@example.com",
+		"GIT_COMMITTER_NAME=Test Committer",
+		"GIT_COMMITTER_EMAIL=test@example.com",
+		"GIT_AUTHOR_DATE=2025-01-01T00:00:00Z",
+		"GIT_COMMITTER_DATE=2025-01-01T00:00:00Z",
+	)
+}
+
+// initGitRepo initializes a new git repository in the specified directory.
+func initGitRepo(dir string) error {
+	cmd := exec.Command("git", "init")
+	cmd.Dir = dir
+	err := cmd.Run()
+	if err != nil {
+		return fmt.Errorf("error initializing git repository: %w", err)
+	}
+	// create a single commit out of everything there now
+	cmd = exec.Command("git", "add", ".")
+	cmd.Dir = dir
+	err = cmd.Run()
+	if err != nil {
+		return fmt.Errorf("error staging files: %w", err)
+	}
+	cmd = exec.Command("git", "commit", "-m", "create repo")
+	cmd.Dir = dir
+	cmd.Env = gitCommitEnv()
+	err = cmd.Run()
+	if err != nil {
+		return fmt.Errorf("error making initial commit: %w", err)
+	}
+	return nil
+}
+
+// processTestFiles processes the files in the txtar archive in sequence.
+func processTestFiles(t *testing.T, archive *txtar.Archive, dir string, update bool) error {
+	var initialCommit string
+	filesForNextCommit := make(map[string]bool)
+
+	for i, file := range archive.Files {
+		switch file.Name {
+		case ".commit":
+			commit, err := makeGitCommit(dir, string(file.Data), filesForNextCommit)
+			if err != nil {
+				return fmt.Errorf("error making git commit: %w", err)
+			}
+			clear(filesForNextCommit)
+			initialCommit = cmp.Or(initialCommit, commit)
+			// fmt.Println("initial commit:", initialCommit)
+			// cmd := exec.Command("git", "log")
+			// cmd.Dir = dir
+			// cmd.Stdout = os.Stdout
+			// cmd.Run()
+
+		case ".run_test":
+			got, err := runDifferentialTest(dir, initialCommit)
+			if err != nil {
+				return fmt.Errorf("error running differential test: %w", err)
+			}
+			want := string(file.Data)
+
+			if update {
+				archive.Files[i].Data = []byte(got)
+				break
+			}
+			if strings.TrimSpace(want) != strings.TrimSpace(got) {
+				t.Errorf("Results don't match.\nExpected:\n%s\n\nActual:\n%s", want, got)
+			}
+
+		case ".run_autoformat":
+			if initialCommit == "" {
+				return fmt.Errorf("initial commit not set, cannot run autoformat")
+			}
+
+			got, err := runAutoformat(dir, initialCommit)
+			if err != nil {
+				return fmt.Errorf("error running autoformat: %w", err)
+			}
+			slices.Sort(got)
+
+			if update {
+				correct := strings.Join(got, "\n") + "\n"
+				archive.Files[i].Data = []byte(correct)
+				break
+			}
+
+			want := strings.Split(strings.TrimSpace(string(file.Data)), "\n")
+			if !slices.Equal(want, got) {
+				t.Errorf("Formatted files don't match.\nExpected:\n%s\n\nActual:\n%s", want, got)
+			}
+
+		default:
+			filePath := filepath.Join(dir, file.Name)
+			if err := os.MkdirAll(filepath.Dir(filePath), 0o700); err != nil {
+				return fmt.Errorf("error creating directory for %s: %w", file.Name, err)
+			}
+			data := file.Data
+			// Remove second trailing newline if present.
+			// An annoyance of the txtar format, messes with gofmt.
+			if bytes.HasSuffix(data, []byte("\n\n")) {
+				data = bytes.TrimSuffix(data, []byte("\n"))
+			}
+			if err := os.WriteFile(filePath, file.Data, 0o600); err != nil {
+				return fmt.Errorf("error writing file %s: %w", file.Name, err)
+			}
+			filesForNextCommit[file.Name] = true
+		}
+	}
+
+	return nil
+}
+
+// makeGitCommit commits the specified files with the given message.
+// Returns the commit hash.
+func makeGitCommit(dir, message string, files map[string]bool) (string, error) {
+	for file := range files {
+		cmd := exec.Command("git", "add", file)
+		cmd.Dir = dir
+		if err := cmd.Run(); err != nil {
+			return "", fmt.Errorf("error staging file %s: %w", file, err)
+		}
+	}
+	message = cmp.Or(message, "Test commit")
+
+	// Make the commit with fixed author, committer, and date for stable hashes
+	cmd := exec.Command("git", "commit", "--allow-empty", "-m", message)
+	cmd.Dir = dir
+	cmd.Env = gitCommitEnv()
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("error making commit: %w", err)
+	}
+
+	// Get the commit hash
+	cmd = exec.Command("git", "rev-parse", "HEAD")
+	cmd.Dir = dir
+	out, err := cmd.Output()
+	if err != nil {
+		return "", fmt.Errorf("error getting commit hash: %w", err)
+	}
+
+	return strings.TrimSpace(string(out)), nil
+}
+
+// runDifferentialTest runs the code review tool on the repository and returns the result.
+func runDifferentialTest(dir, initialCommit string) (string, error) {
+	if initialCommit == "" {
+		return "", fmt.Errorf("initial commit not set, cannot run differential test")
+	}
+
+	// Create a code reviewer for the repository
+	ctx := context.Background()
+	reviewer, err := NewCodeReviewer(ctx, dir, initialCommit)
+	if err != nil {
+		return "", fmt.Errorf("error creating code reviewer: %w", err)
+	}
+
+	// Run the code review
+	result, err := reviewer.Run(ctx, nil)
+	if err != nil {
+		return "", fmt.Errorf("error running code review: %w", err)
+	}
+
+	// Normalize paths in the result
+	normalized := normalizePaths(result, dir)
+	return normalized, nil
+}
+
+// normalizePaths replaces the temp directory paths with a standard placeholder
+func normalizePaths(result string, tempDir string) string {
+	return strings.ReplaceAll(result, tempDir, "/PATH/TO/REPO")
+}
+
+// initGoModule initializes a Go module in the specified directory.
+func initGoModule(dir string) error {
+	cmd := exec.Command("go", "mod", "init", "sketch.dev")
+	cmd.Dir = dir
+	return cmd.Run()
+}
+
+// runAutoformat runs the autoformat function on the repository and returns the list of formatted files.
+func runAutoformat(dir, initialCommit string) ([]string, error) {
+	if initialCommit == "" {
+		return nil, fmt.Errorf("initial commit not set, cannot run autoformat")
+	}
+	ctx := context.Background()
+	reviewer, err := NewCodeReviewer(ctx, dir, initialCommit)
+	if err != nil {
+		return nil, fmt.Errorf("error creating code reviewer: %w", err)
+	}
+	formattedFiles := reviewer.Autoformat(ctx)
+	normalizedFiles := make([]string, len(formattedFiles))
+	for i, file := range formattedFiles {
+		normalizedFiles[i] = normalizePaths(file, dir)
+	}
+	slices.Sort(normalizedFiles)
+	return normalizedFiles, nil
+}
+
+// resolveRealPath follows symlinks and returns the real path
+// This handles platform-specific behaviors like macOS's /private prefix
+func resolveRealPath(path string) string {
+	// Follow symlinks to get the real path
+	realPath, err := filepath.EvalSymlinks(path)
+	if err != nil {
+		// If we can't resolve symlinks, just return the original path
+		return path
+	}
+	return realPath
+}
diff --git a/claudetool/differential.go b/claudetool/differential.go
index 14dce04..014fc13 100644
--- a/claudetool/differential.go
+++ b/claudetool/differential.go
@@ -6,6 +6,7 @@
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/fs"
 	"log/slog"
 	"maps"
 	"os"
@@ -69,14 +70,13 @@
 	// The packages in the initial commit may be different.
 	// Good enough for now.
 	// TODO: do better
-	directPkgs, allPkgs, err := r.packagesForFiles(ctx, changedFiles)
+	allPkgs, err := r.packagesForFiles(ctx, changedFiles)
 	if err != nil {
 		// TODO: log and skip to stuff that doesn't require packages
 		slog.DebugContext(ctx, "CodeReviewer.Run: failed to get packages for files", "err", err)
 		return "", err
 	}
 	allPkgList := slices.Collect(maps.Keys(allPkgs))
-	directPkgList := slices.Collect(maps.Keys(directPkgs))
 
 	var msgs []string
 
@@ -89,16 +89,7 @@
 		msgs = append(msgs, testMsg)
 	}
 
-	vetMsg, err := r.checkVet(ctx, directPkgList)
-	if err != nil {
-		slog.DebugContext(ctx, "CodeReviewer.Run: failed to check vet", "err", err)
-		return "", err
-	}
-	if vetMsg != "" {
-		msgs = append(msgs, vetMsg)
-	}
-
-	goplsMsg, err := r.checkGopls(ctx, changedFiles)
+	goplsMsg, err := r.checkGopls(ctx, changedFiles) // includes vet checks
 	if err != nil {
 		slog.DebugContext(ctx, "CodeReviewer.Run: failed to check gopls", "err", err)
 		return "", err
@@ -111,6 +102,8 @@
 		slog.DebugContext(ctx, "CodeReviewer.Run: no issues found")
 		return "OK", nil
 	}
+
+	msgs = append(msgs, "Please fix before proceeding.")
 	slog.DebugContext(ctx, "CodeReviewer.Run: found issues", "issues", msgs)
 	return strings.Join(msgs, "\n\n"), nil
 }
@@ -139,10 +132,9 @@
 
 	afterTestCmd := exec.CommandContext(ctx, "go", goTestArgs...)
 	afterTestCmd.Dir = r.repoRoot
-	afterTestOut, afterTestErr := afterTestCmd.Output()
-	if afterTestErr == nil {
-		return "", nil // all tests pass, we're good!
-	}
+	afterTestOut, _ := afterTestCmd.Output()
+	// unfortunately, we can't short-circuit here even if all tests pass,
+	// because we need to check for skipped tests.
 
 	err := r.initializeInitialCommitWorktree(ctx)
 	if err != nil {
@@ -162,7 +154,6 @@
 	if afterParseErr != nil {
 		return "", fmt.Errorf("unable to parse test results for current commit: %w\n%s", afterParseErr, afterTestOut)
 	}
-
 	testRegressions, err := r.compareTestResults(beforeResults, afterResults)
 	if err != nil {
 		return "", fmt.Errorf("failed to compare test results: %w", err)
@@ -172,281 +163,6 @@
 	return res, nil
 }
 
-// VetIssue represents a single issue found by go vet
-type VetIssue struct {
-	Position string `json:"posn"`
-	Message  string `json:"message"`
-	// Ignoring suggested_fixes for now as we don't need them for comparison
-}
-
-// VetResult represents the JSON output of go vet -json for a single package
-type VetResult map[string][]VetIssue // category -> issues
-
-// VetResults represents the full JSON output of go vet -json
-type VetResults map[string]VetResult // package path -> result
-
-// checkVet runs go vet on the provided packages in both the current and initial state,
-// compares the results, and reports any new vet issues introduced in the current state.
-func (r *CodeReviewer) checkVet(ctx context.Context, pkgList []string) (string, error) {
-	if len(pkgList) == 0 {
-		return "", nil // no packages to check
-	}
-
-	// Run vet on the current state with JSON output
-	goVetArgs := []string{"vet", "-json"}
-	goVetArgs = append(goVetArgs, pkgList...)
-
-	afterVetCmd := exec.CommandContext(ctx, "go", goVetArgs...)
-	afterVetCmd.Dir = r.repoRoot
-	afterVetOut, afterVetErr := afterVetCmd.CombinedOutput() // ignore error, we'll parse the output regar
-	if afterVetErr != nil {
-		slog.WarnContext(ctx, "CodeReviewer.checkVet: (after) go vet failed", "err", afterVetErr, "output", string(afterVetOut))
-		return "", nil // nothing more we can do here
-	}
-
-	// Parse the JSON output (even if vet returned an error, as it does when issues are found)
-	afterVetResults, err := parseVetJSON(afterVetOut)
-	if err != nil {
-		return "", fmt.Errorf("failed to parse vet output for current state: %w", err)
-	}
-
-	// If no issues were found, we're done
-	if len(afterVetResults) == 0 || !vetResultsHaveIssues(afterVetResults) {
-		return "", nil
-	}
-
-	// Vet detected issues in the current state, check if they existed in the initial state
-	err = r.initializeInitialCommitWorktree(ctx)
-	if err != nil {
-		return "", err
-	}
-
-	beforeVetCmd := exec.CommandContext(ctx, "go", goVetArgs...)
-	beforeVetCmd.Dir = r.initialWorktree
-	beforeVetOut, _ := beforeVetCmd.CombinedOutput() // ignore error, we'll parse the output anyway
-
-	// Parse the JSON output for the initial state
-	beforeVetResults, err := parseVetJSON(beforeVetOut)
-	if err != nil {
-		return "", fmt.Errorf("failed to parse vet output for initial state: %w", err)
-	}
-
-	// Find new issues that weren't present in the initial state
-	vetRegressions := findVetRegressions(beforeVetResults, afterVetResults)
-	if !vetResultsHaveIssues(vetRegressions) {
-		return "", nil // no new issues
-	}
-
-	// Format the results
-	return formatVetRegressions(vetRegressions), nil
-}
-
-// parseVetJSON parses the JSON output from go vet -json
-func parseVetJSON(output []byte) (VetResults, error) {
-	// The output contains multiple JSON objects, one per package
-	// We need to parse them separately
-	results := make(VetResults)
-
-	// Process the output by collecting JSON chunks between # comment lines
-	lines := strings.Split(string(output), "\n")
-	currentChunk := strings.Builder{}
-
-	// Helper function to process accumulated JSON chunks
-	processChunk := func() {
-		chunk := strings.TrimSpace(currentChunk.String())
-		if chunk == "" || !strings.HasPrefix(chunk, "{") {
-			return // Skip empty chunks or non-JSON chunks
-		}
-
-		// Try to parse the chunk as JSON
-		var result VetResults
-		if err := json.Unmarshal([]byte(chunk), &result); err != nil {
-			return // Skip invalid JSON
-		}
-
-		// Merge with our results
-		for pkg, issues := range result {
-			results[pkg] = issues
-		}
-
-		// Reset the chunk builder
-		currentChunk.Reset()
-	}
-
-	// Process lines
-	for _, line := range lines {
-		// If we hit a comment line, process the previous chunk and start a new one
-		if strings.HasPrefix(strings.TrimSpace(line), "#") {
-			processChunk()
-			continue
-		}
-
-		// Add the line to the current chunk
-		currentChunk.WriteString(line)
-		currentChunk.WriteString("\n")
-	}
-
-	// Process the final chunk
-	processChunk()
-
-	return results, nil
-}
-
-// vetResultsHaveIssues checks if there are any actual issues in the vet results
-func vetResultsHaveIssues(results VetResults) bool {
-	for _, pkgResult := range results {
-		for _, issues := range pkgResult {
-			if len(issues) > 0 {
-				return true
-			}
-		}
-	}
-	return false
-}
-
-// findVetRegressions identifies vet issues that are new in the after state
-func findVetRegressions(before, after VetResults) VetResults {
-	regressions := make(VetResults)
-
-	// Go through all packages in the after state
-	for pkgPath, afterPkgResults := range after {
-		beforePkgResults, pkgExistedBefore := before[pkgPath]
-
-		// Initialize package in regressions if it has issues
-		if !pkgExistedBefore {
-			// If the package didn't exist before, all issues are new
-			regressions[pkgPath] = afterPkgResults
-			continue
-		}
-
-		// Compare issues by category
-		for category, afterIssues := range afterPkgResults {
-			beforeIssues, categoryExistedBefore := beforePkgResults[category]
-
-			if !categoryExistedBefore {
-				// If this category didn't exist before, all issues are new
-				if regressions[pkgPath] == nil {
-					regressions[pkgPath] = make(VetResult)
-				}
-				regressions[pkgPath][category] = afterIssues
-				continue
-			}
-
-			// Compare individual issues
-			var newIssues []VetIssue
-			for _, afterIssue := range afterIssues {
-				if !issueExistsIn(afterIssue, beforeIssues) {
-					newIssues = append(newIssues, afterIssue)
-				}
-			}
-
-			// Add new issues to regressions
-			if len(newIssues) > 0 {
-				if regressions[pkgPath] == nil {
-					regressions[pkgPath] = make(VetResult)
-				}
-				regressions[pkgPath][category] = newIssues
-			}
-		}
-	}
-
-	return regressions
-}
-
-// issueExistsIn checks if an issue already exists in a list of issues
-// using a looser comparison that's resilient to position changes
-func issueExistsIn(issue VetIssue, issues []VetIssue) bool {
-	issueFile := extractFilePath(issue.Position)
-
-	for _, existing := range issues {
-		// Main comparison is by message content, which is likely stable
-		if issue.Message == existing.Message {
-			// If messages match exactly, consider it the same issue even if position changed
-			return true
-		}
-
-		// As a secondary check, if the issue is in the same file and has similar message,
-		// it's likely the same issue that might have been slightly reworded or relocated
-		existingFile := extractFilePath(existing.Position)
-		if issueFile == existingFile && messagesSimilar(issue.Message, existing.Message) {
-			return true
-		}
-	}
-	return false
-}
-
-// extractFilePath gets just the file path from a position string like "/path/to/file.go:10:15"
-func extractFilePath(position string) string {
-	parts := strings.Split(position, ":")
-	if len(parts) >= 1 {
-		return parts[0]
-	}
-	return position // fallback to the full position if we can't parse it
-}
-
-// messagesSimilar checks if two messages are similar enough to be considered the same issue
-// This is a simple implementation that could be enhanced with more sophisticated text comparison
-func messagesSimilar(msg1, msg2 string) bool {
-	// For now, simple similarity check: if one is a substring of the other
-	return strings.Contains(msg1, msg2) || strings.Contains(msg2, msg1)
-}
-
-// formatVetRegressions generates a human-readable summary of vet regressions
-func formatVetRegressions(regressions VetResults) string {
-	if !vetResultsHaveIssues(regressions) {
-		return ""
-	}
-
-	var sb strings.Builder
-	sb.WriteString("Go vet issues detected:\n\n")
-
-	// Get sorted list of packages for deterministic output
-	pkgPaths := make([]string, 0, len(regressions))
-	for pkgPath := range regressions {
-		pkgPaths = append(pkgPaths, pkgPath)
-	}
-	slices.Sort(pkgPaths)
-
-	issueCount := 1
-	for _, pkgPath := range pkgPaths {
-		pkgResult := regressions[pkgPath]
-
-		// Get sorted list of categories
-		categories := make([]string, 0, len(pkgResult))
-		for category := range pkgResult {
-			categories = append(categories, category)
-		}
-		slices.Sort(categories)
-
-		for _, category := range categories {
-			issues := pkgResult[category]
-
-			// Skip empty issue lists (shouldn't happen, but just in case)
-			if len(issues) == 0 {
-				continue
-			}
-
-			// Sort issues by position for deterministic output
-			slices.SortFunc(issues, func(a, b VetIssue) int {
-				return strings.Compare(a.Position, b.Position)
-			})
-
-			// Format each issue
-			for _, issue := range issues {
-				sb.WriteString(fmt.Sprintf("%d. [%s] %s: %s\n",
-					issueCount,
-					category,
-					issue.Position,
-					issue.Message))
-				issueCount++
-			}
-		}
-	}
-
-	sb.WriteString("\nPlease fix these issues before proceeding.")
-	return sb.String()
-}
-
 // GoplsIssue represents a single issue reported by gopls check
 type GoplsIssue struct {
 	Position string // File position in format "file:line:col-range"
@@ -496,7 +212,7 @@
 	}
 
 	// Parse the output
-	afterIssues := parseGoplsOutput(afterGoplsOut)
+	afterIssues := parseGoplsOutput(r.repoRoot, afterGoplsOut)
 
 	// If no issues were found, we're done
 	if len(afterIssues) == 0 {
@@ -530,22 +246,18 @@
 	}
 
 	// Run gopls check on the files that existed in the initial commit
-	beforeIssues := []GoplsIssue{}
+	var beforeIssues []GoplsIssue
 	if len(initialFilesToCheck) > 0 {
 		beforeGoplsArgs := append([]string{"check"}, initialFilesToCheck...)
 		beforeGoplsCmd := exec.CommandContext(ctx, "gopls", beforeGoplsArgs...)
 		beforeGoplsCmd.Dir = r.initialWorktree
-		var beforeGoplsOut []byte
-		var beforeCmdErr error
-		beforeGoplsOut, beforeCmdErr = beforeGoplsCmd.CombinedOutput()
+		beforeGoplsOut, beforeCmdErr := beforeGoplsCmd.CombinedOutput()
 		if beforeCmdErr != nil && !looksLikeGoplsIssues(beforeGoplsOut) {
 			// If gopls fails to run properly on the initial commit, log a warning and continue
 			// with empty before issues - this will be conservative and report more issues
-			slog.WarnContext(ctx, "CodeReviewer.checkGopls: gopls check failed on initial commit",
-				"err", err, "output", string(beforeGoplsOut))
-			// Continue with empty beforeIssues
+			slog.WarnContext(ctx, "CodeReviewer.checkGopls: gopls check failed on initial commit", "err", err, "output", string(beforeGoplsOut))
 		} else {
-			beforeIssues = parseGoplsOutput(beforeGoplsOut)
+			beforeIssues = parseGoplsOutput(r.initialWorktree, beforeGoplsOut)
 		}
 	}
 
@@ -556,12 +268,12 @@
 	}
 
 	// Format the results
-	return formatGoplsRegressions(goplsRegressions), nil
+	return r.formatGoplsRegressions(goplsRegressions), nil
 }
 
 // parseGoplsOutput parses the text output from gopls check
 // Each line has the format: '/path/to/file.go:448:22-26: unused parameter: path'
-func parseGoplsOutput(output []byte) []GoplsIssue {
+func parseGoplsOutput(root string, output []byte) []GoplsIssue {
 	var issues []GoplsIssue
 	lines := strings.Split(string(output), "\n")
 
@@ -600,6 +312,10 @@
 
 		// Extract position and message
 		position := line[:messageStart]
+		rel, err := filepath.Rel(root, position)
+		if err == nil {
+			position = rel
+		}
 		message := line[messageStart+2:] // Skip the ': ' separator
 
 		// Verify position has the expected format (at least 2 colons for line:col)
@@ -717,7 +433,7 @@
 }
 
 // formatGoplsRegressions generates a human-readable summary of gopls check regressions
-func formatGoplsRegressions(regressions []GoplsIssue) string {
+func (r *CodeReviewer) formatGoplsRegressions(regressions []GoplsIssue) string {
 	if len(regressions) == 0 {
 		return ""
 	}
@@ -727,11 +443,11 @@
 
 	// Format each issue
 	for i, issue := range regressions {
-		sb.WriteString(fmt.Sprintf("%d. %s: %s\n", i+1, issue.Position, issue.Message))
+		sb.WriteString(fmt.Sprintf("%d. %s: %s\n", i+1, filepath.Join(r.repoRoot, issue.Position), issue.Message))
 	}
 
-	sb.WriteString("\nIMPORTANT: Only fix new gopls check issues in parts of the code that you have already edited. ")
-	sb.WriteString("Do not change existing code that was not part of your current edits.")
+	sb.WriteString("\nIMPORTANT: Only fix new gopls check issues in parts of the code that you have already edited.")
+	sb.WriteString(" Do not change existing code that was not part of your current edits.\n")
 	return sb.String()
 }
 
@@ -748,10 +464,10 @@
 // 2. allPkgs: all packages that might be affected, including downstream packages that depend on the direct packages
 // It may include false positives.
 // Files must be absolute paths!
-func (r *CodeReviewer) packagesForFiles(ctx context.Context, files []string) (directPkgs, allPkgs map[string]*packages.Package, err error) {
+func (r *CodeReviewer) packagesForFiles(ctx context.Context, files []string) (allPkgs map[string]*packages.Package, err error) {
 	for _, f := range files {
 		if !filepath.IsAbs(f) {
-			return nil, nil, fmt.Errorf("path %q is not absolute", f)
+			return nil, fmt.Errorf("path %q is not absolute", f)
 		}
 	}
 	cfg := &packages.Config{
@@ -767,14 +483,12 @@
 	}
 	universe, err := packages.Load(cfg, "./...")
 	if err != nil {
-		return nil, nil, err
+		return nil, err
 	}
 	// Identify packages that directly contain the changed files
-	directPkgs = make(map[string]*packages.Package) // import path -> package
+	directPkgs := make(map[string]*packages.Package) // import path -> package
 	for _, pkg := range universe {
-		// fmt.Println("pkg:", pkg.PkgPath)
 		pkgFiles := allFiles(pkg)
-		// fmt.Println("pkgFiles:", pkgFiles)
 		for _, file := range files {
 			if pkgFiles[file] {
 				// prefer test packages, as they contain strictly more files (right?)
@@ -786,27 +500,35 @@
 		}
 	}
 
-	// Create a copy of directPkgs to expand with dependencies
-	allPkgs = make(map[string]*packages.Package)
-	for k, v := range directPkgs {
-		allPkgs[k] = v
-	}
+	allPkgs = maps.Clone(directPkgs)
 
 	// Add packages that depend on the direct packages
 	addDependentPackages(universe, allPkgs)
-	return directPkgs, allPkgs, nil
+	return allPkgs, nil
 }
 
 // allFiles returns all files that might be referenced by the package.
 // It may contain false positives.
 func allFiles(p *packages.Package) map[string]bool {
 	files := make(map[string]bool)
+	// Add files from package info
 	add := [][]string{p.GoFiles, p.CompiledGoFiles, p.OtherFiles, p.EmbedFiles, p.IgnoredFiles}
 	for _, extra := range add {
 		for _, file := range extra {
 			files[file] = true
 		}
 	}
+	// Add files from testdata directory
+	testdataDir := filepath.Join(p.Dir, "testdata")
+	if _, err := os.Stat(testdataDir); err == nil {
+		fsys := os.DirFS(p.Dir)
+		fs.WalkDir(fsys, "testdata", func(path string, d fs.DirEntry, err error) error {
+			if err == nil && !d.IsDir() {
+				files[filepath.Join(p.Dir, path)] = true
+			}
+			return nil
+		})
+	}
 	return files
 }
 
@@ -816,6 +538,10 @@
 	for {
 		changed := false
 		for _, p := range universe {
+			if strings.HasSuffix(p.PkgPath, ".test") { // ick, but I don't see another way
+				// skip test packages
+				continue
+			}
 			if _, ok := pkgs[p.PkgPath]; ok {
 				// already in pkgs
 				continue
@@ -910,146 +636,151 @@
 // testStatus represents the status of a test in a given commit
 type testStatus int
 
+//go:generate go tool stringer -type=testStatus -trimprefix=testStatus
 const (
 	testStatusUnknown testStatus = iota
 	testStatusPass
 	testStatusFail
 	testStatusBuildFail
 	testStatusSkip
+	testStatusNoTests // no tests exist for this package
 )
 
-// testInfo represents information about a specific test
-type testInfo struct {
-	Package string
-	Test    string // empty for package tests
-}
-
-// String returns a human-readable string representation of the test
-func (t testInfo) String() string {
-	if t.Test == "" {
-		return t.Package
-	}
-	return fmt.Sprintf("%s.%s", t.Package, t.Test)
-}
-
 // testRegression represents a test that regressed between commits
 type testRegression struct {
-	Info         testInfo
+	Package      string
+	Test         string // empty for package tests
 	BeforeStatus testStatus
 	AfterStatus  testStatus
 	Output       string // failure output in the after state
 }
 
-// collectTestStatuses processes a slice of test events and returns a map of test statuses
-func collectTestStatuses(results []testJSON) map[testInfo]testStatus {
-	statuses := make(map[testInfo]testStatus)
-	failedBuilds := make(map[string]bool)      // track packages with build failures
-	testOutputs := make(map[testInfo][]string) // collect output for failing tests
-
-	// First pass: identify build failures
-	for _, result := range results {
-		if result.Action == "fail" && result.FailedBuild != "" {
-			failedBuilds[result.FailedBuild] = true
-		}
+func (r *testRegression) Source() string {
+	if r.Test == "" {
+		return r.Package
 	}
+	return fmt.Sprintf("%s.%s", r.Package, r.Test)
+}
 
-	// Second pass: collect test statuses
-	for _, result := range results {
-		info := testInfo{Package: result.Package, Test: result.Test}
+type packageResult struct {
+	Status     testStatus            // overall status for the package
+	TestStatus map[string]testStatus // name -> status
+	TestOutput map[string][]string   // name -> output parts
+}
 
-		// Skip output events for now, we'll process them in a separate pass
-		if result.Action == "output" {
-			if result.Test != "" { // only collect output for actual tests, not package messages
-				testOutputs[info] = append(testOutputs[info], result.Output)
-			}
-			continue
+// collectTestStatuses processes a slice of test events and returns rich status information
+func collectTestStatuses(results []testJSON) map[string]*packageResult {
+	m := make(map[string]*packageResult)
+
+	for _, event := range results {
+		pkg := event.Package
+		p, ok := m[pkg]
+		if !ok {
+			p = new(packageResult)
+			p.TestStatus = make(map[string]testStatus)
+			p.TestOutput = make(map[string][]string)
+			m[pkg] = p
 		}
 
-		// Handle BuildEvent output
-		if result.Action == "build-fail" {
-			// Mark all tests in this package as build failures
-			for ti := range statuses {
-				if ti.Package == result.ImportPath {
-					statuses[ti] = testStatusBuildFail
-				}
-			}
+		switch event.Action {
+		case "output":
+			p.TestOutput[event.Test] = append(p.TestOutput[event.Test], event.Output)
 			continue
-		}
-
-		// Check if the package has a build failure
-		if _, hasBuildFailure := failedBuilds[result.Package]; hasBuildFailure {
-			statuses[info] = testStatusBuildFail
-			continue
-		}
-
-		// Handle test events
-		switch result.Action {
 		case "pass":
-			statuses[info] = testStatusPass
+			if event.Test == "" {
+				p.Status = testStatusPass
+			} else {
+				p.TestStatus[event.Test] = testStatusPass
+			}
 		case "fail":
-			statuses[info] = testStatusFail
+			if event.Test == "" {
+				if event.FailedBuild != "" {
+					p.Status = testStatusBuildFail
+				} else {
+					p.Status = testStatusFail
+				}
+			} else {
+				p.TestStatus[event.Test] = testStatusFail
+			}
 		case "skip":
-			statuses[info] = testStatusSkip
+			if event.Test == "" {
+				p.Status = testStatusNoTests
+			} else {
+				p.TestStatus[event.Test] = testStatusSkip
+			}
 		}
 	}
 
-	return statuses
+	return m
 }
 
 // compareTestResults identifies tests that have regressed between commits
 func (r *CodeReviewer) compareTestResults(beforeResults, afterResults []testJSON) ([]testRegression, error) {
-	beforeStatuses := collectTestStatuses(beforeResults)
-	afterStatuses := collectTestStatuses(afterResults)
+	before := collectTestStatuses(beforeResults)
+	after := collectTestStatuses(afterResults)
+	var testLevelRegressions []testRegression
+	var packageLevelRegressions []testRegression
 
-	// Collect output for failing tests
-	testOutputMap := make(map[testInfo]string)
-	for _, result := range afterResults {
-		if result.Action == "output" {
-			info := testInfo{Package: result.Package, Test: result.Test}
-			testOutputMap[info] += result.Output
+	afterPkgs := slices.Sorted(maps.Keys(after))
+	for _, pkg := range afterPkgs {
+		afterResult := after[pkg]
+		afterStatus := afterResult.Status
+		// Can't short-circuit here when tests are passing, because we need to check for skipped tests.
+		beforeResult, ok := before[pkg]
+		beforeStatus := testStatusNoTests
+		if ok {
+			beforeStatus = beforeResult.Status
+		}
+		// If things no longer build, stop at the package level.
+		// Otherwise, proceed to the test level, so we have more precise information.
+		if afterStatus == testStatusBuildFail && beforeStatus != testStatusBuildFail {
+			packageLevelRegressions = append(packageLevelRegressions, testRegression{
+				Package:      pkg,
+				BeforeStatus: beforeStatus,
+				AfterStatus:  afterStatus,
+			})
+			continue
+		}
+		tests := slices.Sorted(maps.Keys(afterResult.TestStatus))
+		for _, test := range tests {
+			afterStatus := afterResult.TestStatus[test]
+			switch afterStatus {
+			case testStatusPass:
+				continue
+			case testStatusUnknown:
+				slog.WarnContext(context.Background(), "unknown test status", "package", pkg, "test", test)
+				continue
+			}
+			beforeStatus := beforeResult.TestStatus[test]
+			if isRegression(beforeStatus, afterStatus) {
+				testLevelRegressions = append(testLevelRegressions, testRegression{
+					Package:      pkg,
+					Test:         test,
+					BeforeStatus: beforeStatus,
+					AfterStatus:  afterStatus,
+					Output:       strings.Join(afterResult.TestOutput[test], ""),
+				})
+			}
 		}
 	}
 
+	// If we have test-level regressions, report only those
+	// Otherwise, report package-level regressions
 	var regressions []testRegression
-
-	// Look for tests that regressed
-	for info, afterStatus := range afterStatuses {
-		// Skip tests that are passing or skipped in the after state
-		if afterStatus == testStatusPass || afterStatus == testStatusSkip {
-			continue
-		}
-
-		// Get the before status (default to unknown if not present)
-		beforeStatus, exists := beforeStatuses[info]
-		if !exists {
-			beforeStatus = testStatusUnknown
-		}
-
-		// Log warning if we encounter unexpected unknown status in the 'after' state
-		if afterStatus == testStatusUnknown {
-			slog.WarnContext(context.Background(), "Unexpected unknown test status encountered",
-				"package", info.Package, "test", info.Test)
-		}
-
-		// Check for regressions
-		if isRegression(beforeStatus, afterStatus) {
-			regressions = append(regressions, testRegression{
-				Info:         info,
-				BeforeStatus: beforeStatus,
-				AfterStatus:  afterStatus,
-				Output:       testOutputMap[info],
-			})
-		}
+	if len(testLevelRegressions) > 0 {
+		regressions = testLevelRegressions
+	} else {
+		regressions = packageLevelRegressions
 	}
 
 	// Sort regressions for consistent output
 	slices.SortFunc(regressions, func(a, b testRegression) int {
 		// First by package
-		if c := strings.Compare(a.Info.Package, b.Info.Package); c != 0 {
+		if c := strings.Compare(a.Package, b.Package); c != 0 {
 			return c
 		}
 		// Then by test name
-		return strings.Compare(a.Info.Test, b.Info.Test)
+		return strings.Compare(a.Test, b.Test)
 	})
 
 	return regressions, nil
@@ -1058,13 +789,34 @@
 // badnessLevels maps test status to a badness level
 // Higher values indicate worse status (more severe issues)
 var badnessLevels = map[testStatus]int{
-	testStatusBuildFail: 4, // Worst
-	testStatusFail:      3,
-	testStatusSkip:      2,
+	testStatusBuildFail: 5, // Worst
+	testStatusFail:      4,
+	testStatusSkip:      3,
+	testStatusNoTests:   2,
 	testStatusPass:      1,
 	testStatusUnknown:   0, // Least bad - avoids false positives
 }
 
+// regressionFormatter defines a mapping of before/after state pairs to descriptive messages
+type regressionKey struct {
+	before, after testStatus
+}
+
+var regressionMessages = map[regressionKey]string{
+	{testStatusUnknown, testStatusBuildFail}: "New test has build/vet errors",
+	{testStatusUnknown, testStatusFail}:      "New test is failing",
+	{testStatusUnknown, testStatusSkip}:      "New test is skipped",
+	{testStatusPass, testStatusBuildFail}:    "Was passing, now has build/vet errors",
+	{testStatusPass, testStatusFail}:         "Was passing, now failing",
+	{testStatusPass, testStatusSkip}:         "Was passing, now skipped",
+	{testStatusNoTests, testStatusBuildFail}: "Previously had no tests, now has build/vet errors",
+	{testStatusNoTests, testStatusFail}:      "Previously had no tests, now has failing tests",
+	{testStatusNoTests, testStatusSkip}:      "Previously had no tests, now has skipped tests",
+	{testStatusSkip, testStatusBuildFail}:    "Was skipped, now has build/vet errors",
+	{testStatusSkip, testStatusFail}:         "Was skipped, now failing",
+	{testStatusFail, testStatusBuildFail}:    "Was failing, now has build/vet errors",
+}
+
 // isRegression determines if a test has regressed based on before and after status
 // A regression is defined as an increase in badness level
 func isRegression(before, after testStatus) bool {
@@ -1078,48 +830,18 @@
 		return ""
 	}
 
-	var sb strings.Builder
-	sb.WriteString(fmt.Sprintf("Test regressions detected between initial commit (%s) and HEAD:\n\n", r.initialCommit))
+	buf := new(strings.Builder)
+	fmt.Fprintf(buf, "Test regressions detected between initial commit (%s) and HEAD:\n\n", r.initialCommit)
 
 	for i, reg := range regressions {
-		// Describe the regression
-		sb.WriteString(fmt.Sprintf("%d. %s: ", i+1, reg.Info.String()))
-
-		switch {
-		case reg.BeforeStatus == testStatusUnknown && reg.AfterStatus == testStatusFail:
-			sb.WriteString("New test is failing")
-		case reg.BeforeStatus == testStatusUnknown && reg.AfterStatus == testStatusBuildFail:
-			sb.WriteString("New test has build errors")
-		case reg.BeforeStatus == testStatusPass && reg.AfterStatus == testStatusFail:
-			sb.WriteString("Was passing, now failing")
-		case reg.BeforeStatus == testStatusPass && reg.AfterStatus == testStatusBuildFail:
-			sb.WriteString("Was passing, now has build errors")
-		case reg.BeforeStatus == testStatusSkip && reg.AfterStatus == testStatusFail:
-			sb.WriteString("Was skipped, now failing")
-		case reg.BeforeStatus == testStatusSkip && reg.AfterStatus == testStatusBuildFail:
-			sb.WriteString("Was skipped, now has build errors")
-		default:
-			sb.WriteString("Regression detected")
+		fmt.Fprintf(buf, "%d: %v: ", i+1, reg.Source())
+		key := regressionKey{reg.BeforeStatus, reg.AfterStatus}
+		message, exists := regressionMessages[key]
+		if !exists {
+			message = "Regression detected"
 		}
-		sb.WriteString("\n")
-
-		// Add failure output with indentation for readability
-		if reg.Output != "" {
-			outputLines := strings.Split(strings.TrimSpace(reg.Output), "\n")
-			// Limit output to first 10 lines to avoid overwhelming feedback
-			shownLines := min(len(outputLines), 10)
-
-			sb.WriteString("   Output:\n")
-			for _, line := range outputLines[:shownLines] {
-				sb.WriteString(fmt.Sprintf("   | %s\n", line))
-			}
-			if shownLines < len(outputLines) {
-				sb.WriteString(fmt.Sprintf("   | ... (%d more lines)\n", len(outputLines)-shownLines))
-			}
-		}
-		sb.WriteString("\n")
+		fmt.Fprintf(buf, "%s\n", message)
 	}
 
-	sb.WriteString("Please fix these test failures before proceeding.")
-	return sb.String()
+	return buf.String()
 }
diff --git a/claudetool/testdata/add_skipped_test.txtar b/claudetool/testdata/add_skipped_test.txtar
new file mode 100644
index 0000000..95000cc
--- /dev/null
+++ b/claudetool/testdata/add_skipped_test.txtar
@@ -0,0 +1,27 @@
+Adding a skipped test is a regression
+
+-- p_test.go --
+package p
+
+-- .commit --
+Initial commit with no tests
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.SkipNow()
+}
+
+-- .commit --
+Add skipped test
+
+-- .run_test --
+Test regressions detected between initial commit (2cdfdc5009e364add9a263909472f85eb08480ed) and HEAD:
+
+1: sketch.dev.TestP: New test is skipped
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/basic_gofmt.txtar b/claudetool/testdata/basic_gofmt.txtar
new file mode 100644
index 0000000..c32ebe9
--- /dev/null
+++ b/claudetool/testdata/basic_gofmt.txtar
@@ -0,0 +1,13 @@
+Test autoformatting happens
+
+-- .commit --
+Initial commit
+
+-- p.go --
+    package p
+
+-- .commit --
+Add non-gofmt file
+
+-- .run_autoformat --
+/PATH/TO/REPO/p.go
diff --git a/claudetool/testdata/empty_testdir_add_file.txtar b/claudetool/testdata/empty_testdir_add_file.txtar
new file mode 100644
index 0000000..b754f3d
--- /dev/null
+++ b/claudetool/testdata/empty_testdir_add_file.txtar
@@ -0,0 +1,37 @@
+Test adding a non-empty file to testdata when test expects empty files
+
+-- p_test.go --
+package p
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestEmptyTestdata(t *testing.T) {
+	files, _ := filepath.Glob("testdata/*")
+	for _, path := range files {
+		data, _ := os.ReadFile(path)
+		if len(data) > 0 {
+			t.Fatalf("testdata file is not empty: %s", path)
+		}
+	}
+}
+
+-- testdata/empty --
+-- .commit --
+Initial commit with empty testdata file
+
+-- testdata/nonempty --
+hi
+-- .commit --
+Add non-empty file to testdata
+
+-- .run_test --
+Test regressions detected between initial commit (cdc9ec6dfb8669c11d8aa0df49c72112627784dc) and HEAD:
+
+1: sketch.dev.TestEmptyTestdata: Was passing, now failing
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/failing_to_failing.txtar b/claudetool/testdata/failing_to_failing.txtar
new file mode 100644
index 0000000..eea1b59
--- /dev/null
+++ b/claudetool/testdata/failing_to_failing.txtar
@@ -0,0 +1,28 @@
+Going from failing tests to failing tests is not a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestX(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Initial commit (with failing test)
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestX(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Test still failing
+
+-- .run_test --
+OK
diff --git a/claudetool/testdata/fmt_hierarchy.txtar b/claudetool/testdata/fmt_hierarchy.txtar
new file mode 100644
index 0000000..613d716
--- /dev/null
+++ b/claudetool/testdata/fmt_hierarchy.txtar
@@ -0,0 +1,58 @@
+Test autoformatting hierarchy
+
+-- bad.go --
+// This file is not properly gofmt'd.
+	package main
+-- gofmt.go --
+// This file is properly gofmt'd.
+package main
+-- goimports.go --
+// This file is properly goimport'd, but not gofumpt'd.
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+
+	fmt.Println("hi")
+}
+-- gofumpt.go --
+// This file is properly gofumpt'd.
+package main
+-- .commit --
+Initial commit
+
+-- bad.go --
+// This file is still not gofmt'd, but we won't complain, because it was already not gofmt'd.
+	package main
+
+	import "fmt"
+-- gofmt.go --
+// This file is no longer properly gofmt'd.
+	package main
+-- goimports.go --
+// If we remove the imports, it no longer is goimport'd.
+// It should thus be flagged as a formatting issue, despite being gofmt'd.
+package main
+
+func main() {
+
+	fmt.Println("hi")
+}
+-- gofumpt.go --
+// This file is properly gofmt'd, but no longer gofumpt'd.
+package main
+
+func main() {
+
+	fmt.Println("hi")
+}
+-- .commit --
+Mess with formatting
+
+-- .run_autoformat --
+/PATH/TO/REPO/gofmt.go
+/PATH/TO/REPO/gofumpt.go
+/PATH/TO/REPO/goimports.go
diff --git a/claudetool/testdata/gopls_issue_unchanged.txtar b/claudetool/testdata/gopls_issue_unchanged.txtar
new file mode 100644
index 0000000..d1de43b
--- /dev/null
+++ b/claudetool/testdata/gopls_issue_unchanged.txtar
@@ -0,0 +1,25 @@
+Pre-existing gopls issues should not be reported as regressions
+
+-- p.go --
+package p
+
+func F() {
+	x := 42
+}
+
+-- .commit --
+Initial commit with existing gopls issue
+
+-- p.go --
+package p
+
+func F() {
+	x := 42
+	// unrelated change
+}
+
+-- .commit --
+Make a change but keep the same gopls issue
+
+-- .run_test --
+OK
diff --git a/claudetool/testdata/gopls_issues.txtar b/claudetool/testdata/gopls_issues.txtar
new file mode 100644
index 0000000..56f1557
--- /dev/null
+++ b/claudetool/testdata/gopls_issues.txtar
@@ -0,0 +1,25 @@
+Detect newly introduced gopls issues
+
+-- .commit --
+Initial commit
+
+-- p.go --
+package p
+
+func F() {
+	return
+	panic("unreachable")
+}
+
+-- .commit --
+Add file with gopls issues
+
+-- .run_test --
+Gopls check issues detected:
+
+1. /PATH/TO/REPO/p.go:5:2-22: unreachable code
+
+IMPORTANT: Only fix new gopls check issues in parts of the code that you have already edited. Do not change existing code that was not part of your current edits.
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/gopls_line_number_changed.txtar b/claudetool/testdata/gopls_line_number_changed.txtar
new file mode 100644
index 0000000..c7eeb37
--- /dev/null
+++ b/claudetool/testdata/gopls_line_number_changed.txtar
@@ -0,0 +1,26 @@
+Pre-existing gopls issues with line number changes should not be reported as regressions
+
+-- p.go --
+package p
+
+func F() {
+	x := 42
+}
+
+-- .commit --
+Initial commit with existing gopls issue
+
+-- p.go --
+package p
+
+// Change the line number of the unused variable
+
+func F() {
+	x := 42
+}
+
+-- .commit --
+Add comments that change line numbers
+
+-- .run_test --
+OK
diff --git a/claudetool/testdata/gopls_new_issue_with_existing.txtar b/claudetool/testdata/gopls_new_issue_with_existing.txtar
new file mode 100644
index 0000000..d47fb02
--- /dev/null
+++ b/claudetool/testdata/gopls_new_issue_with_existing.txtar
@@ -0,0 +1,36 @@
+Only new gopls issues should be reported when existing issues are present
+
+-- p.go --
+package p
+
+func F() {
+	x := 42
+}
+
+-- .commit --
+Initial commit with existing gopls issue
+
+-- p.go --
+package p
+
+func F() {
+	x := 42
+}
+
+func G() {
+	panic("i went down down down")
+	panic("burning ring of fire")
+}
+
+-- .commit --
+Add a new function with a new gopls issue
+
+-- .run_test --
+Gopls check issues detected:
+
+1. /PATH/TO/REPO/p.go:9:2-31: unreachable code
+
+IMPORTANT: Only fix new gopls check issues in parts of the code that you have already edited. Do not change existing code that was not part of your current edits.
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/mark_test_skipped.txtar b/claudetool/testdata/mark_test_skipped.txtar
new file mode 100644
index 0000000..bf887e3
--- /dev/null
+++ b/claudetool/testdata/mark_test_skipped.txtar
@@ -0,0 +1,32 @@
+Marking a passing test as skipped test is a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+}
+
+-- .commit --
+Initial commit
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.SkipNow()
+}
+
+-- .commit --
+Skip test
+
+-- .run_test --
+Test regressions detected between initial commit (851ba0628e0e2d93e27e168d6af336f3e4d375c7) and HEAD:
+
+1: sketch.dev.TestP: Was passing, now skipped
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/multi_commit_review.txtar b/claudetool/testdata/multi_commit_review.txtar
new file mode 100644
index 0000000..2286dee
--- /dev/null
+++ b/claudetool/testdata/multi_commit_review.txtar
@@ -0,0 +1,51 @@
+Test that all issues across multiple commits are reported
+
+-- a.go --
+package main
+
+-- .commit --
+Initial commit
+
+-- b.go --
+package main
+
+import "sync"
+
+var x sync.Mutex
+var y = x
+
+-- .commit --
+Add another file
+
+-- c_test.go --
+package main
+
+import "testing"
+
+func TestX(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Add a failing test
+
+-- d.go --
+package main
+
+-- .commit --
+Add yet another file
+
+-- .run_test --
+Test regressions detected between initial commit (a4fd3c2166b35e02a2cde466880e45aea6e6212e) and HEAD:
+
+1: sketch.dev.TestX: New test is failing
+
+
+Gopls check issues detected:
+
+1. /PATH/TO/REPO/b.go:6:9-10: variable declaration copies lock value to y: sync.Mutex
+
+IMPORTANT: Only fix new gopls check issues in parts of the code that you have already edited. Do not change existing code that was not part of your current edits.
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/multiple_format_issues.txtar b/claudetool/testdata/multiple_format_issues.txtar
new file mode 100644
index 0000000..0024a48
--- /dev/null
+++ b/claudetool/testdata/multiple_format_issues.txtar
@@ -0,0 +1,21 @@
+Test that formatting issues are not reported if pre-existing, per-file
+
+-- main.go --
+	package main
+
+-- .commit --
+Initial commit
+
+-- main.go --
+	package main
+
+	import "fmt"
+
+-- file1.go --
+	package main
+
+-- .commit --
+Add multiple files with formatting issues
+
+-- .run_autoformat --
+/PATH/TO/REPO/file1.go
diff --git a/claudetool/testdata/new_test_build_error.txtar b/claudetool/testdata/new_test_build_error.txtar
new file mode 100644
index 0000000..2fe1d34
--- /dev/null
+++ b/claudetool/testdata/new_test_build_error.txtar
@@ -0,0 +1,27 @@
+Adding a new test with build errors is a regression
+
+-- p.go --
+package p
+
+-- .commit --
+Initial commit with no tests
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	(
+}
+
+-- .commit --
+Add test with build error
+
+-- .run_test --
+Test regressions detected between initial commit (cf75ed70e940ce33a17a70b1894ad053b67731c0) and HEAD:
+
+1: sketch.dev: Previously had no tests, now has build/vet errors
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/no_fmt_autogenerated.txtar b/claudetool/testdata/no_fmt_autogenerated.txtar
new file mode 100644
index 0000000..aab3f57
--- /dev/null
+++ b/claudetool/testdata/no_fmt_autogenerated.txtar
@@ -0,0 +1,22 @@
+Test that autogenerated files do not get formatted
+
+-- .commit --
+Initial commit
+
+-- generated.go --
+// Code generated DO NOT EDIT.
+
+package main
+
+import (
+_ "fmt"
+)
+
+-- manual.go --
+	package main
+
+-- .commit --
+Add generated and manual files with poor formatting
+
+-- .run_autoformat --
+/PATH/TO/REPO/manual.go
diff --git a/claudetool/testdata/no_tests.txtar b/claudetool/testdata/no_tests.txtar
new file mode 100644
index 0000000..ceeb2a5
--- /dev/null
+++ b/claudetool/testdata/no_tests.txtar
@@ -0,0 +1,19 @@
+Going from no tests to no tests is not a regression
+
+-- p.go --
+package p
+
+-- .commit --
+Initial commit
+
+-- other.go --
+package p
+
+func F() {
+}
+
+-- .commit --
+Add func
+
+-- .run_test --
+OK
diff --git a/claudetool/testdata/no_tests_to_failing_tests.txtar b/claudetool/testdata/no_tests_to_failing_tests.txtar
new file mode 100644
index 0000000..b0cc956
--- /dev/null
+++ b/claudetool/testdata/no_tests_to_failing_tests.txtar
@@ -0,0 +1,27 @@
+Adding failing tests is a regression
+
+-- p.go --
+package p
+
+-- .commit --
+Initial commit with no tests
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Add failing test
+
+-- .run_test --
+Test regressions detected between initial commit (cf75ed70e940ce33a17a70b1894ad053b67731c0) and HEAD:
+
+1: sketch.dev.TestP: New test is failing
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/passing_to_build_error.txtar b/claudetool/testdata/passing_to_build_error.txtar
new file mode 100644
index 0000000..9558862
--- /dev/null
+++ b/claudetool/testdata/passing_to_build_error.txtar
@@ -0,0 +1,32 @@
+Going from a passing test to a test with build errors is a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+}
+
+-- .commit --
+Initial commit with passing test
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	(
+}
+
+-- .commit --
+Break test with build error
+
+-- .run_test --
+Test regressions detected between initial commit (149798f10aaf06bc4700bb62d46ed6d8d88e35ad) and HEAD:
+
+1: sketch.dev: Was passing, now has build/vet errors
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/passing_to_failing.txtar b/claudetool/testdata/passing_to_failing.txtar
new file mode 100644
index 0000000..31a0556
--- /dev/null
+++ b/claudetool/testdata/passing_to_failing.txtar
@@ -0,0 +1,32 @@
+Going from a passing test to a failing test is a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+}
+
+-- .commit --
+Initial commit with passing test
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Break test
+
+-- .run_test --
+Test regressions detected between initial commit (149798f10aaf06bc4700bb62d46ed6d8d88e35ad) and HEAD:
+
+1: sketch.dev.TestP: Was passing, now failing
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/passing_to_failing_subdir.txtar b/claudetool/testdata/passing_to_failing_subdir.txtar
new file mode 100644
index 0000000..7649554
--- /dev/null
+++ b/claudetool/testdata/passing_to_failing_subdir.txtar
@@ -0,0 +1,32 @@
+Going from a passing test to a failing test in a subdir is a regression
+
+-- foo/p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+}
+
+-- .commit --
+Initial commit with passing test
+
+-- foo/p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Break test
+
+-- .run_test --
+Test regressions detected between initial commit (8c6c71d0beac5e27b87dfc3f2fd9d274b62c3d3a) and HEAD:
+
+1: sketch.dev/foo.TestP: Was passing, now failing
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/skipped_to_build_error.txtar b/claudetool/testdata/skipped_to_build_error.txtar
new file mode 100644
index 0000000..f648572
--- /dev/null
+++ b/claudetool/testdata/skipped_to_build_error.txtar
@@ -0,0 +1,33 @@
+Going from a skipped test to a test with build errors is a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.SkipNow()
+}
+
+-- .commit --
+Initial commit with skipped test
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	(
+}
+
+-- .commit --
+Change skipped test to test with build error
+
+-- .run_test --
+Test regressions detected between initial commit (3635353dfefb2fa652728318567209ccfc4aba42) and HEAD:
+
+1: sketch.dev: Was passing, now has build/vet errors
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/skipped_to_failing.txtar b/claudetool/testdata/skipped_to_failing.txtar
new file mode 100644
index 0000000..50348d2
--- /dev/null
+++ b/claudetool/testdata/skipped_to_failing.txtar
@@ -0,0 +1,33 @@
+Going from a skipped test to a failing test is a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.SkipNow()
+}
+
+-- .commit --
+Initial commit with skipped test
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Change skipped test to failing test
+
+-- .run_test --
+Test regressions detected between initial commit (3635353dfefb2fa652728318567209ccfc4aba42) and HEAD:
+
+1: sketch.dev.TestP: Was skipped, now failing
+
+
+Please fix before proceeding.
diff --git a/claudetool/testdata/vet_error_test.txtar b/claudetool/testdata/vet_error_test.txtar
new file mode 100644
index 0000000..8c9dc64
--- /dev/null
+++ b/claudetool/testdata/vet_error_test.txtar
@@ -0,0 +1,36 @@
+Check that vet errors are detected in a test run
+
+-- p.go --
+package p
+
+-- .commit --
+Initial commit
+
+-- p.go --
+package p
+
+import (
+	"fmt"
+)
+
+func F()  {
+	fmt.Printf("Not a string: %s\n", 10)
+}
+
+-- .commit --
+Add vet error
+
+-- .run_test --
+Test regressions detected between initial commit (656e4d573cb77a30ebce29ebb974533ece282946) and HEAD:
+
+1: sketch.dev: Previously had no tests, now has build/vet errors
+
+
+Gopls check issues detected:
+
+1. /PATH/TO/REPO/p.go:8:2-38: fmt.Printf format %s has arg 10 of wrong type int
+
+IMPORTANT: Only fix new gopls check issues in parts of the code that you have already edited. Do not change existing code that was not part of your current edits.
+
+
+Please fix before proceeding.
diff --git a/claudetool/teststatus_string.go b/claudetool/teststatus_string.go
new file mode 100644
index 0000000..a5b159b
--- /dev/null
+++ b/claudetool/teststatus_string.go
@@ -0,0 +1,28 @@
+// Code generated by "stringer -type=testStatus -trimprefix=testStatus"; DO NOT EDIT.
+
+package claudetool
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[testStatusUnknown-0]
+	_ = x[testStatusPass-1]
+	_ = x[testStatusFail-2]
+	_ = x[testStatusBuildFail-3]
+	_ = x[testStatusSkip-4]
+	_ = x[testStatusNoTests-5]
+}
+
+const _testStatus_name = "UnknownPassFailBuildFailSkipNoTests"
+
+var _testStatus_index = [...]uint8{0, 7, 11, 15, 24, 28, 35}
+
+func (i testStatus) String() string {
+	if i < 0 || i >= testStatus(len(_testStatus_index)-1) {
+		return "testStatus(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _testStatus_name[_testStatus_index[i]:_testStatus_index[i+1]]
+}
diff --git a/go.mod b/go.mod
index a0a8383..9a8ca14 100644
--- a/go.mod
+++ b/go.mod
@@ -10,10 +10,10 @@
 	github.com/richardlehane/crock32 v1.0.1
 	go.skia.org/infra v0.0.0-20250421160028-59e18403fd4a
 	golang.org/x/crypto v0.37.0
-	golang.org/x/net v0.38.0
+	golang.org/x/net v0.39.0
 	golang.org/x/sync v0.13.0
 	golang.org/x/term v0.31.0
-	golang.org/x/tools v0.31.0
+	golang.org/x/tools v0.32.0
 	mvdan.cc/sh/v3 v3.11.0
 )
 
@@ -28,3 +28,5 @@
 	golang.org/x/sys v0.32.0 // indirect
 	golang.org/x/text v0.24.0 // indirect
 )
+
+tool golang.org/x/tools/cmd/stringer
diff --git a/go.sum b/go.sum
index 94f5f98..d503c06 100644
--- a/go.sum
+++ b/go.sum
@@ -43,6 +43,8 @@
 golang.org/x/mod v0.24.0/go.mod h1:IXM97Txy2VM4PJ3gI61r1YEk/gAj6zAHN3AdZt6S9Ww=
 golang.org/x/net v0.38.0 h1:vRMAPTMaeGqVhG5QyLJHqNDwecKTomGeqbnfZyKlBI8=
 golang.org/x/net v0.38.0/go.mod h1:ivrbrMbzFq5J41QOQh0siUuly180yBYtLp+CKbEaFx8=
+golang.org/x/net v0.39.0 h1:ZCu7HMWDxpXpaiKdhzIfaltL9Lp31x/3fCP11bc6/fY=
+golang.org/x/net v0.39.0/go.mod h1:X7NRbYVEA+ewNkCNyJ513WmMdQ3BineSwVtN2zD/d+E=
 golang.org/x/sync v0.13.0 h1:AauUjRAJ9OSnvULf/ARrrVywoJDy0YS2AwQ98I37610=
 golang.org/x/sync v0.13.0/go.mod h1:1dzgHSNfp02xaA81J2MS99Qcpr2w7fw1gpm99rleRqA=
 golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
@@ -56,6 +58,8 @@
 golang.org/x/text v0.24.0/go.mod h1:L8rBsPeo2pSS+xqN0d5u2ikmjtmoJbDBT1b7nHvFCdU=
 golang.org/x/tools v0.31.0 h1:0EedkvKDbh+qistFTd0Bcwe/YLh4vHwWEkiI0toFIBU=
 golang.org/x/tools v0.31.0/go.mod h1:naFTU+Cev749tSJRXJlna0T3WxKvb1kWEx15xA4SdmQ=
+golang.org/x/tools v0.32.0 h1:Q7N1vhpkQv7ybVzLFtTjvQya2ewbwNDZzUgfXGqtMWU=
+golang.org/x/tools v0.32.0/go.mod h1:ZxrU41P/wAbZD8EDa6dDCa6XfpkhJ7HFMjHJXfBDu8s=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 mvdan.cc/sh/v3 v3.11.0 h1:q5h+XMDRfUGUedCqFFsjoFjrhwf2Mvtt1rkMvVz0blw=
diff --git a/loop/agent.go b/loop/agent.go
index 1ce67e5..2f4efe9 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -598,7 +598,10 @@
 <git_root>
 %v
 </git_root>
-`, editPrompt, a.config.ClientGOOS, a.config.ClientGOARCH, a.workingDir, a.repoRoot)
+<HEAD>
+%v
+</HEAD>
+`, editPrompt, a.config.ClientGOOS, a.config.ClientGOARCH, a.workingDir, a.repoRoot, a.initialCommit)
 
 	// Register all tools with the conversation
 	// When adding, removing, or modifying tools here, double-check that the termui tool display
diff --git a/loop/testdata/agent_loop.httprr b/loop/testdata/agent_loop.httprr
index 63f32f1..476c849 100644
--- a/loop/testdata/agent_loop.httprr
+++ b/loop/testdata/agent_loop.httprr
@@ -1,9 +1,9 @@
 httprr trace v1
-9008 1823
+9047 1729
 POST https://api.anthropic.com/v1/messages HTTP/1.1
 Host: api.anthropic.com
 User-Agent: Go-http-client/1.1
-Content-Length: 8811
+Content-Length: 8850
 Anthropic-Version: 2023-06-01
 Content-Type: application/json
 
@@ -231,7 +231,7 @@
  ],
  "system": [
   {
-   "text": "\nYou are an expert coding assistant and architect, with a specialty in Go.\nYou are assisting the user to achieve their goals.\n\nStart by asking concise clarifying questions as needed.\nOnce the intent is clear, work autonomously.\n\nCall the title tool early in the conversation to provide a brief summary of\nwhat the chat is about.\n\nBreak down the overall goal into a series of smaller steps.\n(The first step is often: \"Make a plan.\")\nThen execute each step using tools.\nUpdate the plan if you have encountered problems or learned new information.\n\nWhen in doubt about a step, follow this broad workflow:\n\n- Think about how the current step fits into the overall plan.\n- Do research. Good tool choices: bash, think, keyword_search\n- Make edits.\n- Repeat.\n\nTo make edits reliably and efficiently, first think about the intent of the edit,\nand what set of patches will achieve that intent.\nThen use the patch tool to make those edits. Combine all edits to any given file into a single patch tool call.\n\nFor renames or refactors, consider invoking gopls (via bash).\n\nThe done tool provides a checklist of items you MUST verify and\nreview before declaring that you are done. Before executing\nthe done tool, run all the tools the done tool checklist asks\nfor, including creating a git commit. Do not forget to run tests.\n\n\u003cplatform\u003e\nlinux/amd64\n\u003c/platform\u003e\n\u003cpwd\u003e\n/\n\u003c/pwd\u003e\n\u003cgit_root\u003e\n\n\u003c/git_root\u003e\n",
+   "text": "\nYou are an expert coding assistant and architect, with a specialty in Go.\nYou are assisting the user to achieve their goals.\n\nStart by asking concise clarifying questions as needed.\nOnce the intent is clear, work autonomously.\n\nCall the title tool early in the conversation to provide a brief summary of\nwhat the chat is about.\n\nBreak down the overall goal into a series of smaller steps.\n(The first step is often: \"Make a plan.\")\nThen execute each step using tools.\nUpdate the plan if you have encountered problems or learned new information.\n\nWhen in doubt about a step, follow this broad workflow:\n\n- Think about how the current step fits into the overall plan.\n- Do research. Good tool choices: bash, think, keyword_search\n- Make edits.\n- Repeat.\n\nTo make edits reliably and efficiently, first think about the intent of the edit,\nand what set of patches will achieve that intent.\nThen use the patch tool to make those edits. Combine all edits to any given file into a single patch tool call.\n\nFor renames or refactors, consider invoking gopls (via bash).\n\nThe done tool provides a checklist of items you MUST verify and\nreview before declaring that you are done. Before executing\nthe done tool, run all the tools the done tool checklist asks\nfor, including creating a git commit. Do not forget to run tests.\n\n\u003cplatform\u003e\nlinux/amd64\n\u003c/platform\u003e\n\u003cpwd\u003e\n/\n\u003c/pwd\u003e\n\u003cgit_root\u003e\n\n\u003c/git_root\u003e\n\u003cHEAD\u003e\n\n\u003c/HEAD\u003e\n",
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-{"id":"msg_01T9eHh8JfMCZsCq4BBDF43G","type":"message","role":"assistant","model":"claude-3-7-sonnet-20250219","content":[{"type":"text","text":"Here are the tools available to me:\n\n1. bash - Executes shell commands with optional timeout\n2. keyword_search - Searches for files based on keywords and a query\n3. think - For thinking out loud, taking notes, forming plans\n4. title - Sets a brief title for the conversation\n5. done - Used to indicate task completion with a checklist\n6. codereview - Runs an automated code review\n7. patch - Makes precise text edits to files\n\nI can use these tools to help you accomplish various coding and development tasks, particularly in Go. Is there a specific task you'd like me to help you with?"}],"stop_reason":"end_turn","stop_sequence":null,"usage":{"input_tokens":4,"cache_creation_input_tokens":2183,"cache_read_input_tokens":0,"output_tokens":141}}
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+{"id":"msg_01KJCY38iw9vZgZMutyuHUqk","type":"message","role":"assistant","model":"claude-3-7-sonnet-20250219","content":[{"type":"text","text":"I can access the following tools:\n\n1. bash - Executes shell commands\n2. keyword_search - Searches for files using keywords\n3. think - Records thoughts without external effects\n4. title - Sets a title for the conversation\n5. done - Marks completion with a checklist\n6. codereview - Runs automated code review\n7. patch - Makes precise text edits to files\n\nThese tools help me assist you with coding tasks, particularly in Go development. Is there a specific task you'd like me to help you with?"}],"stop_reason":"end_turn","stop_sequence":null,"usage":{"input_tokens":4,"cache_creation_input_tokens":0,"cache_read_input_tokens":2191,"output_tokens":120}}
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