diff --git a/claudetool/codereview/differential.go b/claudetool/codereview/differential.go
new file mode 100644
index 0000000..6aa8dfe
--- /dev/null
+++ b/claudetool/codereview/differential.go
@@ -0,0 +1,1040 @@
+package codereview
+
+import (
+	"bytes"
+	"cmp"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/fs"
+	"log/slog"
+	"maps"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"slices"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/tools/go/packages"
+	"sketch.dev/llm"
+)
+
+// This file does differential quality analysis of a commit relative to a base commit.
+
+// Tool returns a tool spec for a CodeReview tool backed by r.
+func (r *CodeReviewer) Tool() *llm.Tool {
+	spec := &llm.Tool{
+		Name:        "codereview",
+		Description: `Run an automated code review.`,
+		// If you modify this, update the termui template for prettier rendering.
+		InputSchema: llm.MustSchema(`{"type": "object", "properties": {}}`),
+		Run:         r.Run,
+	}
+	return spec
+}
+
+func (r *CodeReviewer) Run(ctx context.Context, m json.RawMessage) (string, error) {
+	// NOTE: If you add or modify error messages here, update the corresponding UI parsing in:
+	// webui/src/web-components/sketch-tool-card.ts (SketchToolCardCodeReview.getStatusIcon)
+	if err := r.RequireNormalGitState(ctx); err != nil {
+		slog.DebugContext(ctx, "CodeReviewer.Run: failed to check for normal git state", "err", err)
+		return "", err
+	}
+	if err := r.RequireNoUncommittedChanges(ctx); err != nil {
+		slog.DebugContext(ctx, "CodeReviewer.Run: failed to check for uncommitted changes", "err", err)
+		return "", err
+	}
+
+	// Check that the current commit is not the initial commit
+	currentCommit, err := r.CurrentCommit(ctx)
+	if err != nil {
+		slog.DebugContext(ctx, "CodeReviewer.Run: failed to get current commit", "err", err)
+		return "", err
+	}
+	if r.IsInitialCommit(currentCommit) {
+		slog.DebugContext(ctx, "CodeReviewer.Run: current commit is initial commit, nothing to review")
+		return "", fmt.Errorf("no new commits have been added, nothing to review")
+	}
+
+	// No matter what failures happen from here out, we will declare this to have been reviewed.
+	// This should help avoid the model getting blocked by a broken code review tool.
+	r.reviewed = append(r.reviewed, currentCommit)
+
+	changedFiles, err := r.changedFiles(ctx, r.initialCommit, currentCommit)
+	if err != nil {
+		slog.DebugContext(ctx, "CodeReviewer.Run: failed to get changed files", "err", err)
+		return "", err
+	}
+
+	// Prepare to analyze before/after for the impacted files.
+	// We use the current commit to determine what packages exist and are impacted.
+	// The packages in the initial commit may be different.
+	// Good enough for now.
+	// TODO: do better
+	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))
+
+	var errorMessages []string // problems we want the model to address
+	var infoMessages []string  // info the model should consider
+
+	// Find potentially related files that should also be considered
+	// TODO: add some caching here, since this depends only on the initial commit and the changed files, not the details of the changes
+	relatedFiles, err := r.findRelatedFiles(ctx, changedFiles)
+	if err != nil {
+		slog.DebugContext(ctx, "CodeReviewer.Run: failed to find related files", "err", err)
+	} else {
+		relatedMsg := r.formatRelatedFiles(relatedFiles)
+		if relatedMsg != "" {
+			infoMessages = append(infoMessages, relatedMsg)
+		}
+	}
+
+	testMsg, err := r.checkTests(ctx, allPkgList)
+	if err != nil {
+		slog.DebugContext(ctx, "CodeReviewer.Run: failed to check tests", "err", err)
+		return "", err
+	}
+	if testMsg != "" {
+		errorMessages = append(errorMessages, testMsg)
+	}
+
+	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
+	}
+	if goplsMsg != "" {
+		errorMessages = append(errorMessages, goplsMsg)
+	}
+
+	if r.llmReview {
+		llmComments, err := r.doLLMReview(ctx)
+		if err != nil {
+			// Log the error but don't fail the codereview if this check fails
+			slog.WarnContext(ctx, "CodeReviewer.Run: error doing LLM review", "err", err)
+		}
+		if llmComments != "" {
+			infoMessages = append(infoMessages, llmComments)
+		}
+	}
+
+	// NOTE: If you change this output format, update the corresponding UI parsing in:
+	// webui/src/web-components/sketch-tool-card.ts (SketchToolCardCodeReview.getStatusIcon)
+	buf := new(strings.Builder)
+	if len(infoMessages) > 0 {
+		buf.WriteString("# Info\n\n")
+		buf.WriteString(strings.Join(infoMessages, "\n\n"))
+		buf.WriteString("\n\n")
+	}
+	if len(errorMessages) > 0 {
+		buf.WriteString("# Errors\n\n")
+		buf.WriteString(strings.Join(errorMessages, "\n\n"))
+		buf.WriteString("\n\nPlease fix before proceeding.\n")
+	}
+	if buf.Len() == 0 {
+		buf.WriteString("OK")
+	}
+	return buf.String(), nil
+}
+
+func (r *CodeReviewer) initializeInitialCommitWorktree(ctx context.Context) error {
+	if r.initialWorktree != "" {
+		return nil
+	}
+	tmpDir, err := os.MkdirTemp("", "sketch-codereview-worktree")
+	if err != nil {
+		return err
+	}
+	worktreeCmd := exec.CommandContext(ctx, "git", "worktree", "add", "--detach", tmpDir, r.initialCommit)
+	worktreeCmd.Dir = r.repoRoot
+	out, err := worktreeCmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("unable to create worktree for initial commit: %w\n%s", err, out)
+	}
+	r.initialWorktree = tmpDir
+	return nil
+}
+
+func (r *CodeReviewer) checkTests(ctx context.Context, pkgList []string) (string, error) {
+	goTestArgs := []string{"test", "-json", "-v"}
+	goTestArgs = append(goTestArgs, pkgList...)
+
+	afterTestCmd := exec.CommandContext(ctx, "go", goTestArgs...)
+	afterTestCmd.Dir = r.repoRoot
+	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 {
+		return "", err
+	}
+
+	beforeTestCmd := exec.CommandContext(ctx, "go", goTestArgs...)
+	beforeTestCmd.Dir = r.initialWorktree
+	beforeTestOut, _ := beforeTestCmd.Output() // ignore error, interesting info is in the output
+
+	// Parse the jsonl test results
+	beforeResults, beforeParseErr := parseTestResults(beforeTestOut)
+	if beforeParseErr != nil {
+		return "", fmt.Errorf("unable to parse test results for initial commit: %w\n%s", beforeParseErr, beforeTestOut)
+	}
+	afterResults, afterParseErr := parseTestResults(afterTestOut)
+	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)
+	}
+	// TODO: better output formatting?
+	res := r.formatTestRegressions(testRegressions)
+	return res, nil
+}
+
+// GoplsIssue represents a single issue reported by gopls check
+type GoplsIssue struct {
+	Position string // File position in format "file:line:col-range"
+	Message  string // Description of the issue
+}
+
+// checkGopls runs gopls check on the provided files in both the current and initial state,
+// compares the results, and reports any new issues introduced in the current state.
+func (r *CodeReviewer) checkGopls(ctx context.Context, changedFiles []string) (string, error) {
+	if len(changedFiles) == 0 {
+		return "", nil // no files to check
+	}
+
+	// Filter out non-Go files as gopls only works on Go files
+	// and verify they still exist (not deleted)
+	var goFiles []string
+	for _, file := range changedFiles {
+		if !strings.HasSuffix(file, ".go") {
+			continue // not a Go file
+		}
+
+		// Check if the file still exists (not deleted)
+		if _, err := os.Stat(file); os.IsNotExist(err) {
+			continue // file doesn't exist anymore (deleted)
+		}
+
+		goFiles = append(goFiles, file)
+	}
+
+	if len(goFiles) == 0 {
+		return "", nil // no Go files to check
+	}
+
+	// Run gopls check on the current state
+	goplsArgs := append([]string{"check"}, goFiles...)
+
+	afterGoplsCmd := exec.CommandContext(ctx, "gopls", goplsArgs...)
+	afterGoplsCmd.Dir = r.repoRoot
+	afterGoplsOut, err := afterGoplsCmd.CombinedOutput() // gopls returns non-zero if it finds issues
+	if err != nil {
+		// Check if the output looks like real gopls issues or if it's just error output
+		if !looksLikeGoplsIssues(afterGoplsOut) {
+			slog.WarnContext(ctx, "CodeReviewer.checkGopls: gopls check failed to run properly", "err", err, "output", string(afterGoplsOut))
+			return "", nil // Skip rather than failing the entire code review
+		}
+		// Otherwise, proceed with parsing - it's likely just the non-zero exit code due to found issues
+	}
+
+	// Parse the output
+	afterIssues := parseGoplsOutput(r.repoRoot, afterGoplsOut)
+
+	// If no issues were found, we're done
+	if len(afterIssues) == 0 {
+		return "", nil
+	}
+
+	// Gopls detected issues in the current state, check if they existed in the initial state
+	initErr := r.initializeInitialCommitWorktree(ctx)
+	if initErr != nil {
+		return "", err
+	}
+
+	// For each file that exists in the initial commit, run gopls check
+	var initialFilesToCheck []string
+	for _, file := range goFiles {
+		// Get relative path for git operations
+		relFile, err := filepath.Rel(r.repoRoot, file)
+		if err != nil {
+			slog.WarnContext(ctx, "CodeReviewer.checkGopls: failed to get relative path", "repo_root", r.repoRoot, "file", file, "err", err)
+			continue
+		}
+
+		// Check if the file exists in the initial commit
+		checkCmd := exec.CommandContext(ctx, "git", "cat-file", "-e", fmt.Sprintf("%s:%s", r.initialCommit, relFile))
+		checkCmd.Dir = r.repoRoot
+		if err := checkCmd.Run(); err == nil {
+			// File exists in initial commit
+			initialFilePath := filepath.Join(r.initialWorktree, relFile)
+			initialFilesToCheck = append(initialFilesToCheck, initialFilePath)
+		}
+	}
+
+	// Run gopls check on the files that existed in the initial commit
+	var beforeIssues []GoplsIssue
+	if len(initialFilesToCheck) > 0 {
+		beforeGoplsArgs := append([]string{"check"}, initialFilesToCheck...)
+		beforeGoplsCmd := exec.CommandContext(ctx, "gopls", beforeGoplsArgs...)
+		beforeGoplsCmd.Dir = r.initialWorktree
+		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))
+		} else {
+			beforeIssues = parseGoplsOutput(r.initialWorktree, beforeGoplsOut)
+		}
+	}
+
+	// Find new issues that weren't present in the initial state
+	goplsRegressions := findGoplsRegressions(beforeIssues, afterIssues)
+	if len(goplsRegressions) == 0 {
+		return "", nil // no new issues
+	}
+
+	// Format the results
+	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(root string, output []byte) []GoplsIssue {
+	var issues []GoplsIssue
+	for line := range strings.Lines(string(output)) {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+
+		// Skip lines that look like error messages rather than gopls issues
+		if strings.HasPrefix(line, "Error:") ||
+			strings.HasPrefix(line, "Failed:") ||
+			strings.HasPrefix(line, "Warning:") ||
+			strings.HasPrefix(line, "gopls:") {
+			continue
+		}
+
+		// Find the first colon that separates the file path from the line number
+		firstColonIdx := strings.Index(line, ":")
+		if firstColonIdx < 0 {
+			continue // Invalid format
+		}
+
+		// Verify the part before the first colon looks like a file path
+		potentialPath := line[:firstColonIdx]
+		if !strings.HasSuffix(potentialPath, ".go") {
+			continue // Not a Go file path
+		}
+
+		// Find the position of the first message separator ': '
+		// This separates the position info from the message
+		messageStart := strings.Index(line, ": ")
+		if messageStart < 0 || messageStart <= firstColonIdx {
+			continue // Invalid format
+		}
+
+		// 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)
+		colonCount := strings.Count(position, ":")
+		if colonCount < 2 {
+			continue // Not enough position information
+		}
+
+		issues = append(issues, GoplsIssue{
+			Position: position,
+			Message:  message,
+		})
+	}
+
+	return issues
+}
+
+// looksLikeGoplsIssues checks if the output appears to be actual gopls issues
+// rather than error messages about gopls itself failing
+func looksLikeGoplsIssues(output []byte) bool {
+	// If output is empty, it's not valid issues
+	if len(output) == 0 {
+		return false
+	}
+
+	// Check if output has at least one line that looks like a gopls issue
+	// A gopls issue looks like: '/path/to/file.go:123:45-67: message'
+	for line := range strings.Lines(string(output)) {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+
+		// A gopls issue has at least two colons (file path, line number, column)
+		// and contains a colon followed by a space (separating position from message)
+		colonCount := strings.Count(line, ":")
+		hasSeparator := strings.Contains(line, ": ")
+
+		if colonCount >= 2 && hasSeparator {
+			// Check if it starts with a likely file path (ending in .go)
+			parts := strings.SplitN(line, ":", 2)
+			if strings.HasSuffix(parts[0], ".go") {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// normalizeGoplsPosition extracts just the file path from a position string
+func normalizeGoplsPosition(position string) string {
+	// Extract just the file path by taking everything before the first colon
+	parts := strings.Split(position, ":")
+	if len(parts) < 1 {
+		return position
+	}
+	return parts[0]
+}
+
+// findGoplsRegressions identifies gopls issues that are new in the after state
+func findGoplsRegressions(before, after []GoplsIssue) []GoplsIssue {
+	var regressions []GoplsIssue
+
+	// Build map of before issues for easier lookup
+	beforeIssueMap := make(map[string]map[string]bool) // file -> message -> exists
+	for _, issue := range before {
+		file := normalizeGoplsPosition(issue.Position)
+		if _, exists := beforeIssueMap[file]; !exists {
+			beforeIssueMap[file] = make(map[string]bool)
+		}
+		// Store both the exact message and the general issue type for fuzzy matching
+		beforeIssueMap[file][issue.Message] = true
+
+		// Extract the general issue type (everything before the first ':' in the message)
+		generalIssue := issue.Message
+		if colonIdx := strings.Index(issue.Message, ":"); colonIdx > 0 {
+			generalIssue = issue.Message[:colonIdx]
+		}
+		beforeIssueMap[file][generalIssue] = true
+	}
+
+	// Check each after issue to see if it's new
+	for _, afterIssue := range after {
+		file := normalizeGoplsPosition(afterIssue.Position)
+		isNew := true
+
+		if fileIssues, fileExists := beforeIssueMap[file]; fileExists {
+			// Check for exact message match
+			if fileIssues[afterIssue.Message] {
+				isNew = false
+			} else {
+				// Check for general issue type match
+				generalIssue := afterIssue.Message
+				if colonIdx := strings.Index(afterIssue.Message, ":"); colonIdx > 0 {
+					generalIssue = afterIssue.Message[:colonIdx]
+				}
+				if fileIssues[generalIssue] {
+					isNew = false
+				}
+			}
+		}
+
+		if isNew {
+			regressions = append(regressions, afterIssue)
+		}
+	}
+
+	// Sort regressions for deterministic output
+	slices.SortFunc(regressions, func(a, b GoplsIssue) int {
+		return strings.Compare(a.Position, b.Position)
+	})
+
+	return regressions
+}
+
+// formatGoplsRegressions generates a human-readable summary of gopls check regressions
+func (r *CodeReviewer) formatGoplsRegressions(regressions []GoplsIssue) string {
+	if len(regressions) == 0 {
+		return ""
+	}
+
+	var sb strings.Builder
+	sb.WriteString("Gopls check issues detected:\n\n")
+
+	// Format each issue
+	for i, issue := range regressions {
+		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.\n")
+	return sb.String()
+}
+
+func (r *CodeReviewer) HasReviewed(commit string) bool {
+	return slices.Contains(r.reviewed, commit)
+}
+
+func (r *CodeReviewer) IsInitialCommit(commit string) bool {
+	return commit == r.initialCommit
+}
+
+// packagesForFiles returns maps of packages related to the given files:
+// 1. directPkgs: packages that directly contain the changed files
+// 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) (allPkgs map[string]*packages.Package, err error) {
+	for _, f := range files {
+		if !filepath.IsAbs(f) {
+			return nil, fmt.Errorf("path %q is not absolute", f)
+		}
+	}
+	cfg := &packages.Config{
+		Mode:    packages.LoadImports | packages.NeedEmbedFiles,
+		Context: ctx,
+		// Logf: func(msg string, args ...any) {
+		// 	slog.DebugContext(ctx, "loading go packages", "msg", fmt.Sprintf(msg, args...))
+		// },
+		// TODO: in theory, go.mod might not be in the repo root, and there might be multiple go.mod files.
+		// We can cross that bridge when we get there.
+		Dir:   r.repoRoot,
+		Tests: true,
+	}
+	universe, err := packages.Load(cfg, "./...")
+	if err != nil {
+		return nil, err
+	}
+	// Identify packages that directly contain the changed files
+	directPkgs := make(map[string]*packages.Package) // import path -> package
+	for _, pkg := range universe {
+		pkgFiles := allFiles(pkg)
+		for _, file := range files {
+			if pkgFiles[file] {
+				// prefer test packages, as they contain strictly more files (right?)
+				prev := directPkgs[pkg.PkgPath]
+				if prev == nil || prev.ForTest == "" {
+					directPkgs[pkg.PkgPath] = pkg
+				}
+			}
+		}
+	}
+
+	allPkgs = maps.Clone(directPkgs)
+
+	// Add packages that depend on the direct packages
+	addDependentPackages(universe, allPkgs)
+	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
+}
+
+// addDependentPackages adds to pkgs all packages from universe
+// that directly or indirectly depend on any package already in pkgs.
+func addDependentPackages(universe []*packages.Package, pkgs map[string]*packages.Package) {
+	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
+			}
+			for importPath := range p.Imports {
+				if _, ok := pkgs[importPath]; ok {
+					// imports a package dependent on pkgs, add it
+					pkgs[p.PkgPath] = p
+					changed = true
+					break
+				}
+			}
+		}
+		if !changed {
+			break
+		}
+	}
+}
+
+// testJSON is a union of BuildEvent and TestEvent
+type testJSON struct {
+	// TestEvent only:
+	// The Time field holds the time the event happened. It is conventionally omitted
+	// for cached test results.
+	Time time.Time `json:"Time"`
+	// BuildEvent only:
+	// The ImportPath field gives the package ID of the package being built.
+	// This matches the Package.ImportPath field of go list -json and the
+	// TestEvent.FailedBuild field of go test -json. Note that it does not
+	// match TestEvent.Package.
+	ImportPath string `json:"ImportPath"` // BuildEvent only
+	// TestEvent only:
+	// The Package field, if present, specifies the package being tested. When the
+	// go command runs parallel tests in -json mode, events from different tests are
+	// interlaced; the Package field allows readers to separate them.
+	Package string `json:"Package"`
+	// Action is used in both BuildEvent and TestEvent.
+	// It is the key to distinguishing between them.
+	// BuildEvent:
+	// build-output or build-fail
+	// TestEvent:
+	// start, run, pause, cont, pass, bench, fail, output, skip
+	Action string `json:"Action"`
+	// TestEvent only:
+	// The Test field, if present, specifies the test, example, or benchmark function
+	// that caused the event. Events for the overall package test do not set Test.
+	Test string `json:"Test"`
+	// TestEvent only:
+	// The Elapsed field is set for "pass" and "fail" events. It gives the time elapsed in seconds
+	// for the specific test or the overall package test that passed or failed.
+	Elapsed float64
+	// TestEvent:
+	// The Output field is set for Action == "output" and is a portion of the
+	// test's output (standard output and standard error merged together). The
+	// output is unmodified except that invalid UTF-8 output from a test is coerced
+	// into valid UTF-8 by use of replacement characters. With that one exception,
+	// the concatenation of the Output fields of all output events is the exact output
+	// of the test execution.
+	// BuildEvent:
+	// The Output field is set for Action == "build-output" and is a portion of
+	// the build's output. The concatenation of the Output fields of all output
+	// events is the exact output of the build. A single event may contain one
+	// or more lines of output and there may be more than one output event for
+	// a given ImportPath. This matches the definition of the TestEvent.Output
+	// field produced by go test -json.
+	Output string `json:"Output"`
+	// TestEvent only:
+	// The FailedBuild field is set for Action == "fail" if the test failure was caused
+	// by a build failure. It contains the package ID of the package that failed to
+	// build. This matches the ImportPath field of the "go list" output, as well as the
+	// BuildEvent.ImportPath field as emitted by "go build -json".
+	FailedBuild string `json:"FailedBuild"`
+}
+
+// parseTestResults converts test output in JSONL format into a slice of testJSON objects
+func parseTestResults(testOutput []byte) ([]testJSON, error) {
+	var results []testJSON
+	dec := json.NewDecoder(bytes.NewReader(testOutput))
+	for {
+		var event testJSON
+		if err := dec.Decode(&event); err != nil {
+			if err == io.EOF {
+				break
+			}
+			return nil, err
+		}
+		results = append(results, event)
+	}
+	return results, nil
+}
+
+// testStatus represents the status of a test in a given commit
+type testStatus int
+
+//go:generate go tool golang.org/x/tools/cmd/stringer -type=testStatus -trimprefix=testStatus
+const (
+	testStatusUnknown testStatus = iota
+	testStatusPass
+	testStatusFail
+	testStatusBuildFail
+	testStatusSkip
+	testStatusNoTests // no tests exist for this package
+)
+
+// testRegression represents a test that regressed between commits
+type testRegression struct {
+	Package      string
+	Test         string // empty for package tests
+	BeforeStatus testStatus
+	AfterStatus  testStatus
+	Output       string // failure output in the after state
+}
+
+func (r *testRegression) Source() string {
+	if r.Test == "" {
+		return r.Package
+	}
+	return fmt.Sprintf("%s.%s", r.Package, r.Test)
+}
+
+type packageResult struct {
+	Status     testStatus            // overall status for the package
+	TestStatus map[string]testStatus // name -> status
+	TestOutput map[string][]string   // name -> output parts
+}
+
+// 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
+		}
+
+		switch event.Action {
+		case "output":
+			p.TestOutput[event.Test] = append(p.TestOutput[event.Test], event.Output)
+			continue
+		case "pass":
+			if event.Test == "" {
+				p.Status = testStatusPass
+			} else {
+				p.TestStatus[event.Test] = testStatusPass
+			}
+		case "fail":
+			if event.Test == "" {
+				if event.FailedBuild != "" {
+					p.Status = testStatusBuildFail
+				} else {
+					p.Status = testStatusFail
+				}
+			} else {
+				p.TestStatus[event.Test] = testStatusFail
+			}
+		case "skip":
+			if event.Test == "" {
+				p.Status = testStatusNoTests
+			} else {
+				p.TestStatus[event.Test] = testStatusSkip
+			}
+		}
+	}
+
+	return m
+}
+
+// compareTestResults identifies tests that have regressed between commits
+func (r *CodeReviewer) compareTestResults(beforeResults, afterResults []testJSON) ([]testRegression, error) {
+	before := collectTestStatuses(beforeResults)
+	after := collectTestStatuses(afterResults)
+	var testLevelRegressions []testRegression
+	var packageLevelRegressions []testRegression
+
+	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
+	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.Package, b.Package); c != 0 {
+			return c
+		}
+		// Then by test name
+		return strings.Compare(a.Test, b.Test)
+	})
+
+	return regressions, nil
+}
+
+// badnessLevels maps test status to a badness level
+// Higher values indicate worse status (more severe issues)
+var badnessLevels = map[testStatus]int{
+	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 {
+	// Higher badness level means worse status
+	return badnessLevels[after] > badnessLevels[before]
+}
+
+// formatTestRegressions generates a human-readable summary of test regressions
+func (r *CodeReviewer) formatTestRegressions(regressions []testRegression) string {
+	if len(regressions) == 0 {
+		return ""
+	}
+
+	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 {
+		fmt.Fprintf(buf, "%d: %v: ", i+1, reg.Source())
+		key := regressionKey{reg.BeforeStatus, reg.AfterStatus}
+		message, exists := regressionMessages[key]
+		if !exists {
+			message = "Regression detected"
+		}
+		fmt.Fprintf(buf, "%s\n", message)
+	}
+
+	return buf.String()
+}
+
+// RelatedFile represents a file historically related to the changed files
+type RelatedFile struct {
+	Path        string  // Path to the file
+	Correlation float64 // Correlation score (0.0-1.0)
+}
+
+// findRelatedFiles identifies files that are historically related to the changed files
+// by analyzing git commit history for co-occurrences.
+func (r *CodeReviewer) findRelatedFiles(ctx context.Context, changedFiles []string) ([]RelatedFile, error) {
+	commits, err := r.getCommitsTouchingFiles(ctx, changedFiles)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get commits touching files: %w", err)
+	}
+	if len(commits) == 0 {
+		return nil, nil
+	}
+
+	relChanged := make(map[string]bool, len(changedFiles))
+	for _, file := range changedFiles {
+		rel, err := filepath.Rel(r.repoRoot, file)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get relative path for %s: %w", file, err)
+		}
+		relChanged[rel] = true
+	}
+
+	historyFiles := make(map[string]int)
+	var historyMu sync.Mutex
+
+	maxWorkers := runtime.GOMAXPROCS(0)
+	semaphore := make(chan bool, maxWorkers)
+	var wg sync.WaitGroup
+
+	for _, commit := range commits {
+		wg.Add(1)
+		semaphore <- true // acquire
+
+		go func(commit string) {
+			defer wg.Done()
+			defer func() { <-semaphore }() // release
+			commitFiles, err := r.getFilesInCommit(ctx, commit)
+			if err != nil {
+				slog.WarnContext(ctx, "Failed to get files in commit", "commit", commit, "err", err)
+				return
+			}
+			incr := 0
+			for _, file := range commitFiles {
+				if relChanged[file] {
+					incr++
+				}
+			}
+			if incr == 0 {
+				return
+			}
+			historyMu.Lock()
+			defer historyMu.Unlock()
+			for _, file := range commitFiles {
+				historyFiles[file] += incr
+			}
+		}(commit)
+	}
+	wg.Wait()
+
+	// normalize
+	maxCount := 0
+	for _, count := range historyFiles {
+		maxCount = max(maxCount, count)
+	}
+	if maxCount == 0 {
+		return nil, nil
+	}
+
+	var relatedFiles []RelatedFile
+	for file, count := range historyFiles {
+		if relChanged[file] {
+			// Don't include inputs in the output.
+			continue
+		}
+		correlation := float64(count) / float64(maxCount)
+		// Require min correlation to avoid noise
+		if correlation >= 0.1 {
+			relatedFiles = append(relatedFiles, RelatedFile{Path: file, Correlation: correlation})
+		}
+	}
+
+	// Highest correlation first
+	slices.SortFunc(relatedFiles, func(a, b RelatedFile) int {
+		return -1 * cmp.Compare(a.Correlation, b.Correlation)
+	})
+
+	// Limit to 1 correlated file per input file.
+	// (Arbitrary limit, to be adjusted.)
+	maxFiles := len(changedFiles)
+	if len(relatedFiles) > maxFiles {
+		relatedFiles = relatedFiles[:maxFiles]
+	}
+
+	// TODO: add an LLM in the mix here (like the keyword search tool) to do a filtering pass,
+	// and then increase the strength of the wording in the relatedFiles message.
+
+	return relatedFiles, nil
+}
+
+// getCommitsTouchingFiles returns all commits that touch any of the specified files
+func (r *CodeReviewer) getCommitsTouchingFiles(ctx context.Context, files []string) ([]string, error) {
+	if len(files) == 0 {
+		return nil, nil
+	}
+	fileArgs := append([]string{"rev-list", "--all", "--date-order", "--max-count=100", "--"}, files...)
+	cmd := exec.CommandContext(ctx, "git", fileArgs...)
+	cmd.Dir = r.repoRoot
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get commits: %w\n%s", err, out)
+	}
+	return nonEmptyTrimmedLines(out), nil
+}
+
+// getFilesInCommit returns all files changed in a specific commit
+func (r *CodeReviewer) getFilesInCommit(ctx context.Context, commit string) ([]string, error) {
+	cmd := exec.CommandContext(ctx, "git", "diff-tree", "--no-commit-id", "--name-only", "-r", commit)
+	cmd.Dir = r.repoRoot
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get files in commit: %w\n%s", err, out)
+	}
+	return nonEmptyTrimmedLines(out), nil
+}
+
+func nonEmptyTrimmedLines(b []byte) []string {
+	var lines []string
+	for line := range strings.Lines(string(b)) {
+		line = strings.TrimSpace(line)
+		if line != "" {
+			lines = append(lines, line)
+		}
+	}
+	return lines
+}
+
+// formatRelatedFiles formats the related files list into a human-readable message
+func (r *CodeReviewer) formatRelatedFiles(files []RelatedFile) string {
+	if len(files) == 0 {
+		return ""
+	}
+
+	buf := new(strings.Builder)
+
+	fmt.Fprintf(buf, "Potentially related files:\n\n")
+
+	for _, file := range files {
+		fmt.Fprintf(buf, "- %s (%0.0f%%)\n", file.Path, 100*file.Correlation)
+	}
+
+	fmt.Fprintf(buf, "\nThese files have historically changed with the files you have modified. Consider whether they require updates as well.\n")
+	return buf.String()
+}