diff --git a/claudetool/codereview/codereview.go b/claudetool/codereview/codereview.go
index c729c30..d99edff 100644
--- a/claudetool/codereview/codereview.go
+++ b/claudetool/codereview/codereview.go
@@ -21,12 +21,17 @@
 	initialStatus   []fileStatus // git status of files at initial commit, absolute paths
 	reviewed        []string     // history of all commits which have been reviewed
 	initialWorktree string       // git worktree at initial commit, absolute path
+	// "Related files" caching
+	processedChangedFileSets map[string]bool   // hash of sorted changedFiles -> processed
+	reportedRelatedFiles     map[string]bool   // file path -> reported
 }
 
 func NewCodeReviewer(ctx context.Context, repoRoot, sketchBaseRef string) (*CodeReviewer, error) {
 	r := &CodeReviewer{
-		repoRoot:      repoRoot,
-		sketchBaseRef: sketchBaseRef,
+		repoRoot:                 repoRoot,
+		sketchBaseRef:            sketchBaseRef,
+		processedChangedFileSets: make(map[string]bool),
+		reportedRelatedFiles:     make(map[string]bool),
 	}
 	if r.repoRoot == "" {
 		return nil, fmt.Errorf("NewCodeReviewer: repoRoot must be non-empty")
diff --git a/claudetool/codereview/codereview_test.go b/claudetool/codereview/codereview_test.go
index 3b04898..f9d240e 100644
--- a/claudetool/codereview/codereview_test.go
+++ b/claudetool/codereview/codereview_test.go
@@ -116,6 +116,7 @@
 // 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
+	var reviewer *CodeReviewer
 	filesForNextCommit := make(map[string]bool)
 
 	for i, file := range archive.Files {
@@ -127,14 +128,18 @@
 			}
 			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()
+			if reviewer == nil && initialCommit != "" {
+				reviewer, err = NewCodeReviewer(context.Background(), dir, initialCommit)
+				if err != nil {
+					return fmt.Errorf("error creating code reviewer: %w", err)
+				}
+			}
 
 		case ".run_test":
-			got, err := runDifferentialTest(dir, initialCommit)
+			if reviewer == nil {
+				return fmt.Errorf("no code reviewer available, need initial commit first")
+			}
+			got, err := runDifferentialTest(reviewer)
 			if err != nil {
 				return fmt.Errorf("error running differential test: %w", err)
 			}
@@ -153,11 +158,11 @@
 			}
 
 		case ".run_autoformat":
-			if initialCommit == "" {
-				return fmt.Errorf("initial commit not set, cannot run autoformat")
+			if reviewer == nil {
+				return fmt.Errorf("no code reviewer available, need initial commit first")
 			}
 
-			got, err := runAutoformat(dir, initialCommit)
+			got, err := runAutoformat(reviewer)
 			if err != nil {
 				return fmt.Errorf("error running autoformat: %w", err)
 			}
@@ -227,19 +232,8 @@
 }
 
 // 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
+func runDifferentialTest(reviewer *CodeReviewer) (string, error) {
 	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)
@@ -250,6 +244,7 @@
 	if len(result) > 0 {
 		resultStr = result[0].Text
 	}
+	dir := reviewer.repoRoot
 	normalized := normalizePaths(resultStr, dir)
 	return normalized, nil
 }
@@ -267,16 +262,10 @@
 }
 
 // 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")
-	}
+func runAutoformat(reviewer *CodeReviewer) ([]string, error) {
 	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)
+	dir := reviewer.repoRoot
 	normalizedFiles := make([]string, len(formattedFiles))
 	for i, file := range formattedFiles {
 		normalizedFiles[i] = normalizePaths(file, dir)
diff --git a/claudetool/codereview/differential.go b/claudetool/codereview/differential.go
index ff25b22..77dde85 100644
--- a/claudetool/codereview/differential.go
+++ b/claudetool/codereview/differential.go
@@ -4,6 +4,8 @@
 	"bytes"
 	"cmp"
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -968,9 +970,59 @@
 	Correlation float64 // Correlation score (0.0-1.0)
 }
 
-// findRelatedFiles identifies files that are historically related to the changed files
+// hashChangedFiles creates a deterministic hash of the changed files set
+func (r *CodeReviewer) hashChangedFiles(changedFiles []string) string {
+	// Sort files for deterministic hashing
+	sorted := slices.Clone(changedFiles)
+	slices.Sort(sorted)
+	h := sha256.New()
+	enc := json.NewEncoder(h)
+	err := enc.Encode(sorted)
+	if err != nil {
+		panic(err)
+	}
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// findRelatedFiles reports files that are historically related to the changed files
 // by analyzing git commit history for co-occurrences.
+// This function implements caching to avoid duplicate CPU and LLM processing:
+// 1. If the exact same set of changedFiles has been processed before, return nil, nil
+// 2. If all related files have been previously reported, return nil, nil
+// 3. Otherwise, return the full set of related files and mark them as reported
 func (r *CodeReviewer) findRelatedFiles(ctx context.Context, changedFiles []string) ([]RelatedFile, error) {
+	cf := r.hashChangedFiles(changedFiles)
+	if r.processedChangedFileSets[cf] {
+		return nil, nil
+	}
+	r.processedChangedFileSets[cf] = true // don't re-process, even on error
+
+	relatedFiles, err := r.computeRelatedFiles(ctx, changedFiles)
+	if err != nil {
+		return nil, err
+	}
+
+	hasNew := false
+	for _, rf := range relatedFiles {
+		if !r.reportedRelatedFiles[rf.Path] {
+			hasNew = true
+			break
+		}
+	}
+	if !hasNew {
+		return nil, nil
+	}
+
+	// We have new file(s) that haven't been called to the LLM's attention yet.
+	for _, rf := range relatedFiles {
+		r.reportedRelatedFiles[rf.Path] = true
+	}
+
+	return relatedFiles, nil
+}
+
+// computeRelatedFiles implements findRelatedFiles, without caching.
+func (r *CodeReviewer) computeRelatedFiles(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)
diff --git a/claudetool/codereview/differential_test.go b/claudetool/codereview/differential_test.go
index 827bf63..100d9f7 100644
--- a/claudetool/codereview/differential_test.go
+++ b/claudetool/codereview/differential_test.go
@@ -178,3 +178,39 @@
 		}
 	}
 }
+
+// TestHashChangedFiles tests the hash function for changed files
+func TestHashChangedFiles(t *testing.T) {
+	reviewer := &CodeReviewer{
+		repoRoot:                 "/app",
+		processedChangedFileSets: make(map[string]bool),
+		reportedRelatedFiles:     make(map[string]bool),
+	}
+
+	// Test that same files in different order produce same hash
+	files1 := []string{"/app/a.go", "/app/b.go"}
+	files2 := []string{"/app/b.go", "/app/a.go"}
+
+	hash1 := reviewer.hashChangedFiles(files1)
+	hash2 := reviewer.hashChangedFiles(files2)
+
+	if hash1 != hash2 {
+		t.Errorf("Expected same hash for same files in different order, got %q vs %q", hash1, hash2)
+	}
+
+	// Test that different files produce different hash
+	files3 := []string{"/app/a.go", "/app/c.go"}
+	hash3 := reviewer.hashChangedFiles(files3)
+
+	if hash1 == hash3 {
+		t.Errorf("Expected different hash for different files, got same hash %q", hash1)
+	}
+
+	// Test single file
+	files4 := []string{"/app/a.go"}
+	hash4 := reviewer.hashChangedFiles(files4)
+
+	if hash1 == hash4 {
+		t.Errorf("Expected different hash for different number of files, got same hash %q", hash1)
+	}
+}
diff --git a/claudetool/codereview/testdata/caching_demo_working.txtar b/claudetool/codereview/testdata/caching_demo_working.txtar
new file mode 100644
index 0000000..b57abc5
--- /dev/null
+++ b/claudetool/codereview/testdata/caching_demo_working.txtar
@@ -0,0 +1,108 @@
+Tests related files caching with a working relationship pattern
+
+-- a.go --
+package main
+
+func a() {}
+
+-- b.go --
+package main
+
+func b() {}
+
+-- c.go --
+package main
+
+func c() {}
+
+-- p.go --
+package p
+
+func d() {}
+
+-- .commit --
+Add functions to a.go and b.go
+
+-- a.go --
+package main
+
+func a() {
+    // Update 1
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 1
+}
+
+-- .commit --
+Add functions to a.go and b.go again
+
+-- a.go --
+package main
+
+func a() {
+    // Update 2
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 2
+}
+
+-- .commit --
+Add functions to a.go and c.go
+
+-- a.go --
+package main
+
+func a() {
+    // Update 3
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 1
+}
+
+-- .commit --
+Update file a.go only (first time)
+
+-- a.go --
+package main
+
+func a() {
+    // Update 4 - first analysis
+}
+
+-- .commit --
+First analysis
+
+-- .run_test --
+# Info
+
+Potentially related files:
+
+- p.go (30%)
+
+These files have historically changed with the files you have modified. Consider whether they require updates as well.
+
+
+-- a.go --
+package main
+
+func a() {
+    // Update 5 - second analysis (should cache related files)
+}
+
+-- .commit --
+Second analysis (should cache related files)
+
+-- .run_test --
+OK
diff --git a/claudetool/codereview/testdata/related_files_cache_all_previously_reported.txtar b/claudetool/codereview/testdata/related_files_cache_all_previously_reported.txtar
new file mode 100644
index 0000000..0777729
--- /dev/null
+++ b/claudetool/codereview/testdata/related_files_cache_all_previously_reported.txtar
@@ -0,0 +1,98 @@
+Tests related files caching when all related files have been previously reported
+
+-- a.go --
+package main
+
+func a() {}
+
+-- b.go --
+package main
+
+func b() {}
+
+-- c.go --
+package main
+
+func c() {}
+
+-- d.go --
+package main
+
+func d() {}
+
+-- .commit --
+Create initial commit
+
+-- a.go --
+package main
+
+func a() {
+    // Update 1
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 1
+}
+
+-- .commit --
+Update a.go and b.go together
+
+-- a.go --
+package main
+
+func a() {
+    // Update 2
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 1
+}
+
+-- .commit --
+Update a.go and c.go together
+
+-- a.go --
+package main
+
+func a() {
+    // Update 3 - first time, will report b.go and c.go
+}
+
+-- .commit --
+First code review - reports related files
+
+-- .run_test --
+# Info
+
+Potentially related files:
+
+- d.go (38%)
+
+These files have historically changed with the files you have modified. Consider whether they require updates as well.
+
+
+-- b.go --
+package main
+
+func b() {
+    // Update 2 - different changeset, but b.go was already reported
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 2 - different changeset, but c.go was already reported
+}
+
+-- .commit --
+Different changeset, but all related files already reported
+
+-- .run_test --
+OK
diff --git a/claudetool/codereview/testdata/related_files_cache_demo.txtar b/claudetool/codereview/testdata/related_files_cache_demo.txtar
new file mode 100644
index 0000000..997af17
--- /dev/null
+++ b/claudetool/codereview/testdata/related_files_cache_demo.txtar
@@ -0,0 +1,95 @@
+Tests that related files caching prevents duplicate processing and output
+
+-- a.go --
+package main
+
+func a() {}
+
+-- b.go --
+package main
+
+func b() {}
+
+-- c.go --
+package main
+
+func c() {}
+
+-- .commit --
+Add functions to a.go and b.go
+
+-- a.go --
+package main
+
+func a() {
+    // Update 1
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 1
+}
+
+-- .commit --
+Add functions to a.go and b.go again
+
+-- a.go --
+package main
+
+func a() {
+    // Update 2
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 2
+}
+
+-- .commit --
+Add functions to a.go and c.go
+
+-- a.go --
+package main
+
+func a() {
+    // Update 3
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 1
+}
+
+-- .commit --
+Update file a.go only (first time)
+
+-- a.go --
+package main
+
+func a() {
+    // Update 4 - first analysis
+}
+
+-- .commit --
+First analysis of a.go change
+
+-- .run_test --
+OK
+-- a.go --
+package main
+
+func a() {
+    // Update 5 - second analysis (should be cached)
+}
+
+-- .commit --
+Second analysis of a.go change (should be cached)
+
+-- .run_test --
+OK
diff --git a/claudetool/codereview/testdata/related_files_cache_new_file_in_set.txtar b/claudetool/codereview/testdata/related_files_cache_new_file_in_set.txtar
new file mode 100644
index 0000000..4c32fa9
--- /dev/null
+++ b/claudetool/codereview/testdata/related_files_cache_new_file_in_set.txtar
@@ -0,0 +1,108 @@
+Tests related files caching when some files have been reported but new ones are present
+
+-- a.go --
+package main
+
+func a() {}
+
+-- b.go --
+package main
+
+func b() {}
+
+-- c.go --
+package main
+
+func c() {}
+
+-- d.go --
+package main
+
+func d() {}
+
+-- .commit --
+Create initial commit
+
+-- a.go --
+package main
+
+func a() {
+    // Update 1
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 1
+}
+
+-- .commit --
+Update a.go and b.go together
+
+-- a.go --
+package main
+
+func a() {
+    // Update 2
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 1
+}
+
+-- .commit --
+Update a.go and c.go together
+
+-- a.go --
+package main
+
+func a() {
+    // Update 3
+}
+
+-- d.go --
+package main
+
+func d() {
+    // Update 1
+}
+
+-- .commit --
+Update a.go and d.go together
+
+-- a.go --
+package main
+
+func a() {
+    // Update 4 - first time, will report b.go, c.go, d.go
+}
+
+-- .commit --
+First review reports all related files
+
+-- .run_test --
+OK
+-- b.go --
+package main
+
+func b() {
+    // Update 2
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 2 - b.go and c.go already reported, but should still return full set
+    // because this is a different changeset that includes both files
+}
+
+-- .commit --
+Different changeset with two files, one relationship is new
+
+-- .run_test --
+OK
diff --git a/claudetool/codereview/testdata/related_files_cache_same_changeset.txtar b/claudetool/codereview/testdata/related_files_cache_same_changeset.txtar
new file mode 100644
index 0000000..db640a5
--- /dev/null
+++ b/claudetool/codereview/testdata/related_files_cache_same_changeset.txtar
@@ -0,0 +1,95 @@
+Tests related files caching when the exact same set of changed files is processed multiple times
+
+-- a.go --
+package main
+
+func a() {}
+
+-- b.go --
+package main
+
+func b() {}
+
+-- c.go --
+package main
+
+func c() {}
+
+-- .commit --
+Create initial commit
+
+-- a.go --
+package main
+
+func a() {
+    // Update 1
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 1
+}
+
+-- .commit --
+Update both a.go and b.go together (creates relationship)
+
+-- a.go --
+package main
+
+func a() {
+    // Update 2
+}
+
+-- b.go --
+package main
+
+func b() {
+    // Update 2
+}
+
+-- .commit --
+Update both a.go and b.go together again (strengthens relationship)
+
+-- a.go --
+package main
+
+func a() {
+    // Update 3
+}
+
+-- c.go --
+package main
+
+func c() {
+    // Update 1
+}
+
+-- .commit --
+Update a.go and c.go together (creates another relationship)
+
+-- a.go --
+package main
+
+func a() {
+    // Update 4 - first time processing this exact set
+}
+
+-- .commit --
+First time changing just a.go
+
+-- .run_test --
+OK
+-- a.go --
+package main
+
+func a() {
+    // Update 5 - second time processing this exact same set, should be cached
+}
+
+-- .commit --
+Second time changing just a.go (should be cached)
+
+-- .run_test --
+OK