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.