diff --git a/go.mod b/go.mod
index c85fc93d..0961edf6 100644
--- a/go.mod
+++ b/go.mod
@@ -7,6 +7,7 @@ require (
 	charm.land/bubbletea/v2 v2.0.6
 	charm.land/lipgloss/v2 v2.0.3
 	github.com/alecthomas/kong v1.15.0
+	github.com/bluekeyes/go-gitdiff v0.9.0
 	github.com/buildkite/shellwords v1.0.1
 	github.com/charmbracelet/colorprofile v0.4.3
 	github.com/charmbracelet/x/ansi v0.11.7
diff --git a/go.sum b/go.sum
index ec549a5c..4386b803 100644
--- a/go.sum
+++ b/go.sum
@@ -16,6 +16,8 @@ github.com/aymanbagabas/go-osc52/v2 v2.0.1 h1:HwpRHbFMcZLEVr42D4p7XBqjyuxQH5SMiE
 github.com/aymanbagabas/go-osc52/v2 v2.0.1/go.mod h1:uYgXzlJ7ZpABp8OJ+exZzJJhRNQ2ASbcXHWsFqH8hp8=
 github.com/aymanbagabas/go-udiff v0.4.1 h1:OEIrQ8maEeDBXQDoGCbbTTXYJMYRCRO1fnodZ12Gv5o=
 github.com/aymanbagabas/go-udiff v0.4.1/go.mod h1:0L9PGwj20lrtmEMeyw4WKJ/TMyDtvAoK9bf2u/mNo3w=
+github.com/bluekeyes/go-gitdiff v0.9.0 h1:w+O6lkRBOqfGcwF0Lf6FFHQrhmxM0hCJW5+rbilGuSs=
+github.com/bluekeyes/go-gitdiff v0.9.0/go.mod h1:WWAk1Mc6EgWarCrPFO+xeYlujPu98VuLW3Tu+B/85AE=
 github.com/buildkite/shellwords v1.0.1 h1:88OjMbEBf+EliVB0tizXJynpAM2CKOvYwepg5n8O70M=
 github.com/buildkite/shellwords v1.0.1/go.mod h1:so0eQnTxgbo58CTYX+4BCx5UuMzvRha9dcKdCKl6NV4=
 github.com/charmbracelet/colorprofile v0.4.3 h1:QPa1IWkYI+AOB+fE+mg/5/4HRMZcaXex9t5KX76i20Q=
diff --git a/internal/reviewdiff/patch.go b/internal/reviewdiff/patch.go
new file mode 100644
index 00000000..1c6b2bde
--- /dev/null
+++ b/internal/reviewdiff/patch.go
@@ -0,0 +1,145 @@
+// Package reviewdiff answers review-comment questions about a Git patch.
+package reviewdiff
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
+)
+
+// Patch describes the files and line ranges represented by a Git patch.
+//
+// Postimage queries use destination paths and line numbers. Deletion queries
+// use source paths and line numbers because those lines no longer exist in the
+// postimage.
+type Patch struct {
+	files     map[string][]lineRange
+	deletions map[string][]lineRange
+}
+
+// Parse parses a Git patch for review-comment queries.
+func Parse(src []byte) (*Patch, error) {
+	files, _, err := gitdiff.Parse(bytes.NewReader(src))
+	if err != nil {
+		return nil, fmt.Errorf("parse Git patch: %w", err)
+	}
+
+	patch := &Patch{
+		files:     make(map[string][]lineRange),
+		deletions: make(map[string][]lineRange),
+	}
+	for _, file := range files {
+		// A destination path is enough to make a file commentable, including
+		// binary, rename-only, and mode-only changes without text fragments.
+		if file.NewName != "" {
+			if _, ok := patch.files[file.NewName]; !ok {
+				patch.files[file.NewName] = nil
+			}
+		}
+
+		for _, fragment := range file.TextFragments {
+			if file.NewName != "" && fragment.NewLines > 0 {
+				patch.files[file.NewName] = append(
+					patch.files[file.NewName],
+					lineRange{
+						start: fragment.NewPosition,
+						end:   fragment.NewPosition + fragment.NewLines - 1,
+					},
+				)
+			}
+
+			// Fragment ranges include context. Walk the old-side cursor so
+			// deletion queries distinguish removed lines from nearby context.
+			oldLine := fragment.OldPosition
+			for _, line := range fragment.Lines {
+				switch line.Op {
+				case gitdiff.OpContext:
+					oldLine++
+				case gitdiff.OpDelete:
+					if file.OldName != "" {
+						patch.deletions[file.OldName] = appendLine(
+							patch.deletions[file.OldName],
+							oldLine,
+						)
+					}
+					oldLine++
+				case gitdiff.OpAdd:
+				}
+			}
+		}
+	}
+
+	return patch, nil
+}
+
+// ContainsFile reports whether path exists in the patch postimage.
+func (p *Patch) ContainsFile(path string) bool {
+	_, ok := p.files[path]
+	return ok
+}
+
+// ContainsLine reports whether the postimage line is in a patch fragment.
+func (p *Patch) ContainsLine(path string, line int) bool {
+	return p.ContainsLineRange(path, line, line)
+}
+
+// ContainsLineRange reports whether every line in the inclusive postimage
+// range is in one patch fragment.
+func (p *Patch) ContainsLineRange(path string, start, end int) bool {
+	if start <= 0 || end < start {
+		return false
+	}
+
+	for _, fragment := range p.files[path] {
+		if fragment.contains(int64(start), int64(end)) {
+			return true
+		}
+	}
+	return false
+}
+
+// DeletesLine reports whether the patch deletes the source line.
+func (p *Patch) DeletesLine(path string, line int) bool {
+	return p.DeletesLineRange(path, line, line)
+}
+
+// DeletesLineRange reports whether the patch deletes any source line in the
+// inclusive range.
+func (p *Patch) DeletesLineRange(path string, start, end int) bool {
+	if start <= 0 || end < start {
+		return false
+	}
+
+	for _, deletion := range p.deletions[path] {
+		if deletion.overlaps(int64(start), int64(end)) {
+			return true
+		}
+	}
+	return false
+}
+
+// lineRange is an inclusive interval in one side of a file diff.
+type lineRange struct {
+	start int64
+	end   int64
+}
+
+func (r lineRange) contains(start, end int64) bool {
+	return r.start <= start && end <= r.end
+}
+
+func (r lineRange) overlaps(start, end int64) bool {
+	return r.start <= end && start <= r.end
+}
+
+// appendLine adds a deleted line while coalescing consecutive lines into a
+// compact range.
+func appendLine(ranges []lineRange, line int64) []lineRange {
+	if len(ranges) == 0 || ranges[len(ranges)-1].end+1 != line {
+		return append(ranges, lineRange{start: line, end: line})
+	}
+
+	ranges[len(ranges)-1].end = line
+	return ranges
+}
diff --git a/internal/reviewdiff/patch_test.go b/internal/reviewdiff/patch_test.go
new file mode 100644
index 00000000..6c237e47
--- /dev/null
+++ b/internal/reviewdiff/patch_test.go
@@ -0,0 +1,103 @@
+package reviewdiff_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.abhg.dev/gs/internal/reviewdiff"
+)
+
+func TestPatchContains(t *testing.T) {
+	patch, err := reviewdiff.Parse([]byte(`diff --git a/main.go b/main.go
+--- a/main.go
++++ b/main.go
+@@ -1,4 +1,5 @@
+ package main
+ 
++import "fmt"
+ func main() {}
+ 
+@@ -10,3 +11,4 @@ func helper() {
+ 	work()
++	fmt.Println("done")
+ }
+ 
+diff --git "a/with space.go" "b/with space.go"
+--- "a/with space.go"
++++ "b/with space.go"
+@@ -1 +1,2 @@
+ package spaced
++var Added = true
+
+diff --git a/old.go b/new.go
+similarity index 100%
+rename from old.go
+rename to new.go
+
+diff --git a/script.sh b/script.sh
+old mode 100644
+new mode 100755
+`))
+	require.NoError(t, err)
+
+	assert.True(t, patch.ContainsFile("main.go"))
+	assert.True(t, patch.ContainsFile("with space.go"))
+	assert.True(t, patch.ContainsFile("new.go"))
+	assert.True(t, patch.ContainsFile("script.sh"))
+	assert.False(t, patch.ContainsFile("old.go"))
+	assert.False(t, patch.ContainsFile("missing.go"))
+
+	assert.True(t, patch.ContainsLine("main.go", 3))
+	assert.True(t, patch.ContainsLine("main.go", 13))
+	assert.False(t, patch.ContainsLine("main.go", 8))
+	assert.False(t, patch.ContainsLine("main.go", 0))
+	assert.True(t, patch.ContainsLine("with space.go", 2))
+
+	assert.True(t, patch.ContainsLineRange("main.go", 1, 5))
+	assert.True(t, patch.ContainsLineRange("main.go", 11, 14))
+	assert.False(t, patch.ContainsLineRange("main.go", 5, 11))
+	assert.False(t, patch.ContainsLineRange("main.go", 4, 2))
+}
+
+func TestPatchDeletes(t *testing.T) {
+	patch, err := reviewdiff.Parse([]byte(`diff --git a/main.go b/main.go
+--- a/main.go
++++ b/main.go
+@@ -2,5 +2,4 @@ package main
+ context
+-delete one
+-delete two
++replacement
+ more context
+ last context
+diff --git a/old.go b/new.go
+similarity index 80%
+rename from old.go
+rename to new.go
+@@ -10,3 +10,2 @@
+ keep
+-removed
+ keep too
+`))
+	require.NoError(t, err)
+
+	assert.True(t, patch.DeletesLine("main.go", 3))
+	assert.True(t, patch.DeletesLine("main.go", 4))
+	assert.False(t, patch.DeletesLine("main.go", 2))
+	assert.False(t, patch.DeletesLine("main.go", 5))
+	assert.True(t, patch.DeletesLineRange("main.go", 1, 3))
+	assert.False(t, patch.DeletesLineRange("main.go", 5, 7))
+	assert.False(t, patch.DeletesLineRange("main.go", 0, 4))
+
+	assert.True(t, patch.DeletesLine("old.go", 11))
+	assert.False(t, patch.DeletesLine("new.go", 11))
+}
+
+func TestParseError(t *testing.T) {
+	_, err := reviewdiff.Parse([]byte(`detached fragment
+@@ -1 +1 @@
+`))
+	require.Error(t, err)
+	assert.ErrorContains(t, err, "parse Git patch")
+}