diff --git a/lsp/mapper/apply_test.go b/lsp/mapper/apply_test.go
new file mode 100644
index 0000000..d6b0fbe
--- /dev/null
+++ b/lsp/mapper/apply_test.go
@@ -0,0 +1,191 @@
+package mapper
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.lsp.dev/protocol"
+)
+
+// pos builds an LSP position on line 0.
+func pos(char uint32) protocol.Position {
+	return protocol.Position{Line: 0, Character: char}
+}
+
+func TestApplyEdits(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		edits   []protocol.TextEdit
+		want    string
+	}{
+		{
+			name:    "single replacement",
+			content: "hello world",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(6), End: pos(11)}, NewText: "there"},
+			},
+			want: "hello there",
+		},
+		{
+			name:    "insertion at empty range",
+			content: "hello world",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(5), End: pos(5)}, NewText: ","},
+			},
+			want: "hello, world",
+		},
+		{
+			name:    "deletion with empty text",
+			content: "hello world",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(5), End: pos(11)}, NewText: ""},
+			},
+			want: "hello",
+		},
+		{
+			name:    "unsorted edits apply correctly",
+			content: "abcdef",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(0), End: pos(1)}, NewText: "X"},
+				{Range: protocol.Range{Start: pos(4), End: pos(5)}, NewText: "Y"},
+			},
+			want: "XbcdYf",
+		},
+		{
+			name:    "edit at document start and end",
+			content: "abc",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(0), End: pos(0)}, NewText: "<"},
+				{Range: protocol.Range{Start: pos(3), End: pos(3)}, NewText: ">"},
+			},
+			want: "<abc>",
+		},
+		{
+			name:    "replace whole document",
+			content: "abc",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(0), End: pos(3)}, NewText: "xyz"},
+			},
+			want: "xyz",
+		},
+		{
+			name:    "no edits is identity",
+			content: "untouched",
+			edits:   nil,
+			want:    "untouched",
+		},
+		{
+			name: "utf16 surrogate pair counts as two units",
+			// a + emoji(4 bytes, 2 units) + b: b sits at char 3.
+			content: "a😀b",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(3), End: pos(4)}, NewText: "B"},
+			},
+			want: "a😀B",
+		},
+		{
+			name: "utf16 cjk counts as one unit",
+			// a + 日(3 bytes, 1 unit) + b: b sits at char 2.
+			content: "a日b",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(2), End: pos(3)}, NewText: "B"},
+			},
+			want: "a日B",
+		},
+		{
+			name:    "utf16 edit spanning wide runes",
+			content: "😀x日y",
+			edits: []protocol.TextEdit{
+				// chars 0..4 span emoji(2) + x(1) + 日(1).
+				{Range: protocol.Range{Start: pos(0), End: pos(4)}, NewText: "ok"},
+			},
+			want: "oky",
+		},
+		{
+			name:    "multiline edit",
+			content: "one\ntwo\nthree\n",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: protocol.Position{Line: 1, Character: 0}, End: protocol.Position{Line: 2, Character: 5}}, NewText: "TWO"},
+			},
+			want: "one\nTWO\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := NewMapper([]byte(tt.content)).ApplyEdits(tt.edits)
+			require.NoError(t, err)
+			assert.Equal(t, tt.want, string(got))
+		})
+	}
+}
+
+func TestApplyEditsErrors(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		edits   []protocol.TextEdit
+	}{
+		{
+			name:    "end past document",
+			content: "abc",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(0), End: pos(4)}, NewText: "x"},
+			},
+		},
+		{
+			name:    "start past document",
+			content: "abc",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(9), End: pos(9)}, NewText: "x"},
+			},
+		},
+		{
+			name:    "start after end",
+			content: "abc",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: pos(2), End: pos(1)}, NewText: "x"},
+			},
+		},
+		{
+			name:    "line past document",
+			content: "abc",
+			edits: []protocol.TextEdit{
+				{Range: protocol.Range{Start: protocol.Position{Line: 5, Character: 0}, End: protocol.Position{Line: 5, Character: 1}}, NewText: "x"},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := NewMapper([]byte(tt.content)).ApplyEdits(tt.edits)
+			assert.Error(t, err)
+		})
+	}
+}
+
+// TestApplyEditsRoundTrip pins the property the format code actions rely
+// on: applying the edits a formatter produced reproduces the formatter's
+// whole-document output.
+func TestApplyEditsRoundTrip(t *testing.T) {
+	content := []byte("struct A { 1: i32 a }\n\nstruct B {\n2: i32 b\n}\n")
+	want := []byte("struct A { 1: i32 a }\n\nstruct B { 2: i32 b }\n")
+
+	m := NewMapper(content)
+
+	edits := []protocol.TextEdit{
+		{
+			Range: protocol.Range{
+				Start: protocol.Position{Line: 2, Character: 0},
+				End:   protocol.Position{Line: 5, Character: 0},
+			},
+			NewText: "struct B { 2: i32 b }\n",
+		},
+	}
+
+	got, err := m.ApplyEdits(edits)
+	require.NoError(t, err)
+	assert.Equal(t, string(want), string(got))
+}