refactor(edit): remove auto-indentation and add line-ending normalization

- [x] Remove auto-indentation from text mode edits (caller controls whitespace)
- [x] Add line-ending detection and normalization for both AST and text modes
- [x] Share edit logic via new `spliceContent` function for both modes
- [x] Fix diff to emit "No newline at end of file" markers
- [x] Fix diff to strip raw CR from CRLF file output
- [x] Remove double-unescape of backslash sequences in new_content
- [x] Fix countDiffLines to be hunk-aware (correctly count lines starting with +/-)
- [x] Fix block-comment stripping to remove standalone lines cleanly
- [x] Fix Python license header stripping to preserve separator blank lines

internal/server/handlers_edit.go

@@ -11,16 +11,6 @@ import (
 	"github.com/mark3labs/mcp-go/mcp"
 )
 
-// unescapeNewlines converts literal \n, \t, \" sequences to actual characters.
-// This handles cases where MCP clients send double-escaped JSON strings.
-func unescapeNewlines(s string) string {
-	s = strings.ReplaceAll(s, "\\n", "\n")
-	s = strings.ReplaceAll(s, "\\t", "\t")
-	s = strings.ReplaceAll(s, "\\\"", "\"")
-	s = strings.ReplaceAll(s, "\\\\", "\\")
-	return s
-}
-
 // handleEditApply handles the edit_apply tool.
 func (s *Server) handleEditApply(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 	return s.handleEdit(ctx, request)
@@ -51,8 +41,12 @@ func (s *Server) handleEdit(ctx context.Context, request mcp.CallToolRequest) (*
 		return mcp.NewToolResultError("file is outside workspace root"), nil
 	}
 
+	// new_content arrives already fully decoded by the JSON-RPC layer (mcp-go):
+	// JSON escapes such as \n, \t, \\, \" have been resolved to their real bytes.
+	// It must therefore be used verbatim — any further unescaping would corrupt
+	// legitimate backslash sequences in source code (string literals, regexes,
+	// Windows paths). See TestEditApplyPreservesBackslashSequences*.
 	newContent := request.GetString("new_content", "")
-	newContent = unescapeNewlines(newContent)
 
 	selectorName := request.GetString("selector_name", "")
 
@@ -114,21 +108,26 @@ func (s *Server) handleEdit(ctx context.Context, request mcp.CallToolRequest) (*
 	}
 }
 
-// countDiffLines counts added (+) and removed (-) lines in a unified diff string.
+// countDiffLines counts added (+) and removed (-) content lines in a unified diff.
+// Only lines inside hunks are counted (everything after the first "@@" header), so the
+// "---"/"+++" file headers are skipped structurally — and content whose own text starts
+// with + or - is counted correctly rather than mistaken for a header. The git-style
+// "\ No newline at end of file" marker is ignored.
 func countDiffLines(diff string) (added, removed int) {
+	inHunk := false
 	for _, line := range strings.Split(diff, "\n") {
-		if len(line) == 0 {
+		if strings.HasPrefix(line, "@@") {
+			inHunk = true
+			continue
+		}
+		if !inHunk || line == "" {
 			continue
 		}
 		switch line[0] {
 		case '+':
-			if !strings.HasPrefix(line, "+++") {
-				added++
-			}
+			added++
 		case '-':
-			if !strings.HasPrefix(line, "---") {
-				removed++
-			}
+			removed++
 		}
 	}
 	return

internal/server/handlers_edit_test.go

@@ -1,47 +1,103 @@
 package server
 
-import "testing"
+import (
+	"context"
+	"os"
+	"strings"
+	"testing"
 
-func TestUnescapeNewlines(t *testing.T) {
-	tests := []struct {
-		name     string
-		input    string
-		expected string
-	}{
-		{
-			name:     "no escapes",
-			input:    "func test() {\n\treturn true;\n}",
-			expected: "func test() {\n\treturn true;\n}",
-		},
-		{
-			name:     "literal backslash n",
-			input:    "func test() {\\n\\treturn true;\\n}",
-			expected: "func test() {\n\treturn true;\n}",
-		},
-		{
-			name:     "literal backslash t",
-			input:    "func test() {\\n\\treturn true;\\n}",
-			expected: "func test() {\n\treturn true;\n}",
-		},
-		{
-			name:     "literal quotes",
-			input:    `func test() {\n\treturn \"true\";\n}`,
-			expected: "func test() {\n\treturn \"true\";\n}",
-		},
+	"github.com/mark3labs/mcp-go/mcp"
+)
 
-		{
-			name:     "mixed content",
-			input:    "line1\\nline2\\tindented\\nline3",
-			expected: "line1\nline2\tindented\nline3",
-		},
+// callEdit invokes handleEditApply with the given args and returns the result.
+func callEdit(t *testing.T, srv *Server, args map[string]any) *mcp.CallToolResult {
+	t.Helper()
+	req := mcp.CallToolRequest{}
+	req.Params.Arguments = args
+	res, err := srv.handleEditApply(context.Background(), req)
+	if err != nil {
+		t.Fatalf("handleEditApply error: %v", err)
+	}
+	if res == nil {
+		t.Fatal("handleEditApply returned nil result")
+	}
+	return res
+}
+
+// TestEditApplyPreservesBackslashSequencesText is the primary regression guard for the
+// double-unescape bug: new_content arrives from the JSON-RPC layer already fully decoded,
+// so the handler must write it to disk verbatim. Literal backslash sequences (\n, \t, \",
+// \\) that legitimately appear in source code must survive byte-for-byte.
+func TestEditApplyPreservesBackslashSequencesText(t *testing.T) {
+	tmpDir := t.TempDir()
+	srv := newTestServer(t, tmpDir)
+
+	f := writeFile(t, tmpDir, "note.txt", "PLACEHOLDER\n")
+
+	// Exactly the bytes a client intends after JSON decoding: backslash sequences are
+	// real source text here, NOT escapes to be interpreted.
+	newContent := `printf("a\nb\tc"); re = \d+; path = "C:\\tmp"; q = \"`
+
+	res := callEdit(t, srv, map[string]any{
+		"file":          f,
+		"operation":     "replace",
+		"selector_text": "PLACEHOLDER",
+		"new_content":   newContent,
+		"response":      "none",
+	})
+	if res.IsError {
+		t.Fatalf("edit returned error: %+v", res.Content)
 	}
 
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			result := unescapeNewlines(tt.input)
-			if result != tt.expected {
-				t.Errorf("unescapeNewlines(%q) = %q, expected %q", tt.input, result, tt.expected)
-			}
-		})
+	got, err := os.ReadFile(f)
+	if err != nil {
+		t.Fatalf("read back: %v", err)
+	}
+	want := newContent + "\n"
+	if string(got) != want {
+		t.Fatalf("new_content not written verbatim (double-unescape regression).\nwant: %q\ngot:  %q", want, string(got))
+	}
+}
+
+// TestCountDiffLinesHunkAware verifies the line counter skips file headers structurally and
+// counts content lines whose own text begins with + or - (previously dropped as "+++"/"---").
+func TestCountDiffLinesHunkAware(t *testing.T) {
+	diff := "--- a.txt\n+++ a.txt\n@@ -1,3 +1,3 @@\n context\n+++plusprefix\n+normal add\n---minusprefix\n-normal del\n"
+	added, removed := countDiffLines(diff)
+	if added != 2 || removed != 2 {
+		t.Fatalf("hunk-aware diff count wrong: added=%d removed=%d (want 2 and 2)", added, removed)
+	}
+}
+
+// TestEditApplyPreservesBackslashSequencesCode covers the AST/code path: the same verbatim
+// guarantee must hold for syntactically-validated code files.
+func TestEditApplyPreservesBackslashSequencesCode(t *testing.T) {
+	tmpDir := t.TempDir()
+	srv := newTestServer(t, tmpDir)
+
+	src := "package main\n\nfunc demo() {\n\tprintln(\"old\")\n}\n"
+	f := writeFile(t, tmpDir, "demo.go", src)
+
+	// Real tab indentation + literal backslash sequences inside string/raw-string literals.
+	newBody := "func demo() {\n\tprintln(\"a\\nb\\tc\")\n\t_ = `\\d+`\n\t_ = \"C:\\\\tmp\"\n}"
+
+	res := callEdit(t, srv, map[string]any{
+		"file":          f,
+		"operation":     "replace",
+		"selector_kind": "function_declaration",
+		"selector_name": "demo",
+		"new_content":   newBody,
+		"response":      "none",
+	})
+	if res.IsError {
+		t.Fatalf("edit returned error: %+v", res.Content)
+	}
+
+	got, err := os.ReadFile(f)
+	if err != nil {
+		t.Fatalf("read back: %v", err)
+	}
+	if !strings.Contains(string(got), newBody) {
+		t.Fatalf("backslash sequences corrupted in code edit.\nwant substring:\n%q\ngot file:\n%q", newBody, string(got))
 	}
 }

internal/server/help_content.go

@@ -137,6 +137,8 @@ const helpEditApply = "# edit_apply — flags and examples\n\n" +
 	"| `none` | Empty response (silent success) |\n\n" +
 	"`compact_response=true` is a deprecated alias for `response=\"count\"` kept for pre-v2 compatibility.\n\n" +
 	"For code files (Go, TypeScript, JavaScript, Python, C, C++, Rust) syntax is validated before writing — the edit is rejected if it would produce invalid syntax.\n\n" +
+	"## new_content handling\n\n" +
+	"`new_content` is inserted **verbatim**. Send it as normal JSON — real newlines (or JSON `\\n`), with quotes JSON-escaped as usual. The server does not re-interpret escape sequences, so literal backslash sequences in source (`\\n`, `\\t`, `\\\\`, regexes, Windows paths) are written exactly as given. Indentation is not auto-applied — include the leading whitespace you want. Line endings are normalized to the file's existing convention (LF or CRLF).\n\n" +
 	"## Selector types\n\n" +
 	"### AST-mode selectors (code files)\n" +
 	"- `selector_kind` — AST node type (e.g. `function_declaration`, `class_declaration`)\n" +

internal/server/server.go

@@ -313,7 +313,7 @@ func (s *Server) registerTools() {
 				mcp.Description("Edit operation: replace, insert_before, insert_after, delete"),
 			),
 			mcp.WithString("new_content",
-				mcp.Description("New content (required for replace/insert operations)"),
+				mcp.Description("New content (required for replace/insert operations). Inserted verbatim: send it as normal JSON (real newlines, quotes JSON-escaped as usual). The server does NOT re-interpret escape sequences, so backslash sequences in source code (e.g. \\n, \\t, \\\\ inside string literals, regexes, or Windows paths) are preserved exactly. Indentation is not auto-applied — include the leading whitespace you want. Line endings are normalized to the file's existing convention (LF or CRLF)."),
 			),
 			// AST-mode selectors (for code files)
 			mcp.WithString("selector_kind",