perf(file_read): compact line-number prefix by default

The default line prefix was the padded '  42| ' form; the compact '42|' form
existed only as opt-in (compact_line_numbers). The padding and trailing space
carry no information an LLM needs, costing ~3 chars/line — ~300 tokens on a
400-line read, the highest-volume path in the server. Flip the default to
compact; compact_line_numbers=false, mode/line_numbers=full, and the session
'full' pref all still restore the padded alignment (explicit override wins).

Updates the two tests that pinned the old default and refreshes stale comments.

internal/server/handlers_file.go

@@ -326,9 +326,13 @@ type lineNumberOpts struct {
 // resolveLineNumberOpts resolves per-call vs session-pref line-number options.
 func (s *Server) resolveLineNumberOpts(request mcp.CallToolRequest) lineNumberOpts {
 	opts := lineNumberOpts{
-		noLineNumbers:   request.GetBool("no_line_numbers", false),
-		lineInterval:    request.GetInt("line_number_interval", 1),
-		compactLineNums: request.GetBool("compact_line_numbers", false),
+		noLineNumbers: request.GetBool("no_line_numbers", false),
+		lineInterval:  request.GetInt("line_number_interval", 1),
+		// Compact prefix ("12│content") is the default — it drops the 4-wide
+		// padding and trailing space of the legacy "  12│ " form, saving ~3
+		// chars/line with no information loss. Pass compact_line_numbers=false
+		// (or mode/line_numbers="full") to restore the padded alignment.
+		compactLineNums: request.GetBool("compact_line_numbers", true),
 	}
 
 	// Apply session line_numbers pref when no explicit per-call override was supplied.

internal/server/handlers_file_test.go

@@ -589,19 +589,23 @@ func TestCompactLineNumbers(t *testing.T) {
 	}
 }
 
-func TestCompactLineNumbersOffByDefault(t *testing.T) {
+func TestCompactLineNumbersDefaultAndOptOut(t *testing.T) {
 	tmpDir := t.TempDir()
 	srv := newTestServer(t, tmpDir)
 
 	content := "line one\nline two\n"
 	f := writeFile(t, tmpDir, "test.txt", content)
 
-	// Default: no compact_line_numbers
-	out := callRead(t, srv, map[string]interface{}{"path": f})
+	// Default is now the compact prefix (token-saving): "1│line one".
+	def := callRead(t, srv, map[string]interface{}{"path": f})
+	if !strings.Contains(def, "1│line one") || strings.Contains(def, "   1│ line one") {
+		t.Errorf("default should use compact prefix, got:\n%s", def)
+	}
 
-	// Should have padded format
-	if !strings.Contains(out, "   1│ line one") {
-		t.Errorf("default should have padded format, got:\n%s", out)
+	// Explicit opt-out restores the padded alignment.
+	padded := callRead(t, srv, map[string]interface{}{"path": f, "compact_line_numbers": false})
+	if !strings.Contains(padded, "   1│ line one") {
+		t.Errorf("compact_line_numbers=false should restore padded format, got:\n%s", padded)
 	}
 }
 
@@ -679,9 +683,9 @@ func Hello() {
 	if !strings.Contains(out, `println("hello")`) {
 		t.Errorf("full mode missing body, got:\n%s", out)
 	}
-	// Padded line numbers
-	if !strings.Contains(out, "   1│ package main") {
-		t.Errorf("default should have padded line numbers, got:\n%s", out)
+	// Default line numbers use the compact prefix.
+	if !strings.Contains(out, "1│package main") {
+		t.Errorf("default should have compact line numbers, got:\n%s", out)
 	}
 	// etag present
 	if !strings.Contains(out, "[etag:") {

internal/server/server.go

@@ -206,7 +206,7 @@ func (s *Server) registerTools() {
 				mcp.Description("Print line numbers only every N lines (default: 1 = every line). E.g. 10 = anchor every 10th line plus first/last. 0 = no line numbers."),
 			),
 			mcp.WithBoolean("compact_line_numbers",
-				mcp.Description("Use compact line prefix '12\u2502' instead of '  12\u2502 ' (no padding, no trailing space). Works with line_number_interval. Default off."),
+				mcp.Description("Use compact line prefix '12\u2502' instead of padded '  12\u2502 ' (no padding, no trailing space). Works with line_number_interval. Default true; set false to restore padded alignment."),
 			),
 			mcp.WithBoolean("collapse_blank_lines",
 				mcp.Description("Collapse runs of consecutive blank lines to a single blank line. Useful for token savings on heavily-spaced code."),

internal/server/session_integration_test.go

@@ -48,7 +48,7 @@ func TestSessionPrefsFileReadLineNumbersNone(t *testing.T) {
 		t.Fatalf("handleFileRead: %v", err)
 	}
 	text := result.Content[0].(mcp.TextContent).Text
-	// Standard line-number format is "   1│ "; with no_line_numbers it's absent.
+	// line_numbers=none suppresses the prefix entirely (no "│" gutter at all).
 	if strings.Contains(text, "   1│") {
 		t.Errorf("session line_numbers=none: expected no line-number prefix, got:\n%s", text)
 	}
@@ -87,7 +87,7 @@ func TestSessionPrefsFileReadLineNumbersCompact(t *testing.T) {
 	req.Params.Arguments = map[string]interface{}{"path": testFile}
 	result, _ := srv.handleFileRead(ctx, req)
 	text := result.Content[0].(mcp.TextContent).Text
-	// Standard padded prefix is "   1│ "; compact is "1│".
+	// Padded prefix is "   1│ "; compact is "1│".
 	if strings.Contains(text, "   1\u2502") {
 		t.Errorf("session line_numbers=compact should use compact prefix, got:\n%s", text)
 	}