diff --git a/gulie/cli.scm b/gulie/cli.scm
index 46f1b15..b6e2ef2 100644
--- a/gulie/cli.scm
+++ b/gulie/cli.scm
@@ -92,6 +92,10 @@
       (let* ((config-path (option-ref options 'config #f))
              (user-config (load-config config-path))
              (config (merge-configs default-config user-config))
+             ;; Wire up --fix
+             (config (if (option-ref options 'fix #f)
+                         (cons '(%fix . #t) config)
+                         config))
              ;; Wire up --pass
              (pass-str (option-ref options 'pass #f))
              (config (if pass-str
diff --git a/gulie/engine.scm b/gulie/engine.scm
index ad4ba22..721287b 100644
--- a/gulie/engine.scm
+++ b/gulie/engine.scm
@@ -127,24 +127,73 @@ and '%min-severity with value 'error, 'warning, or 'info (default)."
     ;; Sort by location
     (sort diagnostics diagnostic<?)))
 
+(define (apply-fixes-to-file file diagnostics)
+  "Apply all fixable diagnostics to FILE. Returns count of fixes applied."
+  (let* ((fixable (filter diagnostic-fix diagnostics))
+         (fixes (map (lambda (d) (cons (diagnostic-line d) (diagnostic-fix d)))
+                     fixable)))
+    (if (null? fixes)
+        0
+        (let* ((text (read-file-to-string file))
+               (lines (list->vector (string-split text #\newline)))
+               (count 0))
+          ;; Apply fixes: each fix is a replace-line, apply by line number
+          ;; Process in reverse line order so indices stay valid
+          (for-each
+           (lambda (fix-pair)
+             (let* ((line-num (car fix-pair))
+                    (fix (cdr fix-pair))
+                    (idx (1- line-num)))
+               (when (and (>= idx 0) (< idx (vector-length lines))
+                          (eq? (fix-type fix) 'replace-line))
+                 (vector-set! lines idx (fix-replacement fix))
+                 (set! count (1+ count)))))
+           (sort fixes (lambda (a b) (> (car a) (car b)))))
+          ;; Write back
+          (when (> count 0)
+            (call-with-output-file file
+              (lambda (port)
+                (let ((len (vector-length lines)))
+                  (let lp ((i 0))
+                    (when (< i len)
+                      (display (vector-ref lines i) port)
+                      (when (< i (1- len))
+                        (newline port))
+                      (lp (1+ i))))))))
+          count))))
 (define (lint-files files config)
   "Lint multiple FILES in parallel. Returns total diagnostic count.
 Uses n-par-map to distribute work across threads, then outputs
-diagnostics sequentially to maintain deterministic file order."
+diagnostics sequentially to maintain deterministic file order.
+When '%fix is #t in config, apply auto-fixes and report unfixed."
   (let* ((ncpus (max 1 (total-processor-count)))
+         (fix-mode? (assq-ref config '%fix))
          (results (n-par-map ncpus
                              (lambda (file)
                                (cons file (lint-file file config)))
                              files))
-         (total 0))
-    ;; Output in original file order (n-par-map preserves it)
+         (total-diags 0)
+         (total-fixed 0))
     (for-each
      (lambda (result)
-       (let ((diags (cdr result)))
-         (set! total (+ total (length diags)))
-         (format-diagnostics diags (current-output-port))))
+       (let* ((file (car result))
+              (diags (cdr result)))
+         (if fix-mode?
+             ;; In fix mode: apply fixes, then report only unfixed diagnostics
+             (let* ((fixed-count (apply-fixes-to-file file diags))
+                    (unfixed (filter (lambda (d) (not (diagnostic-fix d))) diags)))
+               (set! total-fixed (+ total-fixed fixed-count))
+               (set! total-diags (+ total-diags (length unfixed)))
+               (format-diagnostics unfixed (current-output-port)))
+             ;; Normal mode: report everything
+             (begin
+               (set! total-diags (+ total-diags (length diags)))
+               (format-diagnostics diags (current-output-port))))))
      results)
-    total))
+    (when (and fix-mode? (> total-fixed 0))
+      (format (current-error-port) "Fixed ~a issue~a.~%"
+              total-fixed (if (= total-fixed 1) "" "s")))
+    total-diags))
 
 (define (scheme-file? path)
   "Is PATH a Scheme source file?"
diff --git a/gulie/rules/comments.scm b/gulie/rules/comments.scm
index 1ec6629..233f18c 100644
--- a/gulie/rules/comments.scm
+++ b/gulie/rules/comments.scm
@@ -54,18 +54,30 @@
                    ;; Inline comment (after code) should use single ;
                    ;; But we don't enforce this strictly — just flag ;;; or more inline
                    ((and (not own-line?) (>= semis 3))
-                    (list (make-diagnostic
-                           file line-num pos
-                           'info 'comment-semicolons
-                           "inline comments should use ; or ;; not ;;;"
-                           #f)))
+                    (let ((fixed (string-append
+                                  (substring line-text 0 pos)
+                                  ";;"
+                                  (substring line-text (+ pos semis)))))
+                      (list (make-diagnostic
+                             file line-num pos
+                             'info 'comment-semicolons
+                             "inline comments should use ; or ;; not ;;;"
+                             (make-fix 'replace-line line-num 0
+                                       line-num (string-length line-text)
+                                       fixed)))))
                    ;; Own-line comment with single ; (should be ;;)
                    ((and own-line? (= semis 1) (> (string-length line-text) (1+ pos))
                          (not (char=? (string-ref line-text (1+ pos)) #\!)))
-                    (list (make-diagnostic
-                           file line-num pos
-                           'info 'comment-semicolons
-                           "line comments should use ;; not ;"
-                           #f)))
+                    (let ((fixed (string-append
+                                  (substring line-text 0 pos)
+                                  ";;"
+                                  (substring line-text (1+ pos)))))
+                      (list (make-diagnostic
+                             file line-num pos
+                             'info 'comment-semicolons
+                             "line comments should use ;; not ;"
+                             (make-fix 'replace-line line-num 0
+                                       line-num (string-length line-text)
+                                       fixed)))))
                    (else '()))))))))
   #f))