diff --git a/TeXmacs/plugins/mgs/progs/data/mgs.scm b/TeXmacs/plugins/mgs/progs/data/mgs.scm
deleted file mode 100644
index 20652e3572..0000000000
--- a/TeXmacs/plugins/mgs/progs/data/mgs.scm
+++ /dev/null
@@ -1,142 +0,0 @@
-
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;
-;; MODULE      : data/mgs.scm
-;; DESCRIPTION : Mogan Scheme (.mgs) data format
-;; COPYRIGHT   : (C) 2003  Joris van der Hoeven
-;;
-;; This software falls under the GNU general public license version 3 or later.
-;; It comes WITHOUT ANY WARRANTY WHATSOEVER. For details, see the file LICENSE
-;; in the root directory or <http://www.gnu.org/licenses/gpl-3.0.html>.
-;;
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
-(texmacs-module (data mgs))
-
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;; Scheme format for TeXmacs (no information loss)
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
-(define (mgs-recognizes? s)
-  (and (string? s) (string-starts? s "(document (TeXmacs")))
-
-(define-format mgs
-  (:name "Mogan Scheme")
-  (:suffix "mgs")
-  (:must-recognize mgs-recognizes?))
-
-(define (format-mgs-string str)
-  ;; Parsing the string to an S-expression
-  (define (parse-string s)
-    (call-with-input-string s (lambda (port) (read port))))
-  
-  (define indent-cache (make-vector 50 #f))
-  
-  (define (get-indent n)
-    (if (< n 50)
-        (let ((cached (vector-ref indent-cache n)))
-          (if cached
-              cached
-              (let ((spaces (make-string n #\space)))
-                (vector-set! indent-cache n spaces)
-                spaces)))
-        (make-string n #\space)))
-  
-  (define (has-more-than-2? lst)
-    (and (pair? lst) (pair? (cdr lst))))
-  
-  (define (every pred lst)
-    (or (null? lst)
-        (and (pred (car lst))
-             (every pred (cdr lst)))))
-  
-  (define (simple-structure? expr)
-    (define (simple-element? x)
-      (or (not (pair? x)) 
-          (every (lambda (e) (not (pair? e))) (cdr x))))
-    (cond
-      ((not (pair? expr)) #t)  
-      (else (every simple-element? (cdr expr)))))  
-  
-  (define (expr-total-string-length expr)
-    (cond
-      ((null? expr) 0)
-      ((string? expr) (string-length expr))
-      ((pair? expr) (+ (expr-total-string-length (car expr))
-                       (expr-total-string-length (cdr expr))))
-      (else 0)))
-  
-  (define (format-expr expr indent)
-    (cond
-      ((null? expr) "()")
-      ((string? expr) (format #f "~s" expr))
-      ((number? expr) (number->string expr))
-      ((symbol? expr) (symbol->string expr))
-      ((pair? expr)
-        (let* ((head (car expr))
-              (body (cdr expr))
-              (is-symbol-head? (symbol? head))
-              (has-enough-elems? (has-more-than-2? body))
-              (is-simple-struct? (simple-structure? expr))
-              (simple-and-short? (and is-simple-struct?
-                                     (<= (expr-total-string-length expr) 100)))              
-              (should-format? (and is-symbol-head? 
-                                  has-enough-elems?
-                                  (not simple-and-short?))))
-          (string-append
-            "(" (format-expr head indent)
-            (if (null? body) 
-                ")" 
-                (let ((new-indent (+ indent 2)))
-                  (string-append
-                    (if should-format? "\n" " ")
-                    (format-body body should-format? new-indent)
-                    ")"))))))))
-  
-  (define (format-body body should-format? indent)
-    (string-join 
-      (map 
-        (lambda (rest-expr)
-          (string-append
-            (if should-format? (get-indent indent) "")
-            (format-expr rest-expr indent)))
-        body) 
-      (if should-format? "\n" " ")))
-
-  ;; Return the original string when an error occurs
-  (catch #t
-    (lambda () 
-      (let ((expr (parse-string str)))
-        (format-expr expr 0)))
-    (lambda args str)))  
-
-(define (texmacs->mgs t . opts)
-  (if (null? opts) (set! opts '()))
-  (let* ((options (if (list-1? opts) (car opts) opts))
-         (do-format? (== (and (assoc-ref options "texmacs->mgs:formatted")
-                             (get-preference "texmacs->mgs:formatted"))
-                        "on"))
-         (stm-str (texmacs->stm (herk-tree->utf8-tree t))))
-    (if do-format?
-        (format-mgs-string stm-str)
-        stm-str)))
-
-(define (mgs->texmacs text)
-  (utf8-tree->herk-tree (stm->texmacs text)))
-
-(define (mgs-snippet->texmacs text)
-  (utf8-tree->herk-tree (stm-snippet->texmacs text)))
-
-(converter texmacs-tree mgs-document
-  (:function-with-options texmacs->mgs)
-  (:option "texmacs->mgs:formatted" "off"))
-
-(converter mgs-document texmacs-tree
-  (:function mgs->texmacs))
-
-(converter texmacs-tree mgs-snippet
-  (:function-with-options texmacs->mgs)
-  (:option "texmacs->mgs:formatted" "off"))
-
-(converter mgs-snippet texmacs-tree
-  (:function mgs-snippet->texmacs))
diff --git a/TeXmacs/progs/init-research.scm b/TeXmacs/progs/init-research.scm
index 057b9bc01e..4ed3a316ba 100644
--- a/TeXmacs/progs/init-research.scm
+++ b/TeXmacs/progs/init-research.scm
@@ -525,7 +525,6 @@
 (lazy-format (data image) postscript pdf svg gif jpeg png ppm tif webp xpm)
 
 (lazy-format (convert rewrite init-rewrite) texmacs verbatim)
-(lazy-format (data mgs) mgs)
 (lazy-format (data stm) stm)
 (lazy-format (data stem) stem)
 (lazy-format (data tmu) tmu)
diff --git a/TeXmacs/progs/texmacs/menus/edit-menu.scm b/TeXmacs/progs/texmacs/menus/edit-menu.scm
index 2ecdf6073b..dd7314b5da 100644
--- a/TeXmacs/progs/texmacs/menus/edit-menu.scm
+++ b/TeXmacs/progs/texmacs/menus/edit-menu.scm
@@ -23,10 +23,7 @@
   (with l
     (filter (lambda (x)
               (or (with-developer-tool?)
-                (and (not (string=? x "mgs"))
-                  (not (string=? x "stm"))
-                  (not (string=? x "stem"))
-                ) ;and
+                (and (not (string=? x "stm")) (not (string=? x "stem")))
               ) ;or
             ) ;lambda
       (cvs "texmacs-snippet" "-snippet" #t)
diff --git a/TeXmacs/progs/texmacs/menus/file-menu.scm b/TeXmacs/progs/texmacs/menus/file-menu.scm
index 46560be45e..9f5f0b8208 100644
--- a/TeXmacs/progs/texmacs/menus/file-menu.scm
+++ b/TeXmacs/progs/texmacs/menus/file-menu.scm
@@ -175,10 +175,7 @@
     (filter (lambda (x)
               (and (not (in? x (image-formats)))
                 (or (with-developer-tool?)
-                  (and (not (string=? x "mgs"))
-                    (not (string=? x "stm"))
-                    (not (string=? x "stem"))
-                  ) ;and
+                  (and (not (string=? x "stm")) (not (string=? x "stem")))
                 ) ;or
               ) ;and
             ) ;lambda
@@ -221,10 +218,7 @@
                   (not (string=? x "latex"))
                   (not (string=? x "latex-class"))
                   (or (with-developer-tool?)
-                    (and (not (string=? x "mgs"))
-                      (not (string=? x "stm"))
-                      (not (string=? x "stem"))
-                    ) ;and
+                    (and (not (string=? x "stm")) (not (string=? x "stem")))
                   ) ;or
                 ) ;and
               ) ;lambda
diff --git a/TeXmacs/progs/texmacs/menus/preferences-menu.scm b/TeXmacs/progs/texmacs/menus/preferences-menu.scm
index 26d670b69e..7357892d0a 100644
--- a/TeXmacs/progs/texmacs/menus/preferences-menu.scm
+++ b/TeXmacs/progs/texmacs/menus/preferences-menu.scm
@@ -243,9 +243,7 @@
             ("1.4" "1.4")
             ("1.5" "1.5")
             ("1.6" "1.6")
-            ("1.7" "1.7"))))
-      (-> "TeXmacs -> Mogan Scheme"
-        (toggle "Formatted Mogan Scheme" "texmacs->mgs:formatted")))
+            ("1.7" "1.7")))))
     (-> "Mathematics"
       (-> "Keyboard"
         (item ("Enforce brackets to match" (toggle-matching-brackets)))
diff --git a/TeXmacs/progs/texmacs/menus/preferences-widgets.scm b/TeXmacs/progs/texmacs/menus/preferences-widgets.scm
index b1ec859df7..9a26dd8333 100644
--- a/TeXmacs/progs/texmacs/menus/preferences-widgets.scm
+++ b/TeXmacs/progs/texmacs/menus/preferences-widgets.scm
@@ -856,18 +856,6 @@
 
 ;; Mogan Scheme ----------
 
-(tm-widget (mogan-scheme-preferences-widget)
-  ======
-  (bold (text "TeXmacs -> Mogan Scheme"))
-  ===
-  (aligned (meti (hlist // (text "Use the Formatted Mogan Scheme"))
-             (toggle (set-boolean-preference "texmacs->mgs:formatted" answer)
-               (get-boolean-preference "texmacs->mgs:formatted")
-             ) ;toggle
-           ) ;meti
-  ) ;aligned
-) ;tm-widget
-
 ;; All converters ----------
 
 (tm-widget (conversion-preferences-widget)
@@ -880,9 +868,6 @@
               (tab (text "Pdf") (centered (dynamic (pdf-preferences-widget))))
             ) ;assuming
             (tab (text "Image") (centered (dynamic (image-preferences-widget))))
-            (tab (text "Mogan Scheme")
-              (centered (dynamic (mogan-scheme-preferences-widget)))
-            ) ;tab
           ) ;tabs
   ) ;padded
   ===
diff --git a/TeXmacs/progs/texmacs/texmacs/tm-files.scm b/TeXmacs/progs/texmacs/texmacs/tm-files.scm
index 8a451259e4..10c68c12c9 100644
--- a/TeXmacs/progs/texmacs/texmacs/tm-files.scm
+++ b/TeXmacs/progs/texmacs/texmacs/tm-files.scm
@@ -740,7 +740,7 @@
 ;;
 ;; 逻辑
 ;; ----
-;; 只允许本地、非 tmfs、非 web 且格式为 texmacs/stm/mgs/tmu 的文档备份；
+;; 只允许本地、非 tmfs、非 web 且格式为 texmacs/stm/tmu 的文档备份；
 ;; 位于 get-texmacs-path 目录或子目录下的内置只读文件直接跳过。
 ;;
 ;; 注意
@@ -752,7 +752,7 @@
     (not (url-rooted-web? name))
     (not (url-rooted-tmfs? name))
     (not (auto-backup-texmacs-path-buffer? name))
-    (in? (auto-backup-format name) '("texmacs" "stm" "mgs" "tmu" "stem"))
+    (in? (auto-backup-format name) '("texmacs" "stm" "tmu" "stem"))
   ) ;and
 ) ;tm-define
 
diff --git a/devel/1131.md b/devel/1131.md
index 98a0f2a89d..f5728ac596 100644
--- a/devel/1131.md
+++ b/devel/1131.md
@@ -80,3 +80,32 @@ S-expression 解析器 `moebius/moebius/data/scheme_der.cpp` 中的 `string_to_s
 - `moebius/moebius/data/scheme_der.cpp`
 - `TeXmacs/plugins/python/packages/code/python.stem`
 - `TeXmacs/tests/stem_python_equiv.scm`（新增等价性测试）
+
+## 2026/06/29 移除 mgs 格式
+
+### What
+彻底移除 `mgs` 文件格式及相关 UI 配置，`stem` 作为继任格式继续保留。
+
+### Why
+- `stem` 已完全替代 `mgs`，两者功能重叠。
+- `mgs` 的格式化转换逻辑不成熟，且已在 `stem` 中去除。
+- 减少菜单、偏好设置和格式白名单中的冗余条目。
+
+### How
+- 删除 `TeXmacs/plugins/mgs/progs/data/mgs.scm` 及空目录 `TeXmacs/plugins/mgs`。
+- 在 `TeXmacs/progs/init-research.scm` 中移除 `(lazy-format (data mgs) mgs)`。
+- 在 `src/Texmacs/Data/new_buffer.cpp` 的 `attach_subformat` 中移除 `"mgs"` 判断。
+- 在 `TeXmacs/progs/texmacs/texmacs/tm-files.scm` 的自动备份白名单与注释中移除 `mgs`。
+- 在 `TeXmacs/progs/texmacs/menus/file-menu.scm`、`edit-menu.scm` 的开发者工具隐藏列表中移除 `"mgs"`。
+- 在 `TeXmacs/progs/texmacs/menus/preferences-menu.scm` 中移除 "TeXmacs -> Mogan Scheme" 子菜单。
+- 在 `TeXmacs/progs/texmacs/menus/preferences-widgets.scm` 中移除 `mogan-scheme-preferences-widget` 及其在 Conversion 标签页中的 Tab。
+
+### 涉及文件
+- `TeXmacs/plugins/mgs/progs/data/mgs.scm`（删除）
+- `TeXmacs/progs/init-research.scm`
+- `src/Texmacs/Data/new_buffer.cpp`
+- `TeXmacs/progs/texmacs/texmacs/tm-files.scm`
+- `TeXmacs/progs/texmacs/menus/file-menu.scm`
+- `TeXmacs/progs/texmacs/menus/edit-menu.scm`
+- `TeXmacs/progs/texmacs/menus/preferences-menu.scm`
+- `TeXmacs/progs/texmacs/menus/preferences-widgets.scm`
diff --git a/src/Texmacs/Data/new_buffer.cpp b/src/Texmacs/Data/new_buffer.cpp
index 145e36ab00..f0152b5643 100644
--- a/src/Texmacs/Data/new_buffer.cpp
+++ b/src/Texmacs/Data/new_buffer.cpp
@@ -460,8 +460,7 @@ attach_buffer_notifier (url name) {
 
 tree
 attach_subformat (tree t, url u, string fm) {
-  if ((fm == "texmacs") || (fm == "stm") || (fm == "mgs") || (fm == "stem"))
-    return t;
+  if ((fm == "texmacs") || (fm == "stm") || (fm == "stem")) return t;
   if (!format_exists (fm)) return t;
 
   string s          = suffix (u);