diff --git a/tp-tests.el b/tp-tests.el
index 88d7934..a7843bb 100644
--- a/tp-tests.el
+++ b/tp-tests.el
@@ -1603,5 +1603,179 @@ Returns list of (START END VALUE) intervals."
     (let ((intervals (tp-get 0 7 '(face :underline (:color :style)) str)))
       (should (= (length intervals) 2)))))
 
+;;; ============================================================
+;;; tp-add-to-layers and tp-add-to-all-layers Tests
+;;; ============================================================
+
+(ert-deftest tp-test-add-to-layers-buffer ()
+  "Test tp-add-to-layers adds properties to specified layers in buffer."
+  (tp-test-with-temp-buffer
+    (insert "Hello")
+    (tp-define-layer layer1 (face bold))
+    (tp-define-layer layer2 (face italic))
+    (tp-define-layer layer3 (face underline))
+    (tp-push-layer 1 6 'layer1)
+    (tp-push-layer 1 6 'layer2)
+    (tp-push-layer 1 6 'layer3)
+    ;; Add help-echo to layer1 and layer3
+    (tp-add-to-layers '(layer1 layer3) 1 6 '(help-echo "test"))
+    ;; layer3 is on top, should have help-echo
+    (should (equal (tp-at 1 'help-echo) "test"))
+    ;; Check layer1 also got help-echo
+    (let ((layer1-props (car (tp-region-layer-props 1 6 'layer1))))
+      (should (equal (plist-get (caddr layer1-props) 'help-echo) "test")))
+    ;; layer2 should NOT have help-echo
+    (let ((layer2-props (car (tp-region-layer-props 1 6 'layer2))))
+      (should (null (plist-get (caddr layer2-props) 'help-echo))))))
+
+(ert-deftest tp-test-add-to-layers-by-index ()
+  "Test tp-add-to-layers with layer indices."
+  (tp-test-with-temp-buffer
+    (insert "Hello")
+    (tp-define-layer layer1 (face bold))
+    (tp-define-layer layer2 (face italic))
+    (tp-define-layer layer3 (face underline))
+    (tp-push-layer 1 6 'layer1)
+    (tp-push-layer 1 6 'layer2)
+    (tp-push-layer 1 6 'layer3)
+    ;; Stack is: layer3 (0), layer2 (1), layer1 (2)
+    ;; Add help-echo to indices 0 and 2 (layer3 and layer1)
+    (tp-add-to-layers '(0 2) 1 6 '(help-echo "indexed"))
+    ;; layer3 (top) should have help-echo
+    (should (equal (tp-at 1 'help-echo) "indexed"))
+    ;; Check layer1 also got help-echo
+    (let ((layer1-props (car (tp-region-layer-props 1 6 'layer1))))
+      (should (equal (plist-get (caddr layer1-props) 'help-echo) "indexed")))
+    ;; layer2 (index 1) should NOT have help-echo
+    (let ((layer2-props (car (tp-region-layer-props 1 6 'layer2))))
+      (should (null (plist-get (caddr layer2-props) 'help-echo))))))
+
+(ert-deftest tp-test-add-to-layers-string ()
+  "Test tp-add-to-layers works on entire string."
+  (let ((str (copy-sequence "Hello")))
+    (setq tp-layer-alist nil)
+    (setq tp-layer-groups nil)
+    (tp-define-layer layer1 (face bold))
+    (tp-define-layer layer2 (face italic))
+    (tp-push-layer str 'layer1)
+    (tp-push-layer str 'layer2)
+    ;; Add help-echo to layer1
+    (tp-add-to-layers '(layer1) str 'help-echo "test")
+    ;; Check layer1 got help-echo
+    (let ((layer1-props (car (tp-region-layer-props 0 5 'layer1 str))))
+      (should (equal (plist-get (caddr layer1-props) 'help-echo) "test")))
+    ;; layer2 (top) should NOT have help-echo
+    (should (null (tp-at 0 'help-echo str)))))
+
+(ert-deftest tp-test-add-to-layers-deep-merge ()
+  "Test tp-add-to-layers deeply merges properties."
+  (tp-test-with-temp-buffer
+    (insert "Hello")
+    (tp-define-layer layer1 (face (:foreground "red")))
+    (tp-push-layer 1 6 'layer1)
+    ;; Add background to layer1 - should merge with existing face
+    (tp-add-to-layers '(layer1) 1 6 '(face (:background "blue")))
+    (let ((face (tp-at 1 'face)))
+      (should (equal (plist-get face :foreground) "red"))
+      (should (equal (plist-get face :background) "blue")))))
+
+(ert-deftest tp-test-add-to-all-layers-buffer ()
+  "Test tp-add-to-all-layers adds properties to all layers in buffer."
+  (tp-test-with-temp-buffer
+    (insert "Hello")
+    (tp-define-layer layer1 (face bold))
+    (tp-define-layer layer2 (face italic))
+    (tp-define-layer layer3 (face underline))
+    (tp-push-layer 1 6 'layer1)
+    (tp-push-layer 1 6 'layer2)
+    (tp-push-layer 1 6 'layer3)
+    ;; Add help-echo to all layers
+    (tp-add-to-all-layers 1 6 '(help-echo "all"))
+    ;; layer3 (top) should have help-echo
+    (should (equal (tp-at 1 'help-echo) "all"))
+    ;; Check all layers got help-echo
+    (let ((layer1-props (car (tp-region-layer-props 1 6 'layer1)))
+          (layer2-props (car (tp-region-layer-props 1 6 'layer2)))
+          (layer3-props (car (tp-region-layer-props 1 6 'layer3))))
+      (should (equal (plist-get (caddr layer1-props) 'help-echo) "all"))
+      (should (equal (plist-get (caddr layer2-props) 'help-echo) "all"))
+      (should (equal (plist-get (caddr layer3-props) 'help-echo) "all")))))
+
+(ert-deftest tp-test-add-to-all-layers-string ()
+  "Test tp-add-to-all-layers works on entire string."
+  (let ((str (copy-sequence "Hello")))
+    (setq tp-layer-alist nil)
+    (setq tp-layer-groups nil)
+    (tp-define-layer layer1 (face bold))
+    (tp-define-layer layer2 (face italic))
+    (tp-push-layer str 'layer1)
+    (tp-push-layer str 'layer2)
+    ;; Add help-echo to all layers
+    (tp-add-to-all-layers str 'help-echo "all")
+    ;; Check all layers got help-echo
+    (let ((layer1-props (car (tp-region-layer-props 0 5 'layer1 str)))
+          (layer2-props (car (tp-region-layer-props 0 5 'layer2 str))))
+      (should (equal (plist-get (caddr layer1-props) 'help-echo) "all"))
+      (should (equal (plist-get (caddr layer2-props) 'help-echo) "all")))))
+
+(ert-deftest tp-test-add-to-all-layers-deep-merge ()
+  "Test tp-add-to-all-layers deeply merges properties."
+  (tp-test-with-temp-buffer
+    (insert "Hello")
+    (tp-define-layer layer1 (face (:foreground "red")))
+    (tp-define-layer layer2 (face (:foreground "blue")))
+    (tp-push-layer 1 6 'layer1)
+    (tp-push-layer 1 6 'layer2)
+    ;; Add background to all layers
+    (tp-add-to-all-layers 1 6 '(face (:background "green")))
+    ;; Top layer (layer2) should have merged face
+    (let ((face (tp-at 1 'face)))
+      (should (equal (plist-get face :foreground) "blue"))
+      (should (equal (plist-get face :background) "green")))
+    ;; layer1 should also have merged face
+    (let* ((layer1-props (car (tp-region-layer-props 1 6 'layer1)))
+           (face (plist-get (caddr layer1-props) 'face)))
+      (should (equal (plist-get face :foreground) "red"))
+      (should (equal (plist-get face :background) "green")))))
+
+(ert-deftest tp-test-add-to-layers-negative-index ()
+  "Test tp-add-to-layers with negative index (-1 means bottom)."
+  (tp-test-with-temp-buffer
+    (insert "Hello")
+    (tp-define-layer layer1 (face bold))
+    (tp-define-layer layer2 (face italic))
+    (tp-push-layer 1 6 'layer1)
+    (tp-push-layer 1 6 'layer2)
+    ;; Stack is: layer2 (0), layer1 (1)
+    ;; Add help-echo to index -1 (bottom = layer1)
+    (tp-add-to-layers '(-1) 1 6 '(help-echo "bottom"))
+    ;; layer2 (top) should NOT have help-echo
+    (should (null (tp-at 1 'help-echo)))
+    ;; layer1 (bottom) should have help-echo
+    (let ((layer1-props (car (tp-region-layer-props 1 6 'layer1))))
+      (should (equal (plist-get (caddr layer1-props) 'help-echo) "bottom")))))
+
+(ert-deftest tp-test-add-to-layers-returns-string ()
+  "Test tp-add-to-layers returns the modified string."
+  (let ((str (copy-sequence "Hello")))
+    (setq tp-layer-alist nil)
+    (setq tp-layer-groups nil)
+    (tp-define-layer layer1 (face bold))
+    (tp-push-layer str 'layer1)
+    (let ((result (tp-add-to-layers '(layer1) str 'help-echo "test")))
+      (should (stringp result))
+      (should (eq result str)))))
+
+(ert-deftest tp-test-add-to-all-layers-returns-string ()
+  "Test tp-add-to-all-layers returns the modified string."
+  (let ((str (copy-sequence "Hello")))
+    (setq tp-layer-alist nil)
+    (setq tp-layer-groups nil)
+    (tp-define-layer layer1 (face bold))
+    (tp-push-layer str 'layer1)
+    (let ((result (tp-add-to-all-layers str 'help-echo "test")))
+      (should (stringp result))
+      (should (eq result str)))))
+
 (provide 'tp-ert-tests)
 ;;; tp-ert-tests.el ends here
diff --git a/tp.el b/tp.el
index 1ac51e3..ba83120 100644
--- a/tp.el
+++ b/tp.el
@@ -2139,6 +2139,127 @@ OBJECT defaults to current buffer."
   (when-let ((intervals (tp-intervals start end object)))
     (plist-get (nth 2 (car intervals)) 'tp-name)))
 
+;;; Layer property manipulation functions
+
+(defun tp-add-to-layers (idx-or-layer-name-list start-or-string &optional end-or-plist plist-or-object object)
+  "Add/merge properties to specified layers.
+
+This function supports two calling conventions:
+
+1. Buffer/string region:
+   (tp-add-to-layers IDX-OR-LAYER-NAME-LIST START END PLIST OBJECT)
+
+2. Entire string:
+   (tp-add-to-layers IDX-OR-LAYER-NAME-LIST STRING &rest PLIST)
+
+IDX-OR-LAYER-NAME-LIST is a list of layer indices (integers) or
+layer names (symbols) specifying which layers to add properties to.
+For indices: 0 means top layer, -1 means bottom layer.
+
+PLIST is a property list to merge into the specified layers.
+Properties are deeply merged (nested plists are merged, not replaced).
+
+OBJECT defaults to current buffer for region form.
+
+Returns the modified object (string) or nil for buffer operations."
+  (let (start end plist obj layer-ids)
+    (setq layer-ids idx-or-layer-name-list)
+    (cond
+     ;; Entire string form: (tp-add-to-layers ids string &rest plist)
+     ((stringp start-or-string)
+      (setq obj start-or-string
+            start 0
+            end (length start-or-string)
+            plist (if (listp end-or-plist)
+                      (if plist-or-object
+                          (cons end-or-plist (cons plist-or-object (when object (list object))))
+                        end-or-plist)
+                    (list end-or-plist plist-or-object))))
+     ;; Region form: (tp-add-to-layers ids start end plist object)
+     ((numberp start-or-string)
+      (setq start start-or-string
+            end end-or-plist
+            plist plist-or-object
+            obj object)))
+
+    ;; Handle plist as list if needed
+    (when (and (listp plist) (listp (car plist)) (keywordp (caar plist)))
+      (setq plist (car plist)))
+
+    ;; Process each interval
+    (tp-intervals-map
+     (lambda (i-start i-end top belows)
+       (let* ((current-stack (tp--layer-stack-to-list top belows))
+              (modified-stack
+               (cl-loop for layer in current-stack
+                        for i from 0
+                        collect
+                        (if (cl-some
+                             (lambda (id)
+                               (let ((found (tp--get-layer-by-idx-or-name current-stack id)))
+                                 (and found (= (car found) i))))
+                             layer-ids)
+                            ;; Merge plist into this layer
+                            (tp--deep-merge-plist layer plist)
+                          ;; Keep layer unchanged
+                          layer))))
+         (set-text-properties
+          (+ start i-start) (+ start i-end)
+          (tp--build-layer-props modified-stack)
+          obj)))
+     start end obj)
+    (if (stringp obj) obj nil)))
+
+(defun tp-add-to-all-layers (start-or-string &optional end-or-plist plist-or-object object)
+  "Add/merge properties to all layers.
+
+This function supports two calling conventions:
+
+1. Buffer/string region:
+   (tp-add-to-all-layers START END PLIST OBJECT)
+
+2. Entire string:
+   (tp-add-to-all-layers STRING &rest PLIST)
+
+PLIST is a property list to merge into all layers.
+Properties are deeply merged (nested plists are merged, not replaced).
+
+OBJECT defaults to current buffer for region form.
+
+This function uses `tp-add-to-layers' internally, collecting all
+layer indices and passing them to add the plist to every layer.
+
+Returns the modified object (string) or nil for buffer operations."
+  (let (start end plist obj)
+    (cond
+     ;; Entire string form: (tp-add-to-all-layers string &rest plist)
+     ((stringp start-or-string)
+      (setq obj start-or-string
+            start 0
+            end (length start-or-string)
+            plist (if (listp end-or-plist)
+                      (if plist-or-object
+                          (cons end-or-plist (cons plist-or-object (when object (list object))))
+                        end-or-plist)
+                    (list end-or-plist plist-or-object))))
+     ;; Region form: (tp-add-to-all-layers start end plist object)
+     ((numberp start-or-string)
+      (setq start start-or-string
+            end end-or-plist
+            plist plist-or-object
+            obj object)))
+
+    ;; Handle plist as list if needed
+    (when (and (listp plist) (listp (car plist)) (keywordp (caar plist)))
+      (setq plist (car plist)))
+
+    ;; Get the maximum layer count in the region to build a list of all indices
+    (let ((max-count (tp-layer-count start end obj)))
+      (when (> max-count 0)
+        (let ((all-indices (cl-loop for i from 0 below max-count collect i)))
+          (tp-add-to-layers all-indices start end plist obj))))
+    (if (stringp obj) obj nil)))
+
 
 (provide 'tp)
 ;;; tp.el ends here