diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 6d782cc0e4..7c6b0e06a0 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3557,8 +3557,8 @@ nvim_open_tabpage({buffer}, {config})                    *nvim_open_tabpage()*
       • {config}  (`vim.api.keyset.tabpage_config`) Configuration for the new
                   tabpage. Keys:
                   • enter: Whether to enter the new tabpage (default: true)
-                  • after: Position to insert tabpage (default: 0). 0 = after
-                    current, 1 = first, N = before Nth.
+                  • after: Position to insert tabpage (default: -1; after
+                    current). 0 = first, N = after Nth.
 
     Return: ~
         (`integer`) Tabpage handle of the created tabpage
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index b49a547d63..b96f977c96 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -1679,8 +1679,8 @@ function vim.api.nvim_notify(msg, log_level, opts) end
 --- Use 0 for current buffer.
 --- @param config vim.api.keyset.tabpage_config Configuration for the new tabpage. Keys:
 --- - enter: Whether to enter the new tabpage (default: true)
---- - after: Position to insert tabpage (default: 0).
----          0 = after current, 1 = first, N = before Nth.
+--- - after: Position to insert tabpage (default: -1; after current).
+---          0 = first, N = after Nth.
 --- @return integer # Tabpage handle of the created tabpage
 function vim.api.nvim_open_tabpage(buffer, config) end
 
diff --git a/src/nvim/api/tabpage.c b/src/nvim/api/tabpage.c
index 89b445e5c7..28c701f0c5 100644
--- a/src/nvim/api/tabpage.c
+++ b/src/nvim/api/tabpage.c
@@ -5,7 +5,6 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/api/private/dispatch.h"
 #include "nvim/api/private/helpers.h"
-#include "nvim/api/private/validate.h"
 #include "nvim/api/tabpage.h"
 #include "nvim/api/vim.h"
 #include "nvim/autocmd.h"
@@ -196,8 +195,8 @@ Boolean nvim_tabpage_is_valid(Tabpage tabpage)
 ///               Use 0 for current buffer.
 /// @param config Configuration for the new tabpage. Keys:
 ///   - enter: Whether to enter the new tabpage (default: true)
-///   - after: Position to insert tabpage (default: 0).
-///            0 = after current, 1 = first, N = before Nth.
+///   - after: Position to insert tabpage (default: -1; after current).
+///            0 = first, N = after Nth.
 /// @param[out] err Error details, if any
 /// @return Tabpage handle of the created tabpage
 Tabpage nvim_open_tabpage(Buffer buffer, Dict(tabpage_config) *config, Error *err)
@@ -218,19 +217,15 @@ Tabpage nvim_open_tabpage(Buffer buffer, Dict(tabpage_config) *config, Error *er
     return 0;
   }
 
-  int after = 0;  // Default to after current tabpage
+  int after = -1;  // Default to after current tabpage
   if (HAS_KEY_X(config, after)) {
     after = (int)config->after;
-    VALIDATE_INT(after >= 0, "after", config->after, {
-      return 0;
-    });
-    // NOTE: win_new_tabpage will handle after > count by appending to the end.
   }
 
   tabpage_T *tp;
   win_T *wp;
   TRY_WRAP(err, {
-    tp = win_new_tabpage(after, NULL, enter, &wp);
+    tp = win_new_tabpage(after + 1, NULL, enter, &wp);
   });
   if (!tp) {
     if (!ERROR_SET(err)) {  // set error maybe more specific
diff --git a/test/functional/api/tabpage_spec.lua b/test/functional/api/tabpage_spec.lua
index f978c15d46..8d364216e5 100644
--- a/test/functional/api/tabpage_spec.lua
+++ b/test/functional/api/tabpage_spec.lua
@@ -242,8 +242,8 @@ describe('api/tabpage', function()
       eq(4, #newtabs2)
       eq({
         tab1,
-        new_tab,
         tab2,
+        new_tab,
         tab3,
       }, newtabs2)
       eq(api.nvim_get_current_tabpage(), tab3)
@@ -375,20 +375,20 @@ describe('api/tabpage', function()
       eq(3, #initial_tabs)
       eq({ tab1, tab2, tab3 }, initial_tabs)
 
-      -- Test after=1: should become first tab
-      local first_tab = api.nvim_open_tabpage(0, { enter = false, after = 1 })
+      -- Test after=0: should become first tab
+      local first_tab = api.nvim_open_tabpage(0, { enter = false, after = 0 })
       local tabs_after_first = api.nvim_list_tabpages()
       eq(4, #tabs_after_first)
       eq({ first_tab, tab1, tab2, tab3 }, tabs_after_first)
 
-      -- Test after=0: should insert after current tab (tab3)
-      local explicit_after_current = api.nvim_open_tabpage(0, { enter = false, after = 0 })
+      -- Test after=-1: should insert after current tab (tab3)
+      local explicit_after_current = api.nvim_open_tabpage(0, { enter = false, after = -1 })
       local tabs_after_current = api.nvim_list_tabpages()
       eq(5, #tabs_after_current)
       eq({ first_tab, tab1, tab2, tab3, explicit_after_current }, tabs_after_current)
 
-      -- Test inserting before a middle tab (before tab2, which is now position 3)
-      local before_middle = api.nvim_open_tabpage(0, { enter = false, after = 3 })
+      -- Test inserting before a middle tab (before tab2, which is now number 3)
+      local before_middle = api.nvim_open_tabpage(0, { enter = false, after = 2 })
       local tabs_after_middle = api.nvim_list_tabpages()
       eq(6, #tabs_after_middle)
       eq({ first_tab, tab1, before_middle, tab2, tab3, explicit_after_current }, tabs_after_middle)
@@ -408,8 +408,6 @@ describe('api/tabpage', function()
         default_after_current,
         explicit_after_current,
       }, final_tabs)
-
-      eq("Invalid 'after': -1", pcall_err(api.nvim_open_tabpage, 0, { after = -1 }))
     end)
 
     it('handles position beyond last tab', function()
@@ -477,7 +475,7 @@ describe('api/tabpage', function()
       api.nvim_set_current_tabpage(tabs[3])
 
       -- Insert after=0 (after current, which is tab 3)
-      local new_after_current = api.nvim_open_tabpage(0, { enter = false, after = 0 })
+      local new_after_current = api.nvim_open_tabpage(0, { enter = false })
       local result_tabs = api.nvim_list_tabpages()
       eq(6, #result_tabs)
       eq({
@@ -489,8 +487,8 @@ describe('api/tabpage', function()
         tabs[5],
       }, result_tabs)
 
-      -- Insert at position 2 (before tab2, which becomes new position 2)
-      local new_at_pos2 = api.nvim_open_tabpage(0, { enter = false, after = 2 })
+      -- Insert as number 2 (before tab2, which will become number 3)
+      local new_at_pos2 = api.nvim_open_tabpage(0, { enter = false, after = 1 })
       local final_result = api.nvim_list_tabpages()
       eq(7, #final_result)
       eq({
@@ -512,7 +510,7 @@ describe('api/tabpage', function()
       -- Create new tab with enter=true, should insert after current (tab2)
       local tab3 = api.nvim_open_tabpage(0, {
         enter = true,
-        after = 0,
+        after = -1,
       })
 
       local tabs = api.nvim_list_tabpages()
@@ -524,7 +522,7 @@ describe('api/tabpage', function()
       api.nvim_set_current_tabpage(tab1)
       local tab4 = api.nvim_open_tabpage(0, {
         enter = true,
-        after = 1, -- Should become first tab
+        after = 0, -- Should become first tab
       })
 
       local final_tabs = api.nvim_list_tabpages()