fix(api): nvim_cmd{cmd="win_getid"} parsed as :winsize #24181

Problem:
`:lua vim.cmd.win_getid(30,10)` is interpreted as `:win[size] 30 10`.
User intention was to call `vim.fn.win_getid(30,10)`.

Solution:
Check that the `cmd` actually matches the resolved command.

3 files changed, 15 insertions, 9 deletions

diff --git a/src/nvim/README.md b/src/nvim/README.md
index 75155fb9c6..0de0fb9a3f 100644
--- a/src/nvim/README.md
+++ b/src/nvim/README.md
@@ -208,15 +208,11 @@ To debug the main process, you can debug the nvim binary with the `--headless`
 flag which does not launch the TUI and will allow you to set breakpoints in code
 not related to TUI rendering like so:
 
-```
-lldb -- ./build/bin/nvim --headless --listen ~/.cache/nvim/debug-server.pipe
-```
+    lldb -- ./build/bin/nvim --headless --listen ~/.cache/nvim/debug-server.pipe
 
 You can then attach to the headless process to interact with the editor like so:
 
-```
-./build/bin/nvim --remote-ui --server ~/.cache/nvim/debug-server.pipe
-```
+    ./build/bin/nvim --remote-ui --server ~/.cache/nvim/debug-server.pipe
 
 Conversely for debugging TUI rendering, you can start a headless process and
 debug the remote-ui process multiple times without losing editor state.
diff --git a/src/nvim/api/command.c b/src/nvim/api/command.c
index eba209b424..3157132256 100644
--- a/src/nvim/api/command.c
+++ b/src/nvim/api/command.c
@@ -393,6 +393,12 @@ String nvim_cmd(uint64_t channel_id, Dict(cmd) *cmd, Dict(cmd_opts) *opts, Error
   VALIDATE(!is_cmd_ni(ea.cmdidx), "Command not implemented: %s", cmdname, {
     goto end;
   });
+  const char *fullname = IS_USER_CMDIDX(ea.cmdidx)
+    ? get_user_command_name(ea.useridx, ea.cmdidx)
+    : get_command_name(NULL, ea.cmdidx);
+  VALIDATE(strncmp(fullname, cmdname, strlen(cmdname)) == 0, "Invalid command: \"%s\"", cmdname, {
+    goto end;
+  });
 
   // Get the command flags so that we can know what type of arguments the command uses.
   // Not required for a user command since `find_ex_command` already deals with it in that case.
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index d56f780ca6..1ac732b128 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -335,8 +335,7 @@ describe('API', function()
       nvim('command', 'edit '..fname)
       nvim('command', 'normal itesting\napi')
       nvim('command', 'w')
-      local f = io.open(fname)
-      ok(f ~= nil)
+      local f = assert(io.open(fname))
       if is_os('win') then
         eq('testing\r\napi\r\n', f:read('*a'))
       else
@@ -3950,7 +3949,7 @@ describe('API', function()
         }
       }, meths.parse_cmd('MyCommand test it', {}))
     end)
-    it('errors for invalid command', function()
+    it('validates command', function()
       eq('Error while parsing command line', pcall_err(meths.parse_cmd, '', {}))
       eq('Error while parsing command line', pcall_err(meths.parse_cmd, '" foo', {}))
       eq('Error while parsing command line: E492: Not an editor command: Fubar',
@@ -4097,6 +4096,11 @@ describe('API', function()
       eq("Invalid 'reg': expected single character, got xx",
         pcall_err(meths.cmd, { cmd = "put", args = {}, reg = 'xx' }, {}))
 
+      -- #20681
+      eq('Invalid command: "win_getid"', pcall_err(meths.cmd, { cmd = 'win_getid'}, {}))
+      eq('Invalid command: "echo "hi""', pcall_err(meths.cmd, { cmd = 'echo "hi"'}, {}))
+      eq('Invalid command: "win_getid"', pcall_err(exec_lua, [[return vim.cmd.win_getid{}]]))
+
       -- Lua call allows empty {} for dict item.
       eq('', exec_lua([[return vim.cmd{ cmd = "set", args = {}, magic = {} }]]))
       eq('', exec_lua([[return vim.cmd{ cmd = "set", args = {}, mods = {} }]]))