diff --git a/runtime/doc/develop.txt b/runtime/doc/develop.txt
index deb40445e9..2d486b6ba9 100644
--- a/runtime/doc/develop.txt
+++ b/runtime/doc/develop.txt
@@ -1,4 +1,4 @@
-*develop.txt*   For Vim version 9.1.  Last change: 2025 Aug 11
+*develop.txt*   For Vim version 9.1.  Last change: 2025 Sep 01
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -362,6 +362,34 @@ We follow POSIX.1-2001 (SUSv3) for type sizes, which in practice means:
 	unsigned    32-bit or larger unsigned
 
 
+FUNCTION PROTOTYPES				*assumptions-prototypes*
+
+Vim currently does not use conventional header files (`.h`) for most internal
+function prototypes.  Instead, the current architecture uses individual `.pro`
+files in the `src/proto/` directory, with one `.pro` file per `.c` file.
+
+Unlike traditional self-contained header files, these `.pro` files do not
+contain API documentation, struct and enum definitions, or other declaration;
+only function prototypes.
+
+The bundling of these files is not automated.  The `src/proto.h` header is
+composed of a list of manual `#include` directives, one for each individual
+`.pro` file.
+
+Due to this design, integrating a new source file within this architecture
+involves creating a corresponding `.pro` file, manually adding a new `#include`
+directive to `proto.h` and manually adding this file to both `src/Makefile` and
+`src/Make_mvc.mak`.
+
+A `make proto` target exists in `src/Makefile` with the original goal of
+automating the process of updating the `.pro` files.  However, this target is
+unreliable, fails on major platforms (e.g., macOS), and is only part of the
+new release workflow.  In practice, contributors edit the relevant `.pro` files
+by hand when adding, removing, or modifying a function signatures.
+
+This system has been in place since at least v1.24, when Vim’s functions were
+still written in K&R style.
+
 ==============================================================================
 4. Coding style						*coding-style*
 
diff --git a/runtime/doc/tags b/runtime/doc/tags
index 617753221c..e988ffb578 100644
--- a/runtime/doc/tags
+++ b/runtime/doc/tags
@@ -6228,6 +6228,7 @@ assert_report()	testing.txt	/*assert_report()*
 assert_true()	testing.txt	/*assert_true()*
 assumptions-C-compiler	develop.txt	/*assumptions-C-compiler*
 assumptions-makefiles	develop.txt	/*assumptions-makefiles*
+assumptions-prototypes	develop.txt	/*assumptions-prototypes*
 assumptions-variables	develop.txt	/*assumptions-variables*
 astro.vim	syntax.txt	/*astro.vim*
 asy.vim	syntax.txt	/*asy.vim*