diff --git a/README.md b/README.md
index 946be37..52c0b93 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,13 @@ required capability.
      scripts and programs that rely on long-standing glibc behavior.
   5. *(default)* — **vlibc**: glibc-extended, without the spoofing layer, but
      with extended standard features and high glibc compatibility.
+- **Profiles 1-3 are functional.** `--enable-onlyposix`, `--enable-muslmimic`,
+  and `--enable-muslext` build with real build-time profile gating: a generated
+  `include/vlibc/features.h` exposes `VLIBC_LEVEL`, the public `<stddef.h>` and
+  `<string.h>` headers are gated by it, and an initial string slice (`memcpy`,
+  `memmove`, `memset`, `strlen`, `strcmp`, `strlcpy`, `strlcat`,
+  `strcasestr`) is compiled per level. Profiles 4 and 5 (`spoof`, `vlibc`)
+  remain configure-level declarations in this increment.
 - **Two install methods** (`--with-install=`):
   - `alongside` (default) — install next to the system libc, under a
     vlibc-specific tree; the system libc is left untouched.
diff --git a/STYLEGUIDE.md b/STYLEGUIDE.md
index 8b5c852..008cb52 100644
--- a/STYLEGUIDE.md
+++ b/STYLEGUIDE.md
@@ -76,6 +76,34 @@ linked binaries (see `docs/overview.md`):
 
 Every public function carries the tightest correct attribute.
 
+## Profile gating
+
+Profile-gated code follows these formatting and naming conventions (the
+semantic classification of functions into levels lives in
+`docs/compatibility.md`, not here):
+
+- Public headers gate declarations with `#if VLIBC_LEVEL >= N`. Prefer the
+  `VLIBC_LEVEL_GE(n)` helper from `include/vlibc/features.h`; fall back to the
+  raw comparison only when the level is a literal constant:
+
+  ```c
+  #if VLIBC_LEVEL_GE(2)
+  size_t strlcpy(char *dst, const char *src, size_t n);
+  #endif
+  ```
+
+- Group implementation sources by level in `Makefile.am` using the per-level
+  conditionals `if PROFILE_GE_2` / `if PROFILE_GE_3`, mirroring the
+  `#if VLIBC_LEVEL >= N` rule in headers.
+- Standard headers use `VLIBC_`-prefixed include guards
+  (`VLIBC_STDDEF_H`, `VLIBC_STRING_H`), never the `_STDDEF_H`-style reserved
+  forms. A leading underscore is reserved for the implementation and the
+  C/POSIX standards (see Naming); the `VLIBC_` prefix stays out of the
+  reserved namespace while remaining unambiguous.
+- Intent attributes respect the gating: `pure` / `const` are only valid on
+  side-effect-free functions. Never annotate a mutating function (`memcpy`,
+  `memmove`, `memset`, `strlcpy`, `strlcat`) with either.
+
 ## Error handling
 
 - No empty blocks; no silent failure.
diff --git a/docs/compatibility.md b/docs/compatibility.md
index 80af717..66e4ae6 100644
--- a/docs/compatibility.md
+++ b/docs/compatibility.md
@@ -11,6 +11,51 @@ mutually exclusive; the default is `vlibc`.
 | 4     | `--enable-spoof`     | `spoof`     | A glibc replica: emulates glibc for drop-in compatibility, higher than `muslext` or `vlibc` full. |
 | 5     | *(default)*          | `vlibc`     | glibc-extended: glibc minus its baggage, plus extended standard features. High (but not spoof-level) glibc compatibility. |
 
+## Profile mechanism
+
+The profiles map onto a single integer level, `VLIBC_LEVEL`. Levels are
+**cumulative**: a profile at level N exposes everything whose minimum level is
+`<= N`.
+
+| Level | Classification                          |
+|-------|-----------------------------------------|
+| 1     | ISO C + POSIX base                      |
+| 2     | + BSD / XSI extensions                  |
+| 3     | + GNU extensions                        |
+
+Public headers gate declarations with `#if VLIBC_LEVEL >= N`; a function whose
+minimum level is 2 is only declared when the active profile is level 2 or
+higher. The build applies the same rule to sources via
+`AM_CONDITIONAL([PROFILE_GE_2])` and `AM_CONDITIONAL([PROFILE_GE_3])`, so each
+level's implementation files are only compiled when the selected profile
+reaches that level.
+
+The generated installed header `include/vlibc/features.h` exposes the macros
+the gating is based on:
+
+- `VLIBC_LEVEL` — the active profile's integer level.
+- `VLIBC_PROFILE` — the selected profile name.
+- `VLIBC_LEVEL_GE(n)` — expands to 1 when `VLIBC_LEVEL >= n`, 0 otherwise.
+
+### Slice manifest
+
+The initial implementation covers eight string/memory functions, classified as
+follows:
+
+| Function     | Level | Origin  |
+|--------------|-------|---------|
+| `memcpy`     | L1    | ISO C   |
+| `memmove`    | L1    | ISO C   |
+| `memset`     | L1    | ISO C   |
+| `strlen`     | L1    | ISO C   |
+| `strcmp`     | L1    | ISO C   |
+| `strlcpy`    | L2    | BSD     |
+| `strlcat`    | L2    | BSD     |
+| `strcasestr` | L3    | GNU     |
+
+Profiles 4 and 5 (`spoof`, `vlibc`) build on the same level mechanism but are
+not implemented in this increment beyond their configure declarations.
+
 ## Differences from glibc
 
 - **No legacy baggage.** vlibc targets modern, standard-conforming behavior and