diff --git a/docs/extension_api.rst b/docs/extension_api.rst
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+Extension API
+=============
+
+This page documents the Bazel *module extension* that provisions GCC toolchains and (optionally) related
+software development packages (SDP archives) via ``http_archive``.
+
+Scope
+-----
+
+The extension provides:
+
+- A module extension named ``gcc``.
+- Two tag classes:
+  - ``toolchain``: declares a toolchain repository and its configuration.
+  - ``sdp``: declares an archive (tarball) containing toolchain binaries and metadata needed for repository setup.
+
+Where it lives
+--------------
+
+The implementation is a Starlark module extension defined in a ``.bzl`` file and exported as::
+
+  gcc = module_extension(...)
+
+Tag classes
+-----------
+
+toolchain
+~~~~~~~~~
+
+Defines a toolchain repository and how it should be provisioned (default package, custom SDP, or system toolchain).
+
+**Required attributes**
+- ``name`` (string): Toolchain repo name.
+- ``target_cpu`` (string): Target CPU. Allowed: ``x86_64``, ``aarch64``.
+- ``target_os`` (string): Target OS. Allowed: ``linux``, ``qnx``.
+
+**Optional attributes**
+- ``sdp_to_link`` (string): Name of the SDP package repo to link to this toolchain.
+- ``use_default_package`` (bool): Use the default package from the version matrix.
+- ``use_system_toolchain`` (bool): If true, do not download/link SDP archives (system toolchain mode).
+- ``runtime_ecosystem`` (string): Runtime ecosystem identifier (default ``posix``).
+- ``version`` (string): GCC toolchain version.
+- ``license_info_url`` (string): URL of the license server (used for QNX flows).
+- ``license_info_variable`` (string): Environment variable name for license info (used for QNX flows).
+- ``sdp_version`` (string): SDP version (default ``8.0.0``; used for QNX SDP selection).
+- ``license_path`` (string): Path to shared license file (default ``/opt/score_qnx/license/licenses``).
+- ``extra_compile_flags`` (list of strings): Additional compiler flags.
+- ``extra_link_flags`` (list of strings): Additional linker flags.
+
+sdp
+~~~
+
+Defines an archive (tarball) that can be fetched via ``http_archive`` to provide toolchain binaries.
+
+**Optional attributes**
+- ``name`` (string): Package name. If empty, the extension may derive one depending on usage patterns.
+- ``build_file`` (string): Path to the BUILD file used when creating the repository.
+- ``url`` (string): Archive URL.
+- ``strip_prefix`` (string): Prefix to strip when extracting.
+- ``sha256`` (string): Archive checksum.
+
+Behavior
+--------
+
+The extension gathers tags from the root module (non-root usage is rejected).
+
+Provisioning logic summary:
+
+- ``sdp`` tags are translated into ``http_archive(...)`` invocations.
+- ``toolchain`` tags create a toolchain repository via ``gcc_toolchain(...)``.
+- If ``use_system_toolchain`` is true, the extension skips SDP linking for that toolchain.
+- If ``use_default_package`` is true, the extension derives an SDP archive from ``VERSION_MATRIX`` and links it.
+
+Implementation reference
+------------------------
+
+If you keep the extension implementation in your repository, include it directly so documentation stays in sync:
+
+.. literalinclude:: ../path/to/gcc_extension.bzl
+   :language: python
+   :linenos:
diff --git a/docs/index.rst b/docs/index.rst
index 30d397b..40bdadc 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -15,7 +15,7 @@
 Bazel C/C++ Toolchain Config Documentation
 ============================
 
-TBD
+Welcome to Bazel C/C++ Toolchain Config documentation. This repository containts Bazel toolchain configurations for different platforms used within S-CORE modules.
 
 .. contents:: Table of Contents
    :depth: 2