diff --git a/BUILD.md b/BUILD.md
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+# Building clayterm from source
+
+This guide is for maintainers and builders working on clayterm itself.
+
+It covers:
+
+- cloning the repo correctly,
+- initializing the `clay` git submodule,
+- installing the toolchain needed to compile the C sources to WebAssembly,
+- building the local development artifacts, and
+- verifying that the repo is ready for development.
+
+It does **not** cover npm/JSR packaging or publishing.
+
+## What the local build produces
+
+The local source build is driven by `make`.
+
+It generates:
+
+- `clayterm.wasm` — the compiled WebAssembly module built from the C sources
+- `wasm.ts` — a generated TypeScript file derived from `clayterm.wasm`
+
+`wasm.ts` is generated output, not hand-maintained source.
+
+## Clone the repo with submodules
+
+The build depends on the `clay` git submodule.
+
+Preferred fresh clone:
+
+```sh
+git clone --recurse-submodules https://github.com/thefrontside/clayterm.git
+cd clayterm
+```
+
+If you already cloned without submodules:
+
+```sh
+git submodule update --init --recursive
+```
+
+Quick check:
+
+```sh
+git submodule status --recursive
+```
+
+You should also see a populated `clay/` directory. If `clay/` is missing or
+empty, fix the submodule state before building.
+
+## Required tools
+
+You need:
+
+- `git`
+- `make`
+- `clang` with wasm32-capable support
+- `deno`
+
+Equivalent packages are fine if your package manager uses different names.
+
+## Install the toolchain
+
+### macOS
+
+Install Apple's command line tools first. They provide the base developer tools,
+including `git` and `make`.
+
+```sh
+xcode-select --install
+```
+
+Then install LLVM and Deno with Homebrew:
+
+```sh
+brew install llvm deno
+```
+
+Use Homebrew LLVM before Apple's system `clang` when building clayterm:
+
+```sh
+echo 'export PATH="$(brew --prefix llvm)/bin:$PATH"' >> ~/.zshrc
+source ~/.zshrc
+```
+
+If you do not already have `git` available after installing the command line
+tools, install it with Homebrew:
+
+```sh
+brew install git
+```
+
+### Debian / Ubuntu
+
+Install the build toolchain and Git:
+
+```sh
+sudo apt-get update
+sudo apt-get install -y build-essential clang lld git curl
+```
+
+Install Deno with the official installer:
+
+```sh
+curl -fsSL https://deno.land/install.sh | sh
+```
+
+Add Deno to your shell path:
+
+```sh
+echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.bashrc
+source ~/.bashrc
+```
+
+### Fedora / RHEL
+
+Install the build toolchain and Git:
+
+```sh
+sudo dnf install -y clang lld make git curl
+```
+
+Install Deno with the official installer:
+
+```sh
+curl -fsSL https://deno.land/install.sh | sh
+```
+
+Add Deno to your shell path:
+
+```sh
+echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.bashrc
+source ~/.bashrc
+```
+
+### Windows
+
+The recommended Windows build-host path is **WSL2 with Ubuntu**.
+
+From an elevated PowerShell prompt:
+
+```powershell
+wsl --install -d Ubuntu
+```
+
+Then open the Ubuntu environment and follow the **Debian / Ubuntu** instructions
+above.
+
+The build host runs inside WSL2, but the resulting WebAssembly artifacts are
+intended to run on **native Windows** at runtime.
+
+## Verify the toolchain
+
+Before building, confirm the required tools are available:
+
+```sh
+git --version
+make --version
+clang --version
+deno --version
+```
+
+For a quick wasm-target smoke test, make sure `clang` can compile for `wasm32`:
+
+```sh
+clang --target=wasm32 -c -x c /dev/null -o /tmp/clayterm-wasm-test.o
+rm -f /tmp/clayterm-wasm-test.o
+```
+
+On macOS, if `which clang` still points to `/usr/bin/clang` and the wasm test
+fails, make sure the Homebrew LLVM `bin/` directory is at the front of your
+`PATH`.
+
+## Build from source
+
+Run the local source build from the repository root:
+
+```sh
+make
+```
+
+This should produce:
+
+- `clayterm.wasm`
+- `wasm.ts`
+
+For a clean rebuild:
+
+```sh
+make clean && make
+```
+
+## When to rebuild
+
+Re-run `make` when:
+
+- you change files under `src/`
+- you update the `clay` submodule
+- `clayterm.wasm` or `wasm.ts` is missing
+- generated outputs look stale after switching branches or pulling changes
+
+When in doubt, use a clean rebuild:
+
+```sh
+make clean && make
+```
+
+## Verify the build
+
+After `make` succeeds, run the test suite:
+
+```sh
+deno task test
+```
+
+Before opening a PR, it is also a good idea to run the same checks CI runs:
+
+```sh
+deno task fmt:check
+deno lint
+```
+
+## Troubleshooting
+
+### `clay/` is missing or empty
+
+Symptoms may include build failures such as:
+
+- `fatal error: '../clay/clay.h' file not found`
+
+Recovery:
+
+```sh
+git submodule update --init --recursive
+```
+
+Then verify the submodule state and rebuild:
+
+```sh
+git submodule status --recursive
+make clean && make
+```
+
+### `clang` cannot target `wasm32`
+
+Symptoms may include:
+
+- target-related `clang` errors mentioning `wasm32`
+- linker failures while producing `clayterm.wasm`
+
+Recovery:
+
+- make sure you are using an LLVM/Clang build with wasm support
+- on macOS, prefer the Homebrew `llvm` toolchain over `/usr/bin/clang`
+- on Linux/WSL2, make sure both `clang` and `lld` are installed
+- rerun the wasm smoke test:
+
+```sh
+clang --target=wasm32 -c -x c /dev/null -o /tmp/clayterm-wasm-test.o
+rm -f /tmp/clayterm-wasm-test.o
+```
+
+If the smoke test fails, fix the toolchain first and only then rerun `make`.
+
+### Generated artifacts are missing or stale
+
+Symptoms may include:
+
+- `clayterm.wasm` is missing
+- `wasm.ts` is missing
+- you changed `src/` or updated `clay/`, but the generated outputs do not match
+
+Recovery:
+
+```sh
+make clean && make
+```
+
+Then verify the repo is in a good state:
+
+```sh
+deno task test
+```
+
+## Scope note
+
+This document is intentionally limited to local source builds for development.
+
+Out of scope:
+
+- `deno task build:npm`
+- `deno task build:jsr`
+- `npm publish`
+- `deno publish`
+- release tagging and package publishing workflows
diff --git a/README.md b/README.md
index 6af34a3..e6b853b 100644
--- a/README.md
+++ b/README.md
@@ -212,16 +212,12 @@ process.stdin.on("data", (buf) => {
 
 ## Development
 
-Requires `clang` with wasm32 target support.
+For local source builds, toolchain setup, and `clay` submodule instructions, see
+[BUILD.md](BUILD.md).
 
-First build the `.wasm`
+Quick local validation:
 
 ```sh
 make
-```
-
-run tests
-
-```sh
 deno task test
 ```