diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 5dbd411..d0036df 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -29,14 +29,16 @@ jobs:
         uses: actions/cache@v4
         with:
           path: /opt/chez
-          key: chez-${{ runner.os }}-v10.4.1
+          key: chez-${{ runner.os }}-v10.4.1-x11off
 
       - name: Build Chez Scheme from source
         if: steps.cache-chez.outputs.cache-hit != 'true'
         run: |
           git clone --depth 1 --branch v10.4.1 https://github.com/cisco/ChezScheme.git /tmp/chez-src
           cd /tmp/chez-src
-          ./configure --installprefix=/opt/chez --threads
+          # --disable-x11: the expression editor's X11 clipboard isn't needed and
+          # would pull an X11 build/link dep.
+          ./configure --installprefix=/opt/chez --threads --disable-x11
           make -j"$(nproc)"
           sudo make install
           sudo chown -R "$USER" /opt/chez
diff --git a/README.md b/README.md
index 092c07e..c935ac1 100644
--- a/README.md
+++ b/README.md
@@ -45,6 +45,27 @@ $ bin/joltc -e '(/ 1 2)'
 1/2
 ```
 
+## Compile a binary
+
+`bin/joltc build` ahead-of-time compiles a project into a single self-contained
+executable — the runtime, `clojure.core`, the standard library, the app, and its
+`deps.edn` dependencies are linked in, so the result needs no Chez install, no
+JVM, and no source on disk to run.
+
+```bash
+bin/joltc build -m myapp.core -o myapp   # compile myapp.core's -main into ./myapp
+./myapp arg1 arg2                        # runs anywhere; args reach -main
+```
+
+Modes trade dynamism for speed: the default (release) build uses the proven code
+generator; `--opt` also runs the inference + scalar-replacement passes over the
+closed-world program; `--dev` is unoptimized.
+
+This needs Chez's kernel development files (`libkernel.a`, `scheme.h`) and a C
+compiler. They come with a from-source Chez install; a distro `chezscheme`
+package ships only the runtime, so `build` won't link a binary there.
+RFC 0007 (`docs/rfc/`) covers the design and the three-mode model.
+
 ## Architecture
 
 A small Chez runtime (`host/chez/*.ss`: value model, persistent collections, seqs,