diff --git a/README.md b/README.md
index 170bf75..7114d42 100644
--- a/README.md
+++ b/README.md
@@ -74,7 +74,7 @@ To work on codegraph itself, run it from a clone instead — Node 22+, pnpm
 (`corepack enable pnpm`), then the toolchain of each extractor you build:
 
 ```bash
-git clone https://github.com/defsquare/codegraph.git && cd codegraph
+git clone --recurse-submodules https://github.com/defsquare/codegraph.git && cd codegraph
 pnpm install && pnpm -r build                    # the CLI (bin/codegraph) and the TypeScript extractor
 (cd extractors/java && ./mvnw -B package)        # Java: a JDK 17+; the wrapper, no local Maven needed
 ./build.sh --csharp                              # C#: the .NET 10 SDK
@@ -82,6 +82,11 @@ pnpm install && pnpm -r build                    # the CLI (bin/codegraph) and t
 ln -s "$PWD/bin/codegraph" ~/.local/bin/codegraph   # optional, works from a symlink
 ```
 
+Already cloned without `--recurse-submodules`? Run
+`git submodule update --init --recursive` first: the website's theme lives in
+`website/themes/hextra` as a submodule, and with it missing Hugo aborts
+`pnpm -r build` with `unknown output format "llms" for kind "home"`.
+
 Now point it at some Java. Any tree of `.java` files works; it does not have
 to build, and no JDK is needed.
 
diff --git a/website/README.md b/website/README.md
index 10c6282..4600657 100644
--- a/website/README.md
+++ b/website/README.md
@@ -4,10 +4,16 @@
 documentation at `/docs/`.
 
 ```bash
+git submodule update --init --recursive   # once per clone: fetches themes/hextra
 hugo server -D --port 1314   # http://localhost:1314/  and  /docs/
 hugo --minify --gc           # → public/
 ```
 
+The theme is a git submodule, so a plain `git clone` leaves `themes/hextra`
+empty and Hugo fails with `unknown output format "llms" for kind "home"` —
+that format is declared by Hextra, not by this site. `pnpm -r build` runs the
+Hugo build too, so the submodule must be initialised before it.
+
 The two halves look nothing alike and that is deliberate: the landing page is
 themeless and runs on the Defsquare Design System, the documentation runs on
 [Hextra](https://github.com/imfing/hextra) (vendored at `themes/hextra` as a