diff --git a/.devcontainer/devcontainer-lock.json b/.devcontainer/devcontainer-lock.json
new file mode 100644
index 0000000..2f86d4b
--- /dev/null
+++ b/.devcontainer/devcontainer-lock.json
@@ -0,0 +1,9 @@
+{
+  "features": {
+    "ghcr.io/devcontainers/features/nix:1": {
+      "version": "1.3.1",
+      "resolved": "ghcr.io/devcontainers/features/nix@sha256:8953a9da475055d140db0ede314e8e1b117d28c1eabf289c1b07c7619ce6c078",
+      "integrity": "sha256:8953a9da475055d140db0ede314e8e1b117d28c1eabf289c1b07c7619ce6c078"
+    }
+  }
+}
diff --git a/Makefile b/Makefile
index 3031e1e..2de1924 100644
--- a/Makefile
+++ b/Makefile
@@ -13,7 +13,8 @@ EPUB_NAME := book.epub
 EPUB_PATH := output/$(EPUB_NAME)
 
 ASCIIDOC_FLAGS = \
-  --doctype book
+  --doctype book \
+  --failure-level ERROR
 
 .PHONY: all count clean lint epubcheck
 
diff --git a/README.md b/README.md
index 981a40a..a478206 100644
--- a/README.md
+++ b/README.md
@@ -37,8 +37,8 @@ Everything you need to get started is included in the package
 ## Development environment
 
 Letterpress ships a [Nix flake](flake.nix) that pins the entire toolchain
-(Asciidoctor, asciidoctor-pdf/epub3/diagram/mathematical, PlantUML, Graphviz,
-a JRE, Vale and EPUBCheck). This is the same environment used in CI, so builds
+(Asciidoctor, asciidoctor-pdf/epub3/diagram/mathematical, PlantUML, Mermaid,
+Graphviz, a JRE, Vale and EPUBCheck). This is the same environment used in CI, so builds
 are reproducible everywhere.
 
 ### Enter the dev shell
diff --git a/flake.nix b/flake.nix
index 516d6c0..ae34864 100644
--- a/flake.nix
+++ b/flake.nix
@@ -31,12 +31,13 @@
         };
 
         # Everything needed to render the book: the gem env plus the external
-        # binaries asciidoctor-diagram shells out to (plantuml -> java + graphviz).
+        # binaries asciidoctor-diagram shells out to.
         runtimeInputs = with pkgs; [
           gems
           gems.wrappedRuby
           gnumake
           plantuml
+          mermaid-cli
           graphviz
           jre
           vale
@@ -48,6 +49,7 @@
           version = "0.1.0";
           src = ./.;
           nativeBuildInputs = runtimeInputs;
+          DIAGRAM_PLANTUML_CLASSPATH = "${pkgs.plantuml}/lib/plantuml.jar";
           # asciidoctor-diagram caches into $HOME
           buildPhase = ''
             export HOME=$TMPDIR
@@ -68,6 +70,7 @@
 
         devShells.default = pkgs.mkShell {
           packages = runtimeInputs ++ (with pkgs; [ bundix ]);
+          DIAGRAM_PLANTUML_CLASSPATH = "${pkgs.plantuml}/lib/plantuml.jar";
           shellHook = ''
             echo "letterpress dev shell"
             echo "  make            build PDF + EPUB"