diff --git a/.gitignore b/.gitignore
index 873a5132b8..afce5d8dc1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -81,6 +81,7 @@ web_modules/
 .netlify
 /packages/frontend/docs/
 /target/
+/build.ninja
 
 # Sentry files
 instrument.mjs
diff --git a/build-targets.json b/build-targets.json
new file mode 100644
index 0000000000..f4bb7505d4
--- /dev/null
+++ b/build-targets.json
@@ -0,0 +1,50 @@
+{
+    "document-types-wasm": {
+        "crate": "packages/document-types",
+        "command": "wasm-pack build packages/document-types",
+        "inputs": [
+            "Cargo.lock",
+            "Cargo.toml",
+            "rust-toolchain.toml",
+            "packages/document-types/.cargo/config.toml",
+            "packages/document-types/Cargo.toml",
+            "packages/document-types/README.md",
+            "packages/document-types/src"
+        ],
+        "outputs": [
+            "packages/document-types/pkg/package.json",
+            "packages/document-types/pkg/catcolab_document_types.js",
+            "packages/document-types/pkg/catcolab_document_types_bg.js",
+            "packages/document-types/pkg/catcolab_document_types.d.ts",
+            "packages/document-types/pkg/catcolab_document_types_bg.wasm",
+            "packages/document-types/pkg/catcolab_document_types_bg.wasm.d.ts",
+            "packages/document-types/pkg/README.md"
+        ]
+    },
+    "catlog-wasm-browser": {
+        "crate": "packages/catlog-wasm",
+        "command": "wasm-pack build packages/catlog-wasm --target browser -d dist/pkg-browser --debug",
+        "inputs": [
+            "Cargo.lock",
+            "Cargo.toml",
+            "rust-toolchain.toml",
+            "packages/catlog/Cargo.toml",
+            "packages/catlog/examples",
+            "packages/catlog/src",
+            "packages/catlog-wasm/.cargo/config.toml",
+            "packages/catlog-wasm/Cargo.toml",
+            "packages/catlog-wasm/src",
+            "packages/document-types/.cargo/config.toml",
+            "packages/document-types/Cargo.toml",
+            "packages/document-types/src"
+        ],
+        "outputs": [
+            "packages/catlog-wasm/dist/pkg-browser/package.json",
+            "packages/catlog-wasm/dist/pkg-browser/catlog_wasm.js",
+            "packages/catlog-wasm/dist/pkg-browser/catlog_wasm_bg.js",
+            "packages/catlog-wasm/dist/pkg-browser/catlog_wasm.d.ts",
+            "packages/catlog-wasm/dist/pkg-browser/catlog_wasm_bg.wasm",
+            "packages/catlog-wasm/dist/pkg-browser/catlog_wasm_bg.wasm.d.ts"
+        ]
+    }
+}
diff --git a/build.py b/build.py
new file mode 100755
index 0000000000..c75735d5fd
--- /dev/null
+++ b/build.py
@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+
+from io import StringIO
+from pathlib import Path
+import json
+import os
+
+import ninja
+from ninja.ninja_syntax import Writer
+
+
+ROOT = Path(__file__).resolve().parent
+MANIFEST = ROOT / "build-targets.json"
+BUILD_FILE = ROOT / "build.ninja"
+BUILD_DIR = "target/ninja"
+
+# Inputs shared by every target; changing the build definition rebuilds everything.
+COMMON_INPUTS = ["build.py", "build-targets.json"]
+
+
+def expand_inputs(inputs: list[str]) -> list[str]:
+    """Expand directory inputs to the files they contain, as ninja needs files."""
+    files: set[Path] = set()
+    for rel in inputs:
+        path = ROOT / rel
+        if path.is_dir():
+            files.update(p for p in path.rglob("*") if p.is_file())
+        else:
+            files.add(path)
+    return sorted(p.relative_to(ROOT).as_posix() for p in files)
+
+
+targets = json.loads(MANIFEST.read_text())
+
+buffer = StringIO()
+w = Writer(buffer)
+
+w.variable("builddir", BUILD_DIR)
+w.newline()
+
+w.pool("wasm_pool", depth=1)
+w.newline()
+
+w.rule(
+    "wasm_pack",
+    command=f"mkdir -p {BUILD_DIR} && $wasm_pack_command && touch $out",
+    description="WASM-PACK $target_name",
+    pool="wasm_pool",
+    restat=True,
+)
+w.newline()
+
+for name, target in targets.items():
+    stamp = f"{BUILD_DIR}/{name}.stamp"
+    w.build(
+        stamp,
+        "wasm_pack",
+        inputs=expand_inputs(COMMON_INPUTS + target["inputs"]),
+        implicit_outputs=target["outputs"],
+        variables={
+            "wasm_pack_command": target["command"],
+            "target_name": name,
+        },
+    )
+    w.build(name, "phony", stamp)
+    w.newline()
+
+w.build("wasm", "phony", list(targets))
+w.default("wasm")
+
+contents = buffer.getvalue()
+if not BUILD_FILE.exists() or BUILD_FILE.read_text() != contents:
+    BUILD_FILE.write_text(contents)
+
+os.chdir(ROOT)
+ninja.ninja()
diff --git a/flake.nix b/flake.nix
index 5606d923ab..ce4d0dea25 100644
--- a/flake.nix
+++ b/flake.nix
@@ -178,13 +178,14 @@
               esbuild
               lld
               netcat
+              ninja
               nodejs_24
               nix
               openssl
               pkg-config
               pnpm
               postgresql
-              python3
+              (python3.withPackages (ps: [ ps.ninja ]))
               python312Packages.ipykernel
               python312Packages.jupyter-core
               python312Packages.jupyter-server
@@ -286,8 +287,12 @@
             in
             {
               catcolabApi = pkgs.callPackage ./infrastructure/catcolab-api.nix craneArgs;
-              catlog-wasm-browser = pkgs.callPackage ./packages/catlog-wasm/default.nix craneArgs;
-              document-types-wasm = pkgs.callPackage ./packages/document-types/default.nix craneArgs;
+              catlog-wasm-browser = pkgs.callPackage ./infrastructure/ninja-target.nix (
+                craneArgs // { target = "catlog-wasm-browser"; }
+              );
+              document-types-wasm = pkgs.callPackage ./infrastructure/ninja-target.nix (
+                craneArgs // { target = "document-types-wasm"; }
+              );
               frontend = frontendPackage.package;
               frontend-tests = frontendPackage.tests;
             };
diff --git a/infrastructure/ninja-target.nix b/infrastructure/ninja-target.nix
new file mode 100644
index 0000000000..32066d9a4b
--- /dev/null
+++ b/infrastructure/ninja-target.nix
@@ -0,0 +1,60 @@
+# Builds a target from build-targets.json via build.py (ninja). The source
+# fileset is derived from the target's `inputs`, so ninja and nix share one
+# dependency listing.
+{
+  craneLib,
+  cargoArtifacts,
+  pkgs,
+  target,
+}:
+let
+  root = ../.;
+  targets = builtins.fromJSON (builtins.readFile (root + "/build-targets.json"));
+  spec = targets.${target};
+  toPath = rel: root + "/${rel}";
+in
+craneLib.buildPackage {
+  inherit cargoArtifacts;
+  inherit (craneLib.crateNameFromCargoToml { cargoToml = toPath "${spec.crate}/Cargo.toml"; })
+    version
+    pname
+    ;
+  doCheck = false;
+
+  nativeBuildInputs = [
+    (pkgs.python3.withPackages (ps: [ ps.ninja ]))
+    pkgs.wasm-pack
+    pkgs.wasm-bindgen-cli
+    pkgs.binaryen
+    pkgs.nodejs
+  ];
+
+  buildInputs = [
+    pkgs.openssl
+  ];
+
+  src = pkgs.lib.fileset.toSource {
+    inherit root;
+    fileset = pkgs.lib.fileset.unions (
+      map toPath (
+        [
+          "build.py"
+          "build-targets.json"
+        ]
+        ++ spec.inputs
+      )
+    );
+  };
+
+  # wasm-pack expects a wasm-bindgen-cli in the environment matching the version in Cargo.lock;
+  # see the overlay in flake.nix. It also needs a writeable $HOME (https://github.com/ipetkov/crane/issues/362).
+  buildPhase = ''
+    HOME=$(mktemp -d) python3 build.py ${target}
+  '';
+
+  installPhase = ''
+    mkdir -p $out
+    cp ${pkgs.lib.concatStringsSep " " spec.outputs} $out/
+    ls $out/
+  '';
+}
diff --git a/packages/catlog-wasm/default.nix b/packages/catlog-wasm/default.nix
deleted file mode 100644
index b4ff0f2743..0000000000
--- a/packages/catlog-wasm/default.nix
+++ /dev/null
@@ -1,59 +0,0 @@
-{
-  craneLib,
-  cargoArtifacts,
-  pkgs,
-}:
-craneLib.buildPackage {
-  inherit cargoArtifacts;
-  inherit (craneLib.crateNameFromCargoToml { cargoToml = ./Cargo.toml; }) version pname;
-  doCheck = false;
-
-  cargoExtraArgs = "-p catlog-wasm";
-
-  nativeBuildInputs = [
-    pkgs.wasm-pack
-    pkgs.wasm-bindgen-cli
-    pkgs.binaryen
-    pkgs.nodejs
-  ];
-
-  buildInputs = [
-    pkgs.openssl
-  ];
-
-  src = pkgs.lib.fileset.toSource {
-    root = ../..;
-    fileset = pkgs.lib.fileset.unions [
-      ../../Cargo.toml
-      ../../Cargo.lock
-      (craneLib.fileset.commonCargoSources ./.)
-      (craneLib.fileset.commonCargoSources ../catlog)
-      (craneLib.fileset.commonCargoSources ../document-types)
-      ./package.json
-    ];
-  };
-
-  # run wasm-pack instead of the default cargo
-  buildPhase = ''
-    cd packages/catlog-wasm
-    # Run the wasm-pack command. wasm-pack will expect to find version of wasm-bindgen-cli in the
-    # environment that must matches the version wasm-bindgen used in the Cargo.toml. The wasm-bindgen-cli
-    # in the nix environment is defined an overlay in flake.nix.
-    #
-    # If the versions do not match there will be a build error when building with nix:
-    # Error: Not able to find or install a local wasm-bindgen.
-    #
-    # With RUST_LOG=debug set there should be a log like indicating the exact problem:
-    # Checking installed `wasm-bindgen` version == expected version: 0.2.105 == 0.2.106
-    # 
-    # wasm-pack needs a writeable $HOME
-    # https://github.com/ipetkov/crane/issues/362
-    HOME=$(mktemp -d) npm run build:browser
-  '';
-
-  installPhase = ''
-    mkdir -p $out
-    cp -r dist/pkg-browser/* $out/
-    ls $out/
-  '';
-}
diff --git a/packages/catlog-wasm/package.json b/packages/catlog-wasm/package.json
index 1a8046b398..fce3f49f75 100644
--- a/packages/catlog-wasm/package.json
+++ b/packages/catlog-wasm/package.json
@@ -5,6 +5,6 @@
     "version": "0.1.0",
     "type": "module",
     "scripts": {
-        "build:browser": "wasm-pack build ./. --target browser -d ./dist/pkg-browser --debug"
+        "build:browser": "../../build.py catlog-wasm-browser"
     }
 }
diff --git a/packages/document-methods/package.json b/packages/document-methods/package.json
index a796f16cac..119670994e 100644
--- a/packages/document-methods/package.json
+++ b/packages/document-methods/package.json
@@ -9,7 +9,7 @@
         "./*": "./src/*"
     },
     "scripts": {
-        "build:wasm": "wasm-pack build ../document-types",
+        "build:wasm": "../../build.py document-types-wasm",
         "format": "oxfmt .",
         "lint": "oxlint --fix && oxfmt .",
         "check": "pnpm run build:wasm && tsc && pnpm run lint",
diff --git a/packages/document-types/default.nix b/packages/document-types/default.nix
deleted file mode 100644
index bb055271bd..0000000000
--- a/packages/document-types/default.nix
+++ /dev/null
@@ -1,56 +0,0 @@
-{
-  craneLib,
-  cargoArtifacts,
-  pkgs,
-}:
-craneLib.buildPackage {
-  inherit cargoArtifacts;
-  inherit (craneLib.crateNameFromCargoToml { cargoToml = ./Cargo.toml; }) version pname;
-  doCheck = false;
-
-  cargoExtraArgs = "-p catcolab-document-types";
-
-  nativeBuildInputs = [
-    pkgs.wasm-pack
-    pkgs.wasm-bindgen-cli
-    pkgs.binaryen
-    pkgs.nodejs
-  ];
-
-  buildInputs = [
-    pkgs.openssl
-  ];
-
-  src = pkgs.lib.fileset.toSource {
-    root = ../..;
-    fileset = pkgs.lib.fileset.unions [
-      ../../Cargo.toml
-      ../../Cargo.lock
-      (craneLib.fileset.commonCargoSources ./.)
-    ];
-  };
-
-  # run wasm-pack instead of the default cargo
-  buildPhase = ''
-    cd packages/document-types
-    # Run the wasm-pack command. wasm-pack will expect to find version of wasm-bindgen-cli in the
-    # environment that must matches the version wasm-bindgen used in the Cargo.toml. The wasm-bindgen-cli
-    # in the nix environment is defined an overlay in flake.nix.
-    #
-    # If the versions do not match there will be a build error when building with nix:
-    # Error: Not able to find or install a local wasm-bindgen.
-    #
-    # With RUST_LOG=debug set there should be a log like indicating the exact problem:
-    # Checking installed `wasm-bindgen` version == expected version: 0.2.105 == 0.2.106
-    # 
-    # wasm-pack needs a writeable $HOME
-    # https://github.com/ipetkov/crane/issues/362
-    HOME=$(mktemp -d) wasm-pack build ./. -d ./pkg
-  '';
-
-  installPhase = ''
-    mkdir -p $out
-    cp -r pkg/* $out/
-    ls $out/
-  '';
-}
diff --git a/packages/frontend/package.json b/packages/frontend/package.json
index b6c35898f8..e278cef667 100644
--- a/packages/frontend/package.json
+++ b/packages/frontend/package.json
@@ -5,8 +5,8 @@
     "license": "MIT",
     "type": "module",
     "scripts": {
-        "build:doc-types": "pnpm --prefix ../document-methods run build:wasm",
-        "build:wasm": "pnpm --prefix ../catlog-wasm run build:browser",
+        "build:doc-types": "../../build.py document-types-wasm",
+        "build:wasm": "../../build.py catlog-wasm-browser",
         "build:bindings": "cargo run -p backend -- generate-bindings",
         "build:tcm": "tcm -p 'src/**/*.module.css' . && pnpm --prefix ../ui-components run build:tcm",
         "build:deps": "pnpm run build:doc-types && pnpm run build:wasm && pnpm run build:bindings && pnpm run build:tcm",