diff --git a/package.json b/package.json
index 5ea86417..dd80f192 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,9 @@
 {
   "name": "@observablehq/runtime",
-  "version": "4.4.7",
+  "version": "4.5.0-rc.4",
+  "publishConfig": {
+    "tag": "next"
+  },
   "license": "ISC",
   "main": "dist/runtime.umd.js",
   "module": "src/index.js",
@@ -35,7 +38,7 @@
   ],
   "dependencies": {
     "@observablehq/inspector": "^3.1.0",
-    "@observablehq/stdlib": "^3.0.0",
+    "@observablehq/stdlib": "3.2.0-rc.5",
     "esm": "^3.0.84"
   },
   "devDependencies": {
diff --git a/src/module.js b/src/module.js
index 37567c9f..d8c6df12 100644
--- a/src/module.js
+++ b/src/module.js
@@ -6,14 +6,14 @@ import rethrow from "./rethrow";
 import {variable_invalidation, variable_visibility} from "./runtime";
 import Variable, {TYPE_DUPLICATE, TYPE_IMPLICIT, TYPE_NORMAL, no_observer} from "./variable";
 
-export default function Module(runtime, special) {
+export default function Module(runtime, builtins = []) {
   Object.defineProperties(this, {
     _runtime: {value: runtime},
     _scope: {value: new Map},
-    _special: {value: new Map([
+    _builtins: {value: new Map([
       ["invalidation", variable_invalidation],
       ["visibility", variable_visibility],
-      ...special
+      ...builtins
     ])},
     _source: {value: null, writable: true}
   });
@@ -27,7 +27,8 @@ Object.defineProperties(Module.prototype, {
   derive: {value: module_derive, writable: true, configurable: true},
   import: {value: module_import, writable: true, configurable: true},
   value: {value: module_value, writable: true, configurable: true},
-  variable: {value: module_variable, writable: true, configurable: true}
+  variable: {value: module_variable, writable: true, configurable: true},
+  builtin: {value: module_builtin, writable: true, configurable: true}
 });
 
 function module_redefine(name) {
@@ -63,7 +64,7 @@ async function module_value(name) {
 }
 
 function module_derive(injects, injectModule) {
-  var copy = new Module(this._runtime, this._special);
+  var copy = new Module(this._runtime, this._builtins);
   copy._source = this;
   forEach.call(injects, function(inject) {
     if (typeof inject !== "object") inject = {name: inject + ""};
@@ -101,7 +102,7 @@ function module_copy(copy, map) {
           sourceModule = sourceInput._module;
       copy.import(sourceInput._name, name, map.get(sourceModule)
         || (sourceModule._source
-           ? sourceModule._copy(new Module(copy._runtime, copy._special), map) // import-with
+           ? sourceModule._copy(new Module(copy._runtime, copy._builtins), map) // import-with
            : sourceModule));
     } else {
       copy.define(name, source._inputs.map(variable_name), source._definition);
@@ -114,8 +115,8 @@ function module_resolve(name) {
   var variable = this._scope.get(name), value;
   if (!variable) {
     variable = new Variable(TYPE_IMPLICIT, this);
-    if (this._special.has(name)) {
-      variable.define(name, this._special.get(name));
+    if (this._builtins.has(name)) {
+      variable.define(name, constant(this._builtins.get(name)));
     } else if (this._runtime._builtin._scope.has(name)) {
       variable.import(name, this._runtime._builtin);
     } else {
@@ -134,6 +135,10 @@ function module_resolve(name) {
   return variable;
 }
 
+function module_builtin(name, value) {
+  this._builtins.set(name, value);
+}
+
 function variable_name(variable) {
   return variable._name;
 }
diff --git a/src/runtime.js b/src/runtime.js
index 3e1ada92..6e396c5c 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -1,4 +1,4 @@
-import {Library} from "@observablehq/stdlib";
+import {Library, FileAttachments} from "@observablehq/stdlib";
 import {RuntimeError} from "./errors";
 import generatorish from "./generatorish";
 import load from "./load";
@@ -38,7 +38,8 @@ Object.defineProperties(Runtime.prototype, {
   _computeSoon: {value: runtime_computeSoon, writable: true, configurable: true},
   _computeNow: {value: runtime_computeNow, writable: true, configurable: true},
   dispose: {value: runtime_dispose, writable: true, configurable: true},
-  module: {value: runtime_module, writable: true, configurable: true}
+  module: {value: runtime_module, writable: true, configurable: true},
+  fileAttachments: {value: FileAttachments, writable: true, configurable: true}
 });
 
 function runtime_dispose() {
@@ -50,18 +51,18 @@ function runtime_dispose() {
   });
 }
 
-function runtime_module(define, observer = noop, special = {}) {
+function runtime_module(define, observer = noop) {
   let module;
   if (define === undefined) {
     if (module = this._init) {
       this._init = null;
       return module;
     }
-    return new Module(this, Object.entries(special));
+    return new Module(this);
   }
   module = this._modules.get(define);
   if (module) return module;
-  this._init = module = new Module(this, Object.entries(special));
+  this._init = module = new Module(this);
   this._modules.set(define, module);
   try {
     define(this, observer);
diff --git a/test/module/builtin-test.js b/test/module/builtin-test.js
new file mode 100644
index 00000000..d18964d6
--- /dev/null
+++ b/test/module/builtin-test.js
@@ -0,0 +1,23 @@
+import {Runtime} from "../../src";
+import tape from "../tape";
+import valueof from "../variable/valueof";
+
+tape("module.builtin(name, value) defines a module-specific builtin variable", async test => {
+  const runtime = new Runtime();
+  const module = runtime.module();
+  module.builtin("foo", 42);
+  const bar = module.variable(true).define("bar", ["foo"], foo => foo + 1);
+  await new Promise(setImmediate);
+  test.deepEqual(await valueof(bar), {value: 43});
+});
+
+tape("module.builtin(name, value) can be overridden by a normal variable", async test => {
+  const runtime = new Runtime();
+  const module = runtime.module();
+  module.builtin("foo", 0);
+  const foo = module.variable(true).define("foo", [], () => 42);
+  const bar = module.variable(true).define("bar", ["foo"], foo => foo + 1);
+  await new Promise(setImmediate);
+  test.deepEqual(await valueof(foo), {value: 42});
+  test.deepEqual(await valueof(bar), {value: 43});
+});
diff --git a/test/module/special-test.js b/test/module/special-test.js
deleted file mode 100644
index f7a9fb3b..00000000
--- a/test/module/special-test.js
+++ /dev/null
@@ -1,30 +0,0 @@
-import {Runtime} from "../../src";
-import tape from "../tape";
-import valueof from "../variable/valueof";
-import noop from "../../src/noop";
-
-tape("runtime.module(…, special) defines a module special variable", async test => {
-  const runtime = new Runtime();
-  const module = runtime.module(noop, undefined, {foo: () => 42});
-  const bar = module.variable(true).define("bar", ["foo"], foo => foo + 1);
-  await new Promise(setImmediate);
-  test.deepEqual(await valueof(bar), {value: 43});
-});
-
-tape("runtime.module(…, special) can be a constant", async test => {
-  const runtime = new Runtime();
-  const module = runtime.module(noop, undefined, {foo: 42});
-  const bar = module.variable(true).define("bar", ["foo"], foo => foo + 1);
-  await new Promise(setImmediate);
-  test.deepEqual(await valueof(bar), {value: 43});
-});
-
-tape("runtime.module(…, special) can be overridden by a normal variable", async test => {
-  const runtime = new Runtime();
-  const module = runtime.module(noop, undefined, {foo: 0});
-  const foo = module.variable(true).define("foo", [], () => 42);
-  const bar = module.variable(true).define("bar", ["foo"], foo => foo + 1);
-  await new Promise(setImmediate);
-  test.deepEqual(await valueof(foo), {value: 42});
-  test.deepEqual(await valueof(bar), {value: 43});
-});
diff --git a/test/variable/derive-test.js b/test/variable/derive-test.js
index 88a6f879..e514d2fa 100644
--- a/test/variable/derive-test.js
+++ b/test/variable/derive-test.js
@@ -1,7 +1,6 @@
 import {Runtime} from "../../src/";
 import tape from "../tape";
 import valueof from "./valueof";
-import noop from "../../src/noop";
 
 tape("module.derive(overrides, module) injects variables into a copied module", async test => {
   const runtime = new Runtime();
@@ -20,9 +19,10 @@ tape("module.derive(overrides, module) injects variables into a copied module",
   test.deepEqual(await valueof(d1), {value: 42});
 });
 
-tape("module.derive(…) copies special variables", async test => {
+tape("module.derive(…) copies module-specific builtins", async test => {
   const runtime = new Runtime();
-  const module0 = runtime.module(noop, undefined, {a: 1});
+  const module0 = runtime.module();
+  module0.builtin("a", 1);
   const b0 = module0.variable(true).define("b", ["a"], a => a + 1);
   const module1_0 = module0.derive([], module0);
   const c1 = module1_0.variable(true).define("c", ["a"], a => a + 2);
diff --git a/yarn.lock b/yarn.lock
index 9f1a2da3..65b9b1e7 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -25,13 +25,12 @@
   dependencies:
     esm "^3.0.84"
 
-"@observablehq/stdlib@^3.0.0":
-  version "3.1.1"
-  resolved "https://registry.yarnpkg.com/@observablehq/stdlib/-/stdlib-3.1.1.tgz#65e4368cba5920aa649c8c2a0c9d8f23bb51150f"
-  integrity sha512-lwt+ThlV5QIFg7WCIKn7IvCnGpEsGw8BkASPd5yyNZ7hU8gxubE7jOChVte8Lf+yMQmZsju55bCsk813+u4GxQ==
+"@observablehq/stdlib@3.2.0-rc.5":
+  version "3.2.0-rc.5"
+  resolved "https://registry.yarnpkg.com/@observablehq/stdlib/-/stdlib-3.2.0-rc.5.tgz#e7b001d320ee8eb8d41f69fb37933e227d261dcc"
+  integrity sha512-qfNiq5il0jQiR9/gihZSBCI/kLaNBDHIrBcUI0aqYspXkz0D7ZJ45QZNwF/siLZVMYhUfmXWqjZb+JZ+034iMA==
   dependencies:
-    d3-require "^1.2.0"
-    esm "^3.0.84"
+    d3-require "^1.2.4"
     marked "https://github.com/observablehq/marked.git#94c6b946f462fd25db4465d71a6859183f86c57f"
 
 "@types/estree@*":
@@ -329,10 +328,10 @@ cssstyle@^1.2.2:
   dependencies:
     cssom "0.3.x"
 
-d3-require@^1.2.0:
-  version "1.2.3"
-  resolved "https://registry.yarnpkg.com/d3-require/-/d3-require-1.2.3.tgz#65a1c2137e69c5c442f1ca2a04f72a3965124c16"
-  integrity sha512-zfat3WTZRZmh7jCrTIhg4zZvivA0DZvez8lZq0JwYG9aW8LcSUFO4eiPnL5F2MolHcLE8CLfEt06sPP8N7y3AQ==
+d3-require@^1.2.4:
+  version "1.2.4"
+  resolved "https://registry.npmjs.org/d3-require/-/d3-require-1.2.4.tgz#59afc591d5089f99fecd8c45ef7539e1fee112b3"
+  integrity sha512-8UseEGCkBkBxIMouLMPONUBmU8DUPC1q12LARV1Lk/2Jwa32SVgmRfX8GdIeR06ZP+CG85YD3N13K2s14qCNyA==
 
 dashdash@^1.12.0:
   version "1.14.1"