diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index bcdbc8c7..cc5fbeb4 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -74,9 +74,9 @@ These dependencies are made available in different ways depending on context.
 ### Local Development
 
 When developing locally, you can download all of these dependencies by running
-`npm run init`. This provides the following options for `compiler` (for the
-embedded compiler), `protocol` (for the embedded protocol), and `api` (for the
-JS API):
+`npm install` and then `npm run init`. This provides the following options for
+`compiler` (for the embedded compiler), `protocol` (for the embedded protocol),
+and `api` (for the JS API):
 
 * `--<type>-path`: The local filesystem path of the package to use. This is
   useful when doing local development on both the host and its dependencies at
@@ -85,6 +85,9 @@ JS API):
 * `--<type>-ref`: A Git reference for the GitHub repository of the package to
   clone.
 
+If developing locally, you will need to specify both the compiler and language.
+For example: `npm run init -- --compiler-path=dart-sass --language-path=language`.
+
 By default:
 
 * This uses the version of the embedded protocol and compiler specified by
diff --git a/lib/index.mjs b/lib/index.mjs
index 374e98e9..ff94ed4e 100644
--- a/lib/index.mjs
+++ b/lib/index.mjs
@@ -13,6 +13,7 @@ export const SassBoolean = sass.SassBoolean;
 export const SassCalculation = sass.SassCalculation
 export const SassColor = sass.SassColor;
 export const SassFunction = sass.SassFunction;
+export const SassMixin = sass.SassMixin;
 export const SassList = sass.SassList;
 export const SassMap = sass.SassMap;
 export const SassNumber = sass.SassNumber;
@@ -95,6 +96,10 @@ export default {
     defaultExportDeprecation();
     return sass.SassFunction;
   },
+  get SassMixin() {
+    defaultExportDeprecation();
+    return sass.SassMixin;
+  },
   get SassList() {
     defaultExportDeprecation();
     return sass.SassList;
diff --git a/lib/index.ts b/lib/index.ts
index c79ffec2..69005262 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -12,6 +12,7 @@ export {sassFalse, sassTrue} from './src/value/boolean';
 export {SassColor} from './src/value/color';
 export {SassFunction} from './src/value/function';
 export {SassMap} from './src/value/map';
+export {SassMixin} from './src/value/mixin';
 export {SassNumber} from './src/value/number';
 export {SassString} from './src/value/string';
 export {Value} from './src/value';
diff --git a/lib/src/protofier.ts b/lib/src/protofier.ts
index 0092cc11..e2d7f951 100644
--- a/lib/src/protofier.ts
+++ b/lib/src/protofier.ts
@@ -24,6 +24,7 @@ import {
   CalculationOperation,
   CalculationOperator,
 } from './value/calculations';
+import {SassMixin} from './value/mixin';
 
 /**
  * A class that converts [Value] objects into protobufs.
@@ -119,6 +120,10 @@ export class Protofier {
         fn.signature = value.signature!;
         result.value = {case: 'hostFunction', value: fn};
       }
+    } else if (value instanceof SassMixin) {
+      const mixin = new proto.Value_CompilerMixin();
+      mixin.id = value.id;
+      result.value = {case: 'compilerMixin', value: mixin};
     } else if (value instanceof SassCalculation) {
       result.value = {
         case: 'calculation',
@@ -322,6 +327,9 @@ export class Protofier {
           'The compiler may not send Value.host_function.'
         );
 
+      case 'compilerMixin':
+        return new SassMixin(value.value.value.id);
+
       case 'calculation':
         return this.deprotofyCalculation(value.value.value);
 
diff --git a/lib/src/value/index.ts b/lib/src/value/index.ts
index d7c95f8e..6e5129fe 100644
--- a/lib/src/value/index.ts
+++ b/lib/src/value/index.ts
@@ -12,6 +12,7 @@ import {SassNumber} from './number';
 import {SassString} from './string';
 import {valueError} from '../utils';
 import {SassCalculation} from './calculations';
+import {SassMixin} from './mixin';
 
 /**
  * A SassScript value.
@@ -139,6 +140,17 @@ export abstract class Value implements ValueObject {
     // TODO(awjin): Narrow the return type to SassFunction.
   }
 
+  /**
+   * Casts `this` to `SassMixin`; throws if `this` isn't a mixin
+   * reference.
+   *
+   * If `this` came from a function argument, `name` is the argument name
+   * (without the `$`) and is used for error reporting.
+   */
+  assertMixin(name?: string): SassMixin {
+    throw valueError(`${this} is not a mixin reference`, name);
+  }
+
   /**
    * Casts `this` to `SassMap`; throws if `this` isn't a map.
    *
diff --git a/lib/src/value/mixin.ts b/lib/src/value/mixin.ts
new file mode 100644
index 00000000..1d8ecd86
--- /dev/null
+++ b/lib/src/value/mixin.ts
@@ -0,0 +1,41 @@
+// Copyright 2021 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import {hash} from 'immutable';
+
+import {Value} from './index';
+
+/** A first-class SassScript mixin. */
+export class SassMixin extends Value {
+  /**
+   * This is the unique ID that the compiler uses to determine which mixin it
+   * refers to.
+   *
+   * This is marked as public so that the protofier can access it, but it's not
+   * part of the package's public API and should not be accessed by user code.
+   * It may be renamed or removed without warning in the future.
+   */
+  readonly id: number;
+
+  constructor(id: number) {
+    super();
+    this.id = id;
+  }
+
+  equals(other: Value): boolean {
+    return other instanceof SassMixin && other.id === this.id;
+  }
+
+  hashCode(): number {
+    return hash(this.id);
+  }
+
+  toString(): string {
+    return `<compiler mixin ${this.id}>`;
+  }
+
+  assertMixin(): SassMixin {
+    return this;
+  }
+}
diff --git a/package.json b/package.json
index 32bdafa6..567126ac 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "sass-embedded",
   "version": "1.68.0",
-  "protocol-version": "2.2.0",
+  "protocol-version": "2.3.0",
   "compiler-version": "1.68.0",
   "description": "Node.js library that communicates with Embedded Dart Sass using the Embedded Sass protocol",
   "repository": "sass/embedded-host-node",
diff --git a/tool/get-embedded-compiler.ts b/tool/get-embedded-compiler.ts
index 2b702a2a..cb8b7245 100644
--- a/tool/get-embedded-compiler.ts
+++ b/tool/get-embedded-compiler.ts
@@ -8,7 +8,7 @@ import * as shell from 'shelljs';
 import * as utils from './utils';
 
 /**
- * Downlaods and builds the Embedded Dart Sass compiler.
+ * Downloads and builds the Embedded Dart Sass compiler.
  *
  * Can check out and build the source from a Git `ref` or build from the source
  * at `path`. By default, checks out the latest revision from GitHub.