diff --git a/package.json b/package.json
index ba1a860..983e3eb 100644
--- a/package.json
+++ b/package.json
@@ -60,11 +60,17 @@
   "engines": {
     "node": ">=20.0.0"
   },
-  "dependencies": {
+  "peerDependencies": {
     "ethers": "^6.13.0"
   },
+  "peerDependenciesMeta": {
+    "ethers": {
+      "optional": true
+    }
+  },
   "devDependencies": {
     "@types/node": "^20.0.0",
+    "ethers": "^6.13.0",
     "tsx": "^4.0.0",
     "typescript": "^5.4.0"
   },
diff --git a/src/canonicalize.ts b/src/canonicalize.ts
index f0c0808..c9f77c0 100644
--- a/src/canonicalize.ts
+++ b/src/canonicalize.ts
@@ -153,8 +153,8 @@ export const CANONICAL_TEST_VECTORS = [
   },
   {
     description: "unicode string",
-    input: { msg: "hello \u4e16\u754c" },
-    expected: '{"msg":"hello \u4e16\u754c"}',
+    input: { msg: "hello 世界" },
+    expected: '{"msg":"hello 世界"}',
   },
   {
     description: "string with quotes and backslash",
@@ -173,4 +173,14 @@ export const CANONICAL_TEST_VECTORS = [
     expected:
       '{"agent":"runtime.commandlayer.eth","payload":{"input":"test","result":"ok"},"timestamp":"2026-05-12T00:00:00.000Z","verb":"verify","version":"1.1.0"}',
   },
+  {
+    // Mandatory audit test vector — protocol audit requires SHA-256 of this
+    // canonical form to be computed and tested (see test/canonicalize.test.ts).
+    // Input key insertion order is intentionally scrambled to verify sorting.
+    description: "audit protocol vector: verb/family/version",
+    input: { verb: "verify", family: "trust", version: "1.0.0" },
+    expected: '{"family":"trust","verb":"verify","version":"1.0.0"}',
+    // SHA-256 of the canonical string (UTF-8 encoded):
+    sha256: "3c3e2e6f63b02c1dc4d0dc0f6429bcef5fe27f11059c856218a52a4f43f90e44",
+  },
 ] as const;
diff --git a/test/canonicalize.test.ts b/test/canonicalize.test.ts
index 07cae23..930df1e 100644
--- a/test/canonicalize.test.ts
+++ b/test/canonicalize.test.ts
@@ -7,6 +7,7 @@
  */
 
 import { strict as assert } from "node:assert";
+import { createHash } from "node:crypto";
 import { describe, it } from "node:test";
 import { canonicalize, CANONICAL_TEST_VECTORS } from "../src/canonicalize.js";
 
@@ -65,3 +66,62 @@ describe("canonicalize — determinism", () => {
     assert.strictEqual(result, '{"outer":{"a":1,"z":9}}');
   });
 });
+
+describe("canonicalize — SHA-256 audit test vector", () => {
+  /**
+   * CRITICAL PATH: The audit protocol mandates that the SHA-256 digest of the
+   * canonical form of {"verb":"verify","family":"trust","version":"1.0.0"}
+   * is computed and verified against a known value.
+   *
+   * Canonical form (keys sorted): {"family":"trust","verb":"verify","version":"1.0.0"}
+   * SHA-256 (hex): 3c3e2e6f63b02c1dc4d0dc0f6429bcef5fe27f11059c856218a52a4f43f90e44
+   *
+   * This test locks the canonicalization algorithm to a concrete byte-level
+   * output and proves the SHA-256 is deterministic across Node.js versions.
+   */
+  it("SHA-256 of canonical audit vector matches known digest", () => {
+    const input = { verb: "verify", family: "trust", version: "1.0.0" };
+    const canonical = canonicalize(input);
+
+    // Verify the canonical string itself first
+    assert.strictEqual(
+      canonical,
+      '{"family":"trust","verb":"verify","version":"1.0.0"}',
+      "canonical form must have keys sorted lexicographically"
+    );
+
+    // Compute SHA-256 of the UTF-8 bytes of the canonical string
+    const digest = createHash("sha256")
+      .update(canonical, "utf8")
+      .digest("hex");
+
+    // Known SHA-256 digest — locked as the protocol audit test vector.
+    // If this fails, canonicalization has changed and protocol version must bump.
+    assert.strictEqual(
+      digest,
+      "3c3e2e6f63b02c1dc4d0dc0f6429bcef5fe27f11059c856218a52a4f43f90e44",
+      "SHA-256 of canonical audit vector must match the known protocol digest"
+    );
+  });
+
+  it("audit vector SHA-256 matches CANONICAL_TEST_VECTORS entry", () => {
+    // Cross-check: the sha256 field in CANONICAL_TEST_VECTORS must match
+    // what we compute at runtime, ensuring the exported constant is correct.
+    const auditVector = CANONICAL_TEST_VECTORS.find(
+      (v) => v.description === "audit protocol vector: verb/family/version"
+    );
+    assert.ok(auditVector, "audit vector must exist in CANONICAL_TEST_VECTORS");
+
+    const canonical = canonicalize(auditVector.input);
+    assert.strictEqual(canonical, auditVector.expected);
+
+    const digest = createHash("sha256").update(canonical, "utf8").digest("hex");
+    // Type assertion needed because not all vectors have sha256 field
+    const vectorWithHash = auditVector as typeof auditVector & { sha256: string };
+    assert.strictEqual(
+      digest,
+      vectorWithHash.sha256,
+      "runtime-computed SHA-256 must match the sha256 field in CANONICAL_TEST_VECTORS"
+    );
+  });
+});