diff --git a/package.json b/package.json
index 57a8efc..c0423bb 100644
--- a/package.json
+++ b/package.json
@@ -52,7 +52,7 @@
     "generate:server-json": "tsx scripts/sync-release-metadata.ts",
     "publish:mcp-registry": "tsx scripts/publish-mcp-registry.ts",
     "lint": "eslint .",
-    "test": "node --test --test-concurrency=1 dist/test/coordinator-socket-error.integration.test.js dist/test/identity-store.test.js dist/test/identity-cert.test.js dist/test/broadcast-window.integration.test.js dist/test/identity-restart.integration.test.js dist/test/mesh-e2e.integration.test.js dist/test/tls-transport.integration.test.js dist/test/peer-id-verification.integration.test.js dist/test/become-coordinator-actual-port.integration.test.js dist/test/state-sync-convergence.test.js dist/test/downtime-replay.test.js dist/test/downtime-replay.integration.test.js dist/test/filestore.test.js dist/test/handshake.test.js dist/test/approval.integration.test.js dist/test/listener-policy.integration.test.js dist/test/mesh-smoke.integration.test.js",
+    "test": "node --test --test-concurrency=1 dist/test/coordinator-socket-error.integration.test.js dist/test/identity-store.test.js dist/test/identity-cert.test.js dist/test/identity-device-id.test.js dist/test/broadcast-window.integration.test.js dist/test/identity-restart.integration.test.js dist/test/mesh-e2e.integration.test.js dist/test/tls-transport.integration.test.js dist/test/peer-id-verification.integration.test.js dist/test/become-coordinator-actual-port.integration.test.js dist/test/state-sync-convergence.test.js dist/test/downtime-replay.test.js dist/test/downtime-replay.integration.test.js dist/test/filestore.test.js dist/test/handshake.test.js dist/test/approval.integration.test.js dist/test/listener-policy.integration.test.js dist/test/mesh-smoke.integration.test.js",
     "test:visibility": "node --test dist/test/visibility.integration.test.js",
     "test:delivery": "node dist/test/delivery-receipt.runner.js",
     "test:federation": "node dist/test/federation.integration.test.js",
diff --git a/src/core/identity-store.ts b/src/core/identity-store.ts
index 6614e7f..4a518de 100644
--- a/src/core/identity-store.ts
+++ b/src/core/identity-store.ts
@@ -12,6 +12,7 @@ import * as path from "node:path";
 import {
   generateIdentity,
   getCertificateFingerprint,
+  deriveDeviceId,
   CERTIFICATE_VALIDITY_MS,
 } from "./identity.js";
 import type { PeerIdentity } from "./identity.js";
@@ -145,6 +146,7 @@ function loadStoredIdentity(identityFile: string): PeerIdentity | undefined {
       privateKey: parsed.privateKey,
       certificate: parsed.certificate,
       fingerprint: getCertificateFingerprint(parsed.certificate),
+      deviceId: deriveDeviceId(parsed.privateKey),
     };
   } catch (err) {
     console.error(
diff --git a/src/core/identity.ts b/src/core/identity.ts
index 8b2b27e..3cde880 100644
--- a/src/core/identity.ts
+++ b/src/core/identity.ts
@@ -19,6 +19,9 @@ import {
   randomBytes,
 } from "node:crypto";
 
+/** The uncompressed SEC1 point tag byte (RFC 5480 §2.2): 0x04 marks what follows as raw X||Y, never compressed or hybrid encoding. */
+const UNCOMPRESSED_POINT_TAG = 0x04;
+
 // ─── Types ──────────────────────────────────────────────────────────────────
 
 /** Self-signed certificate validity. Exported so identity persistence can derive its renewal margin. */
@@ -31,6 +34,10 @@ export interface PeerIdentity {
   certificate: string;
   /** SHA-256 fingerprint of the certificate (DER), hex-encoded with colons. */
   fingerprint: string;
+  /**
+   * SHA-256 of the raw, uncompressed SEC1 public-key point -- wire-mesh's own device-id derivation (never the certificate's DER encoding, which embeds a serial number and validity window that change on every reissue even for an identical key; this is the exact instability fingerprint already has). Additive: nothing in this codebase reads it yet. `fingerprint` remains what MeshStore.peerId is set to until the substrate migration cuts over to this field instead.
+   */
+  deviceId: Uint8Array;
 }
 
 // ─── ASN.1 DER helpers ─────────────────────────────────────────────────────
@@ -312,6 +319,7 @@ export function generateIdentity(): PeerIdentity {
     privateKey: keyPair.privateKey,
     certificate,
     fingerprint: getCertificateFingerprint(certificate),
+    deviceId: deriveDeviceId(keyPair.privateKey),
   };
 }
 
@@ -345,6 +353,21 @@ function pemToDer(pem: string): Buffer {
   return Buffer.from(b64, "base64");
 }
 
+/** Derives wire-mesh's own device-id (SHA-256 of the raw, uncompressed SEC1 public-key point) from a PEM-encoded EC private key. */
+export function deriveDeviceId(privateKeyPem: string): Uint8Array {
+  const publicKey = createPublicKey(privateKeyPem);
+  const jwk = publicKey.export({ format: "jwk" });
+  if (jwk.x === undefined || jwk.y === undefined) {
+    throw new Error("expected an EC JWK with x/y coordinates");
+  }
+  const rawPublicKey = Buffer.concat([
+    Buffer.from([UNCOMPRESSED_POINT_TAG]),
+    Buffer.from(jwk.x, "base64url"),
+    Buffer.from(jwk.y, "base64url"),
+  ]);
+  return createHash("sha256").update(rawPublicKey).digest();
+}
+
 /** Encode DER bytes as a PEM certificate string. */
 function derToCertificatePem(der: Buffer): string {
   const b64 = der.toString("base64").match(/.{1,64}/g);
diff --git a/src/test/identity-device-id.test.ts b/src/test/identity-device-id.test.ts
new file mode 100644
index 0000000..26c4f4a
--- /dev/null
+++ b/src/test/identity-device-id.test.ts
@@ -0,0 +1,43 @@
+/**
+ * The device-id wire-mesh's own migration will key peer identity on is SHA-256 of the raw public-key bytes -- never the certificate's own DER encoding, which embeds a serial number and validity window that change on every reissue even for an identical key (the same bug this project's certificate-fingerprint peerId already has, and the reason the migration plan treats it as provably unstable). This is purely additive: nothing yet reads identity.deviceId, so this only proves the value itself is correct, not that anything consumes it.
+ */
+
+import { createHash, createPublicKey } from "node:crypto";
+import * as assert from "node:assert/strict";
+import { test } from "node:test";
+import { generateIdentity } from "../core/identity.js";
+
+/** Re-derives the raw uncompressed SEC1 point independently of identity.ts's own implementation, via the JWK x/y coordinates every Node public KeyObject can export -- an oracle the test checks identity.ts's own derivation against, not a copy of it. */
+function rawPublicKeyFromCertificate(certificatePem: string): Buffer {
+  const publicKey = createPublicKey(certificatePem);
+  const jwk = publicKey.export({ format: "jwk" });
+  if (jwk.x === undefined || jwk.y === undefined) {
+    throw new Error("expected an EC JWK with x/y coordinates");
+  }
+  const UNCOMPRESSED_POINT_TAG = 0x04;
+  return Buffer.concat([
+    Buffer.from([UNCOMPRESSED_POINT_TAG]),
+    Buffer.from(jwk.x, "base64url"),
+    Buffer.from(jwk.y, "base64url"),
+  ]);
+}
+
+const SHA256_BYTE_LENGTH = 32;
+
+void test("deviceId is SHA-256 of the raw public key, independent of the certificate", () => {
+  const identity = generateIdentity();
+  assert.equal(identity.deviceId.length, SHA256_BYTE_LENGTH);
+
+  const rawPublicKey = rawPublicKeyFromCertificate(identity.certificate);
+  const expected = createHash("sha256").update(rawPublicKey).digest();
+  assert.equal(Buffer.from(identity.deviceId).equals(expected), true);
+});
+
+void test("deviceId differs between two freshly generated identities", () => {
+  const a = generateIdentity();
+  const b = generateIdentity();
+  assert.notEqual(
+    Buffer.from(a.deviceId).toString("hex"),
+    Buffer.from(b.deviceId).toString("hex"),
+  );
+});