diff --git a/packages/api/src/controllers/token.test.ts b/packages/api/src/controllers/token.test.ts
index caa6926..d12850b 100644
--- a/packages/api/src/controllers/token.test.ts
+++ b/packages/api/src/controllers/token.test.ts
@@ -1,10 +1,12 @@
 import assert from "node:assert/strict";
 import { test } from "node:test";
-import { InvalidRequestError, UnauthorizedClientError } from "../errors.js";
+import { InvalidGrantError, InvalidRequestError, UnauthorizedClientError } from "../errors.js";
 import {
   assertClientSecretMatches,
+  assertRefreshTokenClientBinding,
   buildUserIdTokenClaims,
   resolveGrantedScopes,
+  resolveSessionClientId,
   TokenRequestSchema,
 } from "./user/token.js";
 
@@ -93,3 +95,38 @@ test("buildUserIdTokenClaims omits nonce when not provided", () => {
 
   assert.equal(claims.nonce, undefined);
 });
+
+test("resolveSessionClientId returns client id for valid session data", () => {
+  assert.equal(resolveSessionClientId({ clientId: "demo-client" }), "demo-client");
+});
+
+test("resolveSessionClientId returns null when client id is missing", () => {
+  assert.equal(resolveSessionClientId({}), null);
+});
+
+test("resolveSessionClientId returns null for invalid shapes", () => {
+  assert.equal(resolveSessionClientId(null), null);
+  assert.equal(resolveSessionClientId("bad"), null);
+  assert.equal(resolveSessionClientId({ clientId: 42 }), null);
+});
+
+test("assertRefreshTokenClientBinding accepts matching client ids", () => {
+  assert.doesNotThrow(() => assertRefreshTokenClientBinding("demo-client", "demo-client"));
+});
+
+test("assertRefreshTokenClientBinding rejects mismatched client ids", () => {
+  assert.throws(
+    () => assertRefreshTokenClientBinding("demo-client", "other-client"),
+    (error: unknown) =>
+      error instanceof InvalidGrantError &&
+      error.message === "Refresh token was not issued to this client"
+  );
+});
+
+test("assertRefreshTokenClientBinding rejects missing binding", () => {
+  assert.throws(
+    () => assertRefreshTokenClientBinding(null, "demo-client"),
+    (error: unknown) =>
+      error instanceof InvalidGrantError && error.message === "Invalid or expired refresh token"
+  );
+});
diff --git a/packages/api/src/controllers/user/opaqueLoginFinish.ts b/packages/api/src/controllers/user/opaqueLoginFinish.ts
index 5f82f73..39d98d7 100644
--- a/packages/api/src/controllers/user/opaqueLoginFinish.ts
+++ b/packages/api/src/controllers/user/opaqueLoginFinish.ts
@@ -144,6 +144,7 @@ export const postOpaqueLoginFinish = withRateLimit("opaque", (body) => {
         sub: user.sub,
         email: user.email || undefined,
         name: user.name || undefined,
+        clientId: "demo-public-client",
         otpRequired: otpRequired,
         otpVerified: false,
       });
diff --git a/packages/api/src/controllers/user/token.ts b/packages/api/src/controllers/user/token.ts
index 5e64a92..a00edc9 100644
--- a/packages/api/src/controllers/user/token.ts
+++ b/packages/api/src/controllers/user/token.ts
@@ -4,16 +4,13 @@ import { InvalidGrantError, InvalidRequestError, UnauthorizedClientError } from
 import { genericErrors } from "../../http/openapi-helpers.js";
 import { getCachedBody, withRateLimit } from "../../middleware/rateLimit.js";
 import { signJWT } from "../../services/jwks.js";
-import {
-  createSession,
-  getActorFromRefreshToken,
-  refreshSessionWithToken,
-} from "../../services/sessions.js";
+import { createSession, refreshSessionWithToken } from "../../services/sessions.js";
 import { getSetting } from "../../services/settings.js";
 import type {
   Context,
   ControllerSchema,
   IdTokenClaims,
+  SessionData,
   TokenRequest,
   TokenResponse,
 } from "../../types.js";
@@ -96,6 +93,23 @@ export function buildUserIdTokenClaims(data: {
   };
 }
 
+export function resolveSessionClientId(sessionData: unknown): string | null {
+  if (!sessionData || typeof sessionData !== "object") return null;
+  const maybeClientId = (sessionData as { clientId?: unknown }).clientId;
+  if (typeof maybeClientId !== "string" || maybeClientId.length === 0) return null;
+  return maybeClientId;
+}
+
+export function assertRefreshTokenClientBinding(
+  issuedClientId: string | null,
+  authenticatedClientId: string | undefined
+): void {
+  if (!issuedClientId) throw new InvalidGrantError("Invalid or expired refresh token");
+  if (issuedClientId !== authenticatedClientId) {
+    throw new InvalidGrantError("Refresh token was not issued to this client");
+  }
+}
+
 export const TokenRequestSchema = z.union([
   z.object({
     grant_type: z.literal("authorization_code"),
@@ -187,11 +201,17 @@ export const postToken = withRateLimit("token")(
           clientAuthOk = true;
         }
         if (!clientAuthOk) throw new UnauthorizedClientError("Client authentication failed");
-        const actor = await getActorFromRefreshToken(context, tokenRequest.refresh_token);
-        if (!actor || !actor.userSub)
+        const { sessions } = await import("../../db/schema.js");
+        const { eq } = await import("drizzle-orm");
+        const sessionRow = await context.db.query.sessions.findFirst({
+          where: eq(sessions.refreshToken, tokenRequest.refresh_token),
+        });
+        if (!sessionRow || !sessionRow.userSub)
           throw new InvalidGrantError("Invalid or expired refresh token");
+        const issuedClientId = resolveSessionClientId(sessionRow.data);
+        assertRefreshTokenClientBinding(issuedClientId, providedClientId);
         const { getUserBySub } = await import("../../models/users.js");
-        const user = await getUserBySub(context, actor.userSub);
+        const user = await getUserBySub(context, sessionRow.userSub);
         if (!user) throw new InvalidGrantError("User not found");
         if (!providedClientId) throw new UnauthorizedClientError("Client authentication failed");
         const client = await (await import("../../models/clients.js")).getClient(
@@ -225,12 +245,7 @@ export const postToken = withRateLimit("token")(
             ? client.idTokenLifetimeSeconds
             : defaultIdTtl;
         let amr: string[] | undefined = ["pwd"];
-        const { sessions } = await import("../../db/schema.js");
-        const { eq } = await import("drizzle-orm");
-        const sess = await context.db.query.sessions.findFirst({
-          where: eq(sessions.refreshToken, tokenRequest.refresh_token),
-        });
-        const data = (sess?.data || {}) as Record<string, unknown>;
+        const data = (sessionRow.data || {}) as Record<string, unknown>;
         if (data && data.otpVerified === true) amr = ["pwd", "otp"];
         const issuer = await resolveIssuer(context);
         const idTokenClaims = buildUserIdTokenClaims({
@@ -527,7 +542,8 @@ export const postToken = withRateLimit("token")(
         sub: user.sub,
         email: user.email || undefined,
         name: user.name || undefined,
-      };
+        clientId: authenticatedClientId,
+      } satisfies SessionData;
       const s = await createSession(context, "user", sessionData);
       tokenResponse.refresh_token = s.refreshToken;
 
diff --git a/packages/api/src/models/registration.ts b/packages/api/src/models/registration.ts
index f96d3f7..b41c147 100644
--- a/packages/api/src/models/registration.ts
+++ b/packages/api/src/models/registration.ts
@@ -46,6 +46,7 @@ export async function userOpaqueRegisterFinish(
     sub,
     email: data.email,
     name: data.name,
+    clientId: "demo-public-client",
   });
   return { sub, accessToken: sessionInfo.sessionId, refreshToken: sessionInfo.refreshToken };
 }
diff --git a/packages/api/src/types.ts b/packages/api/src/types.ts
index fe32075..ae9f0fa 100644
--- a/packages/api/src/types.ts
+++ b/packages/api/src/types.ts
@@ -318,6 +318,7 @@ export interface SessionData {
   sub?: string;
   email?: string;
   name?: string;
+  clientId?: string;
   adminId?: string;
   adminRole?: "read" | "write";
   pendingAuthId?: string;
diff --git a/packages/brochureware/public/whitepaper.generated.md b/packages/brochureware/public/whitepaper.generated.md
index 356041a..cd01528 100644
--- a/packages/brochureware/public/whitepaper.generated.md
+++ b/packages/brochureware/public/whitepaper.generated.md
@@ -149,7 +149,7 @@ Server stores
 - Pending authorization requests; authorization codes with `has_zk`, `zk_pub_kid`, and `drk_hash`.
 - JWKS (public JWKs; private JWKs encrypted at rest when KEK is available).
 - Clients, settings (including runtime flags for OIDC/PKCE, id token TTLs, etc.).
-- Sessions and refresh tokens.
+- Sessions and refresh tokens, including issuing `client_id` binding for refresh token ownership.
 - Audit logs (admin actions).
 
 Server never stores
@@ -159,6 +159,7 @@ Server never stores
 
 Sessions and claims
 - SPA model: `/session` for minimal info; `/refresh-token` rotates session tokens.
+- OIDC refresh grant enforces client binding: cross-client refresh token reuse is rejected.
 - ID tokens may include `permissions` and `groups` claims when configured.
 
 ## 6) Endpoints (as implemented)
diff --git a/specs/0_SECURITY_WHITEPAPER.md b/specs/0_SECURITY_WHITEPAPER.md
index 31fd5c9..70b978f 100644
--- a/specs/0_SECURITY_WHITEPAPER.md
+++ b/specs/0_SECURITY_WHITEPAPER.md
@@ -163,7 +163,7 @@ Server stores
 - Pending authorization requests; authorization codes with `has_zk`, `zk_pub_kid`, and `drk_hash`.
 - JWKS (public JWKs; private JWKs encrypted at rest when KEK is available).
 - Clients, settings (including runtime flags for OIDC/PKCE, id token TTLs, etc.).
-- Sessions and refresh tokens.
+- Sessions and refresh tokens, including issuing `client_id` binding for refresh token ownership.
 - Audit logs (admin actions).
 
 Server never stores
@@ -173,6 +173,7 @@ Server never stores
 
 Sessions and claims
 - SPA model: `/session` for minimal info; `/refresh-token` rotates session tokens.
+- OIDC refresh grant enforces client binding: cross-client refresh token reuse is rejected.
 - ID tokens may include `permissions` and `groups` claims when configured.
 
 ## 6) Endpoints (as implemented)
diff --git a/specs/2_CORE.md b/specs/2_CORE.md
index 4a230da..8f07c5e 100644
--- a/specs/2_CORE.md
+++ b/specs/2_CORE.md
@@ -224,7 +224,8 @@ Same as above **+** `&zk_pub=<base64url(JWK)>` (ephemeral ECDH public key).
 
 - Validates code (unexpired, not consumed), PKCE (S256), client, `redirect_uri`.
 - Mints `id_token` (and `access_token` if enabled by settings).
-- Returns a `refresh_token` bound to the session.
+- Returns a `refresh_token` bound to the session and issuing `client_id`.
+- Refresh grant enforces `client_id` binding: only the original client can rotate that refresh token.
 - Optionally includes user authorization data as custom claims when configured: `permissions` (array of strings) and `groups` (array of strings). These reflect the union of direct user permissions and permissions derived from groups.
 - **SECURITY**: If the code had `has_zk=true`, include ONLY `zk_drk_hash` for verification. Server NEVER returns `zk_drk_jwe` as it doesn't store it.
 - Consume the code.