diff --git a/src/app/oauth/token/route.ts b/src/app/oauth/token/route.ts
index 4d1b7c5..a2df7f5 100644
--- a/src/app/oauth/token/route.ts
+++ b/src/app/oauth/token/route.ts
@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from "next/server";
 import {
   hashOpaqueToken,
   issueTokenPair,
+  refreshClientMatches,
   verifyPkce,
 } from "@/lib/oauth";
 import { prisma } from "@/lib/prisma";
@@ -60,7 +61,7 @@ export async function POST(req: NextRequest) {
 
   if (grantType === "refresh_token") {
     const rawRefresh = params.get("refresh_token") || "";
-    if (!rawRefresh || !clientId) return oauthError("invalid_request", "refresh_token and client_id are required");
+    if (!rawRefresh) return oauthError("invalid_request", "refresh_token is required");
     const refresh = await prisma.oAuthToken.findUnique({
       where: { tokenHash: hashOpaqueToken(rawRefresh) },
       include: { grant: { include: { client: true } } },
@@ -71,7 +72,11 @@ export async function POST(req: NextRequest) {
       refresh.revokedAt ||
       refresh.expiresAt <= new Date() ||
       refresh.grant.revokedAt ||
-      refresh.grant.client.clientId !== clientId
+      !refreshClientMatches({
+        expectedClientId: refresh.grant.client.clientId,
+        tokenEndpointAuthMethod: refresh.grant.client.tokenEndpointAuthMethod,
+        presentedClientId: clientId,
+      })
     ) {
       return oauthError("invalid_grant", "Refresh token is invalid, expired, or revoked");
     }
diff --git a/src/lib/oauth.test.ts b/src/lib/oauth.test.ts
index da3357b..eb561dc 100644
--- a/src/lib/oauth.test.ts
+++ b/src/lib/oauth.test.ts
@@ -4,6 +4,7 @@ import {
   isAllowedRedirectUri,
   normalizeScope,
   pkceChallenge,
+  refreshClientMatches,
   verifyPkce,
 } from "./oauth";
 
@@ -26,4 +27,33 @@ describe("agent OAuth primitives", () => {
     expect(isAllowedRedirectUri("https://user:pass@example.com/callback")).toBe(false);
     expect(isAllowedRedirectUri("https://example.com/callback#fragment")).toBe(false);
   });
+
+  it("allows a public PKCE client to refresh without repeating client_id", () => {
+    expect(refreshClientMatches({
+      expectedClientId: "gc_client_chatgpt",
+      tokenEndpointAuthMethod: "none",
+      presentedClientId: "",
+    })).toBe(true);
+  });
+
+  it("still enforces client binding when client_id is supplied", () => {
+    expect(refreshClientMatches({
+      expectedClientId: "gc_client_chatgpt",
+      tokenEndpointAuthMethod: "none",
+      presentedClientId: "gc_client_chatgpt",
+    })).toBe(true);
+    expect(refreshClientMatches({
+      expectedClientId: "gc_client_chatgpt",
+      tokenEndpointAuthMethod: "none",
+      presentedClientId: "another-client",
+    })).toBe(false);
+  });
+
+  it("does not allow a client that requires token-endpoint authentication to omit identity", () => {
+    expect(refreshClientMatches({
+      expectedClientId: "confidential-client",
+      tokenEndpointAuthMethod: "client_secret_basic",
+      presentedClientId: "",
+    })).toBe(false);
+  });
 });
diff --git a/src/lib/oauth.ts b/src/lib/oauth.ts
index 891d9b4..207dba6 100644
--- a/src/lib/oauth.ts
+++ b/src/lib/oauth.ts
@@ -144,6 +144,21 @@ export function isAllowedRedirectUri(value: string): boolean {
   }
 }
 
+export function refreshClientMatches(input: {
+  expectedClientId: string;
+  tokenEndpointAuthMethod: string;
+  presentedClientId?: string | null;
+}) {
+  const presented = String(input.presentedClientId || "").trim();
+
+  // Public OAuth clients using token_endpoint_auth_method=none cannot
+  // authenticate themselves at refresh time. The refresh token is already
+  // bound to its grant/client in GroundControl, so client_id is optional.
+  // If a client_id is presented, it must still match exactly.
+  if (!presented) return input.tokenEndpointAuthMethod === "none";
+  return presented === input.expectedClientId;
+}
+
 export function parseRedirectUris(value: string): string[] {
   try {
     const parsed = JSON.parse(value);