diff --git a/src/core/oauth-provider.ts b/src/core/oauth-provider.ts
index 8b4ac91d3..ba89beb8e 100644
--- a/src/core/oauth-provider.ts
+++ b/src/core/oauth-provider.ts
@@ -389,10 +389,18 @@ export class GBrainOAuthProvider implements OAuthServerProvider {
     // as a fully-admin access token. Mirrors the filter pattern already used
     // by exchangeClientCredentials (this file) and exchangeRefreshToken's F3
     // subset enforcement (RFC 6749 §6) so all three grant entry points clamp
-    // consistently. Empty/omitted requested scope inherits the empty-stored
-    // shape (existing behavior; not a security boundary).
+    // consistently. When the client requests NO scope, RFC 6749 §3.3 lets the
+    // server fall back to a default — we default to the client's full
+    // registered scope (matching exchangeClientCredentials, which already does
+    // `requestedScope ? ... : allowedScopes`). Previously an omitted request
+    // granted the empty set, which then propagated into the access+refresh
+    // tokens and never self-healed: every op failed `insufficient_scope` even
+    // though the client was registered with `read write`. Clients that omit
+    // `scope` on /authorize (e.g. some MCP connectors) hit this. Still clamped
+    // to the allowed set, so an explicit over-broad request can't escalate.
     const allowedScopes = parseScopeString(client.scope);
-    const grantedScopes = (params.scopes || []).filter(s => hasScope(allowedScopes, s));
+    const requestedScopes = (params.scopes && params.scopes.length) ? params.scopes : allowedScopes;
+    const grantedScopes = requestedScopes.filter(s => hasScope(allowedScopes, s));
 
     await this.sql`
       INSERT INTO oauth_codes (code_hash, client_id, scopes, code_challenge,
diff --git a/test/oauth-authorize-scope-default.test.ts b/test/oauth-authorize-scope-default.test.ts
new file mode 100644
index 000000000..4dc2c3d81
--- /dev/null
+++ b/test/oauth-authorize-scope-default.test.ts
@@ -0,0 +1,89 @@
+/**
+ * Authorize-grant scope default (RFC 6749 §3.3).
+ *
+ * When a client omits `scope` on /authorize, the granted scope must default to
+ * the client's full registered scope — NOT the empty set. Regression guard for
+ * the bug where an omitted request granted [], which then propagated into the
+ * access + refresh tokens and never self-healed: every op failed
+ * `insufficient_scope` even though the client was registered `read write`
+ * (some MCP connectors omit `scope` on /authorize). The clamp must still hold —
+ * an explicit over-broad request cannot escalate past the client's allowed set.
+ */
+import { describe, test, expect, beforeAll, afterAll, beforeEach } from 'bun:test';
+import { PGLiteEngine } from '../src/core/pglite-engine.ts';
+import { GBrainOAuthProvider } from '../src/core/oauth-provider.ts';
+import { sqlQueryForEngine } from '../src/core/sql-query.ts';
+
+let engine: PGLiteEngine;
+let provider: GBrainOAuthProvider;
+let sql: ReturnType<typeof sqlQueryForEngine>;
+
+beforeAll(async () => {
+  engine = new PGLiteEngine();
+  await engine.connect({});
+  await engine.initSchema();
+  sql = sqlQueryForEngine(engine);
+  provider = new GBrainOAuthProvider({ sql });
+});
+
+afterAll(async () => {
+  await engine.disconnect();
+});
+
+beforeEach(async () => {
+  await (engine as any).db.exec('DELETE FROM oauth_tokens');
+  await (engine as any).db.exec('DELETE FROM oauth_codes');
+  await (engine as any).db.exec('DELETE FROM oauth_clients');
+});
+
+// authorize() writes the granted scope into oauth_codes then redirects; we
+// assert on the stored grant directly, so the redirect is a no-op.
+const noopRes = { redirect() {} } as any;
+
+async function authorizeAndReadScopes(
+  scope: string,
+  requested: string[] | undefined,
+): Promise<string[]> {
+  const reg = await provider.registerClientManual(
+    'authz-test', ['authorization_code'], scope, ['https://example.test/cb'],
+  );
+  const client = await provider.clientsStore.getClient(reg.clientId);
+  expect(client).toBeTruthy();
+  await provider.authorize(
+    client!,
+    {
+      scopes: requested,
+      codeChallenge: 'test-challenge',
+      redirectUri: 'https://example.test/cb',
+      state: 'xyz',
+    } as any,
+    noopRes,
+  );
+  const rows = (await sql`
+    SELECT scopes FROM oauth_codes WHERE client_id = ${reg.clientId}
+  `) as Array<{ scopes: string[] }>;
+  expect(rows.length).toBe(1);
+  return rows[0].scopes ?? [];
+}
+
+describe('authorize() scope default — omitted scope inherits client grant', () => {
+  test('omitted scope → inherits full registered scope', async () => {
+    expect((await authorizeAndReadScopes('read write', undefined)).sort()).toEqual(['read', 'write']);
+  });
+
+  test('empty scope array → inherits full registered scope', async () => {
+    expect((await authorizeAndReadScopes('read write', [])).sort()).toEqual(['read', 'write']);
+  });
+
+  test('explicit subset is honored (not overridden to full)', async () => {
+    expect(await authorizeAndReadScopes('read write admin', ['read'])).toEqual(['read']);
+  });
+
+  test('clamp preserved: over-broad request cannot escalate', async () => {
+    expect(await authorizeAndReadScopes('read', ['read', 'admin'])).toEqual(['read']);
+  });
+
+  test('clamp preserved: requesting only a disallowed scope grants nothing (no inheritance)', async () => {
+    expect(await authorizeAndReadScopes('read write', ['admin'])).toEqual([]);
+  });
+});