fix(browse): CLI always sends explicit pair scopes via shared DEFAULT_PAIR_SCOPES

The effective pairing default lived in two places: the CLI omitted scopes
unless --restrict was passed, and the server filled in its own literal.
handlePairAgent now always sends an explicit scopes list and both sides
reference one exported constant, DEFAULT_PAIR_SCOPES, so the default cannot
silently drift again (pinned by a server-auth source tripwire).

Three input traps closed in the same surface:
- Bare --restrict (or --restrict swallowing the next flag) parsed as "no
  restriction" and silently granted FULL access, the opposite of the user's
  intent. validatePairAgentFlags rejects it pre-server, before any consent
  gate, so an arg error never boots a daemon.
- A scopes list could smuggle the control scope past the explicit flag:
  --restrict "read,control" minted a control-scoped session with no
  --control. /pair now 400s on control in a scopes list without the control
  flag, and the CLI points the user at --control.
- Option typos validated only at exchange time: createSetupKey stored any
  scope string and any rateLimit, so /pair returned 200 with a poisoned
  setup key whose failure surfaced to the REMOTE agent at /connect as a
  misleading "Invalid request body". Shared validation now runs in both
  creators and throws typed InvalidScopeError; /pair and /token 400 with the
  message, naming the bad scope or negative rateLimit. Also
  `opts.rateLimit || 10` became `?? 10` so the documented "0 = unlimited"
  survives the /pair path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/cli.ts

@@ -18,6 +18,9 @@ import { resolveConfig, ensureStateDir, readVersionHash, isPairAgentEnabled } fr
 import { parseProxyConfig, computeConfigHash, ProxyConfigError } from './proxy-config';
 import { redactProxyUrl } from './proxy-redact';
 import { spawnTerminalAgent } from './terminal-agent-control';
+// Zero side effects on import (documented invariant in token-registry.ts) —
+// safe to pull the shared pairing default into the CLI.
+import { DEFAULT_PAIR_SCOPES } from './token-registry';
 
 const config = resolveConfig();
 const IS_WINDOWS = process.platform === 'win32';
@@ -1216,6 +1219,29 @@ async function tunnelAgents(): Promise<number> {
   return 0;
 }
 
+/** Reject pair-agent scope-flag misuse BEFORE any consent or server work.
+ * Bare `--restrict` (or a flag-shaped value from a forgotten argument) used
+ * to parse as "no restriction" and silently grant FULL access — the exact
+ * opposite of the user's intent. And `control` never rides in via --restrict:
+ * browser-wide destructive ops stay behind the explicit --control flag. */
+function validatePairAgentFlags(args: string[]): void {
+  if (!hasFlag(args, '--restrict')) return;
+  const restrict = parseFlag(args, '--restrict');
+  if (!restrict || !restrict.trim() || restrict.startsWith('--')) {
+    console.error('[browse] --restrict needs a scope list, e.g. --restrict read or --restrict "read,write". Bare --restrict would silently grant FULL access.');
+    process.exit(1);
+  }
+  if (hasFlag(args, '--control') || hasFlag(args, '--admin')) {
+    // Server-side, the control flag wins and the scopes list is ignored.
+    console.warn('[browse] --restrict is ignored when --control/--admin is set (control implies full access).');
+    return;
+  }
+  if (restrict.split(',').map(s => s.trim()).includes('control')) {
+    console.error('[browse] The control scope is not granted via --restrict. Re-run with --control.');
+    process.exit(1);
+  }
+}
+
 async function handleTunnel(args: string[]): Promise<never> {
   const sub = args[0];
   if (sub === 'revoke' && args.length === 2 && args[1].trim()) {
@@ -1236,8 +1262,12 @@ async function handlePairAgent(state: ServerState, args: string[]): Promise<void
   const localHost = parseFlag(args, '--local');
 
   // Call POST /pair to create a setup key
-  // Default: full access (read+write+admin+meta). --control adds browser-wide ops.
+  // Default: DEFAULT_PAIR_SCOPES (full page access). --control adds browser-wide ops.
   // --restrict limits: --restrict read (read-only), --restrict "read,write" (no admin)
+  // Scopes are ALWAYS sent explicitly so the effective default lives in one
+  // place (token-registry) instead of drifting between CLI omission and
+  // server fallback. Flag misuse was rejected pre-server by
+  // validatePairAgentFlags.
   const pairResp = await fetch(`http://127.0.0.1:${state.port}/pair`, {
     method: 'POST',
     headers: {
@@ -1248,7 +1278,9 @@ async function handlePairAgent(state: ServerState, args: string[]): Promise<void
       domains,
       clientId: clientName,
       control,
-      ...(restrict ? { scopes: restrict.split(',').map(s => s.trim()) } : {}),
+      scopes: restrict
+        ? restrict.split(',').map(s => s.trim())
+        : [...DEFAULT_PAIR_SCOPES],
     }),
     signal: AbortSignal.timeout(5000),
   });
@@ -1791,6 +1823,9 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
   // state, so replacing it kills nothing the user had.
   let pairAgentPreexistingDaemonAlive = false;
   if (command === 'pair-agent') {
+    // Scope-flag misuse is rejected before consent gates and ensureServer —
+    // an arg error must never boot a daemon.
+    validatePairAgentFlags(commandArgs);
     const preState = readState();
     pairAgentPreexistingDaemonAlive = Boolean(preState?.pid && isProcessAlive(preState.pid));
   }

browse/src/server.ts

@@ -31,7 +31,8 @@ import {
   initRegistry, validateToken as validateScopedToken, checkScope, checkDomain,
   checkRate, createToken, createSetupKey, exchangeSetupKey, revokeToken,
   listTokens, recordCommand,
-  isRootToken, checkConnectRateLimit, type TokenInfo,
+  isRootToken, checkConnectRateLimit, type TokenInfo, type ScopeCategory,
+  DEFAULT_PAIR_SCOPES, InvalidScopeError,
 } from './token-registry';
 import { validateTempPath } from './path-security';
 import { resolveConfig, ensureStateDir, readVersionHash, resolveChromiumProfile, cleanSingletonLocks, isPairAgentEnabled } from './config';
@@ -2316,7 +2317,14 @@ export function buildFetchHandler(cfg: ServerConfig): ServerHandle {
             scopes: session.scopes,
             agent: session.clientId,
           }), { status: 200, headers: { 'Content-Type': 'application/json' } });
-        } catch {
+        } catch (err) {
+          // Name the caller's typo (bad scope, negative rateLimit) instead of
+          // hiding it behind the generic body error.
+          if (err instanceof InvalidScopeError) {
+            return new Response(JSON.stringify({ error: err.message }), {
+              status: 400, headers: { 'Content-Type': 'application/json' },
+            });
+          }
           return new Response(JSON.stringify({ error: 'Invalid request body' }), {
             status: 400, headers: { 'Content-Type': 'application/json' },
           });
@@ -2384,12 +2392,20 @@ export function buildFetchHandler(cfg: ServerConfig): ServerHandle {
         }
         try {
           const pairBody = await req.json() as any;
-          // Default: full access (read+write+admin+meta). The trust boundary is
-          // the pairing ceremony itself, not the scope. --control adds browser-wide
-          // destructive commands (stop, restart, disconnect). --restrict limits scope.
+          // Default: DEFAULT_PAIR_SCOPES (full page access). The trust boundary
+          // is the pairing ceremony itself, not the scope. --control adds
+          // browser-wide destructive commands (stop, restart, disconnect).
+          // --restrict limits scope — but can never grant control: that scope
+          // stays behind the explicit control flag.
+          if (!pairBody.control && !pairBody.admin
+              && Array.isArray(pairBody.scopes) && pairBody.scopes.includes('control')) {
+            return new Response(JSON.stringify({
+              error: 'The control scope requires the control flag (--control); it cannot be granted via a scopes list.',
+            }), { status: 400, headers: { 'Content-Type': 'application/json' } });
+          }
           const scopes = pairBody.control || pairBody.admin
-            ? ['read', 'write', 'admin', 'meta', 'control'] as const
-            : (pairBody.scopes || ['read', 'write', 'admin', 'meta']) as const;
+            ? [...DEFAULT_PAIR_SCOPES, 'control' as const]
+            : ((pairBody.scopes || [...DEFAULT_PAIR_SCOPES]) as ScopeCategory[]);
           const setupKey = createSetupKey({
             clientId: pairBody.clientId,
             scopes: [...scopes],
@@ -2425,7 +2441,14 @@ export function buildFetchHandler(cfg: ServerConfig): ServerHandle {
             tunnel_url: verifiedTunnelUrl,
             server_url: `http://127.0.0.1:${browsePort}`,
           }), { status: 200, headers: { 'Content-Type': 'application/json' } });
-        } catch {
+        } catch (err) {
+          // Name the caller's typo (bad scope, negative rateLimit) instead of
+          // hiding it behind the generic body error.
+          if (err instanceof InvalidScopeError) {
+            return new Response(JSON.stringify({ error: err.message }), {
+              status: 400, headers: { 'Content-Type': 'application/json' },
+            });
+          }
           return new Response(JSON.stringify({ error: 'Invalid request body' }), {
             status: 400, headers: { 'Content-Type': 'application/json' },
           });

browse/src/token-registry.ts

@@ -82,6 +82,35 @@ const SCOPE_MAP: Record<ScopeCategory, Set<string>> = {
   meta: SCOPE_META,
 };
 
+/**
+ * Scopes granted by POST /pair when nothing narrower is requested.
+ * Deliberately full page access (b73f3644 / #907): the trust boundary is the
+ * pairing ceremony, not the scope. 'control' (browser-wide destructive ops)
+ * is the only scope that stays opt-in via the control flag. Referenced by
+ * BOTH server.ts (/pair default) and cli.ts (explicit send) so the two
+ * defaults cannot silently drift apart again.
+ */
+export const DEFAULT_PAIR_SCOPES: readonly ScopeCategory[] = ['read', 'write', 'admin', 'meta'];
+
+/**
+ * Typed error for caller-supplied token options (unknown scope, negative
+ * rateLimit). HTTP handlers catch it to 400 with the message at the endpoint
+ * where the typo happened — pre-fix, a bad scope sailed through /pair into a
+ * poisoned setup key and surfaced as a misleading "Invalid request body" to
+ * the remote agent at /connect.
+ */
+export class InvalidScopeError extends Error {}
+
+function assertValidTokenOptions(scopes: readonly string[], rateLimit: number): void {
+  const validScopes: ScopeCategory[] = ['read', 'write', 'admin', 'meta', 'control'];
+  for (const s of scopes) {
+    if (!validScopes.includes(s as ScopeCategory)) {
+      throw new InvalidScopeError(`Invalid scope: ${s}. Valid: ${validScopes.join(', ')}`);
+    }
+  }
+  if (rateLimit < 0) throw new InvalidScopeError('rateLimit must be >= 0');
+}
+
 // ─── Types ──────────────────────────────────────────────────────
 
 export interface TokenInfo {
@@ -200,13 +229,7 @@ export function createToken(opts: CreateTokenOptions): TokenInfo {
   } = opts;
 
   // Validate inputs
-  const validScopes: ScopeCategory[] = ['read', 'write', 'admin', 'meta', 'control'];
-  for (const s of scopes) {
-    if (!validScopes.includes(s as ScopeCategory)) {
-      throw new Error(`Invalid scope: ${s}. Valid: ${validScopes.join(', ')}`);
-    }
-  }
-  if (rateLimit < 0) throw new Error('rateLimit must be >= 0');
+  assertValidTokenOptions(scopes, rateLimit);
   if (expiresSeconds !== null && expiresSeconds !== undefined && expiresSeconds < 0) {
     throw new Error('expiresSeconds must be >= 0 or null');
   }
@@ -248,6 +271,13 @@ export function createToken(opts: CreateTokenOptions): TokenInfo {
  * Setup keys expire in 5 minutes and can only be exchanged once.
  */
 export function createSetupKey(opts: Omit<CreateTokenOptions, 'clientId'> & { clientId?: string }): TokenInfo {
+  const scopes = opts.scopes || ['read', 'write'];
+  // ?? not ||: rateLimit 0 is documented as "unlimited" and must survive.
+  const rateLimit = opts.rateLimit ?? 10;
+  // Validate HERE, not only at exchange time in createToken — otherwise a
+  // typo mints a poisoned setup key whose failure surfaces to the wrong
+  // party (the remote agent, at /connect, as "Invalid request body").
+  assertValidTokenOptions(scopes, rateLimit);
   const token = generateToken('gsk_setup_');
   const now = new Date();
   const expiresAt = new Date(now.getTime() + 5 * 60 * 1000).toISOString(); // 5 min
@@ -256,10 +286,10 @@ export function createSetupKey(opts: Omit<CreateTokenOptions, 'clientId'> & { cl
     token,
     clientId: opts.clientId || `remote-${Date.now()}`,
     type: 'setup',
-    scopes: opts.scopes || ['read', 'write'],
+    scopes,
     domains: opts.domains,
     tabPolicy: opts.tabPolicy || 'own-only',
-    rateLimit: opts.rateLimit || 10,
+    rateLimit,
     expiresAt,
     createdAt: now.toISOString(),
     usesRemaining: 1,