diff --git a/packages/cli/src/bridge-errors.ts b/packages/cli/src/bridge-errors.ts
index d749750..dfd311a 100644
--- a/packages/cli/src/bridge-errors.ts
+++ b/packages/cli/src/bridge-errors.ts
@@ -18,6 +18,16 @@ export function mapBridgeError(err: unknown, io: Io): number {
   }
   const kind = classifyBridgeError(err);
   const msg = err instanceof Error ? err.message : String(err);
+  // The server speaks to its library callers ("pass { domain: '<one of them>' }").
+  // A CLI user has no object to pass — name the flag instead. Verbs that can
+  // prove the gap up front (read, dom) refuse before connecting; this catches
+  // the rest, e.g. `fpx session` on a multi-domain profile.
+  if (msg.includes('declared multiple domains')) {
+    io.err(
+      `${msg.replace(/ — pass \{ domain.*$/, '')} — pick one with --storage-domain <domain>`,
+    );
+    return EXIT.USAGE;
+  }
   const hints: Record<string, string> = {
     bridge_down: 'is Chrome running with the Transporter extension installed?',
     timeout: 'is a tab open on the declared domain and signed in?',
diff --git a/packages/cli/src/profiles.ts b/packages/cli/src/profiles.ts
index 540072b..8758a37 100644
--- a/packages/cli/src/profiles.ts
+++ b/packages/cli/src/profiles.ts
@@ -53,6 +53,35 @@ export function emptyProfile(domains: string[]): Profile {
 const isStringArray = (v: unknown): v is string[] =>
   Array.isArray(v) && v.every((s) => typeof s === 'string' && s.length > 0);
 
+const isRecord = (v: unknown): v is Record<string, unknown> =>
+  v !== null && typeof v === 'object' && !Array.isArray(v);
+
+const hasStrings = (e: Record<string, unknown>, keys: readonly string[]): boolean =>
+  keys.every((k) => typeof e[k] === 'string' && (e[k] as string).length > 0);
+
+const optionalString = (v: unknown): boolean =>
+  v === undefined || (typeof v === 'string' && v.length > 0);
+
+/**
+ * Per-element shape checks for the object arrays. These arrays are hand-edited
+ * into profiles.json (only --capture-header and --dom-selector have flags), so
+ * `Array.isArray` alone let a typo through here and surface much later as a
+ * ProtocolError from the extension's hello validator, mid-pair.
+ *
+ * Deliberately structural: required fields present and string-typed. The
+ * protocol's format rules (origin scheme, key/selector character sets, JSON
+ * pointer syntax) stay in @fetchproxy/protocol's validator — duplicating them
+ * here would give two definitions of valid, and they would drift.
+ */
+const ELEMENT_SHAPE: Record<string, (e: Record<string, unknown>) => boolean> = {
+  captureHeaders: (e) => hasStrings(e, ['host', 'headerName']) && optionalString(e.path),
+  indexedDb: (e) =>
+    hasStrings(e, ['origin', 'database', 'store']) && isStringArray(e.keys) && e.keys.length > 0,
+  localStoragePointers: (e) => hasStrings(e, ['outputKey', 'storageKey', 'jsonPointer']),
+  sessionStoragePointers: (e) => hasStrings(e, ['outputKey', 'storageKey', 'jsonPointer']),
+  domSelectors: (e) => hasStrings(e, ['name', 'selector']) && optionalString(e.attribute),
+};
+
 function validateProfile(name: string, raw: unknown): Profile {
   const fail = (field: string): never => {
     throw new UsageError(
@@ -69,7 +98,12 @@ function validateProfile(name: string, raw: unknown): Profile {
   for (const k of [
     'captureHeaders', 'indexedDb', 'localStoragePointers', 'sessionStoragePointers', 'domSelectors',
   ] as const) {
-    if (p[k] !== undefined && !Array.isArray(p[k])) fail(k);
+    if (p[k] === undefined) continue;
+    if (!Array.isArray(p[k])) fail(k);
+    const shapeOk = ELEMENT_SHAPE[k]!;
+    (p[k] as unknown[]).forEach((entry, i) => {
+      if (!isRecord(entry) || !shapeOk(entry)) fail(`${k}[${i}]`);
+    });
   }
   if (p.download !== undefined && typeof p.download !== 'boolean') fail('download');
   return { ...emptyProfile(p.domains as string[]), ...(p as Partial<Profile>) } as Profile;
diff --git a/packages/cli/src/storage-scope.ts b/packages/cli/src/storage-scope.ts
new file mode 100644
index 0000000..cc3251c
--- /dev/null
+++ b/packages/cli/src/storage-scope.ts
@@ -0,0 +1,20 @@
+import type { Profile } from './profiles.js';
+import { UsageError } from './output.js';
+
+/**
+ * Storage reads (cookies / localStorage / sessionStorage / indexedDb / DOM)
+ * target exactly ONE declared domain. With several declared and none picked,
+ * the server refuses — but its message names the library option
+ * (`pass { domain: '<one of them>' }`), which is not something a CLI user can
+ * pass. Refuse here instead, before connecting, naming the flag that fixes it.
+ *
+ * `fpx pair` does the same check against `--domain`; only the flag differs.
+ */
+export function requireStorageDomain(profile: Profile, storageDomain: string | undefined): void {
+  if (storageDomain !== undefined || profile.domains.length <= 1) return;
+  throw new UsageError(
+    `profile declares multiple domains (${profile.domains.join(', ')}) — ` +
+      'storage reads target one of them: pick it with --storage-domain <domain>',
+    'add --storage-subdomain <sub> too if the signed-in tab is on a subdomain.',
+  );
+}
diff --git a/packages/cli/src/verbs/dom.ts b/packages/cli/src/verbs/dom.ts
index 533b085..771a5ba 100644
--- a/packages/cli/src/verbs/dom.ts
+++ b/packages/cli/src/verbs/dom.ts
@@ -1,6 +1,7 @@
 import type { Command } from '../args.js';
 import type { Profile } from '../profiles.js';
 import { serverOptsFor } from '../server-opts.js';
+import { requireStorageDomain } from '../storage-scope.js';
 import { EXIT, UsageError, printJson, type Io } from '../output.js';
 import { mapBridgeError } from '../bridge-errors.js';
 import { defaultServerFactory, pairCodePrinter, type VerbServerFactory } from './fetch.js';
@@ -32,6 +33,7 @@ export async function runDom(
 ): Promise<number> {
   // Validate scope narrowing BEFORE connecting — usage errors must not
   // cost a bridge round-trip (and must not trigger pairing).
+  requireStorageDomain(profile, cmd.storageDomain);
   const names = narrowDomNames(cmd.names, profile.domSelectors.map((d) => d.name));
 
   const server = makeServer({
diff --git a/packages/cli/src/verbs/read.ts b/packages/cli/src/verbs/read.ts
index 86cb1e7..d37b640 100644
--- a/packages/cli/src/verbs/read.ts
+++ b/packages/cli/src/verbs/read.ts
@@ -1,6 +1,7 @@
 import type { Command } from '../args.js';
 import type { Profile } from '../profiles.js';
 import { serverOptsFor } from '../server-opts.js';
+import { requireStorageDomain } from '../storage-scope.js';
 import { EXIT, UsageError, printJson, type Io } from '../output.js';
 import { mapBridgeError } from '../bridge-errors.js';
 import { defaultServerFactory, pairCodePrinter, type VerbServerFactory } from './fetch.js';
@@ -50,6 +51,7 @@ export async function runRead(
 ): Promise<number> {
   // Validate scope narrowing BEFORE connecting — usage errors must not
   // cost a bridge round-trip (and must not trigger pairing).
+  requireStorageDomain(profile, cmd.storageDomain);
   const scope = { domain: cmd.storageDomain, subdomain: cmd.storageSubdomain };
   let keys: string[] = [];
   if (cmd.bucket === 'indexedDb') {
diff --git a/packages/cli/tests/dom.test.ts b/packages/cli/tests/dom.test.ts
index b363a3b..e47c97c 100644
--- a/packages/cli/tests/dom.test.ts
+++ b/packages/cli/tests/dom.test.ts
@@ -28,6 +28,14 @@ const PROFILE = {
 };
 
 describe('runDom', () => {
+  it('multi-domain profile without --storage-domain → UsageError naming the flag', async () => {
+    const server = stubServer();
+    await expect(runDom({ kind: 'dom', profile: 'r', names: [] },
+      { ...PROFILE, domains: ['resy.com', 'resy.io'] }, memIo(), () => server))
+      .rejects.toThrow(/--storage-domain/);
+    expect(server.listen).not.toHaveBeenCalled();
+  });
+
   it('narrows to requested declared names and reads via readDom', async () => {
     const server = stubServer();
     const code = await runDom(
diff --git a/packages/cli/tests/profiles.test.ts b/packages/cli/tests/profiles.test.ts
index 550f78f..25b4da7 100644
--- a/packages/cli/tests/profiles.test.ts
+++ b/packages/cli/tests/profiles.test.ts
@@ -36,6 +36,50 @@ describe('profiles', () => {
     expect(() => loadProfiles(home)).toThrow(/bad.*domains/);
   });
 
+  it('rejects a malformed element inside an object array, naming its index', () => {
+    saveProfiles({
+      bad: {
+        ...emptyProfile(['x.com']),
+        indexedDb: [
+          { origin: 'https://x.com', database: 'db', store: 's', keys: ['k'] },
+          { database: 'db', store: 's', keys: ['k'] },
+        ],
+      } as never,
+    }, home);
+    expect(() => loadProfiles(home)).toThrow(UsageError);
+    // The index matters: with several scopes declared, "invalid indexedDb"
+    // alone leaves the user hunting for which entry is wrong.
+    expect(() => loadProfiles(home)).toThrow(/indexedDb\[1\]/);
+  });
+
+  it('rejects a non-object element, an empty key list, and a bad optional field', () => {
+    const cases: Array<[string, unknown]> = [
+      ['captureHeaders', ['not-an-object']],
+      ['captureHeaders', [{ host: 'x.com', headerName: 'x-tok', path: 7 }]],
+      ['indexedDb', [{ origin: 'https://x.com', database: 'db', store: 's', keys: [] }]],
+      ['localStoragePointers', [{ outputKey: 'o', storageKey: 's' }]],
+      ['sessionStoragePointers', [{ outputKey: 'o', storageKey: 's', jsonPointer: 3 }]],
+      ['domSelectors', [{ name: 'n', selector: '#a', attribute: '' }]],
+    ];
+    for (const [field, value] of cases) {
+      saveProfiles({ bad: { ...emptyProfile(['x.com']), [field]: value } as never }, home);
+      expect(() => loadProfiles(home), field).toThrow(new RegExp(`${field}\\[0\\]`));
+    }
+  });
+
+  it('accepts well-formed elements, including the optional fields', () => {
+    saveProfiles({
+      ok: {
+        ...emptyProfile(['x.com']),
+        captureHeaders: [{ host: 'x.com', headerName: 'x-tok', path: '/api/*' }],
+        indexedDb: [{ origin: 'https://x.com', database: 'db', store: 's', keys: ['k'] }],
+        localStoragePointers: [{ outputKey: 'o', storageKey: 'auth', jsonPointer: '/token' }],
+        domSelectors: [{ name: 'csrf', selector: 'meta[name=csrf]', attribute: 'content' }],
+      } as never,
+    }, home);
+    expect(loadProfiles(home).ok.domSelectors[0].attribute).toBe('content');
+  });
+
   it('getProfile throws a UsageError listing known profiles', () => {
     saveProfiles({ trip: emptyProfile(['tripadvisor.com']) }, home);
     expect(() => getProfile('nope', home)).toThrow(/known profiles: trip/);
diff --git a/packages/cli/tests/read.test.ts b/packages/cli/tests/read.test.ts
index 01f90c4..c37daf2 100644
--- a/packages/cli/tests/read.test.ts
+++ b/packages/cli/tests/read.test.ts
@@ -48,6 +48,27 @@ describe('runRead', () => {
       PROFILE, memIo(), () => stubServer())).rejects.toThrow(/profile declare/);
   });
 
+  it('multi-domain profile without --storage-domain → UsageError naming the flag', async () => {
+    const multi = { ...PROFILE, domains: ['resy.com', 'resy.io'] };
+    const server = stubServer();
+    await expect(runRead({ kind: 'read', profile: 'r', bucket: 'cookies', keys: [] },
+      multi, memIo(), () => server)).rejects.toThrow(/--storage-domain/);
+    // Refused before connecting: a usage error must not cost a bridge
+    // round-trip, and must not trigger pairing.
+    expect(server.listen).not.toHaveBeenCalled();
+  });
+
+  it('multi-domain profile WITH --storage-domain proceeds', async () => {
+    const multi = { ...PROFILE, domains: ['resy.com', 'resy.io'] };
+    const server = stubServer();
+    const code = await runRead(
+      { kind: 'read', profile: 'r', bucket: 'cookies', keys: [], storageDomain: 'resy.io' },
+      multi, memIo(), () => server);
+    expect(code).toBe(EXIT.OK);
+    expect(server.readCookies).toHaveBeenCalledWith(
+      { keys: ['a', 'b'], domain: 'resy.io', subdomain: undefined });
+  });
+
   it('empty declared bucket → UsageError with the declare flag', async () => {
     await expect(runRead({ kind: 'read', profile: 'r', bucket: 'sessionStorage', keys: [] },
       PROFILE, memIo(), () => stubServer())).rejects.toThrow(/--session-storage/);
diff --git a/packages/cli/tests/session.test.ts b/packages/cli/tests/session.test.ts
index 5bd88d6..ce59be2 100644
--- a/packages/cli/tests/session.test.ts
+++ b/packages/cli/tests/session.test.ts
@@ -42,6 +42,27 @@ describe('runSession', () => {
     expect(io.errs.join('\n')).toMatch(/waiting: trigger a request/);
   });
 
+  it("rewrites the server's multi-domain error to name --storage-domain", async () => {
+    const io = memIo();
+    // `fpx session` can't pre-check like read/dom do: a captureHeaders-only
+    // profile never does a domain-scoped read, so a multi-domain profile with
+    // no --storage-domain is legitimate there. The server's message is what
+    // reaches the user, and it speaks to library callers.
+    const boot = vi.fn(async () => {
+      throw new Error(
+        'FetchproxyServer: this MCP declared multiple domains ["honeybook.com", "hbportal.co"]'
+        + " — pass { domain: '<one of them>' } on every per-call request",
+      );
+    });
+    const code = await runSession(
+      { kind: 'session', profile: 'hb', storageDomain: undefined, storageSubdomain: undefined },
+      { ...emptyProfile(['honeybook.com', 'hbportal.co']), cookies: ['a'] }, io, boot as never);
+    expect(code).toBe(EXIT.USAGE);
+    expect(io.errs[0]).toContain('--storage-domain');
+    expect(io.errs[0]).not.toContain('pass { domain');
+    expect(io.errs[0]).toContain('["honeybook.com", "hbportal.co"]');
+  });
+
   it('bridge failure → exit 2', async () => {
     const io = memIo();
     const boot = vi.fn(async () => { throw new Error('extension gone'); });