diff --git a/packages/pen-core/src/__tests__/element-builders-edge-cases.test.ts b/packages/pen-core/src/__tests__/element-builders-edge-cases.test.ts
index 0a199fe82..50ced536d 100644
--- a/packages/pen-core/src/__tests__/element-builders-edge-cases.test.ts
+++ b/packages/pen-core/src/__tests__/element-builders-edge-cases.test.ts
@@ -1,16 +1,21 @@
 import { describe, it, expect } from 'vitest';
 import {
+  buildActivityLog,
   buildBadge,
   buildBodyText,
   buildCalendarGrid,
+  buildCallout,
   buildCardRow,
   buildChartBars,
   buildEmptyState,
   buildHeading,
+  buildInviteRow,
   buildKbd,
+  buildMemberRow,
   buildPrice,
   buildRatingStars,
   buildStepper,
+  buildTag,
   buildTimeline,
 } from '../element-builders/index.js';
 
@@ -172,3 +177,97 @@ describe('buildHeading — invalid level rejection', () => {
     expect(() => buildHeading({ content: 'no level' })).not.toThrow();
   });
 });
+
+// Same defensive pattern as buildHeading — the MCP tool defs declare
+// tone / status enums, but ab-corpus + production both call these
+// builders with raw JSON args, bypassing schema validation. A model
+// inventing an out-of-enum string used to crash the dispatch with
+// `undefined is not an object`. Each builder now rejects at the entry
+// boundary so the surrounding batch keeps going through the rest.
+describe('buildTag — invalid tone rejection', () => {
+  it('throws for an unknown tone', () => {
+    expect(() => buildTag({ label: 'Active', tone: 'critical' as never })).toThrow(
+      /invalid tone "critical"/,
+    );
+  });
+  it('accepts every valid tone + omitted', () => {
+    for (const tone of ['default', 'accent', 'success', 'warning', 'error'] as const) {
+      expect(() => buildTag({ label: 'x', tone })).not.toThrow();
+    }
+    expect(() => buildTag({ label: 'x' })).not.toThrow();
+  });
+});
+
+describe('buildCallout — invalid tone rejection', () => {
+  it('throws for an unknown tone', () => {
+    expect(() => buildCallout({ body: 'x', tone: 'fancy' as never })).toThrow(
+      /invalid tone "fancy"/,
+    );
+  });
+  it('accepts every valid tone + omitted', () => {
+    for (const tone of ['info', 'success', 'warning', 'danger', 'note'] as const) {
+      expect(() => buildCallout({ body: 'x', tone })).not.toThrow();
+    }
+    expect(() => buildCallout({ body: 'x' })).not.toThrow();
+  });
+});
+
+describe('buildActivityLog — invalid tone rejection', () => {
+  it('throws for an unknown tone', () => {
+    expect(() =>
+      buildActivityLog({
+        actor: 'Sarah',
+        action: 'merged PR',
+        timestamp: '2h ago',
+        tone: 'critical' as never,
+      }),
+    ).toThrow(/invalid tone "critical"/);
+  });
+  it('accepts every valid tone + omitted', () => {
+    const base = { actor: 'a', action: 'b', timestamp: '1m' };
+    for (const tone of ['info', 'success', 'warning', 'danger', 'neutral'] as const) {
+      expect(() => buildActivityLog({ ...base, tone })).not.toThrow();
+    }
+    expect(() => buildActivityLog(base)).not.toThrow();
+  });
+});
+
+describe('buildInviteRow — invalid status rejection', () => {
+  it('throws for an unknown status', () => {
+    expect(() => buildInviteRow({ email: 'a@b.com', status: 'revoked' as never })).toThrow(
+      /invalid status "revoked"/,
+    );
+  });
+  it('accepts every valid status + omitted', () => {
+    for (const status of ['pending', 'expired', 'accepted'] as const) {
+      expect(() => buildInviteRow({ email: 'x@y.com', status })).not.toThrow();
+    }
+    expect(() => buildInviteRow({ email: 'x@y.com' })).not.toThrow();
+  });
+});
+
+describe('buildMemberRow — invalid trailing.tone rejection (status_dot variant)', () => {
+  it('throws when trailing.kind=status_dot has an unknown tone', () => {
+    expect(() =>
+      buildMemberRow({
+        name: 'Sarah',
+        trailing: { kind: 'status_dot', tone: 'unknown' as never },
+      }),
+    ).toThrow(/invalid trailing\.tone "unknown"/);
+  });
+  it('accepts every valid status tone', () => {
+    for (const tone of ['online', 'busy', 'away', 'offline'] as const) {
+      expect(() =>
+        buildMemberRow({ name: 'x', trailing: { kind: 'status_dot', tone } }),
+      ).not.toThrow();
+    }
+    // omitted tone defaults to 'online'
+    expect(() => buildMemberRow({ name: 'x', trailing: { kind: 'status_dot' } })).not.toThrow();
+  });
+  it('does not validate when trailing is role_badge or menu (no tone field)', () => {
+    expect(() =>
+      buildMemberRow({ name: 'x', trailing: { kind: 'role_badge', value: 'Owner' } }),
+    ).not.toThrow();
+    expect(() => buildMemberRow({ name: 'x', trailing: { kind: 'menu' } })).not.toThrow();
+  });
+});
diff --git a/packages/pen-core/src/element-builders/activity-log.ts b/packages/pen-core/src/element-builders/activity-log.ts
index 77fd4b4e4..bc69f9b6f 100644
--- a/packages/pen-core/src/element-builders/activity-log.ts
+++ b/packages/pen-core/src/element-builders/activity-log.ts
@@ -13,6 +13,14 @@ export interface ActivityLogParams {
   tone?: 'info' | 'success' | 'warning' | 'danger' | 'neutral';
 }
 
+const VALID_ACTIVITY_LOG_TONES = new Set<string>([
+  'info',
+  'success',
+  'warning',
+  'danger',
+  'neutral',
+]);
+
 const ACTOR_FG = '#0F172A';
 const ACTION_FG = '#475569';
 const TS_FG = '#94A3B8';
@@ -46,7 +54,13 @@ const TONE_FG: Record<NonNullable<ActivityLogParams['tone']>, string> = {
  * "审计日志条目", "活动记录".
  */
 export function buildActivityLog(params: ActivityLogParams): ElementTree {
-  const tone = params.tone ?? 'info';
+  const requestedTone = (params.tone ?? 'info') as string;
+  if (!VALID_ACTIVITY_LOG_TONES.has(requestedTone)) {
+    throw new Error(
+      `add_activity_log_v0: invalid tone "${requestedTone}"; expected one of: info, success, warning, danger, neutral`,
+    );
+  }
+  const tone = requestedTone as NonNullable<ActivityLogParams['tone']>;
   const rowChildren: ElementTree[] = [];
 
   if (params.icon) {
diff --git a/packages/pen-core/src/element-builders/callout.ts b/packages/pen-core/src/element-builders/callout.ts
index 5bf4872f3..b6fbc25c3 100644
--- a/packages/pen-core/src/element-builders/callout.ts
+++ b/packages/pen-core/src/element-builders/callout.ts
@@ -2,6 +2,8 @@ import type { ElementTree } from './helpers.js';
 
 export type CalloutTone = 'info' | 'success' | 'warning' | 'danger' | 'note';
 
+const VALID_CALLOUT_TONES = new Set<string>(['info', 'success', 'warning', 'danger', 'note']);
+
 export interface CalloutParams {
   /** Body text. Required. */
   body: string;
@@ -33,7 +35,13 @@ const TONES: Record<CalloutTone, ToneSpec> = {
  * onboarding notes, "did you know" panels.
  */
 export function buildCallout(params: CalloutParams): ElementTree {
-  const tone = TONES[params.tone ?? 'note'];
+  const requestedTone = (params.tone ?? 'note') as string;
+  if (!VALID_CALLOUT_TONES.has(requestedTone)) {
+    throw new Error(
+      `add_callout_v0: invalid tone "${requestedTone}"; expected one of: info, success, warning, danger, note`,
+    );
+  }
+  const tone = TONES[requestedTone as CalloutTone];
   const stackChildren: ElementTree[] = [];
   if (params.title) {
     stackChildren.push({
diff --git a/packages/pen-core/src/element-builders/invite-row.ts b/packages/pen-core/src/element-builders/invite-row.ts
index 35c77fc19..57eaefc5f 100644
--- a/packages/pen-core/src/element-builders/invite-row.ts
+++ b/packages/pen-core/src/element-builders/invite-row.ts
@@ -2,6 +2,8 @@ import type { ElementTree } from './helpers.js';
 
 export type InviteStatus = 'pending' | 'expired' | 'accepted';
 
+const VALID_INVITE_STATUSES = new Set<string>(['pending', 'expired', 'accepted']);
+
 export interface InviteRowParams {
   /** Invitee email (e.g. "sarah@acme.com"). */
   email: string;
@@ -33,7 +35,13 @@ const ACTION_FG = '#2563EB';
  * invite row", "邀请列表行", "待接受邀请".
  */
 export function buildInviteRow(params: InviteRowParams): ElementTree {
-  const status = params.status ?? 'pending';
+  const requestedStatus = (params.status ?? 'pending') as string;
+  if (!VALID_INVITE_STATUSES.has(requestedStatus)) {
+    throw new Error(
+      `add_invite_row_v0: invalid status "${requestedStatus}"; expected one of: pending, expired, accepted`,
+    );
+  }
+  const status = requestedStatus as InviteStatus;
   const tone = STATUS_TONE[status];
   const actionLabel = params.action_label ?? 'Resend';
   const initial = (params.email.charAt(0) ?? '?').toUpperCase();
diff --git a/packages/pen-core/src/element-builders/member-row.ts b/packages/pen-core/src/element-builders/member-row.ts
index de878273b..d6e03d780 100644
--- a/packages/pen-core/src/element-builders/member-row.ts
+++ b/packages/pen-core/src/element-builders/member-row.ts
@@ -26,6 +26,8 @@ const MENU_FG = '#94A3B8';
 
 type StatusTone = 'online' | 'busy' | 'away' | 'offline';
 
+const VALID_STATUS_TONES = new Set<string>(['online', 'busy', 'away', 'offline']);
+
 const STATUS_TONE_FILL: Record<StatusTone, string> = {
   online: '#10B981',
   busy: '#EF4444',
@@ -69,7 +71,13 @@ function buildTrailing(t: MemberRowTrailing): ElementTree {
       fill: [{ type: 'solid', color: MENU_FG }],
     };
   }
-  const tone = t.tone ?? 'online';
+  const requestedTone = (t.tone ?? 'online') as string;
+  if (!VALID_STATUS_TONES.has(requestedTone)) {
+    throw new Error(
+      `add_member_row_v0: invalid trailing.tone "${requestedTone}"; expected one of: online, busy, away, offline`,
+    );
+  }
+  const tone = requestedTone as StatusTone;
   return {
     type: 'frame',
     name: 'Status Dot',
diff --git a/packages/pen-core/src/element-builders/tag.ts b/packages/pen-core/src/element-builders/tag.ts
index dcf19832b..199eee876 100644
--- a/packages/pen-core/src/element-builders/tag.ts
+++ b/packages/pen-core/src/element-builders/tag.ts
@@ -2,6 +2,8 @@ import type { ElementTree } from './helpers.js';
 
 export type TagTone = 'default' | 'accent' | 'success' | 'warning' | 'error';
 
+const VALID_TAG_TONES = new Set<string>(['default', 'accent', 'success', 'warning', 'error']);
+
 export interface TagParams {
   label: string;
   /** Render the trailing × close icon. Default true — that's what makes
@@ -36,7 +38,13 @@ const TONES: Record<TagTone, ToneSpec> = {
  * clicking the ×: "Status: Active ×", "Plan: Pro ×", "Tag: design ×".
  */
 export function buildTag(params: TagParams): ElementTree {
-  const tone = TONES[params.tone ?? 'default'];
+  const requestedTone = (params.tone ?? 'default') as string;
+  if (!VALID_TAG_TONES.has(requestedTone)) {
+    throw new Error(
+      `add_tag_v0: invalid tone "${requestedTone}"; expected one of: default, accent, success, warning, error`,
+    );
+  }
+  const tone = TONES[requestedTone as TagTone];
   const removable = params.removable ?? true;
   const children: ElementTree[] = [
     {