diff --git a/apps/gateway/src/main.ts b/apps/gateway/src/main.ts
index d1eacf5f..cb066d18 100644
--- a/apps/gateway/src/main.ts
+++ b/apps/gateway/src/main.ts
@@ -14,10 +14,16 @@ import { mountMcpHandler } from './mcp/mcp.controller.js';
 import { McpService } from './mcp/mcp.service.js';
 import { detectAndAssertTier, TierDetectionError } from '@mosaicstack/storage';
 import { resolveGatewayConfigPath } from './env.js';
+import { assertValidationPipeSeesDtoDecorators } from './validation-pipe-check.js';
 
 async function bootstrap(): Promise<void> {
   const logger = new Logger('Bootstrap');
 
+  // Fail loud BEFORE anything else if the global ValidationPipe cannot see
+  // the guarded DTOs' decorated properties (#1391): a broken metatype turns
+  // every request body into a 400 at first use; this surfaces it at boot.
+  assertValidationPipeSeesDtoDecorators();
+
   if (!process.env['BETTER_AUTH_SECRET']) {
     throw new Error('BETTER_AUTH_SECRET is required');
   }
diff --git a/apps/gateway/src/validation-pipe-check.spec.ts b/apps/gateway/src/validation-pipe-check.spec.ts
new file mode 100644
index 00000000..3d6ca7d1
--- /dev/null
+++ b/apps/gateway/src/validation-pipe-check.spec.ts
@@ -0,0 +1,104 @@
+/**
+ * Boot-time ValidationPipe metatype self-check (#1391).
+ *
+ * The check exists to fail loud at boot when the global pipe cannot see a
+ * guarded DTO's decorated properties — the #436 class-erasure signature and
+ * its dependency-graph cousins. Red/green arms:
+ *
+ *   GREEN  real module state: BootstrapSetupDto's three properties are
+ *          decorated and visible through the globalThis-shared storage.
+ *   RED    a control class with NO decorators (the erasure shape): the
+ *          check throws PipeMetatypeCheckError naming every property.
+ *   RED-2  a control where one property is decorated and two are not: the
+ *          error names exactly the missing two — the miss list is precise,
+ *          not a blanket failure.
+ */
+import { describe, expect, it } from 'vitest';
+import { IsString } from 'class-validator';
+import {
+  assertValidationPipeSeesDtoDecorators,
+  PipeMetatypeCheckError,
+} from './validation-pipe-check.js';
+
+describe('assertValidationPipeSeesDtoDecorators (#1391 boot check)', () => {
+  it('GREEN: passes on real module state (decorated DTO visible to the pipe)', () => {
+    expect(() => assertValidationPipeSeesDtoDecorators()).not.toThrow();
+  });
+
+  it('RED control: a class whose properties lost their decorators throws, naming them', async () => {
+    // Simulate metatype erasure: an undecorated class standing where a
+    // decorated DTO should be. Redefine the guard table for the test by
+    // importing the module and pointing its table at the eroded class —
+    // the check reads the table at call time, so a fresh module instance
+    // with a swapped table reproduces the boot failure deterministically.
+    const { PIPE_GUARDED_DTOS } = await import('./validation-pipe-check.js');
+
+    class ErodedDto {
+      name?: string;
+      email?: string;
+      password?: string;
+    }
+
+    const original = PIPE_GUARDED_DTOS[0];
+    expect(original).toBeDefined();
+    // Swap in the eroded target (same declared properties, zero decorators).
+    (
+      PIPE_GUARDED_DTOS as unknown as Array<{ name: string; target: object; properties: string[] }>
+    ).splice(0, PIPE_GUARDED_DTOS.length, {
+      name: 'ErodedDto',
+      target: ErodedDto,
+      properties: ['name', 'email', 'password'],
+    });
+
+    try {
+      expect(() => assertValidationPipeSeesDtoDecorators()).toThrow(PipeMetatypeCheckError);
+      try {
+        assertValidationPipeSeesDtoDecorators();
+      } catch (err) {
+        const message = err instanceof Error ? err.message : '';
+        expect(message).toContain('ErodedDto.name');
+        expect(message).toContain('ErodedDto.email');
+        expect(message).toContain('ErodedDto.password');
+      }
+    } finally {
+      // Restore real module state for any later test in this file.
+      (PIPE_GUARDED_DTOS as unknown as unknown[]).splice(0, PIPE_GUARDED_DTOS.length, original);
+    }
+    // And confirm the restore is real.
+    expect(() => assertValidationPipeSeesDtoDecorators()).not.toThrow();
+  });
+
+  it('RED-2 control: a partially decorated class names exactly the missing properties', async () => {
+    const { PIPE_GUARDED_DTOS } = await import('./validation-pipe-check.js');
+
+    class HalfErodedDto {
+      @IsString()
+      name?: string;
+      email?: string;
+      password?: string;
+    }
+
+    const original = PIPE_GUARDED_DTOS[0];
+    (
+      PIPE_GUARDED_DTOS as unknown as Array<{ name: string; target: object; properties: string[] }>
+    ).splice(0, PIPE_GUARDED_DTOS.length, {
+      name: 'HalfErodedDto',
+      target: HalfErodedDto,
+      properties: ['name', 'email', 'password'],
+    });
+
+    try {
+      try {
+        assertValidationPipeSeesDtoDecorators();
+        expect.unreachable('partially decorated DTO must fail the boot check');
+      } catch (err) {
+        const message = err instanceof Error ? err.message : '';
+        expect(message).toContain('HalfErodedDto.email');
+        expect(message).toContain('HalfErodedDto.password');
+        expect(message).not.toContain('HalfErodedDto.name has no');
+      }
+    } finally {
+      (PIPE_GUARDED_DTOS as unknown as unknown[]).splice(0, PIPE_GUARDED_DTOS.length, original);
+    }
+  });
+});
diff --git a/apps/gateway/src/validation-pipe-check.ts b/apps/gateway/src/validation-pipe-check.ts
new file mode 100644
index 00000000..61af929a
--- /dev/null
+++ b/apps/gateway/src/validation-pipe-check.ts
@@ -0,0 +1,94 @@
+import 'reflect-metadata';
+import { getMetadataStorage } from 'class-validator';
+import { BootstrapSetupDto } from './admin/bootstrap.dto.js';
+
+/**
+ * Boot-time self-check: the global ValidationPipe must be able to SEE the
+ * decorated properties of the DTOs it guards (#1391, #436 class).
+ *
+ * WHY THIS EXISTS. When Nest resolves a @Body() metatype to Object — via
+ * `import type` class erasure (#436), or a dependency graph where the
+ * controller's decorators and the application's route enhancers disagree
+ * (#1391's hypothesized dual-@nestjs/common on a mixed install) — the
+ * ValidationPipe's whitelist treats every property as forbidden. The first
+ * symptom is a 400 on the FIRST bootstrap attempt of a fresh install, the
+ * worst place to discover wiring damage: the operator cannot tell a broken
+ * payload from a broken daemon.
+ *
+ * This check fails LOUD at boot instead: if the pipe cannot see the DTO's
+ * decorated properties, the gateway refuses to start with a named cause.
+ * It catches the whole class — erasure, decorator metadata loss — on every
+ * host, at the moment the damage exists rather than at first use.
+ *
+ * Storage sharing note: class-validator keys its metadata storage on
+ * globalThis, so duplicate package copies do NOT hide metadata (measured,
+ * #1391 diagnosis). What hides it is losing the metatype itself, which is
+ * what this asserts against.
+ */
+
+/**
+ * DTOs the global pipe guards, mapped to the properties the whitelist must
+ * admit. Target is the CONSTRUCTOR (the object class itself): class-validator
+ * decorators register metadata keyed on the constructor, and its executor
+ * looks up `object.constructor` (ValidationExecutor.js:50) — the probe
+ * through `prototype` returns zero. Extend when adding DTOs to the app.
+ */
+export const PIPE_GUARDED_DTOS: Array<{
+  name: string;
+  target: abstract new (...args: never[]) => unknown;
+  properties: string[];
+}> = [
+  {
+    name: 'BootstrapSetupDto',
+    target: BootstrapSetupDto,
+    properties: ['name', 'email', 'password'],
+  },
+];
+
+export class PipeMetatypeCheckError extends Error {
+  constructor(missing: string[]) {
+    super(
+      'ValidationPipe metatype check failed: ' +
+        missing.join('; ') +
+        '. The global ValidationPipe cannot see decorated DTO properties — ' +
+        'every request body would be rejected as non-whitelisted. ' +
+        'Check for import-type erasure or decorator metadata loss in the ' +
+        'dependency graph (see issues #436, #1391).',
+    );
+    this.name = 'PipeMetatypeCheckError';
+  }
+}
+
+/**
+ * Assert the pipe's whitelist can see every guarded DTO's decorated
+ * properties. Throws PipeMetatypeCheckError (fail-loud at boot) listing
+ * each miss. Pure function of module state: no I/O, safe to call twice.
+ */
+export function assertValidationPipeSeesDtoDecorators(): void {
+  const storage = getMetadataStorage();
+  const missing: string[] = [];
+
+  for (const dto of PIPE_GUARDED_DTOS) {
+    // class-validator records constraints keyed on the DTO's constructor
+    // (decorators run on the class), and its executor resolves them via
+    // object.constructor. A property with no recorded metadata is invisible
+    // to the whitelist — whatever the cause — and fails here.
+    // Signature mirrors ValidationExecutor.js:50 — (constructor, schema, always,
+    // strictGroups, groups?). No schema, always=true, no groups: every
+    // constraint regardless of grouping, which is what the whitelist sees.
+    const metadatas = storage.getTargetValidationMetadatas(dto.target, '', true, false);
+    const decorated = new Set(metadatas.map((m) => m.propertyName));
+
+    for (const property of dto.properties) {
+      if (!decorated.has(property)) {
+        missing.push(
+          `${dto.name}.${property} has no class-validator constraints visible to the pipe`,
+        );
+      }
+    }
+  }
+
+  if (missing.length > 0) {
+    throw new PipeMetatypeCheckError(missing);
+  }
+}