diff --git a/packages/iios-service/src/app.module.ts b/packages/iios-service/src/app.module.ts
index 31bae1c..1212e62 100644
--- a/packages/iios-service/src/app.module.ts
+++ b/packages/iios-service/src/app.module.ts
@@ -1,9 +1,10 @@
 import { Module } from '@nestjs/common';
 import { PrismaModule } from './prisma/prisma.module';
+import { PlatformModule } from './platform/platform.module';
 import { HealthController } from './health.controller';
 
 @Module({
-  imports: [PrismaModule],
+  imports: [PrismaModule, PlatformModule],
   controllers: [HealthController],
 })
 export class AppModule {}
diff --git a/packages/iios-service/src/platform/fail-closed.test.ts b/packages/iios-service/src/platform/fail-closed.test.ts
new file mode 100644
index 0000000..ee7bf2b
--- /dev/null
+++ b/packages/iios-service/src/platform/fail-closed.test.ts
@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { makeFakePorts } from '@insignia/iios-testkit';
+import { PolicyDeniedError } from '@insignia/iios-contracts';
+import { decideOrThrow } from './fail-closed';
+
+describe('decideOrThrow (fail-closed gate)', () => {
+  it('returns the decision when OPA allows', async () => {
+    const ports = makeFakePorts();
+    const decision = await decideOrThrow(ports, {});
+    expect(decision.allow).toBe(true);
+  });
+
+  it('throws PolicyDeniedError when OPA denies', async () => {
+    const ports = makeFakePorts();
+    ports.opa.deny('blocked');
+    await expect(decideOrThrow(ports, {})).rejects.toBeInstanceOf(PolicyDeniedError);
+  });
+
+  it('throws PolicyDeniedError when OPA times out (never implicit-allow)', async () => {
+    const ports = makeFakePorts();
+    ports.opa.timeout();
+    await expect(decideOrThrow(ports, {})).rejects.toBeInstanceOf(PolicyDeniedError);
+  });
+});
diff --git a/packages/iios-service/src/platform/fail-closed.ts b/packages/iios-service/src/platform/fail-closed.ts
new file mode 100644
index 0000000..77c88cd
--- /dev/null
+++ b/packages/iios-service/src/platform/fail-closed.ts
@@ -0,0 +1,21 @@
+import { IiosPlatformPorts, PolicyDecision, PolicyDeniedError } from '@insignia/iios-contracts';
+
+/**
+ * Fail-closed authorization (Bottom-Up hard gate; Critics KG-03).
+ *
+ * Calls the OPA port. The kernel proceeds ONLY on an explicit allow. A denial,
+ * a thrown error, or a timeout all become a PolicyDeniedError — never an
+ * implicit allow. Callers must let this propagate so nothing is written.
+ */
+export async function decideOrThrow(ports: IiosPlatformPorts, input: unknown): Promise<PolicyDecision> {
+  let decision: PolicyDecision;
+  try {
+    decision = await ports.opa.decide(input);
+  } catch (err) {
+    throw new PolicyDeniedError(`policy port unavailable: ${(err as Error).message}`);
+  }
+  if (!decision.allow) {
+    throw new PolicyDeniedError(decision.obligations[0]?.reason ?? 'policy denied');
+  }
+  return decision;
+}
diff --git a/packages/iios-service/src/platform/platform-ports.ts b/packages/iios-service/src/platform/platform-ports.ts
new file mode 100644
index 0000000..d8ff652
--- /dev/null
+++ b/packages/iios-service/src/platform/platform-ports.ts
@@ -0,0 +1,55 @@
+import type {
+  IiosPlatformPorts,
+  PlatformPrincipal,
+  PolicyDecision,
+  ContextDecisionBundle,
+  SourceHandleResolution,
+} from '@insignia/iios-contracts';
+
+/** DI token for the platform ports (Session/OPA/CMP/MDM/CRRE/SAS/Capability). */
+export const PLATFORM_PORTS = Symbol('PLATFORM_PORTS');
+
+/**
+ * P1 default binding: a permissive, in-service implementation so `nest start`
+ * runs without the real platform fabric (which other teams own) and without a
+ * runtime dependency on the testkit. Tests inject @insignia/iios-testkit fakes
+ * to exercise deny / timeout / ambiguous paths. Real adapters replace this from
+ * P5 onward.
+ */
+export class LocalDevPorts implements IiosPlatformPorts {
+  session = {
+    async verify(_token: string): Promise<PlatformPrincipal> {
+      return { principalRef: 'local-dev', orgId: 'org_demo', appId: 'portal-demo', assurance: 'service' };
+    },
+  };
+  opa = {
+    async decide(_input: unknown): Promise<PolicyDecision> {
+      return { decisionRef: 'local-allow', allow: true, obligations: [], ttlSeconds: 60 };
+    },
+  };
+  cmp = {
+    async checkPurpose(_input: unknown): Promise<{ receiptRef?: string; status: 'ALLOW' | 'DENY' | 'NOT_REQUIRED' }> {
+      return { status: 'NOT_REQUIRED' };
+    },
+  };
+  mdm = {
+    async resolveSourceHandle(_input: unknown): Promise<SourceHandleResolution> {
+      return { confidence: 0, status: 'UNRESOLVED' };
+    },
+  };
+  crre = {
+    async resolve(_input: unknown): Promise<ContextDecisionBundle> {
+      return { bundleRef: 'local', scopeHash: 'local', resourceSet: [], constraints: [] };
+    },
+  };
+  sas = {
+    async tokenizeParts(parts: unknown[]): Promise<unknown[]> {
+      return parts;
+    },
+  };
+  capability = {
+    async invoke(_input: unknown): Promise<{ providerRef: string; outcome: string }> {
+      return { providerRef: 'local', outcome: 'SENT' };
+    },
+  };
+}
diff --git a/packages/iios-service/src/platform/platform.module.ts b/packages/iios-service/src/platform/platform.module.ts
new file mode 100644
index 0000000..c69c2a1
--- /dev/null
+++ b/packages/iios-service/src/platform/platform.module.ts
@@ -0,0 +1,10 @@
+import { Global, Module } from '@nestjs/common';
+import { PLATFORM_PORTS, LocalDevPorts } from './platform-ports';
+
+/** Binds the platform ports (P1: the permissive in-service default). */
+@Global()
+@Module({
+  providers: [{ provide: PLATFORM_PORTS, useClass: LocalDevPorts }],
+  exports: [PLATFORM_PORTS],
+})
+export class PlatformModule {}