diff --git a/packages/gateway/src/routes/guard.ts b/packages/gateway/src/routes/guard.ts
new file mode 100644
index 0000000..30bf673
--- /dev/null
+++ b/packages/gateway/src/routes/guard.ts
@@ -0,0 +1,136 @@
+/**
+ * MAGATAMA Guard scan endpoint (Layer-3 injection defense as a service).
+ *
+ * Exposes the gateway's prompt-injection defense to trusted internal
+ * clients (Flightdeck, Magatama Core, Switchblade) as a standalone
+ * scan API, independent of the inline completion-pipeline defense:
+ *
+ *   GET  /v1/guard/health → mode + availability
+ *   POST /v1/guard/scan   → { status: clear|review|blocked, findings, judge }
+ *
+ * The endpoint always returns a verdict, even when the inline defense
+ * mode is `off` — callers ask explicitly, so scanning is always wanted.
+ */
+import type { FastifyInstance, FastifyRequest, FastifyReply } from 'fastify';
+import { z } from 'zod';
+import {
+  scanForInjection,
+  decideAction,
+  getInjectionMode,
+  llmJudge,
+  type InjectionMode,
+  type InjectionScanResult,
+} from '../modules/injection-defense.js';
+import { callOllama } from '../pipeline/llm-client.js';
+import { logger } from '../observability/logger.js';
+
+const ScanRequestSchema = z.object({
+  text: z.string().min(1).max(20_000),
+  projectId: z.string().max(200).optional(),
+  source: z.string().max(100).optional().default('external'),
+});
+
+type GuardStatus = 'clear' | 'review' | 'blocked';
+
+interface GuardFinding {
+  severity: 'low' | 'medium' | 'high' | 'critical';
+  kind: string;
+  evidence: string;
+  patternId: string;
+  preview: string;
+}
+
+function toFindings(scan: InjectionScanResult): GuardFinding[] {
+  return scan.matches.map((match) => ({
+    severity: match.severity,
+    kind: match.category,
+    evidence: match.description,
+    patternId: match.id,
+    preview: match.matchPreview,
+  }));
+}
+
+export async function guardRoute(fastify: FastifyInstance): Promise<void> {
+  fastify.get('/health', async (_request: FastifyRequest, reply: FastifyReply) => {
+    return reply.send({
+      status: 'ok',
+      service: 'magatama-guard',
+      layer: 'gateway-injection-defense',
+      mode: getInjectionMode(),
+      judgeModel: process.env['LLM_JUDGE_MODEL'] || 'qwen2.5:3b',
+      time: new Date().toISOString(),
+    });
+  });
+
+  fastify.post('/scan', async (request: FastifyRequest, reply: FastifyReply) => {
+    let body: z.infer<typeof ScanRequestSchema>;
+    try {
+      body = ScanRequestSchema.parse(request.body);
+    } catch (err) {
+      return reply.status(400).send({
+        statusCode: 400,
+        error: 'Bad Request',
+        message: err instanceof z.ZodError ? err.errors[0]?.message : 'Invalid request body',
+      });
+    }
+
+    const startMs = Date.now();
+    const configuredMode = getInjectionMode();
+    // Explicit scan requests always get a verdict, even with inline defense off.
+    const mode: InjectionMode = configuredMode === 'off' ? 'block' : configuredMode;
+    const scan = scanForInjection(body.text);
+    const action = decideAction(mode, scan);
+
+    let judge: Awaited<ReturnType<typeof llmJudge>> | null = null;
+    let status: GuardStatus;
+    if (action === 'block') {
+      status = 'blocked';
+    } else if (action === 'llm_judge') {
+      try {
+        judge = await llmJudge(body.text, {
+          model: process.env['LLM_JUDGE_MODEL'] || 'qwen2.5:3b',
+          callLLM: async (req) => {
+            const resp = await callOllama(
+              {
+                model: req.model,
+                prompt: req.prompt,
+                system: req.system,
+                stream: false,
+                options: { temperature: 0, num_predict: 8, ...(req.options ?? {}) },
+              },
+              'fast',
+            );
+            return { response: resp.response };
+          },
+        });
+        status = judge.verdict === 'injection' ? 'blocked' : judge.verdict === 'uncertain' ? 'review' : 'clear';
+      } catch (err) {
+        logger.warn({ err }, 'guard scan: LLM judge failed, downgrading to review');
+        status = 'review';
+      }
+    } else {
+      // Below the block threshold: medium/high evidence still deserves review.
+      status = scan.matches.some((m) => m.severity === 'medium' || m.severity === 'high') ? 'review' : 'clear';
+    }
+
+    if (status !== 'clear') {
+      logger.info(
+        { source: body.source, projectId: body.projectId, status, score: scan.score, matches: scan.matches.map((m) => m.id) },
+        'guard scan verdict',
+      );
+    }
+
+    return reply.send({
+      source: 'llm-gateway-injection-defense',
+      service: 'magatama-layer3',
+      status,
+      blocked: status === 'blocked',
+      score: scan.score,
+      mode,
+      findings: toFindings(scan),
+      judge,
+      latencyMs: Date.now() - startMs,
+      checkedAt: new Date().toISOString(),
+    });
+  });
+}
diff --git a/packages/gateway/src/server.ts b/packages/gateway/src/server.ts
index 0d5626e..d7c30b7 100644
--- a/packages/gateway/src/server.ts
+++ b/packages/gateway/src/server.ts
@@ -5,6 +5,7 @@ import fastifyHelmet from '@fastify/helmet';
 import { completionRoute } from './routes/completion.js';
 import { batchRoute } from './routes/batch.js';
 import { classifyRoute } from './routes/classify.js';
+import { guardRoute } from './routes/guard.js';
 import { healthRoute } from './routes/health.js';
 import { metricsRoute } from './routes/metrics.js';
 import { reviewRoute } from './routes/review.js';
@@ -141,6 +142,7 @@ async function buildServer() {
   });
   await server.register(batchRoute, { prefix: '/v1' });
   await server.register(classifyRoute, { prefix: '/v1' });
+  await server.register(guardRoute, { prefix: '/v1/guard' });
   await server.register(reviewRoute, { prefix: '/v1' });
   await server.register(learningInsightsRoute, { prefix: '/v1' });
   await server.register(healthRoute);