diff --git a/src/routes/api/spark/run/+server.ts b/src/routes/api/spark/run/+server.ts
index e9a11c23..dbf1e07b 100644
--- a/src/routes/api/spark/run/+server.ts
+++ b/src/routes/api/spark/run/+server.ts
@@ -53,6 +53,25 @@ interface SparkRunBody {
 	};
 }
 
+export function _createOrderedMissionRelayQueue<T>(
+	deliver: (event: T) => Promise<void>,
+	onError: () => void = () => console.warn('[SparkRun] Mission relay delivery failed.')
+) {
+	let tail = Promise.resolve();
+	return {
+		enqueue(event: T) {
+			tail = tail.then(() => deliver(event)).catch(() => onError());
+		},
+		async flush() {
+			for (;;) {
+				const observedTail = tail;
+				await observedTail;
+				if (tail === observedTail) return;
+			}
+		}
+	};
+}
+
 export const GET: RequestHandler = async (event) => {
 	const unauthorized = requireControlAuth(event, {
 		surface: 'SparkRunHealth',
@@ -218,6 +237,7 @@ export const POST: RequestHandler = async (event) => {
 		windowMs: 60_000
 	});
 	if (rateLimited) return rateLimited;
+	const relayQueue = _createOrderedMissionRelayQueue(relayMissionControlEvent);
 
 	try {
 		const body = (await event.request.json().catch(() => ({}))) as SparkRunBody;
@@ -309,7 +329,7 @@ export const POST: RequestHandler = async (event) => {
 				}
 			};
 			eventBridge.emit(bridgeEvent);
-			void relayMissionControlEvent(bridgeEvent);
+			relayQueue.enqueue(bridgeEvent);
 		};
 
 		emitMissionEvent('mission_created', `Mission created (${mission.id}).`);
@@ -381,7 +401,7 @@ export const POST: RequestHandler = async (event) => {
 					}
 				};
 				eventBridge.emit(relayEvent);
-				void relayMissionControlEvent(relayEvent);
+				relayQueue.enqueue(relayEvent);
 				if (bridgeEvent.type === 'dispatch_started' && !missionStartedEmitted) {
 					missionStartedEmitted = true;
 					emitMissionEvent('mission_started', `Mission started (${mission.id}).`, {
@@ -391,6 +411,7 @@ export const POST: RequestHandler = async (event) => {
 				}
 			}
 		});
+		await relayQueue.flush();
 
 		return json({
 			success: true,
@@ -406,6 +427,7 @@ export const POST: RequestHandler = async (event) => {
 			audit: capability
 		});
 	} catch (error) {
+		await relayQueue.flush();
 		if (error instanceof HarnessAuthorityError) {
 			return json({ success: false, error: error.message, code: error.code, authority: error.verdict }, { status: error.status });
 		}
diff --git a/src/routes/api/spark/run/spark-run.integration.test.ts b/src/routes/api/spark/run/spark-run.integration.test.ts
index 3194e05d..5ad7ff6c 100644
--- a/src/routes/api/spark/run/spark-run.integration.test.ts
+++ b/src/routes/api/spark/run/spark-run.integration.test.ts
@@ -23,7 +23,7 @@ vi.mock('$lib/server/provider-runtime', () => ({
 	}
 }));
 
-import { GET, POST } from './+server';
+import { _createOrderedMissionRelayQueue, GET, POST } from './+server';
 import { providerRuntime } from '$lib/server/provider-runtime';
 import { eventBridge } from '$lib/services/event-bridge';
 import { getMissionControlPersistPath, getMissionControlRelaySnapshot } from '$lib/server/mission-control-relay';
@@ -348,6 +348,84 @@ describe('/api/spark/run integration', () => {
 		}
 	});
 
+	it('serializes relay delivery and waits for a slow first event before flush resolves', async () => {
+		let releaseFirst!: () => void;
+		const firstGate = new Promise<void>((resolve) => {
+			releaseFirst = resolve;
+		});
+		const started: string[] = [];
+		const completed: string[] = [];
+		const queue = _createOrderedMissionRelayQueue(async (event: { type: string }) => {
+			started.push(event.type);
+			if (event.type === 'mission_created') await firstGate;
+			completed.push(event.type);
+		});
+
+		queue.enqueue({ type: 'mission_created' });
+		queue.enqueue({ type: 'dispatch_started' });
+		queue.enqueue({ type: 'mission_started' });
+		queue.enqueue({ type: 'task_failed' });
+		queue.enqueue({ type: 'mission_failed' });
+		let flushSettled = false;
+		const flush = queue.flush().then(() => {
+			flushSettled = true;
+		});
+		await Promise.resolve();
+
+		expect(started).toEqual(['mission_created']);
+		expect(completed).toEqual([]);
+		expect(flushSettled).toBe(false);
+
+		releaseFirst();
+		await flush;
+		expect(started).toEqual([
+			'mission_created', 'dispatch_started', 'mission_started', 'task_failed', 'mission_failed'
+		]);
+		expect(completed).toEqual(started);
+	});
+
+	it('keeps flush open for relay events appended while an earlier delivery is draining', async () => {
+		let releaseFirst!: () => void;
+		let releaseSecond!: () => void;
+		let markSecondStarted!: () => void;
+		const firstGate = new Promise<void>((resolve) => {
+			releaseFirst = resolve;
+		});
+		const secondGate = new Promise<void>((resolve) => {
+			releaseSecond = resolve;
+		});
+		const secondStarted = new Promise<void>((resolve) => {
+			markSecondStarted = resolve;
+		});
+		const completed: string[] = [];
+		const queue = _createOrderedMissionRelayQueue(async (event: { type: string }) => {
+			if (event.type === 'mission_created') await firstGate;
+			if (event.type === 'mission_failed') {
+				markSecondStarted();
+				await secondGate;
+			}
+			completed.push(event.type);
+		});
+
+		queue.enqueue({ type: 'mission_created' });
+		let flushSettled = false;
+		const flush = queue.flush().then(() => {
+			flushSettled = true;
+		});
+		await Promise.resolve();
+		queue.enqueue({ type: 'mission_failed' });
+		releaseFirst();
+		await secondStarted;
+
+		expect(completed).toEqual(['mission_created']);
+		expect(flushSettled).toBe(false);
+
+		releaseSecond();
+		await flush;
+		expect(completed).toEqual(['mission_created', 'mission_failed']);
+		expect(flushSettled).toBe(true);
+	});
+
 	it('derives one mission_started when provider runtime repeats dispatch_started', async () => {
 		const dispatch = vi.mocked(providerRuntime.dispatch);
 		const emitted: Array<{ type?: string; missionId?: string }> = [];