diff --git a/.server-changes/v3-engine-retirement-messaging.md b/.server-changes/v3-engine-retirement-messaging.md
new file mode 100644
index 0000000000..ca7aceb472
--- /dev/null
+++ b/.server-changes/v3-engine-retirement-messaging.md
@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+When the v3 engine is retired, triggering a v3 task and connecting the v3 dev CLI now fail with a clear message pointing to the v4 migration guide instead of failing opaquely. Enforcement is off by default, so self-hosted instances still running v3 are unaffected until they migrate.
diff --git a/apps/webapp/app/env.server.ts b/apps/webapp/app/env.server.ts
index f793654dd3..c04a52cff2 100644
--- a/apps/webapp/app/env.server.ts
+++ b/apps/webapp/app/env.server.ts
@@ -535,6 +535,16 @@ const EnvironmentSchema = z
     // log-only mode before enforcement.
     DEPRECATE_V3_CLI_DEPLOYS_ENABLED: z.string().default("0"),
 
+    // Master switch for the v3 engine (RunEngineVersion.V1) shutdown. When
+    // enabled it: rejects triggers that resolve to V1 (single, batch, schedule,
+    // replay, triggerAndWait) with a graceful error pointing at the v4 migration
+    // guide; closes the legacy `trigger dev` websocket used by v3 CLIs; and turns
+    // the V1 run-lifecycle background jobs (heartbeat timeout, TTL expiry, retry,
+    // resume, scheduled fires) into no-ops so abandoned V1 runs stop generating
+    // database load. v4 (V2) is never affected (every gate also checks the run is
+    // V1). Defaults to off so self-hosted instances still on V1 keep working.
+    DEPRECATE_V3_ENABLED: z.string().default("0"),
+
     OBJECT_STORE_BASE_URL: z.string().optional(),
     OBJECT_STORE_BUCKET: z.string().optional(),
     OBJECT_STORE_ACCESS_KEY_ID: z.string().optional(),
diff --git a/apps/webapp/app/v3/engineDeprecation.server.ts b/apps/webapp/app/v3/engineDeprecation.server.ts
new file mode 100644
index 0000000000..7e7dad9654
--- /dev/null
+++ b/apps/webapp/app/v3/engineDeprecation.server.ts
@@ -0,0 +1,34 @@
+import { env } from "~/env.server";
+
+/**
+ * Graceful sunset of the v3 engine (RunEngineVersion.V1).
+ *
+ * v3 maps to engine V1 (MarQS + Graphile); v4 is engine V2 (run-engine). A
+ * single master flag (DEPRECATE_V3_ENABLED, default off) gates every shutdown
+ * behaviour so the cloud can flip the switch while self-hosted instances still
+ * on V1 keep working until they migrate. This mirrors
+ * DEPRECATE_V3_CLI_DEPLOYS_ENABLED, which already gates deploys.
+ *
+ * The flag controls three surfaces:
+ *   1. Triggers that resolve to V1 are rejected with a graceful error.
+ *   2. The legacy `trigger dev` websocket (v3 CLIs only) is closed.
+ *   3. V1 run-lifecycle background jobs become no-ops to shed database load.
+ *
+ * Every call site also checks the run/project is actually V1, so v4 (V2) is
+ * never affected.
+ */
+
+export const V3_MIGRATION_URL = "https://trigger.dev/docs/migrating-from-v3";
+
+export const V3_TRIGGER_DEPRECATION_MESSAGE = `Trigger.dev v3 is no longer supported. Please upgrade your project to v4 to keep triggering tasks: ${V3_MIGRATION_URL}`;
+
+// Sent as a websocket close reason, which is capped at 123 bytes, so keep it short.
+export const V3_DEV_DEPRECATION_MESSAGE = `Trigger.dev v3 is no longer supported. Upgrade to v4: ${V3_MIGRATION_URL}`;
+
+/**
+ * Whether the v3 (engine V1) shutdown is being enforced. Guard every V1-only
+ * code path with `isV3Disabled() && <run/project is V1>` so v4 is untouched.
+ */
+export function isV3Disabled(): boolean {
+  return env.DEPRECATE_V3_ENABLED === "1";
+}
diff --git a/apps/webapp/app/v3/handleSocketIo.server.ts b/apps/webapp/app/v3/handleSocketIo.server.ts
index aec9ded6f4..2e7fd211bb 100644
--- a/apps/webapp/app/v3/handleSocketIo.server.ts
+++ b/apps/webapp/app/v3/handleSocketIo.server.ts
@@ -36,6 +36,7 @@ import { ResumeAttemptService } from "./services/resumeAttempt.server";
 import { UpdateFatalRunErrorService } from "./services/updateFatalRunError.server";
 import { WorkerGroupTokenService } from "./services/worker/workerGroupTokenService.server";
 import { SharedSocketConnection } from "./sharedSocketConnection";
+import { isV3Disabled } from "./engineDeprecation.server";
 
 export const socketIo = singleton("socketIo", initalizeIoServer);
 
@@ -426,6 +427,16 @@ function createSharedQueueConsumerNamespace(io: Server) {
     clientMessages: ClientToSharedQueueMessages,
     serverMessages: SharedQueueToClientMessages,
     onConnection: async (socket, handler, sender, logger) => {
+      // v3 (engine V1) shutdown: don't start the MarQS shared-queue consumer, so no
+      // deployed V1 runs are dequeued. This namespace is V1-only; v4 dequeues through
+      // the run-engine worker path. This is the code-level equivalent of taking the
+      // v3 coordinator offline.
+      if (isV3Disabled()) {
+        logger.warn("Refusing /shared-queue connection: v3 engine is shut down");
+        socket.disconnect(true);
+        return;
+      }
+
       const sharedSocketConnection = new SharedSocketConnection({
         // @ts-ignore - for some reason the built ZodNamespace Server type is not compatible with the Server type here, but only when doing typechecking
         namespace: sharedQueue.namespace,
diff --git a/apps/webapp/app/v3/handleWebsockets.server.ts b/apps/webapp/app/v3/handleWebsockets.server.ts
index 2e3b3c05ec..ee7c97d079 100644
--- a/apps/webapp/app/v3/handleWebsockets.server.ts
+++ b/apps/webapp/app/v3/handleWebsockets.server.ts
@@ -6,6 +6,7 @@ import { singleton } from "../utils/singleton";
 import { AuthenticatedSocketConnection } from "./authenticatedSocketConnection.server";
 import { Gauge } from "prom-client";
 import { metricsRegister } from "~/metrics.server";
+import { isV3Disabled, V3_DEV_DEPRECATION_MESSAGE } from "./engineDeprecation.server";
 
 export const wss = singleton("wss", initalizeWebSocketServer);
 
@@ -58,6 +59,19 @@ async function handleWebSocketConnection(ws: WebSocket, req: IncomingMessage) {
 
   const authenticatedEnv = authenticationResult.environment;
 
+  // This legacy websocket is only used by the v3 `trigger dev` CLI (v4 uses a
+  // different dev transport). When the v3 shutdown is on, close it with a
+  // graceful reason instead of letting the CLI sit connected with no work.
+  if (isV3Disabled()) {
+    logger.warn("Rejected deprecated v3 dev CLI websocket connection", {
+      environmentId: authenticatedEnv.id,
+      projectId: authenticatedEnv.projectId,
+      organizationId: authenticatedEnv.organizationId,
+    });
+    ws.close(1008, V3_DEV_DEPRECATION_MESSAGE);
+    return;
+  }
+
   const authenticatedConnection = new AuthenticatedSocketConnection(
     ws,
     authenticatedEnv,
diff --git a/apps/webapp/app/v3/scheduleEngine.server.ts b/apps/webapp/app/v3/scheduleEngine.server.ts
index 4a12046a89..1c0040fa11 100644
--- a/apps/webapp/app/v3/scheduleEngine.server.ts
+++ b/apps/webapp/app/v3/scheduleEngine.server.ts
@@ -10,6 +10,7 @@ import { TriggerTaskService } from "./services/triggerTask.server";
 import { meter, tracer } from "./tracer.server";
 import { workerQueue } from "~/services/worker.server";
 import { ServiceValidationError } from "./services/common.server";
+import { isV3Disabled } from "./engineDeprecation.server";
 
 export const scheduleEngine = singleton("ScheduleEngine", createScheduleEngine);
 
@@ -84,6 +85,18 @@ function createScheduleEngine() {
       exactScheduleTime,
     }) => {
       try {
+        // v3 (engine V1) shutdown: skip firing schedules for V1 projects so the
+        // cron doesn't keep doing trigger work just to be rejected. Return success
+        // so the schedule engine treats it as handled and doesn't retry. v4 is
+        // unaffected.
+        if (isV3Disabled() && environment.project.engine === "V1") {
+          logger.debug("[ScheduleEngine] Skipping scheduled fire for shut-down v3 project", {
+            taskIdentifier,
+            scheduleId,
+          });
+          return { success: true };
+        }
+
         // This will trigger either v1 or v2 depending on the engine of the project
         const triggerService = new TriggerTaskService();
 
diff --git a/apps/webapp/app/v3/services/enqueueDelayedRun.server.ts b/apps/webapp/app/v3/services/enqueueDelayedRun.server.ts
index 79cb4fb097..2e9d86916c 100644
--- a/apps/webapp/app/v3/services/enqueueDelayedRun.server.ts
+++ b/apps/webapp/app/v3/services/enqueueDelayedRun.server.ts
@@ -5,6 +5,7 @@ import { commonWorker } from "../commonWorker.server";
 import { BaseService } from "./baseService.server";
 import { enqueueRun } from "./enqueueRun.server";
 import { ExpireEnqueuedRunService } from "./expireEnqueuedRun.server";
+import { isV3Disabled } from "../engineDeprecation.server";
 
 export class EnqueueDelayedRunService extends BaseService {
   public static async enqueue(runId: string, runAt?: Date) {
@@ -75,6 +76,12 @@ export class EnqueueDelayedRunService extends BaseService {
       return;
     }
 
+    // v3 (engine V1) shutdown: don't enqueue delayed V1 runs into MarQS. v4 is unaffected.
+    if (isV3Disabled() && run.engine === "V1") {
+      logger.debug("[EnqueueDelayedRunService] Skipping enqueue for shut-down v3 run", { runId });
+      return;
+    }
+
     if (run.status !== "DELAYED") {
       logger.debug("Delayed run cannot be enqueued because it's not in DELAYED status", {
         run,
diff --git a/apps/webapp/app/v3/services/expireEnqueuedRun.server.ts b/apps/webapp/app/v3/services/expireEnqueuedRun.server.ts
index 12ccddbf2e..27a8e1e8b0 100644
--- a/apps/webapp/app/v3/services/expireEnqueuedRun.server.ts
+++ b/apps/webapp/app/v3/services/expireEnqueuedRun.server.ts
@@ -5,6 +5,7 @@ import { BaseService } from "./baseService.server";
 import { FinalizeTaskRunService } from "./finalizeTaskRun.server";
 import { tryCatch } from "@trigger.dev/core/utils";
 import { getEventRepositoryForStore } from "../eventRepository/index.server";
+import { isV3Disabled } from "../engineDeprecation.server";
 
 export class ExpireEnqueuedRunService extends BaseService {
   public static async ack(runId: string, tx?: PrismaClientOrTransaction) {
@@ -48,6 +49,12 @@ export class ExpireEnqueuedRunService extends BaseService {
       return;
     }
 
+    // v3 (engine V1) shutdown: skip expiring abandoned V1 runs. v4 is unaffected.
+    if (isV3Disabled() && run.engine === "V1") {
+      logger.debug("[ExpireEnqueuedRunService] Skipping expiry for shut-down v3 run", { runId });
+      return;
+    }
+
     if (run.status !== "PENDING") {
       logger.debug("Run cannot be expired because it's not in PENDING status", {
         run,
diff --git a/apps/webapp/app/v3/services/resumeBatchRun.server.ts b/apps/webapp/app/v3/services/resumeBatchRun.server.ts
index d9f5170fc2..a886d12d60 100644
--- a/apps/webapp/app/v3/services/resumeBatchRun.server.ts
+++ b/apps/webapp/app/v3/services/resumeBatchRun.server.ts
@@ -5,6 +5,7 @@ import { BaseService } from "./baseService.server";
 import { logger } from "~/services/logger.server";
 import { BatchTaskRun } from "@trigger.dev/database";
 import { workerQueue } from "~/services/worker.server";
+import { isV3Disabled } from "../engineDeprecation.server";
 
 const finishedBatchRunStatuses = ["COMPLETED", "FAILED", "CANCELED"];
 
@@ -43,6 +44,14 @@ export class ResumeBatchRunService extends BaseService {
       return "ERROR";
     }
 
+    // v3 (engine V1) shutdown: don't resume batches for abandoned V1 projects. v4 is unaffected.
+    if (isV3Disabled() && batchRun.runtimeEnvironment.project.engine === "V1") {
+      logger.debug("[ResumeBatchRunService] Skipping resume for shut-down v3 batch", {
+        batchRunId,
+      });
+      return "ERROR";
+    }
+
     if (batchRun.batchVersion === "v3") {
       return await this.#handleV3BatchRun(batchRun);
     } else {
diff --git a/apps/webapp/app/v3/services/resumeTaskDependency.server.ts b/apps/webapp/app/v3/services/resumeTaskDependency.server.ts
index a831fdd78c..70f6e1e3cd 100644
--- a/apps/webapp/app/v3/services/resumeTaskDependency.server.ts
+++ b/apps/webapp/app/v3/services/resumeTaskDependency.server.ts
@@ -3,6 +3,7 @@ import { logger } from "~/services/logger.server";
 import { marqs } from "~/v3/marqs/index.server";
 import { commonWorker } from "../commonWorker.server";
 import { BaseService } from "./baseService.server";
+import { isV3Disabled } from "../engineDeprecation.server";
 
 export class ResumeTaskDependencyService extends BaseService {
   public async call(dependencyId: string, sourceTaskAttemptId: string) {
@@ -32,6 +33,14 @@ export class ResumeTaskDependencyService extends BaseService {
       return;
     }
 
+    // v3 (engine V1) shutdown: don't resume dependencies for abandoned V1 runs. v4 is unaffected.
+    if (isV3Disabled() && dependency.taskRun.engine === "V1") {
+      logger.debug("[ResumeTaskDependencyService] Skipping resume for shut-down v3 run", {
+        dependencyId,
+      });
+      return;
+    }
+
     if (dependency.taskRun.runtimeEnvironment.type === "DEVELOPMENT") {
       return;
     }
diff --git a/apps/webapp/app/v3/services/retryAttempt.server.ts b/apps/webapp/app/v3/services/retryAttempt.server.ts
index 6ed83c1080..dd1f8bf908 100644
--- a/apps/webapp/app/v3/services/retryAttempt.server.ts
+++ b/apps/webapp/app/v3/services/retryAttempt.server.ts
@@ -2,6 +2,7 @@ import { logger } from "~/services/logger.server";
 import { commonWorker } from "../commonWorker.server";
 import { socketIo } from "../handleSocketIo.server";
 import { BaseService } from "./baseService.server";
+import { isV3Disabled } from "../engineDeprecation.server";
 
 export class RetryAttemptService extends BaseService {
   public async call(runId: string) {
@@ -12,6 +13,12 @@ export class RetryAttemptService extends BaseService {
       return;
     }
 
+    // v3 (engine V1) shutdown: don't retry abandoned V1 runs. v4 is unaffected.
+    if (isV3Disabled() && taskRun.engine === "V1") {
+      logger.debug("[RetryAttemptService] Skipping retry for shut-down v3 run", { runId });
+      return;
+    }
+
     socketIo.coordinatorNamespace.emit("READY_FOR_RETRY", {
       version: "v1",
       runId,
diff --git a/apps/webapp/app/v3/services/triggerTask.server.ts b/apps/webapp/app/v3/services/triggerTask.server.ts
index 7bbaa0dd99..c68171bb34 100644
--- a/apps/webapp/app/v3/services/triggerTask.server.ts
+++ b/apps/webapp/app/v3/services/triggerTask.server.ts
@@ -10,7 +10,8 @@ import { DefaultTriggerTaskValidator } from "~/runEngine/validators/triggerTaskV
 import { AuthenticatedEnvironment } from "~/services/apiAuth.server";
 import { determineEngineVersion } from "../engineVersion.server";
 import { tracer } from "../tracer.server";
-import { WithRunEngine } from "./baseService.server";
+import { isV3Disabled, V3_TRIGGER_DEPRECATION_MESSAGE } from "../engineDeprecation.server";
+import { ServiceValidationError, WithRunEngine } from "./baseService.server";
 import { TriggerTaskServiceV1 } from "./triggerTaskV1.server";
 
 export type TriggerTaskServiceOptions = {
@@ -77,6 +78,14 @@ export class TriggerTaskService extends WithRunEngine {
 
       switch (v) {
         case "V1": {
+          // v3 (engine V1) is being sunset. When the shutdown is on, reject the
+          // trigger with a graceful, actionable error instead of creating a V1
+          // run. Covers single, batch, schedule, replay, and triggerAndWait,
+          // which all route through here.
+          if (isV3Disabled()) {
+            throw new ServiceValidationError(V3_TRIGGER_DEPRECATION_MESSAGE);
+          }
+
           return await this.callV1(taskId, environment, body, options);
         }
         case "V2": {
diff --git a/apps/webapp/app/v3/taskRunHeartbeatFailed.server.ts b/apps/webapp/app/v3/taskRunHeartbeatFailed.server.ts
index c472bff53e..c211b191ea 100644
--- a/apps/webapp/app/v3/taskRunHeartbeatFailed.server.ts
+++ b/apps/webapp/app/v3/taskRunHeartbeatFailed.server.ts
@@ -8,6 +8,7 @@ import { PrismaClientOrTransaction } from "~/db.server";
 import { workerQueue } from "~/services/worker.server";
 import { socketIo } from "./handleSocketIo.server";
 import { TaskRunErrorCodes } from "@trigger.dev/core/v3";
+import { isV3Disabled } from "./engineDeprecation.server";
 
 export class TaskRunHeartbeatFailedService extends BaseService {
   public async call(runId: string) {
@@ -18,6 +19,7 @@ export class TaskRunHeartbeatFailedService extends BaseService {
       {
         select: {
           id: true,
+          engine: true,
           friendlyId: true,
           status: true,
           lockedAt: true,
@@ -49,6 +51,15 @@ export class TaskRunHeartbeatFailedService extends BaseService {
       return;
     }
 
+    // v3 (engine V1) shutdown: leave abandoned V1 runs as-is instead of doing
+    // MarQS/DB work to fail or requeue them. v4 (V2) is unaffected.
+    if (isV3Disabled() && taskRun.engine === "V1") {
+      logger.debug("[TaskRunHeartbeatFailedService] Skipping heartbeat for shut-down v3 run", {
+        runId,
+      });
+      return;
+    }
+
     const service = new FailedTaskRunService();
 
     switch (taskRun.status) {