diff --git a/.server-changes/deprecate-v3-cli-deploys.md b/.server-changes/deprecate-v3-cli-deploys.md
new file mode 100644
index 0000000000..72040b4c5e
--- /dev/null
+++ b/.server-changes/deprecate-v3-cli-deploys.md
@@ -0,0 +1,6 @@
+---
+area: webapp
+type: breaking
+---
+
+Add server-side deprecation gate for deploys from v3 CLI versions (gated by `DEPRECATE_V3_CLI_DEPLOYS_ENABLED`). v4 CLI deploys are unaffected.
diff --git a/apps/webapp/app/env.server.ts b/apps/webapp/app/env.server.ts
index 8d72b2e51b..b839af6eb5 100644
--- a/apps/webapp/app/env.server.ts
+++ b/apps/webapp/app/env.server.ts
@@ -348,6 +348,12 @@ const EnvironmentSchema = z
       .int()
       .default(60 * 1000 * 15), // 15 minutes
 
+    // When enabled, reject deploys made by v3 CLI versions (i.e. payloads that
+    // omit the `type` field). v4 CLI versions always send `type` ("MANAGED" or "V1"),
+    // so they are unaffected. Defaults to off so detection can run in
+    // log-only mode before enforcement.
+    DEPRECATE_V3_CLI_DEPLOYS_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/services/initializeDeployment.server.ts b/apps/webapp/app/v3/services/initializeDeployment.server.ts
index 987925aa70..9ecc25f994 100644
--- a/apps/webapp/app/v3/services/initializeDeployment.server.ts
+++ b/apps/webapp/app/v3/services/initializeDeployment.server.ts
@@ -59,6 +59,28 @@ export class InitializeDeploymentService extends BaseService {
         };
       }
 
+      // v4 CLI versions always send `payload.type` ("MANAGED" or "V1"). v3 CLI
+      // versions never do, so the absence of `type` is a reliable signal that
+      // the request came from a 3.x CLI. Detection always runs (so we can
+      // observe how many deploys are still using v3), enforcement is gated
+      // behind DEPRECATE_V3_CLI_DEPLOYS_ENABLED so it can be rolled out safely.
+      if (!payload.type) {
+        const enforced = env.DEPRECATE_V3_CLI_DEPLOYS_ENABLED === "1";
+
+        logger.warn("Detected deploy from deprecated v3 CLI", {
+          environmentId: environment.id,
+          projectId: environment.projectId,
+          organizationId: environment.project.organizationId,
+          enforced,
+        });
+
+        if (enforced) {
+          throw new ServiceValidationError(
+            "The trigger.dev CLI v3 is no longer supported for deployments. Please upgrade your project to v4: https://trigger.dev/docs/migrating-from-v3"
+          );
+        }
+      }
+
       if (payload.type === "UNMANAGED") {
         throw new ServiceValidationError("UNMANAGED deployments are not supported");
       }