docs(rivetkit): raise serverless start payload limit

Author: NathanFlurry

examples/kitchen-sink/package.json

@@ -12,6 +12,8 @@ use crate::error::EngineProcessError;
 
 const ENGINE_RUNTIME: &str = "engine";
 const RIVETKIT_RUNTIME: &str = "rivetkit";
+const DEFAULT_MALLOC_ARENA_MAX: &str = "2";
+const DEFAULT_MALLOC_TRIM_THRESHOLD: &str = "131072";
 
 #[derive(Debug, Deserialize)]
 struct EngineHealthResponse {
@@ -110,6 +112,7 @@ impl EngineProcessManager {
 			.stdin(Stdio::null())
 			.stdout(Stdio::from(stdout_file))
 			.stderr(Stdio::from(stderr_file));
+		set_default_allocator_env(&mut command);
 
 		// Put the engine in its own process group so terminal signals
 		// (Ctrl+C, Ctrl+Z, SIGHUP on terminal close) targeting our foreground
@@ -176,6 +179,15 @@ impl EngineProcessManager {
 	}
 }
 
+fn set_default_allocator_env(command: &mut Command) {
+	if std::env::var_os("MALLOC_ARENA_MAX").is_none() {
+		command.env("MALLOC_ARENA_MAX", DEFAULT_MALLOC_ARENA_MAX);
+	}
+	if std::env::var_os("MALLOC_TRIM_THRESHOLD_").is_none() {
+		command.env("MALLOC_TRIM_THRESHOLD_", DEFAULT_MALLOC_TRIM_THRESHOLD);
+	}
+}
+
 impl Drop for EngineProcessManager {
 	fn drop(&mut self) {
 		if let Some(handle) = self.watcher.take() {

examples/kitchen-sink/src/index.ts

@@ -25,7 +25,11 @@ import {
 	isDev,
 } from "@/utils/env-vars";
 import { EnvoyConfigSchema } from "./envoy";
-import { ConfigurePoolSchema, ServerlessConfigSchema } from "./serverless";
+import {
+	ConfigurePoolSchema,
+	DEFAULT_SERVERLESS_MAX_START_PAYLOAD_BYTES,
+	ServerlessConfigSchema,
+} from "./serverless";
 
 export const ActorsSchema = z.record(
 	z.string(),
@@ -505,7 +509,7 @@ export const DocServerlessConfigSchema = z
 			.number()
 			.optional()
 			.describe(
-				"Maximum POST /start body size in bytes. Default: 1048576",
+				`Maximum POST /start body size in bytes. Default: ${DEFAULT_SERVERLESS_MAX_START_PAYLOAD_BYTES}`,
 			),
 		publicEndpoint: z
 			.string()

rivetkit-rust/packages/rivetkit-core/src/engine_process.rs

@@ -1,6 +1,8 @@
 import { z } from "zod/v4";
 import { getRivetPublicEndpoint, getRivetPublicToken } from "@/utils/env-vars";
 
+export const DEFAULT_SERVERLESS_MAX_START_PAYLOAD_BYTES = 16 * 1024 * 1024;
+
 export const ConfigurePoolSchema = z
 	.object({
 		name: z.string().optional(),
@@ -17,7 +19,10 @@ export const ConfigurePoolSchema = z
 export const ServerlessConfigSchema = z.object({
 	// MARK: Routing
 	basePath: z.string().optional().default("/api/rivet"),
-	maxStartPayloadBytes: z.number().optional().default(1_048_576),
+	maxStartPayloadBytes: z
+		.number()
+		.optional()
+		.default(DEFAULT_SERVERLESS_MAX_START_PAYLOAD_BYTES),
 
 	// MARK: Public Endpoint Configuration
 	/**

rivetkit-typescript/packages/rivetkit/src/registry/config/index.ts

@@ -93,7 +93,7 @@ These limits apply to the [SQLite database](/docs/actors/state#sqlite-database)
 
 ### KV Preloading
 
-When an actor starts, the engine can pre-fetch KV data declared in the actor name metadata and deliver it alongside the start command. This removes round-trips to storage during actor startup. RivetKit emits the preload manifest from its own key layout and exposes per-actor overrides via `options`. Operators can still enforce a global cap in the [engine config](/docs/self-hosting/configuration) with `pegboard.preload_max_total_bytes`.
+When an actor starts, the engine can pre-fetch KV data declared in the actor name metadata and deliver it alongside the start command. This removes round-trips to storage during actor startup. RivetKit emits the preload manifest from its own key layout and exposes per-actor overrides via `options`. Operators can still enforce a global cap in the [engine config](/docs/self-hosting/configuration) with `pegboard.preload_max_total_bytes`. In serverless mode, this data is serialized into the `/api/rivet/start` request body along with actor config and protocol metadata, so the accepted body size must be larger than the preload budget. RivetKit defaults `serverless.maxStartPayloadBytes` to 16 MiB to leave margin for the default 1 MiB preload budget and larger SQLite startup page preloads.
 
 | Name | Soft Limit | Hard Limit | Description |
 |------|------------|------------|-------------|

rivetkit-typescript/packages/rivetkit/src/registry/config/serverless.ts

@@ -21,6 +21,7 @@ We recommend passing this page to your coding agent to verify your configuration
 ### Serverless
 
 - **Check platform timeouts.** Rivet handles migration between invocations automatically, but shorter timeouts increase migration frequency. See [Timeouts](/docs/general/runtime-modes#timeouts).
+- **Verify `/api/rivet/start` body size limits.** Serverless actor starts carry actor config and preloaded KV or SQLite startup data in the request body. Keep `serverless.maxStartPayloadBytes` and your platform or proxy body limit at **16 MiB or higher**, or lower the preload budget if your platform cannot accept that size. See [Limits](/docs/actors/limits#kv-preloading).
 - **Configure max runners.** Go to Settings > Providers > Edit Provider > Max Runners to set the limit. The default is 100,000 runners. This is effectively your max actor count.
 
 ### Runner