diff --git a/rivetkit-typescript/packages/rivetkit/src/registry/config/index.ts b/rivetkit-typescript/packages/rivetkit/src/registry/config/index.ts
index 4fa5de09fe..00c8f7f449 100644
--- a/rivetkit-typescript/packages/rivetkit/src/registry/config/index.ts
+++ b/rivetkit-typescript/packages/rivetkit/src/registry/config/index.ts
@@ -21,6 +21,7 @@ import {
 	getRivetNamespace,
 	getRivetRunEngine,
 	getRivetRunEngineVersion,
+	getRivetRuntimeMode,
 	getRivetToken,
 	isDev,
 } from "@/utils/env-vars";
@@ -151,6 +152,8 @@ export const RegistryConfigSchema = z
 		 * @experimental
 		 *
 		 * Runtime mode to use when `registry.start()` is called.
+		 *
+		 * Can also be set via RIVET_RUNTIME_MODE.
 		 */
 		mode: RuntimeModeSchema.optional(),
 		/**
@@ -225,11 +228,31 @@ export const RegistryConfigSchema = z
 		const isDevEnv = isDev();
 		const isProductionEnv = getNodeEnv() === "production";
 		const envStartEngine = getRivetRunEngine();
+		const envRuntimeMode =
+			config.mode === undefined ? getRivetRuntimeMode() : undefined;
+		const parsedEnvRuntimeMode =
+			envRuntimeMode === undefined
+				? undefined
+				: RuntimeModeSchema.safeParse(envRuntimeMode);
+		const configuredRuntimeMode =
+			config.mode ??
+			(parsedEnvRuntimeMode?.success
+				? parsedEnvRuntimeMode.data
+				: undefined);
 		const explicitStartEngine =
 			config.startEngine !== undefined || envStartEngine;
 		let startEngine = true;
 		let runtimeMode: RuntimeMode = "envoy";
 
+		if (parsedEnvRuntimeMode && !parsedEnvRuntimeMode.success) {
+			ctx.addIssue({
+				code: "custom",
+				message:
+					"RIVET_RUNTIME_MODE must be either envoy or serverless",
+				path: ["mode"],
+			});
+		}
+
 		// Parse endpoint string (env var fallback is applied via transform above)
 		const parsedEndpoint = config.endpoint
 			? tryParseEndpoint(ctx, {
@@ -250,10 +273,10 @@ export const RegistryConfigSchema = z
 			runtimeMode = "serverless";
 		}
 
-		if (config.mode === "envoy") {
+		if (configuredRuntimeMode === "envoy") {
 			startEngine = false;
 			runtimeMode = "envoy";
-		} else if (config.mode === "serverless") {
+		} else if (configuredRuntimeMode === "serverless") {
 			startEngine = false;
 			runtimeMode = "serverless";
 		}
@@ -580,7 +603,7 @@ export const DocRegistryConfigSchema = z
 			.optional()
 			.describe("Host to bind the local HTTP server to."),
 		mode: RuntimeModeSchema.optional().describe(
-			"Runtime mode for registry.start(). Defaults to 'envoy' for local development and 'serverless' when a Rivet endpoint or production environment is configured.",
+			"Runtime mode for registry.start(). Can also be set via RIVET_RUNTIME_MODE. Defaults to 'envoy' for local development and 'serverless' when a Rivet endpoint or production environment is configured.",
 		),
 		startEngine: z
 			.boolean()
diff --git a/rivetkit-typescript/packages/rivetkit/src/utils/env-vars.ts b/rivetkit-typescript/packages/rivetkit/src/utils/env-vars.ts
index 7a813c7fa5..fc6402bcb8 100644
--- a/rivetkit-typescript/packages/rivetkit/src/utils/env-vars.ts
+++ b/rivetkit-typescript/packages/rivetkit/src/utils/env-vars.ts
@@ -24,6 +24,8 @@ export const getRivetRunEngine = (): boolean =>
 	getEnvUniversal("RIVET_RUN_ENGINE") === "1";
 export const getRivetRunEngineVersion = (): string | undefined =>
 	getEnvUniversal("RIVET_RUN_ENGINE_VERSION");
+export const getRivetRuntimeMode = (): string | undefined =>
+	getEnvUniversal("RIVET_RUNTIME_MODE");
 export const getRivetEnvoyKind = (): string | undefined =>
 	getEnvUniversal("RIVET_ENVOY_KIND");
 export const getRivetEnvoyVersion = (): number | undefined => {
diff --git a/rivetkit-typescript/packages/rivetkit/tests/registry-start-mode.test.ts b/rivetkit-typescript/packages/rivetkit/tests/registry-start-mode.test.ts
index 96154032f8..dfbd46523b 100644
--- a/rivetkit-typescript/packages/rivetkit/tests/registry-start-mode.test.ts
+++ b/rivetkit-typescript/packages/rivetkit/tests/registry-start-mode.test.ts
@@ -8,6 +8,7 @@ const ENV_KEYS = [
 	"RIVET_ENGINE",
 	"RIVET_ENVOY_VERSION",
 	"RIVET_RUN_ENGINE",
+	"RIVET_RUNTIME_MODE",
 ] as const;
 
 type BunGlobal = typeof globalThis & {
@@ -129,6 +130,39 @@ describe("registry start mode config", () => {
 		expect(config.endpoint).toBe("https://api.rivet.dev/");
 	});
 
+	test("RIVET_RUNTIME_MODE overrides endpoint inference", () => {
+		process.env.RIVET_ENDPOINT = "https://api.rivet.dev";
+		process.env.RIVET_RUNTIME_MODE = "envoy";
+
+		const config = parseRegistryConfig();
+
+		expect(config.startEngine).toBe(false);
+		expect(config.runtimeMode).toBe("envoy");
+		expect(config.endpoint).toBe("https://api.rivet.dev/");
+	});
+
+	test("explicit mode overrides RIVET_RUNTIME_MODE", () => {
+		process.env.RIVET_ENDPOINT = "https://api.rivet.dev";
+		process.env.RIVET_RUNTIME_MODE = "local";
+
+		const config = parseRegistryConfig({
+			mode: "envoy",
+		});
+
+		expect(config.startEngine).toBe(false);
+		expect(config.runtimeMode).toBe("envoy");
+		expect(config.endpoint).toBe("https://api.rivet.dev/");
+	});
+
+	test("invalid RIVET_RUNTIME_MODE is rejected", () => {
+		process.env.RIVET_ENDPOINT = "https://api.rivet.dev";
+		process.env.RIVET_RUNTIME_MODE = "local";
+
+		expect(() => parseRegistryConfig()).toThrow(
+			/RIVET_RUNTIME_MODE must be either envoy or serverless/,
+		);
+	});
+
 	test("explicit envoy mode disables local engine startup", () => {
 		const config = parseRegistryConfig({
 			endpoint: "https://api.rivet.dev",
diff --git a/website/src/content/docs/general/environment-variables.mdx b/website/src/content/docs/general/environment-variables.mdx
index ee2d6f651b..a191b58bc2 100644
--- a/website/src/content/docs/general/environment-variables.mdx
+++ b/website/src/content/docs/general/environment-variables.mdx
@@ -34,6 +34,7 @@ These variables configure how clients connect to your actors.
 
 | Environment Variable | Description |
 |---------------------|-------------|
+| `RIVET_RUNTIME_MODE` | Runtime mode for `registry.start()`: `envoy` or `serverless`. |
 | `RIVET_RUN_ENGINE` | Set to `1` to spawn the engine process |
 | `RIVET_RUN_ENGINE_VERSION` | Version of engine to download |