diff --git a/rivetkit-typescript/packages/rivetkit/src/engine-client/api-endpoints.ts b/rivetkit-typescript/packages/rivetkit/src/engine-client/api-endpoints.ts
index 6ebd541395..40975797a1 100644
--- a/rivetkit-typescript/packages/rivetkit/src/engine-client/api-endpoints.ts
+++ b/rivetkit-typescript/packages/rivetkit/src/engine-client/api-endpoints.ts
@@ -146,7 +146,7 @@ export async function getRunnerConfig(
 	return apiCall<never, RunnerConfigsResponse>(
 		config,
 		"GET",
-		`/runner-configs?runner_name=${name}`,
+		`/runner-configs?runner_name=${encodeURIComponent(name)}`,
 	);
 }
 
diff --git a/rivetkit-typescript/packages/rivetkit/src/serverless/configure.ts b/rivetkit-typescript/packages/rivetkit/src/serverless/configure.ts
index 8423064e98..ac1854337c 100644
--- a/rivetkit-typescript/packages/rivetkit/src/serverless/configure.ts
+++ b/rivetkit-typescript/packages/rivetkit/src/serverless/configure.ts
@@ -1,14 +1,19 @@
 import { convertRegistryConfigToClientConfig } from "@/client/config";
+import type { ClientConfig } from "@/client/config";
 import { stringifyError } from "@/common/utils";
+import { RivetError } from "@/actor/errors";
 import {
 	getDatacenters,
+	getRunnerConfig,
 	updateRunnerConfig,
 } from "@/engine-client/api-endpoints";
 import type { RegistryConfig } from "@/registry/config";
 import { logger } from "@/registry/log";
+import { isDev } from "@/utils/env-vars";
 
 const DEFAULT_CONFIGURE_TIMEOUT_MS = 60_000;
 const CONFIGURE_RETRY_DELAY_MS = 1_000;
+const LOCAL_HANDLER_PROBE_TIMEOUT_MS = 1_000;
 
 function sleep(ms: number): Promise<void> {
 	return new Promise((resolve) => setTimeout(resolve, ms));
@@ -62,6 +67,11 @@ export async function configureServerlessPool(
 			const clientConfig = convertRegistryConfigToClientConfig(config);
 			const dcsRes = await getDatacenters(clientConfig);
 			const poolName = customConfig.name ?? "default";
+			await assertLocalServerlessHandlerOwnership(
+				clientConfig,
+				poolName,
+				customConfig.url,
+			);
 			const serverlessToken = config.token ?? config.publicToken;
 			const headers = {
 				...(serverlessToken
@@ -101,6 +111,13 @@ export async function configureServerlessPool(
 			});
 			return;
 		} catch (error) {
+			if (
+				error instanceof RivetError &&
+				error.group === "rivetkit" &&
+				error.code === "local_serverless_handler_conflict"
+			) {
+				throw error;
+			}
 			lastError = error;
 			logger().warn({
 				msg: "serverless pool configuration attempt failed",
@@ -118,3 +135,61 @@ export async function configureServerlessPool(
 	});
 	throw lastError;
 }
+
+async function assertLocalServerlessHandlerOwnership(
+	config: ClientConfig,
+	poolName: string,
+	requestedUrl: string,
+): Promise<void> {
+	if (!isDev()) return;
+
+	const response = await getRunnerConfig(config, poolName);
+	const datacenters = response.runner_configs[poolName]?.datacenters ?? {};
+	const existingUrls = new Set(
+		Object.values(datacenters)
+			.map((runnerConfig) => runnerConfig.serverless?.url)
+			.filter((url): url is string => url !== undefined),
+	);
+
+	for (const existingUrl of existingUrls) {
+		if (normalizeHandlerUrl(existingUrl) === normalizeHandlerUrl(requestedUrl)) {
+			continue;
+		}
+		if (!(await handlerIsLive(existingUrl))) continue;
+
+		throw new RivetError(
+			"rivetkit",
+			"local_serverless_handler_conflict",
+			`namespace \`${config.namespace}\` and pool \`${poolName}\` already use the live serverless handler ${existingUrl}. Stop the other project, set RIVET_NAMESPACE to a different namespace, or set RIVET_RUN_ENGINE_PORT to a different Engine port. See https://rivet.dev/docs/general/environment-variables/`,
+			{
+				public: true,
+				metadata: {
+					namespace: config.namespace,
+					poolName,
+					existingUrl,
+					requestedUrl,
+				},
+			},
+		);
+	}
+}
+
+export function normalizeHandlerUrl(value: string): string {
+	const url = new URL(value);
+	url.hash = "";
+	url.pathname = url.pathname.replace(/\/+$/, "");
+	return url.toString();
+}
+
+async function handlerIsLive(handlerUrl: string): Promise<boolean> {
+	const metadataUrl = new URL(handlerUrl);
+	metadataUrl.pathname = `${metadataUrl.pathname.replace(/\/+$/, "")}/metadata`;
+	try {
+		const response = await fetch(metadataUrl, {
+			signal: AbortSignal.timeout(LOCAL_HANDLER_PROBE_TIMEOUT_MS),
+		});
+		return response.ok;
+	} catch {
+		return false;
+	}
+}
diff --git a/rivetkit-typescript/packages/rivetkit/tests/serverless-configure.test.ts b/rivetkit-typescript/packages/rivetkit/tests/serverless-configure.test.ts
new file mode 100644
index 0000000000..3acb094fe8
--- /dev/null
+++ b/rivetkit-typescript/packages/rivetkit/tests/serverless-configure.test.ts
@@ -0,0 +1,16 @@
+import { describe, expect, test } from "vitest";
+import { normalizeHandlerUrl } from "../src/serverless/configure";
+
+describe("local serverless handler ownership", () => {
+	test("normalizes equivalent hot-reload handler URLs", () => {
+		expect(normalizeHandlerUrl("http://LOCALHOST:3000/api/rivet/")).toBe(
+			normalizeHandlerUrl("http://localhost:3000/api/rivet"),
+		);
+	});
+
+	test("preserves distinct handler ports", () => {
+		expect(normalizeHandlerUrl("http://localhost:3000/api/rivet")).not.toBe(
+			normalizeHandlerUrl("http://localhost:3001/api/rivet"),
+		);
+	});
+});