diff --git a/.changeset/olive-frogs-invent.md b/.changeset/olive-frogs-invent.md
new file mode 100644
index 00000000..7504b375
--- /dev/null
+++ b/.changeset/olive-frogs-invent.md
@@ -0,0 +1,7 @@
+---
+"partyserver": patch
+---
+
+Allow user-defined interfaces as `Props`.
+
+Interfaces do not get implicit index signatures in TypeScript, so the previous `Record<string, unknown>` bound rejected them with "Index signature for type 'string' is missing". `Props` is now bounded by `object` on `Server`, `getServerByName`, and `routePartykitRequest`, and the `T` constraints on the latter two are `Server<Env, object>` so subclasses declaring interface `Props` satisfy them too. Generic defaults stay `Record<string, unknown>`, so untyped usage still reads props values as `unknown`.
diff --git a/packages/partyserver/src/index.ts b/packages/partyserver/src/index.ts
index bec3dd0c..4c2b60e9 100644
--- a/packages/partyserver/src/index.ts
+++ b/packages/partyserver/src/index.ts
@@ -291,8 +291,8 @@ function decodeProps(header: string): unknown {
  */
 export async function getServerByName<
   Env extends Cloudflare.Env = Cloudflare.Env,
-  T extends Server<Env> = Server<Env>,
-  Props extends Record<string, unknown> = Record<string, unknown>
+  T extends Server<Env, object> = Server<Env>,
+  Props extends object = Record<string, unknown>
 >(
   serverNamespace: DurableObjectNamespace<T>,
   name: string,
@@ -428,8 +428,8 @@ function resolveCorsHeaders(
 
 export async function routePartykitRequest<
   Env extends Cloudflare.Env = Cloudflare.Env,
-  T extends Server<Env> = Server<Env>,
-  Props extends Record<string, unknown> = Record<string, unknown>
+  T extends Server<Env, object> = Server<Env>,
+  Props extends object = Record<string, unknown>
 >(
   req: Request,
   env: Env = defaultEnv as Env,
@@ -586,9 +586,15 @@ Did you forget to add a durable object binding to the class ${namespace[0].toUpp
   }
 }
 
+/**
+ * @template Env Environment type containing bindings
+ * @template Props Initial-props type delivered to `onStart()`. Bounded by
+ * `object` rather than `Record<string, unknown>` so user-defined interfaces
+ * qualify — interfaces have no implicit index signature.
+ */
 export class Server<
   Env extends Cloudflare.Env = Cloudflare.Env,
-  Props extends Record<string, unknown> = Record<string, unknown>
+  Props extends object = Record<string, unknown>
 > extends DurableObject<Env> {
   static options: { hibernate?: boolean } = {
     hibernate: false
diff --git a/packages/partyserver/src/tests/props.test-d.ts b/packages/partyserver/src/tests/props.test-d.ts
new file mode 100644
index 00000000..b1d9223a
--- /dev/null
+++ b/packages/partyserver/src/tests/props.test-d.ts
@@ -0,0 +1,53 @@
+/**
+ * Type tests for Server props typing. Compile-time only — this file is
+ * typechecked but never executed (vitest only picks up `*.test.ts`).
+ *
+ * Interfaces do not get implicit index signatures in TypeScript, so a
+ * `Record<string, unknown>` bound rejects user-defined interfaces with
+ * "Index signature for type 'string' is missing". Props bounds must accept
+ * plain interfaces while still rejecting non-object props.
+ */
+import { Server, getServerByName, routePartykitRequest } from "../index";
+
+// A well-defined interface with NO index signature.
+interface AuthProps {
+  userId: string;
+  permissions: string[];
+}
+
+declare const authProps: AuthProps;
+declare const env: Cloudflare.Env;
+declare const request: Request;
+
+// ============================================
+// POSITIVE TESTS - interface props must be accepted
+// ============================================
+
+// Server must be instantiable with interface Props.
+declare class AuthServer extends Server<Cloudflare.Env, AuthProps> {}
+declare const serverNamespace: DurableObjectNamespace<AuthServer>;
+
+// getServerByName must accept interface-typed props.
+getServerByName(serverNamespace, "instance", { props: authProps });
+
+// onStart receives the interface type.
+declare const authServer: AuthServer;
+authServer.onStart(authProps);
+
+// routePartykitRequest must accept interface-typed props.
+routePartykitRequest(request, env, { props: authProps });
+
+// ============================================
+// NEGATIVE TESTS - non-object props stay rejected
+// ============================================
+
+// @ts-expect-error — a primitive is not a props bag
+declare class BadServer extends Server<Cloudflare.Env, string> {}
+
+getServerByName(serverNamespace, "instance", {
+  // @ts-expect-error — a primitive is not a props bag
+  props: "not-an-object"
+});
+
+// Silence unused-declaration noise; this file only exists to typecheck.
+export type {};