diff --git a/src/codec.ts b/src/codec.ts
index 3bfa8a2..0d9c6d3 100644
--- a/src/codec.ts
+++ b/src/codec.ts
@@ -128,7 +128,7 @@ export function decodeBuiltin(raw: string): BuiltinEnvelope {
       throw new CodecError("Missing or invalid 'type' field in envelope");
     }
 
-    return envelope as BuiltinEnvelope;
+    return envelope as unknown as BuiltinEnvelope;
   } catch (cause) {
     if (cause instanceof CodecError) {
       throw cause;
diff --git a/src/types.ts b/src/types.ts
index ea4d261..f7a7618 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -22,7 +22,7 @@ export type BuiltinMessageType = (typeof builtinMessageTypes)[number];
 
 export interface BuiltinEnvelope<
   TType extends BuiltinMessageType = BuiltinMessageType,
-  TPayload extends Record<string, unknown> = Record<string, unknown>
+  TPayload = Record<string, unknown>
 > {
   type: TType;
   requestId?: string;
@@ -30,7 +30,7 @@ export interface BuiltinEnvelope<
   payload?: TPayload;
 }
 
-export interface HelloPayload {
+export interface HelloPayload extends Record<string, unknown> {
   identifier: string;
   hasSecret: boolean;
   hasKeyPair: boolean;
@@ -44,14 +44,14 @@ export type HelloAckNextAction =
   | "rejected"
   | "waiting_pair_confirm";
 
-export interface HelloAckPayload {
+export interface HelloAckPayload extends Record<string, unknown> {
   identifier: string;
   nextAction: HelloAckNextAction;
 }
 
 export type AdminNotificationStatus = "sent" | "failed";
 
-export interface PairRequestPayload {
+export interface PairRequestPayload extends Record<string, unknown> {
   identifier: string;
   expiresAt: number;
   ttlSeconds: number;
@@ -59,12 +59,12 @@ export interface PairRequestPayload {
   codeDelivery: "out_of_band";
 }
 
-export interface PairConfirmPayload {
+export interface PairConfirmPayload extends Record<string, unknown> {
   identifier: string;
   pairingCode: string;
 }
 
-export interface PairSuccessPayload {
+export interface PairSuccessPayload extends Record<string, unknown> {
   identifier: string;
   secret: string;
   pairedAt: number;
@@ -77,12 +77,12 @@ export type PairFailedReason =
   | "admin_notification_failed"
   | "internal_error";
 
-export interface PairFailedPayload {
+export interface PairFailedPayload extends Record<string, unknown> {
   identifier: string;
   reason: PairFailedReason;
 }
 
-export interface AuthRequestPayload {
+export interface AuthRequestPayload extends Record<string, unknown> {
   identifier: string;
   nonce: string;
   proofTimestamp: number;
@@ -90,7 +90,7 @@ export interface AuthRequestPayload {
   publicKey?: string;
 }
 
-export interface AuthSuccessPayload {
+export interface AuthSuccessPayload extends Record<string, unknown> {
   identifier: string;
   authenticatedAt: number;
   status: "online";
@@ -107,33 +107,33 @@ export type AuthFailedReason =
   | "rate_limited"
   | "re_pair_required";
 
-export interface AuthFailedPayload {
+export interface AuthFailedPayload extends Record<string, unknown> {
   identifier: string;
   reason: AuthFailedReason;
 }
 
-export interface RePairRequiredPayload {
+export interface RePairRequiredPayload extends Record<string, unknown> {
   identifier: string;
   reason: "nonce_collision" | "rate_limited" | "trust_revoked";
 }
 
-export interface HeartbeatPayload {
+export interface HeartbeatPayload extends Record<string, unknown> {
   identifier: string;
   status: "alive";
 }
 
-export interface HeartbeatAckPayload {
+export interface HeartbeatAckPayload extends Record<string, unknown> {
   identifier: string;
   status: "online" | "unstable" | "offline";
 }
 
-export interface StatusUpdatePayload {
+export interface StatusUpdatePayload extends Record<string, unknown> {
   identifier: string;
   status: "online" | "unstable" | "offline";
   reason: string;
 }
 
-export interface DisconnectNoticePayload {
+export interface DisconnectNoticePayload extends Record<string, unknown> {
   identifier: string;
   reason: string;
 }
@@ -152,7 +152,7 @@ export type ProtocolErrorCode =
   | "CLIENT_OFFLINE"
   | "INTERNAL_ERROR";
 
-export interface ErrorPayload {
+export interface ErrorPayload extends Record<string, unknown> {
   code: ProtocolErrorCode;
   message?: string;
   details?: Record<string, unknown>;