fix: reject unknown properties on session() and charge() at compile time

Add NoExtraKeys utility type and apply it via overload signatures to
tempo.session() and tempo.charge(). This catches typos like
`stream: { poll: true }` instead of `sse: { poll: true }` that
previously slipped through because generic `extends` constraints
bypass excess property checking.

Fixes the class of bug from PR #159.

Author: brendanjryan

.changeset/no-extra-keys-type-safety.md

@@ -0,0 +1,5 @@
+---
+"mppx": patch
+---
+
+Added `NoExtraKeys` compile-time guard to `tempo.session()` and `tempo.charge()`. Unknown properties (e.g. `stream` instead of `sse`) now cause a type error instead of being silently accepted.

src/internal/types.ts

@@ -412,3 +412,28 @@ export type Flatten<element> = element extends readonly (infer item)[] ? item :
  * @internal
  */
 export type ValueOf<T> = T[keyof T]
+
+/**
+ * Rejects objects with keys not present in `Shape`.
+ *
+ * TypeScript's `extends` constraint on generics does not perform excess-property
+ * checking, so typos like `{ stream: … }` instead of `{ sse: … }` silently
+ * pass.  Wrapping the parameter type with `NoExtraKeys` maps every extra key
+ * to `never`, surfacing a compile-time error.
+ *
+ * @example
+ * ```ts
+ * type Opts = { sse?: boolean }
+ * declare function f<T extends Opts>(p: NoExtraKeys<T, Opts>): void
+ * f({ sse: true })          // ✅
+ * f({ stream: true })       // ❌  — 'stream' mapped to never
+ * ```
+ *
+ * @internal
+ */
+export type NoExtraKeys<T, Shape> = [T] extends [Shape]
+  ? T & { [K in Exclude<keyof T, KeysOfUnion<Shape>>]: never }
+  : never
+
+/** @internal */
+type KeysOfUnion<T> = T extends unknown ? keyof T : never

src/tempo/internal/account.ts

@@ -33,18 +33,11 @@ export function resolve(parameters: resolve.Parameters) {
 }
 
 export declare namespace resolve {
-  type Parameters = { recipient?: Address | undefined } & (
-    | {
-        /** Account that performs payment operations. */
-        account?: Account | undefined
-        /** When true, the account also sponsors (pays) transaction fees. */
-        feePayer?: true | undefined
-      }
-    | {
-        /** Address that receives payment. */
-        account?: Address | undefined
-        /** Optional fee payer account or fee payer URL for covering transaction fees. */
-        feePayer?: Account | string | undefined
-      }
-  )
+  type Parameters = {
+    recipient?: Address | undefined
+    /** Account or address that performs payment operations / receives payment. */
+    account?: Account | Address | undefined
+    /** When `true`, the account also sponsors fees. An `Account` object or URL string can also be provided as a dedicated fee payer. */
+    feePayer?: Account | string | true | undefined
+  }
 }

src/tempo/server/Charge.ts

@@ -9,7 +9,7 @@ import {
 import { tempo as tempo_chain } from 'viem/chains'
 import { Abis, Transaction } from 'viem/tempo'
 import { PaymentExpiredError } from '../../Errors.js'
-import type { LooseOmit } from '../../internal/types.js'
+import type { LooseOmit, NoExtraKeys } from '../../internal/types.js'
 import * as Method from '../../Method.js'
 import * as Client from '../../viem/Client.js'
 import * as Account from '../internal/account.js'
@@ -29,6 +29,10 @@ import * as Methods from '../Methods.js'
  * const charge = tempo.charge()
  * ```
  */
+export function charge<const parameters extends charge.Parameters>(
+  parameters?: NoExtraKeys<parameters, charge.Parameters>,
+): Method.Server<typeof Methods.charge, charge.DeriveDefaults<parameters>>
+/** @internal */
 export function charge<const parameters extends charge.Parameters>(
   parameters: parameters = {} as parameters,
 ) {

src/tempo/server/Methods.ts

@@ -14,7 +14,10 @@ import { session as session_, settle as settle_ } from './Session.js'
  * ```
  */
 export function tempo<const parameters extends tempo.Parameters>(parameters?: parameters) {
-  return [tempo.charge(parameters), tempo.session(parameters)] as const
+  return [
+    tempo.charge(parameters as charge_.Parameters as never),
+    tempo.session(parameters as session_.Parameters as never),
+  ] as const
 }
 
 export namespace tempo {

src/tempo/server/Session.ts

@@ -29,7 +29,7 @@ import {
   VerificationFailedError,
 } from '../../Errors.js'
 import type { Challenge, Credential } from '../../index.js'
-import type { LooseOmit } from '../../internal/types.js'
+import type { LooseOmit, NoExtraKeys } from '../../internal/types.js'
 import * as Method from '../../Method.js'
 import * as Store from '../../Store.js'
 import * as Client from '../../viem/Client.js'
@@ -81,6 +81,14 @@ type SessionMethodDetails = {
  * })
  * ```
  */
+export function session<const parameters extends session.Parameters>(
+  p?: NoExtraKeys<parameters, session.Parameters>,
+): Method.Server<
+  typeof Methods.session,
+  session.DeriveDefaults<parameters>,
+  parameters['sse'] extends false | undefined ? undefined : Transport.Sse
+>
+/** @internal */
 export function session<const parameters extends session.Parameters>(p?: parameters) {
   const parameters = p as parameters
   const {