diff --git a/src/__tests__/definition.test.ts b/src/__tests__/definition.test.ts
index 786b4f2..10fbeb8 100644
--- a/src/__tests__/definition.test.ts
+++ b/src/__tests__/definition.test.ts
@@ -13,12 +13,14 @@ import * as Exit from "effect/Exit";
 import * as Fiber from "effect/Fiber";
 import * as Option from "effect/Option";
 import * as Result from "effect/Result";
+import * as Schema from "effect/Schema";
 import { TestClock } from "effect/testing";
 import * as WorkflowEngine from "effect/unstable/workflow/WorkflowEngine";
 import { describe, expect, expectTypeOf, it } from "vitest";
 import { handle, implementActivities, type ActivityRunner } from "../activities.js";
 import {
   continueAsNew,
+  defineActivity,
   executeChild,
   sleepUntil,
   version,
@@ -80,6 +82,38 @@ const _types = () => {
   // @ts-expect-error wrong payload shape
   Charge({ orderId: 1 });
 
+  // Omitted schemas retain their runtime defaults in the type channels.
+  const Defaults = defineActivity("defaults", { payload: { id: Schema.String } });
+  const defaults = Defaults({ id: "x" });
+  expectTypeOf<Effect.Success<typeof defaults>>().toEqualTypeOf<void>();
+  expectTypeOf<Effect.Error<typeof defaults>>().toEqualTypeOf<never>();
+
+  // An error schema can still be inferred while success defaults to void.
+  const ErrorOnly = defineActivity("errorOnly", {
+    payload: { id: Schema.String },
+    error: CardDeclined,
+  });
+  const errorOnly = ErrorOnly({ id: "x" });
+  expectTypeOf<Effect.Success<typeof errorOnly>>().toEqualTypeOf<void>();
+  expectTypeOf<Effect.Error<typeof errorOnly>>().toEqualTypeOf<typeof CardDeclined.Type>();
+
+  defineActivity<"missingSuccess", { id: typeof Schema.String }, typeof Schema.String>(
+    "missingSuccess",
+    // @ts-expect-error a non-default success generic requires its runtime schema
+    { payload: { id: Schema.String } },
+  );
+
+  defineActivity<
+    "missingError",
+    { id: typeof Schema.String },
+    Schema.Void,
+    typeof CardDeclined
+  >(
+    "missingError",
+    // @ts-expect-error a non-default error generic requires its runtime schema
+    { payload: { id: Schema.String } },
+  );
+
   // Messages: deferred success, mailbox payload, update request typing.
   expectTypeOf<Effect.Success<typeof Approval.await>>().toEqualTypeOf<string>();
   expectTypeOf<Effect.Success<typeof Priority.take>>().toEqualTypeOf<{
diff --git a/src/definition.ts b/src/definition.ts
index 61b2fa9..bc2c6c9 100644
--- a/src/definition.ts
+++ b/src/definition.ts
@@ -133,6 +133,22 @@ export type SuccessOf<A> =
 export type ErrorOf<A> =
   A extends TypedActivity<string, Schema.Top, Schema.Top, infer E> ? E["Type"] : never;
 
+type IsExactly<A, B> = [A] extends [B] ? ([B] extends [A] ? true : false) : false;
+
+type ActivityDeclaration<
+  Payload extends Schema.Struct.Fields | Schema.Top,
+  Success extends Schema.Top,
+  Error extends Schema.Top,
+> = {
+  readonly payload: Payload;
+  readonly options?: TypedActivityOptions;
+} & (IsExactly<Success, Schema.Void> extends true
+  ? { readonly success?: Success }
+  : { readonly success: Success })
+  & (IsExactly<Error, Schema.Never> extends true
+    ? { readonly error?: Error }
+    : { readonly error: Error });
+
 /** Build the serializable projection of an activity declaration. */
 const makeTypedActivity = <
   const Name extends string,
@@ -141,12 +157,7 @@ const makeTypedActivity = <
   Error extends Schema.Top = Schema.Never,
 >(
   name: Name,
-  definition: {
-    readonly payload: Payload;
-    readonly success?: Success;
-    readonly error?: Error;
-    readonly options?: TypedActivityOptions;
-  },
+  definition: ActivityDeclaration<Payload, Success, Error>,
 ): TypedActivity<
   Name,
   Payload extends Schema.Struct.Fields ? Schema.Struct<Payload> : Payload,
@@ -319,12 +330,7 @@ export const defineActivity = <
   Error extends Schema.Top = Schema.Never,
 >(
   name: Name,
-  decl: {
-    readonly payload: Payload;
-    readonly success?: Success;
-    readonly error?: Error;
-    readonly options?: TypedActivityOptions;
-  },
+  decl: ActivityDeclaration<Payload, Success, Error>,
 ): DefinedActivity<
   Name,
   Payload extends Schema.Struct.Fields ? Schema.Struct<Payload> : Payload,