diff --git a/packages/iter-fest/src/Observable.fromOf.spec.ts b/packages/iter-fest/src/Observable.fromOf.spec.ts
index 0bc87aa..ead3827 100644
--- a/packages/iter-fest/src/Observable.fromOf.spec.ts
+++ b/packages/iter-fest/src/Observable.fromOf.spec.ts
@@ -6,7 +6,7 @@ import {
   type Observer,
   type StartFunction,
   type Subscription
-} from './Observable.js';
+} from './Observable.ts';
 import type { JestMockOf } from './private/JestMockOf.js';
 
 describe('comprehensive', () => {
diff --git a/packages/iter-fest/src/Observable.spec.ts b/packages/iter-fest/src/Observable.spec.ts
index a629132..b925b8f 100644
--- a/packages/iter-fest/src/Observable.spec.ts
+++ b/packages/iter-fest/src/Observable.spec.ts
@@ -7,8 +7,8 @@ import {
   type StartFunction,
   type SubscriberFunction,
   type Subscription
-} from './Observable.js';
-import { SymbolObservable } from './SymbolObservable.js';
+} from './Observable.ts';
+import { SymbolObservable } from './SymbolObservable.ts';
 import type { JestMockOf } from './private/JestMockOf.js';
 
 describe('comprehensive', () => {
diff --git a/packages/iter-fest/src/Observable.ts b/packages/iter-fest/src/Observable.ts
index 44d16c7..2f4e52f 100644
--- a/packages/iter-fest/src/Observable.ts
+++ b/packages/iter-fest/src/Observable.ts
@@ -1,7 +1,7 @@
 // @ts-expect-error core-js is not typed.
 import CoreJSObservable from 'core-js-pure/full/observable/index.js';
 
-import { SymbolObservable } from './SymbolObservable.js';
+import { SymbolObservable } from './SymbolObservable.ts';
 
 export interface SubscriptionObserver<T> {
   /** Sends the next value in the sequence */
diff --git a/packages/iter-fest/src/observableFromAsync.spec.ts b/packages/iter-fest/src/observableFromAsync.spec.ts
index 2656e60..4380a9f 100644
--- a/packages/iter-fest/src/observableFromAsync.spec.ts
+++ b/packages/iter-fest/src/observableFromAsync.spec.ts
@@ -1,8 +1,8 @@
-import type { CompleteFunction, ErrorFunction, NextFunction, Observable, StartFunction } from './Observable.js';
-import { observableFromAsync } from './observableFromAsync.js';
+import type { CompleteFunction, ErrorFunction, NextFunction, Observable, StartFunction } from './Observable.ts';
+import { observableFromAsync } from './observableFromAsync.ts';
 
 import { type JestMockOf } from './private/JestMockOf.js';
-import withResolvers from './private/withResolvers.js';
+import withResolvers from './private/withResolvers.ts';
 
 describe('comprehensive', () => {
   let complete: JestMockOf<CompleteFunction>;
diff --git a/packages/iter-fest/src/observableFromAsync.ts b/packages/iter-fest/src/observableFromAsync.ts
index 0020ae3..48b5150 100644
--- a/packages/iter-fest/src/observableFromAsync.ts
+++ b/packages/iter-fest/src/observableFromAsync.ts
@@ -1,4 +1,4 @@
-import { Observable } from './Observable.js';
+import { Observable } from './Observable.ts';
 
 export function observableFromAsync<T>(iterable: AsyncIterable<T>): Observable<T> {
   return new Observable(subscriber => {
diff --git a/packages/iter-fest/src/observableSubscribeAsReadable.spec.ts b/packages/iter-fest/src/observableSubscribeAsReadable.spec.ts
index c45d507..62c73bf 100644
--- a/packages/iter-fest/src/observableSubscribeAsReadable.spec.ts
+++ b/packages/iter-fest/src/observableSubscribeAsReadable.spec.ts
@@ -1,7 +1,7 @@
-import { Observable, type SubscriberFunction, type SubscriptionObserver } from './Observable.js';
-import { observableSubscribeAsReadable } from './observableSubscribeAsReadable.js';
+import { Observable, type SubscriberFunction, type SubscriptionObserver } from './Observable.ts';
+import { observableSubscribeAsReadable } from './observableSubscribeAsReadable.ts';
 import type { JestMockOf } from './private/JestMockOf.js';
-import hasResolved from './private/hasResolved.js';
+import hasResolved from './private/hasResolved.ts';
 
 describe('comprehensive', () => {
   let observable: Observable<number>;
diff --git a/packages/iter-fest/src/observableValues.spec.ts b/packages/iter-fest/src/observableValues.spec.ts
index e5c05a6..194bee4 100644
--- a/packages/iter-fest/src/observableValues.spec.ts
+++ b/packages/iter-fest/src/observableValues.spec.ts
@@ -1,5 +1,5 @@
-import { Observable, type SubscriberFunction, type SubscriptionObserver } from './Observable.js';
-import { observableValues } from './observableValues.js';
+import { Observable, type SubscriberFunction, type SubscriptionObserver } from './Observable.ts';
+import { observableValues } from './observableValues.ts';
 import type { JestMockOf } from './private/JestMockOf.js';
 
 describe('comprehensive', () => {
diff --git a/packages/iter-fest/src/observableValues.ts b/packages/iter-fest/src/observableValues.ts
index b7c04b5..d5a210a 100644
--- a/packages/iter-fest/src/observableValues.ts
+++ b/packages/iter-fest/src/observableValues.ts
@@ -1,6 +1,6 @@
 import type { Observable } from './Observable.js';
-import { observableSubscribeAsReadable } from './observableSubscribeAsReadable.js';
-import { readableStreamValues } from './readableStreamValues.js';
+import { observableSubscribeAsReadable } from './observableSubscribeAsReadable.ts';
+import { readableStreamValues } from './readableStreamValues.ts';
 
 export function observableValues<T>(observable: Observable<T>): AsyncIterableIterator<T> {
   const readable = observableSubscribeAsReadable(observable);
diff --git a/packages/iter-fest/src/tsconfig.json b/packages/iter-fest/src/tsconfig.json
index ebe1e5a..2221ed4 100644
--- a/packages/iter-fest/src/tsconfig.json
+++ b/packages/iter-fest/src/tsconfig.json
@@ -1,6 +1,7 @@
 // This configuration file is for VSCode only.
 {
   "compilerOptions": {
+    "allowImportingTsExtensions": true,
     "esModuleInterop": true,
     "jsx": "react",
     "module": "ESNext",
diff --git a/packages/iter-fest/src/tsconfig.precommit.production.json b/packages/iter-fest/src/tsconfig.precommit.production.json
index 40cc24f..da317fd 100644
--- a/packages/iter-fest/src/tsconfig.precommit.production.json
+++ b/packages/iter-fest/src/tsconfig.precommit.production.json
@@ -1,5 +1,6 @@
 {
   "compilerOptions": {
+    "allowImportingTsExtensions": true,
     "esModuleInterop": true,
     "jsx": "react",
     "module": "ESNext",
diff --git a/packages/iter-fest/src/tsconfig.precommit.test.json b/packages/iter-fest/src/tsconfig.precommit.test.json
index 7f4ffb7..8854991 100644
--- a/packages/iter-fest/src/tsconfig.precommit.test.json
+++ b/packages/iter-fest/src/tsconfig.precommit.test.json
@@ -1,5 +1,6 @@
 {
   "compilerOptions": {
+    "allowImportingTsExtensions": true,
     "esModuleInterop": true,
     "jsx": "react",
     "module": "ESNext",