diff --git a/packages/web-components/test/display/display.test.js b/packages/web-components/test/display/display.test.js
index bb8428b..24de81e 100644
--- a/packages/web-components/test/display/display.test.js
+++ b/packages/web-components/test/display/display.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../installSesLockdown.js';
 import { expect, test } from 'vitest';
-import { parseAsValue } from '../../src/display/display.js';
+import { parseAsValue } from '../../src/display/display.ts';
 
 test(parseAsValue, () => {
   expect(parseAsValue('30')).toBe(30n);
diff --git a/packages/web-components/test/display/natValue/captureNum.test.js b/packages/web-components/test/display/natValue/captureNum.test.js
index eefa082..d20fa40 100644
--- a/packages/web-components/test/display/natValue/captureNum.test.js
+++ b/packages/web-components/test/display/natValue/captureNum.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../../installSesLockdown.js';
 import { describe, expect, it } from 'vitest';
-import { captureNum } from '../../../src/display/natValue/helpers/captureNum.js';
+import { captureNum } from '../../../src/display/natValue/helpers/captureNum.ts';
 
 describe(captureNum, () => {
   it('can capture positive numbers', () => {
diff --git a/packages/web-components/test/display/natValue/parseAsNat.test.js b/packages/web-components/test/display/natValue/parseAsNat.test.js
index 83cc630..c12e953 100644
--- a/packages/web-components/test/display/natValue/parseAsNat.test.js
+++ b/packages/web-components/test/display/natValue/parseAsNat.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../../installSesLockdown.js';
 import { describe, expect, it } from 'vitest';
-import { parseAsNat } from '../../../src/display/natValue/parseAsNat.js';
+import { parseAsNat } from '../../../src/display/natValue/parseAsNat.ts';
 
 describe(parseAsNat, () => {
   it('can parse cents to dollars', () => {
diff --git a/packages/web-components/test/display/natValue/roundToDecimalPlaces.test.js b/packages/web-components/test/display/natValue/roundToDecimalPlaces.test.js
index 4c2ce05..ca4e69d 100644
--- a/packages/web-components/test/display/natValue/roundToDecimalPlaces.test.js
+++ b/packages/web-components/test/display/natValue/roundToDecimalPlaces.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../../installSesLockdown.js';
 import { describe, expect, it } from 'vitest';
-import { roundToDecimalPlaces as round } from '../../../src/display/natValue/helpers/roundToDecimalPlaces.js';
+import { roundToDecimalPlaces as round } from '../../../src/display/natValue/helpers/roundToDecimalPlaces.ts';
 
 describe(round, () => {
   it('can round', () => {
diff --git a/packages/web-components/test/display/natValue/stringifyNat.test.js b/packages/web-components/test/display/natValue/stringifyNat.test.js
index 6aa39de..c87783e 100644
--- a/packages/web-components/test/display/natValue/stringifyNat.test.js
+++ b/packages/web-components/test/display/natValue/stringifyNat.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../../installSesLockdown.js';
 import { describe, expect, it } from 'vitest';
-import { stringifyNat } from '../../../src/display/natValue/stringifyNat.js';
+import { stringifyNat } from '../../../src/display/natValue/stringifyNat.ts';
 
 describe(stringifyNat, () => {
   it('can stringify cents to dollars', () => {
diff --git a/packages/web-components/test/display/natValue/stringifyRatio.test.js b/packages/web-components/test/display/natValue/stringifyRatio.test.js
index 6faebce..4ebb44a 100644
--- a/packages/web-components/test/display/natValue/stringifyRatio.test.js
+++ b/packages/web-components/test/display/natValue/stringifyRatio.test.js
@@ -6,8 +6,8 @@ import {
   stringifyRatio,
   stringifyRatioAsFraction,
   stringifyRatioAsPercent,
-} from '../../../src/display/index.js';
-import { makeRatio } from '../../../src/display/natValue/ratio.js';
+} from '../../../src/display/index.ts';
+import { makeRatio } from '../../../src/display/natValue/ratio.ts';
 
 describe('stringifyRatio', () => {
   const ethBrand = Far('eth brand', {});
diff --git a/packages/web-components/test/display/setValue/stringifySet.test.js b/packages/web-components/test/display/setValue/stringifySet.test.js
index 446056f..57ca547 100644
--- a/packages/web-components/test/display/setValue/stringifySet.test.js
+++ b/packages/web-components/test/display/setValue/stringifySet.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../../installSesLockdown.js';
 import { expect, test } from 'vitest';
-import { stringifySet } from '../../../src/display/setValue/stringifySet.js';
+import { stringifySet } from '../../../src/display/setValue/stringifySet.ts';
 
 test(stringifySet, () => {
   expect(stringifySet(['foo', 'bar'])).toBe('2');
diff --git a/packages/web-components/test/wallet-connection/walletConnection.test.js b/packages/web-components/test/wallet-connection/walletConnection.test.js
index 510191a..4f3df0d 100644
--- a/packages/web-components/test/wallet-connection/walletConnection.test.js
+++ b/packages/web-components/test/wallet-connection/walletConnection.test.js
@@ -1,7 +1,7 @@
 // @ts-check
 import '../installSesLockdown.js';
 import { describe, vi, expect, it } from 'vitest';
-import { makeAgoricWalletConnection } from '../../src/wallet-connection/walletConnection.js';
+import { makeAgoricWalletConnection } from '../../src/wallet-connection/walletConnection.ts';
 import {
   makeAgoricSigner,
   // @ts-expect-error exported by mock below
@@ -10,8 +10,8 @@ import {
   // @ts-expect-error exported by mock below
   // eslint-disable-next-line import/named
   provisionSmartWallet as mockProvisionSmartWallet,
-} from '../../src/wallet-connection/makeAgoricSigner.js';
-import { Errors } from '../../src/wallet-connection/errors.js';
+} from '../../src/wallet-connection/makeAgoricSigner.ts';
+import { Errors } from '../../src/wallet-connection/errors.ts';
 
 const testAddress = 'agoric123test';
 const rpc = 'https://fake.rpc';
@@ -20,7 +20,7 @@ const rpc = 'https://fake.rpc';
 // eslint-disable-next-line no-undef
 global.window = { keplr: {} };
 
-vi.mock('../../src/wallet-connection/makeAgoricSigner.js', () => {
+vi.mock('../../src/wallet-connection/makeAgoricSigner.ts', () => {
   const submitSpendAction = vi.fn();
   const provisionSmartWallet = vi.fn();
 
@@ -35,7 +35,7 @@ vi.mock('../../src/wallet-connection/makeAgoricSigner.js', () => {
   };
 });
 
-vi.mock('../../src/queryBankBalances.js', () => {
+vi.mock('../../src/queryBankBalances.ts', () => {
   return {
     queryBankBalances: vi.fn(),
   };
diff --git a/packages/web-components/tsconfig.build.json b/packages/web-components/tsconfig.build.json
index 5d195ee..1741d4b 100644
--- a/packages/web-components/tsconfig.build.json
+++ b/packages/web-components/tsconfig.build.json
@@ -1,6 +1,7 @@
 {
   "extends": "./tsconfig.json",
   "compilerOptions": {
+    "allowImportingTsExtensions": false,
     "noEmit": false,
     "declaration": true,
     "declarationMap": true
diff --git a/packages/web-components/tsconfig.json b/packages/web-components/tsconfig.json
index 4339750..5ef692e 100644
--- a/packages/web-components/tsconfig.json
+++ b/packages/web-components/tsconfig.json
@@ -2,6 +2,7 @@
 {
   "extends": "../../tsconfig.json",
   "compilerOptions": {
+    "allowImportingTsExtensions": true, // for tests
     "outDir": "./dist",
     "checkJs": true
   },