diff --git a/.changeset/tender-walls-hang.md b/.changeset/tender-walls-hang.md
new file mode 100644
index 00000000000..a9307b2eaa8
--- /dev/null
+++ b/.changeset/tender-walls-hang.md
@@ -0,0 +1,5 @@
+---
+"@firebase/auth": patch
+---
+
+Fix wrongly-typed tenantId fields in requests to some endpoints
diff --git a/packages/auth/src/api/account_management/mfa.test.ts b/packages/auth/src/api/account_management/mfa.test.ts
index f05c393413e..a2974eceb72 100644
--- a/packages/auth/src/api/account_management/mfa.test.ts
+++ b/packages/auth/src/api/account_management/mfa.test.ts
@@ -60,10 +60,7 @@ describe('api/account_management/startEnrollPhoneMfa', () => {
 
     const response = await startEnrollPhoneMfa(auth, request);
     expect(response.phoneSessionInfo.sessionInfo).to.eq('session-info');
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
     expect(mock.calls[0].method).to.eq('POST');
     expect(mock.calls[0].headers!.get(HttpHeader.CONTENT_TYPE)).to.eq(
       'application/json'
@@ -94,10 +91,7 @@ describe('api/account_management/startEnrollPhoneMfa', () => {
       FirebaseError,
       "Firebase: This user's credential isn't valid for this project. This can happen if the user's token has been tampered with, or if the user isn't for the project associated with this API key. (auth/invalid-user-token)."
     );
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
   });
 });
 
@@ -130,10 +124,7 @@ describe('api/account_management/finalizeEnrollPhoneMfa', () => {
     const response = await finalizeEnrollPhoneMfa(auth, request);
     expect(response.idToken).to.eq('id-token');
     expect(response.refreshToken).to.eq('refresh-token');
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
     expect(mock.calls[0].method).to.eq('POST');
     expect(mock.calls[0].headers!.get(HttpHeader.CONTENT_TYPE)).to.eq(
       'application/json'
@@ -164,10 +155,7 @@ describe('api/account_management/finalizeEnrollPhoneMfa', () => {
       FirebaseError,
       'Firebase: The verification ID used to create the phone auth credential is invalid. (auth/invalid-verification-id).'
     );
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
   });
 });
 
@@ -195,10 +183,7 @@ describe('api/account_management/withdrawMfa', () => {
     const response = await withdrawMfa(auth, request);
     expect(response.idToken).to.eq('id-token');
     expect(response.refreshToken).to.eq('refresh-token');
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
     expect(mock.calls[0].method).to.eq('POST');
     expect(mock.calls[0].headers!.get(HttpHeader.CONTENT_TYPE)).to.eq(
       'application/json'
@@ -229,9 +214,6 @@ describe('api/account_management/withdrawMfa', () => {
       FirebaseError,
       "Firebase: This user's credential isn't valid for this project. This can happen if the user's token has been tampered with, or if the user isn't for the project associated with this API key. (auth/invalid-user-token)."
     );
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
   });
 });
diff --git a/packages/auth/src/api/account_management/mfa.ts b/packages/auth/src/api/account_management/mfa.ts
index f2e8e2008cb..83c4c710c8a 100644
--- a/packages/auth/src/api/account_management/mfa.ts
+++ b/packages/auth/src/api/account_management/mfa.ts
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import { Endpoint, HttpMethod, _performApiRequest } from '../index';
+import { Endpoint, HttpMethod, _addTidIfNecessary, _performApiRequest } from '../index';
 import { SignInWithPhoneNumberRequest } from '../authentication/sms';
 import { FinalizeMfaResponse } from '../authentication/mfa';
 import { AuthInternal } from '../../model/auth';
@@ -47,7 +47,7 @@ export interface StartPhoneMfaEnrollmentRequest {
     phoneNumber: string;
     recaptchaToken: string;
   };
-  tenantId: string | null;
+  tenantId?: string;
 }
 
 export interface StartPhoneMfaEnrollmentResponse {
@@ -58,22 +58,19 @@ export interface StartPhoneMfaEnrollmentResponse {
 
 export function startEnrollPhoneMfa(
   auth: AuthInternal,
-  request: Omit<StartPhoneMfaEnrollmentRequest, 'tenantId'>
+  request: StartPhoneMfaEnrollmentRequest
 ): Promise<StartPhoneMfaEnrollmentResponse> {
   return _performApiRequest<
     StartPhoneMfaEnrollmentRequest,
     StartPhoneMfaEnrollmentResponse
-  >(auth, HttpMethod.POST, Endpoint.START_PHONE_MFA_ENROLLMENT, {
-    tenantId: auth.tenantId,
-    ...request
-  });
+  >(auth, HttpMethod.POST, Endpoint.START_PHONE_MFA_ENROLLMENT, _addTidIfNecessary(auth, request));
 }
 
 export interface FinalizePhoneMfaEnrollmentRequest {
   idToken: string;
   phoneVerificationInfo: SignInWithPhoneNumberRequest;
   displayName?: string | null;
-  tenantId: string | null;
+  tenantId?: string;
 }
 
 export interface FinalizePhoneMfaEnrollmentResponse
@@ -81,36 +78,30 @@ export interface FinalizePhoneMfaEnrollmentResponse
 
 export function finalizeEnrollPhoneMfa(
   auth: AuthInternal,
-  request: Omit<FinalizePhoneMfaEnrollmentRequest, 'tenantId'>
+  request: FinalizePhoneMfaEnrollmentRequest
 ): Promise<FinalizePhoneMfaEnrollmentResponse> {
   return _performApiRequest<
     FinalizePhoneMfaEnrollmentRequest,
     FinalizePhoneMfaEnrollmentResponse
-  >(auth, HttpMethod.POST, Endpoint.FINALIZE_PHONE_MFA_ENROLLMENT, {
-    tenantId: auth.tenantId,
-    ...request
-  });
+  >(auth, HttpMethod.POST, Endpoint.FINALIZE_PHONE_MFA_ENROLLMENT, _addTidIfNecessary(auth, request));
 }
 
 export interface WithdrawMfaRequest {
   idToken: string;
   mfaEnrollmentId: string;
-  tenantId: string | null;
+  tenantId?: string;
 }
 
 export interface WithdrawMfaResponse extends FinalizeMfaResponse {}
 
 export function withdrawMfa(
   auth: AuthInternal,
-  request: Omit<WithdrawMfaRequest, 'tenantId'>
+  request: WithdrawMfaRequest
 ): Promise<WithdrawMfaResponse> {
   return _performApiRequest<WithdrawMfaRequest, WithdrawMfaResponse>(
     auth,
     HttpMethod.POST,
     Endpoint.WITHDRAW_MFA,
-    {
-      tenantId: auth.tenantId,
-      ...request
-    }
+    _addTidIfNecessary(auth, request)
   );
 }
diff --git a/packages/auth/src/api/authentication/mfa.test.ts b/packages/auth/src/api/authentication/mfa.test.ts
index ebb4b8f6045..d3ba40cc394 100644
--- a/packages/auth/src/api/authentication/mfa.test.ts
+++ b/packages/auth/src/api/authentication/mfa.test.ts
@@ -56,10 +56,7 @@ describe('api/authentication/startSignInPhoneMfa', () => {
 
     const response = await startSignInPhoneMfa(auth, request);
     expect(response.phoneResponseInfo.sessionInfo).to.eq('session-info');
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
     expect(mock.calls[0].method).to.eq('POST');
     expect(mock.calls[0].headers!.get(HttpHeader.CONTENT_TYPE)).to.eq(
       'application/json'
@@ -90,10 +87,7 @@ describe('api/authentication/startSignInPhoneMfa', () => {
       FirebaseError,
       'Firebase: The supplied auth credential is malformed or has expired. (auth/invalid-credential).'
     );
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
   });
 });
 
@@ -126,10 +120,7 @@ describe('api/authentication/finalizeSignInPhoneMfa', () => {
     const response = await finalizeSignInPhoneMfa(auth, request);
     expect(response.idToken).to.eq('id-token');
     expect(response.refreshToken).to.eq('refresh-token');
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
     expect(mock.calls[0].method).to.eq('POST');
     expect(mock.calls[0].headers!.get(HttpHeader.CONTENT_TYPE)).to.eq(
       'application/json'
@@ -160,9 +151,6 @@ describe('api/authentication/finalizeSignInPhoneMfa', () => {
       FirebaseError,
       'Firebase: The SMS verification code used to create the phone auth credential is invalid. Please resend the verification code sms and be sure to use the verification code provided by the user. (auth/invalid-verification-code).'
     );
-    expect(mock.calls[0].request).to.eql({
-      tenantId: null,
-      ...request
-    });
+    expect(mock.calls[0].request).to.eql(request);
   });
 });
diff --git a/packages/auth/src/api/authentication/mfa.ts b/packages/auth/src/api/authentication/mfa.ts
index 75ad048c7ea..3c79376a66e 100644
--- a/packages/auth/src/api/authentication/mfa.ts
+++ b/packages/auth/src/api/authentication/mfa.ts
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import { _performApiRequest, Endpoint, HttpMethod } from '../index';
+import { _performApiRequest, Endpoint, HttpMethod, _addTidIfNecessary } from '../index';
 import { Auth } from '../../model/public_types';
 import { IdTokenResponse } from '../../model/id_token';
 import { MfaEnrollment } from '../account_management/mfa';
@@ -44,7 +44,7 @@ export interface StartPhoneMfaSignInRequest {
   phoneSignInInfo: {
     recaptchaToken: string;
   };
-  tenantId: string | null;
+  tenantId?: string;
 }
 
 export interface StartPhoneMfaSignInResponse {
@@ -55,36 +55,30 @@ export interface StartPhoneMfaSignInResponse {
 
 export function startSignInPhoneMfa(
   auth: Auth,
-  request: Omit<StartPhoneMfaSignInRequest, 'tenantId'>
+  request: StartPhoneMfaSignInRequest
 ): Promise<StartPhoneMfaSignInResponse> {
   return _performApiRequest<
     StartPhoneMfaSignInRequest,
     StartPhoneMfaSignInResponse
-  >(auth, HttpMethod.POST, Endpoint.START_PHONE_MFA_SIGN_IN, {
-    tenantId: auth.tenantId,
-    ...request
-  });
+  >(auth, HttpMethod.POST, Endpoint.START_PHONE_MFA_SIGN_IN, _addTidIfNecessary(auth, request));
 }
 
 export interface FinalizePhoneMfaSignInRequest {
   mfaPendingCredential: string;
   phoneVerificationInfo: SignInWithPhoneNumberRequest;
-  tenantId: string | null;
+  tenantId?: string;
 }
 
 export interface FinalizePhoneMfaSignInResponse extends FinalizeMfaResponse {}
 
 export function finalizeSignInPhoneMfa(
   auth: Auth,
-  request: Omit<FinalizePhoneMfaSignInRequest, 'tenantId'>
+  request: FinalizePhoneMfaSignInRequest,
 ): Promise<FinalizePhoneMfaSignInResponse> {
   return _performApiRequest<
     FinalizePhoneMfaSignInRequest,
     FinalizePhoneMfaSignInResponse
-  >(auth, HttpMethod.POST, Endpoint.FINALIZE_PHONE_MFA_SIGN_IN, {
-    tenantId: auth.tenantId,
-    ...request
-  });
+  >(auth, HttpMethod.POST, Endpoint.FINALIZE_PHONE_MFA_SIGN_IN, _addTidIfNecessary(auth, request));
 }
 
 /**
diff --git a/packages/auth/src/mfa/mfa_user.test.ts b/packages/auth/src/mfa/mfa_user.test.ts
index 44188171e66..f5fc504dc44 100644
--- a/packages/auth/src/mfa/mfa_user.test.ts
+++ b/packages/auth/src/mfa/mfa_user.test.ts
@@ -189,7 +189,6 @@ describe('core/mfa/mfa_user/MultiFactorUser', () => {
       expect(withdrawMfaEnrollmentMock.calls[0].request).to.eql({
         idToken: 'access-token',
         mfaEnrollmentId: mfaInfo.uid,
-        tenantId: auth.tenantId
       });
     });
 
@@ -205,7 +204,6 @@ describe('core/mfa/mfa_user/MultiFactorUser', () => {
       expect(withdrawMfaEnrollmentMock.calls[0].request).to.eql({
         idToken: 'access-token',
         mfaEnrollmentId: mfaInfo.uid,
-        tenantId: auth.tenantId
       });
     });
 
diff --git a/packages/auth/src/platform_browser/mfa/assertions/phone.test.ts b/packages/auth/src/platform_browser/mfa/assertions/phone.test.ts
index f1f0bd7223c..29724e5d2d7 100644
--- a/packages/auth/src/platform_browser/mfa/assertions/phone.test.ts
+++ b/packages/auth/src/platform_browser/mfa/assertions/phone.test.ts
@@ -70,7 +70,6 @@ describe('platform_browser/mfa/phone', () => {
       expect(response).to.eql(serverResponse);
       expect(mock.calls[0].request).to.eql({
         idToken: 'enrollment-id-token',
-        tenantId: auth.tenantId,
         phoneVerificationInfo: {
           code: 'verification-code',
           sessionInfo: 'verification-id'
@@ -93,7 +92,6 @@ describe('platform_browser/mfa/phone', () => {
         expect(mock.calls[0].request).to.eql({
           idToken: 'enrollment-id-token',
           displayName: 'display-name',
-          tenantId: auth.tenantId,
           phoneVerificationInfo: {
             code: 'verification-code',
             sessionInfo: 'verification-id'
@@ -119,7 +117,6 @@ describe('platform_browser/mfa/phone', () => {
       expect(response).to.eql(serverResponse);
       expect(mock.calls[0].request).to.eql({
         mfaPendingCredential: 'mfa-pending-credential',
-        tenantId: null,
         phoneVerificationInfo: {
           code: 'verification-code',
           sessionInfo: 'verification-id'
diff --git a/packages/auth/src/platform_browser/strategies/phone.test.ts b/packages/auth/src/platform_browser/strategies/phone.test.ts
index a8a2ce8eac9..6281ff812b1 100644
--- a/packages/auth/src/platform_browser/strategies/phone.test.ts
+++ b/packages/auth/src/platform_browser/strategies/phone.test.ts
@@ -336,7 +336,6 @@ describe('platform_browser/strategies/phone', () => {
         );
         expect(sessionInfo).to.eq('session-info');
         expect(endpoint.calls[0].request).to.eql({
-          tenantId: auth.tenantId,
           idToken: session.credential,
           phoneEnrollmentInfo: {
             phoneNumber: 'number',
@@ -369,7 +368,6 @@ describe('platform_browser/strategies/phone', () => {
         );
         expect(sessionInfo).to.eq('session-info');
         expect(endpoint.calls[0].request).to.eql({
-          tenantId: auth.tenantId,
           mfaPendingCredential: 'mfa-pending-credential',
           mfaEnrollmentId: 'mfa-enrollment-id',
           phoneSignInInfo: {