diff --git a/app/controllers/api/v1/accounts/microsoft/authorizations_controller.rb b/app/controllers/api/v1/accounts/microsoft/authorizations_controller.rb
index c65a3031d..07decfde0 100644
--- a/app/controllers/api/v1/accounts/microsoft/authorizations_controller.rb
+++ b/app/controllers/api/v1/accounts/microsoft/authorizations_controller.rb
@@ -6,7 +6,10 @@ class Api::V1::Accounts::Microsoft::AuthorizationsController < Api::V1::Accounts
       {
         redirect_uri: "#{base_url}/microsoft/callback",
         scope: scope,
-        state: state
+        state: state,
+        # Force the Microsoft account picker so an already-signed-in account does not
+        # silently authorize and re-bind to an existing inbox in the new-inbox flow.
+        prompt: 'select_account'
       }
     )
     if redirect_url
diff --git a/spec/controllers/api/v1/accounts/microsoft/authorization_controller_spec.rb b/spec/controllers/api/v1/accounts/microsoft/authorization_controller_spec.rb
index 18a26a393..ea88756a5 100644
--- a/spec/controllers/api/v1/accounts/microsoft/authorization_controller_spec.rb
+++ b/spec/controllers/api/v1/accounts/microsoft/authorization_controller_spec.rb
@@ -43,7 +43,7 @@ RSpec.describe 'Microsoft Authorization API', type: :request do
         ]
         expect(params['scope']).to eq(expected_scope)
         expect(params['redirect_uri']).to eq(["#{ENV.fetch('FRONTEND_URL', 'http://localhost:3000')}/microsoft/callback"])
-        expect(url).not_to match(/(?:\?|&)prompt=/)
+        expect(params['prompt']).to eq(['select_account'])
 
         # Validate state parameter exists and can be decoded back to the account
         expect(params['state']).to be_present