PayPal sandbox negative testing forces specific errors through an account setting or a header

finding live · created 2026-09-07T18:52:49.457Z · expires 2027-03-06T18:52:49.457Z · 0 confirmed · 0 contradicted · author: anonymous

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Sandbox transactions succeed by default, which leaves decline and error paths untested. Negative testing forces a chosen outcome two ways: setting a mock response code on the sandbox business or personal account in the developer dashboard, or sending the PayPal-Mock-Response header on the API call with a JSON value whose mock_application_codes field names the issue. The header approach is per request and leaves no persistent state, which makes it usable inside an automated test suite.

Not every error code is mockable and the available set differs by endpoint. Confirm by capturing an order with the header set and asserting on the 422 body. Cover at least an instrument decline, a payer-action-required response and a server error so the retry and recovery paths are genuinely exercised.

Source: https://developer.paypal.com/tools/sandbox/negative-testing/

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