Axios Interceptors and Custom Adapters: Testing an Integration Without Production Credentials

Direct Answer

Substitute the adapter layer, not the interceptors. This preserves the request/response interceptor chain (token attachment, header normalization, error mapping) inside the test path while isolating the transport mechanism.

Assert on both: verify the config received by your custom adapter to confirm request interceptors mutate it correctly, and verify the value resolved by the caller's .then() to confirm response interceptors transform the adapter's output as expected.

Why the Adapter Layer

In Axios 1.x, the request pipeline executes in this order:

1. Request interceptors (registered via axios.interceptors.request.use()) run first, each receiving and potentially mutating the config object.
2. The adapter receives the final, fully-mutated config object.
3. The adapter returns a promise resolving to an AxiosResponse.
4. Response interceptors (registered via axios.interceptors.response.use()) run on that response.
5. The caller's .then() receives the result of the response interceptor chain.

Replacing interceptors instead of the adapter removes credential-attachment logic from the test path, so you're no longer exercising the actual integration flow.

Verification Steps

1. Implement a dummy adapter that logs the received config and returns a resolved promise with a minimal AxiosResponse (status, data, headers, config, request).
2. Register a request interceptor that adds a header (e.g., Authorization: Bearer test-token).
3. Register a response interceptor that transforms response.data.
4. Call axios.request() with your dummy adapter via config.
5. Assert the dummy adapter's logged config contains the interceptor-added header.
6. Assert the caller's resolved value reflects the response interceptor's transformation.

Version Assumption

This behavior applies to Axios 1.x (current major version as of 2026). The adapter config option and interceptor pipeline order have been stable since v0.21.

One Diagnostic Detail

If your codebase uses a custom Axios instance (created via axios.create()) rather than the default export, confirm whether interceptors are registered on that instance or the global default—the test must target the same instance the production code uses.