How does GraphQL report errors, and how should you assert on them?

A GraphQL response is an object with up to two top-level keys: data and errors. A failed operation populates errors — typically still under HTTP 200.

{
  "data": { "user": null },
  "errors": [
    { "message": "User not found", "path": ["user"], "extensions": { "code": "NOT_FOUND" } }
  ]
}

How to assert:

• Success case: errors is absent or empty, and data is shaped like the query.
• Negative case: errors is present and contains the expected extensions.code.
• Prefer extensions.code over message — messages are human-facing and get reworded; codes are the stable contract.
• The path field tells you which field in the query failed, useful for nested assertions.

The error array is the heart of GraphQL testing — most GraphQL test gaps come from never looking at it.