Diagnose Apex Errors

1. Reproduces the reported symptom and compares the failing component with a similar one that works.
2. Inspects settings, controller code, guest user permissions, debug logs and API versions to narrow the cause.
3. States the root cause and delivers a fix plan, often with more than one option.
4. Works around platform limits in code when a fix is needed, such as moving work into a Queueable to get past a deserialization bug and a session token limit.
5. Follows the cause past the code when that is where it lies, such as demo phone numbers that are not carrier-connected.
6. On larger builds, writes Apex and Flow, deploys it and verifies it with tests.

Writing or changing the code itself continues in Engineer Apex Classes, and test coverage in Test Apex Classes.