Error Codes

Every user-facing B4.run failure carries a stable B4_Exxxx code. The code is printed on the surface where the failure appears — CLI stderr, HTTP/SSE error bodies, and permission-denial tool results — so a failure is searchable and linkable regardless of how you hit it.

Codes are grouped by range: E1xxx config / b4 check, E2xxx sandbox, E3xxx permissions, E4xxx model / provider, and E5xxx runtime / import. A code may omit a docs link and still be a valid, searchable identifier.