Errors

Prefer structured handling. Many “errors” are tool JSON ({"error":"…"} inside MCP content), not HTTP status codes.

HTTP layer (`/mcp`)

Status	`error`	Meaning	Action
`401`	`signature_invalid`	Bad/missing signature, or user paused assistants	Fix signing / wait
`413`	`payload_too_large`	Body > 256 KiB	Shrink request

Code	Meaning	Action
`approval_required`	Phone prompt pending	Show match code; wait; retry rules
`not_approved`	Excerpt/send retry without a valid `approval_id`	Re-prompt or stop
`denied`	Policy refuse or blocked domain	Stop that ask; do not hammer
`rate_limited`	Prompts / mute / daily cap	Back off (`retry_after_seconds`)
`no_answer`	Model/extractive path found nothing safe	Narrow the question
`unavailable`	Store locked / transient	Retry later
`unknown_tool`	Name not offered	`tools/list`
`bad_request`	Bad arguments	Fix args

Relay 429 does not say which limiter fired.

TCP accept + TLS hang is offline or decoy — same class. Do not treat it as a distinct “unregistered” signal.