Fallback chains that keep the conversation alive.

Give any agent an ordered list of backends. The router tries each in turn and falls through on any failure — HTTP error, unreachable host, out of credits, missing key — logging a warning each time. Set it once for chat, TTS, or a shared defaults block.

One chain, inherited by every agent that needs it.

Any agent that omits llm/a2a inherits defaults.llm; any agent that omits tts inherits defaults.tts. An agent can still carry its own chain instead of the shared one — an agent with no chat backend anywhere is a config error at startup, not a silent failure.