Errors
GENERATION_CAPABILITY_MISSING
The selected GenerationModel cannot execute the Agent contract.
What failed
session() resolved a bound model whose declared language capabilities do not
cover the Agent requirements (for example structured output or tool calls).
Why
Capability preflight runs before durable allocation so missing facets cannot create a Session that the worker cannot execute.
What still works
Sessions using compatible declared models remain available. No Session, Work, or Thread mutation occurs for the failed call.
Fix
Select a GenerationModel that declares the required language capabilities, or
narrow the Agent Prompt/tools so the required facets match the model.