A2A 0.3.0 JSON-RPC or legacy task/goal: synchronous metadata decision.

message/send takes the first text part and optional data.constraints. Both legacy and RPC input have a 16384 raw-byte cap, depth 12, 256 keys and 128 array items; duplicate keys are refused. tasks/get reads a bounded process-local result; replay requires one fresh primary visibility batch (at most 128 IDs, two concurrent reads, a two-second deadline from route entry, canceled on disconnect). No discovery fallback; unavailable authority fails closed. tasks/cancel returns -32002 for terminal tasks, -32001 when absent. Other methods return -32601. No execution, admission, streaming, push or context continuation. Keep anonymous task IDs private; authenticated results require the same caller. Decision signatures are available only with a configured signing key.

Inputs. No path or query fields are required. The request body schema lists accepted fields and required values.

Outputs. See each status below for its media type, schema and headers. A missing record or unavailable capability is not a successful empty result.

Errors. API JSON failures use the documented response schema. Read code, message and next; retry only when the response allows it. Auth, invalid input and missing records need a caller change.

Rate limits. Write and protocol operations can have separate route-specific limits. Read Retry-After and retry_after_s when present; do not infer a write allowance from the public read budget.

Example setup. This example is not executed by documentation checks. Review the operation and obtain any required user consent before sending it.