R.5
Glossary
Each term below has one meaning across the manual, and its link names the section that defines it.
| Term | Meaning in this manual | Defined in |
|---|---|---|
| A2A error | One of the nine errors the specification defines and maps into every binding, such as TaskNotFoundError | 5.5 |
| Agent card | The JSON document that describes an agent, usually at /.well-known/agent-card.json | 2.1 |
| Artifact | An output of a task, made of parts. A long one arrives in chunks. | 3.3 |
| Blocking call | A send that waits for a terminal or interrupted state, the default | 4.1 |
| Capability | A card flag, such as streaming, that allows a group of operations | 2.1 |
| Card signature | An entry in signatures: a JSON Web Signature over the card's RFC 8785 canonical form | 2.3 |
| Context | A conversation that groups tasks and messages under one contextId | 3.5 |
| Direct reply | A Message that is the whole result of a send, with no task | 4.1 |
| Extended card | A fuller card that GetExtendedAgentCard returns to an authenticated client | 2.3 |
| Extension | A protocol addition named by a URI, declared in capabilities.extensions and activated with A2A-Extensions | 6.2 |
| History | The messages a task keeps, capped by historyLength | 4.3 |
| In-task authorization | An agent's request for permission partway through a task. The task waits in TASK_STATE_AUTH_REQUIRED for an answer that arrives outside A2A. | 6.1 |
| Interface | One entry in supportedInterfaces: a URL, a protocol binding, a protocol version, and an optional tenant | 2.1 |
| Interrupted state | TASK_STATE_INPUT_REQUIRED or TASK_STATE_AUTH_REQUIRED. The task waits. | 4.2 |
| Message | One turn, with a messageId, a role, and parts | 3.1 |
| Opaque | Hidden from the caller. An opaque agent shows only what it sends back. | 1.1 |
| Part | The smallest unit of content: one of text, raw, url, or data | 3.2 |
| Polling | Calling GetTask again and again to follow a task | 4.1 |
| Protocol binding | One concrete form of the operations: JSON-RPC, gRPC, or HTTP+JSON | 5.1 |
| Push notification | An HTTP POST of a StreamResponse to a webhook the client registered | 4.6 |
| Remote agent | The agent behind an A2A endpoint that receives messages and runs tasks | 1.1 |
| Security requirement | One entry in securityRequirements, which maps scheme names to the scopes a request needs | 6.1 |
| Security scheme | A named way to authenticate, declared in a card's securitySchemes | 6.1 |
| Service parameter | A per-request key, such as A2A-Version, sent as an HTTP header or as gRPC metadata | 5.4 |
| Skill | One entry in a card's skills, such as run-tests | 2.1 |
| Status message | The one message a TaskStatus can hold, such as a question or a reason | 3.1 |
| Stream | Server-Sent Events, or a gRPC stream, of StreamResponse frames. SubscribeToTask opens one on a task. | 4.5 |
| Task | The unit of work a server creates for a message, with an id and a TaskState | 3.4 |
| Tenant | An optional routing string on an interface, copied into every request sent to that interface | 5.2 |
| Terminal state | TASK_STATE_COMPLETED, TASK_STATE_FAILED, TASK_STATE_CANCELED, or TASK_STATE_REJECTED. The task never changes again. | 4.4 |
A2A protocol 1.0.1 · 3303592 · 2026-05-28 · Edition 2026.10