Objects and fields

The proto defines every object in a request, a response, or a stream frame, and each field has one lowerCamelCase JSON name, one type, and a presence rule.

Bold rows name proto messages, the normative source for data objects, and the rows under each are its fields. Fields carry their lowerCamelCase JSON names, although the §5.5 example converts push_notification_config, a field the proto names task_push_notification_config (conflict D22).

Yes in the Req. column marks a REQUIRED field, which must be present and set in every valid message. A required array holds one element or more, yet an empty tasks list is legitimate (conflict D19). A type that starts with optional uses the proto keyword that records whether the field was set.

The tables leave out some optional fields. Task, Message, Part, Artifact, and both stream events carry a metadata object, and Message and Artifact carry extensions, a list of URIs. A part can name a filename, an artifact a description, a skill its examples, and a card a documentationUrl and an iconUrl. Every request message, from SendMessageRequest to GetExtendedAgentCardRequest, carries an optional tenant string that must match the chosen interface's tenant when that is set. Operations by binding lists the request and response messages.

Task and message objects

Message, Part, Artifact and chunks, Task, TaskStatus, and TaskState, SendMessage, and push notification configs define these objects.

Field	Type	Req.	Meaning
Task			The unit of work, created by the server
id	string	yes	Minted by the server
contextId	string		The context. Both stream events require it.
status	TaskStatus	yes	The state, status message, and time
artifacts	repeated Artifact		The outputs. `ListTasks` needs `includeArtifacts` for them.
history	repeated Message		The kept messages, capped by `historyLength`
TaskStatus			The status of a task
state	TaskState	yes	A value from Task states
message	Message		The agent's note, such as a question
timestamp	google.protobuf.Timestamp		When it was recorded, in ISO 8601 and UTC
Message			One turn of conversation
messageId	string	yes	Minted by the sender. Agents can use it to detect duplicates.
contextId	string		The context. Every server message has it.
taskId	string		The task. A client value names an existing task that matches `contextId`.
role	Role	yes	`ROLE_USER` from the client, `ROLE_AGENT` from the server
parts	repeated Part	yes	The content, one part or more
referenceTaskIds	repeated string		Other tasks the message refers to
Part			Exactly one of `text`, `raw`, `url`, and `data`
text	string		Text content
raw	bytes		File content, base64 in JSON
url	string		A URL to the file content
data	google.protobuf.Value		Any JSON value
mediaType	string		The media type, for every kind of part
Artifact			One output of a task
artifactId	string	yes	Unique in the task. Chunks match on it.
name	string		A readable name, such as `test-log.txt`
parts	repeated Part	yes	The content, one part or more
TaskStatusUpdateEvent			A change of task status
taskId, contextId	string	yes	The task and its context
status	TaskStatus	yes	The new status
TaskArtifactUpdateEvent			A new artifact, or one chunk of it
taskId, contextId	string	yes	The task and its context
artifact	Artifact	yes	The artifact or the chunk
append	bool		When true, add to the artifact with this `artifactId`
lastChunk	bool		When true, this is the last chunk
StreamResponse			A frame or push body, one of four
task	Task		The task now. It opens task streams and subscriptions.
message	Message		A message-only stream holds one, then closes.
statusUpdate	TaskStatusUpdateEvent		A change of task status
artifactUpdate	TaskArtifactUpdateEvent		A new artifact or chunk
SendMessageConfiguration			The `configuration` of a send
acceptedOutputModes	repeated string		Media types the client accepts
taskPushNotificationConfig	TaskPushNotificationConfig		A webhook to register, with no `taskId`
historyLength	optional int32		Unset means no limit, and `0` means none.
returnImmediately	bool		True returns once the task exists. The default waits.
TaskPushNotificationConfig			A webhook for one task
tenant	string		Must match the interface `tenant`, if set
id	string		The configuration id, assigned on create
taskId	string		The task it belongs to
url	string	yes	The webhook URL
token	string		A token for the task or session. No transport is defined.
authentication	AuthenticationInfo		How the agent authenticates: a required `scheme`, such as `Bearer`, and optional `credentials`

Agent card objects

AgentCard fields, Extended cards and signatures, Security schemes, and Extensions define these objects.

Field	Type	Req.	Meaning
AgentCard			What the agent offers, where, and what it demands
name	string	yes	A readable name, such as `test-runner`
description	string	yes	What the agent does
supportedInterfaces	repeated AgentInterface	yes	Where to reach it. The first entry is preferred.
provider	AgentProvider		The provider: a required `url` and `organization`
version	string	yes	The agent's own version, such as `2.3.0`
capabilities	AgentCapabilities	yes	The optional features it supports
securitySchemes	map<string, SecurityScheme>		Accepted authentication schemes, by name
securityRequirements	repeated SecurityRequirement		Scheme names mapped to scopes, such as `{"schemes": {"bearer": {"list": []}}}`
defaultInputModes	repeated string	yes	Media types accepted across all skills
defaultOutputModes	repeated string	yes	Media types produced across all skills
skills	repeated AgentSkill	yes	What the agent does well
signatures	repeated AgentCardSignature		JWS signatures over the canonical card
AgentInterface			One way to reach the agent
url	string	yes	An absolute HTTPS URL in production
protocolBinding	string	yes	`JSONRPC`, `GRPC`, `HTTP+JSON`, or a custom binding URI
tenant	string		A routing value. When set, copy it into each request sent to this interface.
protocolVersion	string	yes	The A2A version, such as `1.0`
AgentCapabilities			Optional features. An absent flag means `false`.
streaming	optional bool		Allows `SendStreamingMessage` and `SubscribeToTask`
pushNotifications	optional bool		Allows the four push configuration operations
extensions	repeated AgentExtension		The protocol extensions it supports
extendedAgentCard	optional bool		Allows `GetExtendedAgentCard`
AgentExtension			One extension the agent supports, in `capabilities.extensions`
uri	string		The URI that identifies the extension
description	string		How this agent uses the extension
required	bool		When true, the client must understand and comply with the extension
params	google.protobuf.Struct		Configuration for the extension, as a JSON object
AgentSkill			One thing the agent does well
id	string	yes	The skill id, such as `run-tests`
name	string	yes	A readable name
description	string	yes	What the skill does
tags	repeated string	yes	Keywords, one or more
inputModes	repeated string		Input media types that override the card's
outputModes	repeated string		Output media types that override the card's
securityRequirements	repeated SecurityRequirement		Requirements for this skill

Names the proto does not define

Older material and stale pages of the docs still use these names, and none exists in the v1.0.1 proto (conflict D10).

Name	Use instead
`kind` on a part or a stream event	The member name, such as `text` or `statusUpdate`
`TextPart`, `FilePart`, `DataPart`	One `Part` with `text`, `raw` or `url`, or `data`
`final` on `TaskStatusUpdateEvent`	The close of the stream
`taskStatusUpdate`, `taskArtifactUpdate`	`statusUpdate`, `artifactUpdate`
`Task.createdAt`, `Task.lastModified`	`status.timestamp`
`TaskArtifactUpdateEvent.index`	`artifactId`, which chunks match on
`PushNotificationConfig`, `CreateTaskPushNotificationConfigRequest`	`TaskPushNotificationConfig`, which the create operation takes as its request (conflict D4)
`configId` on a push configuration	`id`
Card `url`, `protocolVersion`, `preferredTransport`, `additionalInterfaces`	`supportedInterfaces`, with `url` and `protocolVersion` on each entry
Card `supportsAuthenticatedExtendedCard`	`capabilities.extendedAgentCard`
Card `security`	`securityRequirements` (conflict D6)
Card `extensions` at the top level	`capabilities.extensions` (discrepancy N2)
`cursor`, `limit`, `nextCursor`	`pageToken`, `pageSize`, `nextPageToken`

A2A protocol 1.0.1 · 3303592 · 2026-05-28 · Edition 2026.10