Agent2Agent (A2A) Protocol Specification¶

Version: 0.1.0

1. Introduction¶

The Agent2Agent (A2A) Protocol is an open standard designed to facilitate communication and interoperability between independent, potentially opaque AI agent systems. In an ecosystem where agents might be built using different frameworks, languages, or by different vendors, A2A provides a common language and interaction model.

This document provides the detailed technical specification for the A2A protocol. Its primary goal is to enable agents to:

Discover each other's capabilities.
Negotiate interaction modalities (text, files, structured data).
Manage collaborative tasks.
Securely exchange information to achieve user goals without needing access to each other's internal state, memory, or tools.

1.1. Key Goals of A2A¶

Interoperability: Bridge the communication gap between disparate agentic systems.
Collaboration: Enable agents to delegate tasks, exchange context, and work together on complex user requests.
Discovery: Allow agents to dynamically find and understand the capabilities of other agents.
Flexibility: Support various interaction modes including synchronous request/response, streaming for real-time updates, and asynchronous push notifications for long-running tasks.
Security: Facilitate secure communication patterns suitable for enterprise environments, relying on standard web security practices.
Asynchronicity: Natively support long-running tasks and interactions that may involve human-in-the-loop scenarios.

1.2. Guiding Principles¶

Simple: Reuse existing, well-understood standards (HTTP, JSON-RPC 2.0, Server-Sent Events).
Enterprise Ready: Address authentication, authorization, security, privacy, tracing, and monitoring by aligning with established enterprise practices.
Async First: Designed for (potentially very) long-running tasks and human-in-the-loop interactions.
Modality Agnostic: Support exchange of diverse content types including text, audio/video (via file references), structured data/forms, and potentially embedded UI components (e.g., iframes referenced in parts).
Opaque Execution: Agents collaborate based on declared capabilities and exchanged information, without needing to share their internal thoughts, plans, or tool implementations.

For a broader understanding of A2A's purpose and benefits, see What is A2A?.

2. Core Concepts Summary¶

A2A revolves around several key concepts. For detailed explanations, please refer to the Key Concepts guide.

A2A Client: An application or agent that initiates requests to an A2A Server on behalf of a user or another system.
A2A Server (Remote Agent): An agent or agentic system that exposes an A2A-compliant HTTP endpoint, processing tasks and providing responses.
Agent Card: A JSON metadata document published by an A2A Server, describing its identity, capabilities, skills, service endpoint, and authentication requirements.
Task: The fundamental unit of work managed by A2A, identified by a unique ID. Tasks are stateful and progress through a defined lifecycle.
Message: A communication turn within a Task, having a role ("user" or "agent") and containing one or more Parts.
Part: The smallest unit of content within a Message or Artifact (e.g., TextPart, FilePart, DataPart).
Artifact: An output (e.g., a document, image, structured data) generated by the agent as a result of a task, composed of Parts.
Streaming (SSE): Real-time, incremental updates for tasks (status changes, artifact chunks) delivered via Server-Sent Events.
Push Notifications: Asynchronous task updates delivered via server-initiated HTTP POST requests to a client-provided webhook URL, for long-running or disconnected scenarios.
Session: An optional, client-generated identifier to logically group related tasks.

3. Transport and Format¶

3.1. Transport Protocol¶

A2A communication MUST occur over HTTP(S).
The A2A Server exposes its service at a URL defined in its AgentCard.

3.2. Data Format¶

A2A uses JSON-RPC 2.0 as the payload format for all requests and responses (excluding the SSE stream wrapper).

Client requests and server responses MUST adhere to the JSON-RPC 2.0 specification.
The Content-Type header for HTTP requests and responses containing JSON-RPC payloads MUST be application/json.

3.3. Streaming Transport (Server-Sent Events)¶

When streaming is used for methods like tasks/sendSubscribe or tasks/resubscribe:

The server responds with an HTTP 200 OK status and a Content-Type header of text/event-stream.
The body of this HTTP response contains a stream of Server-Sent Events (SSE) as defined by the W3C.
Each SSE data field contains a complete JSON-RPC 2.0 Response object (specifically, a SendTaskStreamingResponse).

4. Authentication and Authorization¶

A2A treats agents as standard enterprise applications, relying on established web security practices. Identity information is not transmitted within A2A JSON-RPC payloads; it is handled at the HTTP transport layer.

For a comprehensive guide on enterprise security aspects, see Enterprise-Ready Features.

4.1. Transport Security¶

As stated in section 3.1, production deployments MUST use HTTPS. Implementations SHOULD use modern TLS configurations (TLS 1.2+ recommended) with strong cipher suites.

4.2. Server Identity Verification¶

A2A Clients SHOULD verify the A2A Server's identity by validating its TLS certificate against trusted certificate authorities (CAs) during the TLS handshake.

4.3. Client/User Identity & Authentication Process¶

Discovery of Requirements: The client discovers the server's required authentication schemes via the authentication field in the AgentCard. Scheme names often align with OpenAPI Authentication methods (e.g., "Bearer" for OAuth 2.0 tokens, "Basic" for Basic Auth, "ApiKey" for API keys).
Credential Acquisition (Out-of-Band): The client obtains the necessary credentials (e.g., API keys, OAuth tokens, JWTs) through an out-of-band process specific to the required authentication scheme and the identity provider. This process is outside the scope of the A2A protocol itself.
Credential Transmission: The client includes these credentials in the appropriate HTTP headers (e.g., Authorization: Bearer <token>, X-API-Key: <value>) of every A2A request sent to the server.

4.4. Server Responsibilities for Authentication¶

The A2A Server:

MUST authenticate every incoming request based on the provided HTTP credentials and its declared authentication requirements from its Agent Card.
SHOULD use standard HTTP status codes like 401 Unauthorized or 403 Forbidden for authentication challenges or rejections.
SHOULD include relevant HTTP headers (e.g., WWW-Authenticate) with 401 Unauthorized responses to indicate the required authentication scheme(s), guiding the client.

4.5. In-Task Authentication (Secondary Credentials)¶

If an agent, during the execution of a task, requires additional credentials for a different system or resource (e.g., to access a specific tool on behalf of the user that requires its own auth):

It SHOULD transition the A2A task to the input-required state (see TaskState).
The accompanying TaskStatus.message (often a DataPart) SHOULD provide details about the required secondary authentication, potentially using an AuthenticationInfo-like structure to describe the need.
The A2A Client then obtains these new credentials out-of-band and provides them in a subsequent tasks/send or tasks/sendSubscribe request. How these credentials are used (e.g., passed as data within the A2A message if the agent is proxying, or used by the client to interact directly with the secondary system) depends on the specific scenario.

4.6. Authorization¶

Once a client is authenticated, the A2A Server is responsible for authorizing the request based on the authenticated client/user identity and its own policies. Authorization logic is implementation-specific and MAY be enforced based on:

The specific skills requested (e.g., as identified by AgentSkill.id from the Agent Card).
The actions attempted within the task.
Data access policies relevant to the resources the agent manages.
OAuth scopes associated with the presented token, if applicable.

Servers should implement the principle of least privilege.

5. Agent Discovery: The Agent Card¶

5.1. Purpose¶

A2A Servers MUST make an Agent Card available. The Agent Card is a JSON document that describes the server's identity, capabilities, skills, service endpoint URL, and how clients should authenticate and interact with it. Clients use this information for discovering suitable agents and for configuring their interactions.

For more on discovery strategies, see the Agent Discovery guide.

5.2. Discovery Mechanisms¶

Clients can find Agent Cards through various methods, including but not limited to:

Well-Known URI: Accessing a predefined path on the agent's domain (see Section 5.3).
Registries/Catalogs: Querying curated catalogs or registries of agents (which might be enterprise-specific, public, or domain-specific).
Direct Configuration: Clients may be pre-configured with the Agent Card URL or the card content itself.

5.3. Recommended Location¶

If using the well-known URI strategy, the recommended location for an agent's Agent Card is: https://{server_domain}/.well-known/agent.json This follows the principles of RFC 8615 for well-known URIs.

5.4. Security of Agent Cards¶

Agent Cards themselves might contain information that is considered sensitive (e.g., the URL of an internal-only agent, or scheme-specific information in authentication.credentials).

If an Agent Card contains sensitive information, the endpoint serving the card MUST be protected by appropriate access controls (e.g., mTLS, network restrictions, authentication required to fetch the card).
It is generally NOT RECOMMENDED to include plaintext secrets (like static API keys) directly in an Agent Card. Prefer authentication schemes where clients obtain dynamic credentials out-of-band. If authentication.credentials is used, it should be for non-secret information like OAuth flow URLs or API key names (not values).

5.5. `AgentCard` Object Structure¶

// An AgentCard conveys key information about an A2A Server:
// - Overall identity and descriptive details.
// - Service endpoint URL.
// - Supported A2A protocol capabilities (streaming, push notifications).
// - Authentication requirements.
// - Default input/output content types (MIME types).
// - A list of specific skills the agent offers.
interface AgentCard {
  // Human-readable name of the agent (e.g., "Recipe Advisor Agent").
  name: string;
  // A human-readable description of the agent and its general purpose.
  // [CommonMark](https://commonmark.org/) MAY be used for rich text formatting.
  // (e.g., "This agent helps users find recipes, plan meals, and get cooking instructions.")
  description?: string | null;
  // The base URL endpoint for the agent's A2A service (where JSON-RPC requests are sent).
  // Must be an absolute HTTPS URL for production (e.g., `https://agent.example.com/a2a/api`).
  // HTTP MAY be used for local development/testing only.
  url: string;
  // Information about the organization or entity providing the agent.
  provider?: AgentProvider | null;
  // Version string for the agent or its A2A implementation
  // (format is defined by the provider, e.g., "1.0.0", "2023-10-26-beta").
  version: string;
  // URL pointing to human-readable documentation for the agent (e.g., API usage, detailed skill descriptions).
  documentationUrl?: string | null;
  // Specifies optional A2A protocol features supported by this agent.
  capabilities: AgentCapabilities;
  // Authentication schemes required to interact with the agent's `url` endpoint.
  // If `null`, omitted, or an empty `schemes` array, no A2A-level authentication is explicitly advertised
  // (NOT recommended for production; other security like network ACLs might still apply).
  authentication?: AgentAuthentication | null;
  // Array of [MIME types](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types)
  // the agent generally accepts as input across all skills, unless overridden by a specific skill.
  // Default if omitted: `["text/plain"]`. Example: `["text/plain", "image/png"]`.
  defaultInputModes?: string[];
  // Array of MIME types the agent generally produces as output across all skills, unless overridden by a specific skill.
  // Default if omitted: `["text/plain"]`. Example: `["text/plain", "application/json"]`.
  defaultOutputModes?: string[];
  // An array of specific skills or capabilities the agent offers.
  // Must contain at least one skill if the agent is expected to perform actions beyond simple presence.
  skills: AgentSkill[];
}

Field Name	Type	Required	Description
`name`	`string`	Yes	Human-readable name of the agent.
`description`	`string` \| `null`	No	Human-readable description. CommonMark MAY be used.
`url`	`string`	Yes	Base URL for the agent's A2A service. Must be absolute. HTTPS for production.
`provider`	`AgentProvider` \| `null`	No	Information about the agent's provider.
`version`	`string`	Yes	Agent or A2A implementation version string.
`documentationUrl`	`string` \| `null`	No	URL to human-readable documentation for the agent.
`capabilities`	`AgentCapabilities`	Yes	Specifies optional A2A protocol features supported (e.g., streaming, push notifications).
`authentication`	`AgentAuthentication` \| `null`	No	Authentication schemes required. `null` or empty implies no A2A-advertised auth (not recommended for production).
`defaultInputModes`	`string[]`	No	Default accepted input MIME types. Defaults to `["text/plain"]` if omitted.
`defaultOutputModes`	`string[]`	No	Default produced output MIME types. Defaults to `["text/plain"]` if omitted.
`skills`	`AgentSkill[]`	Yes	Array of skills. Must have at least one if the agent performs actions.

5.5.1. `AgentProvider` Object¶

Information about the organization or entity providing the agent.

interface AgentProvider {
  // Name of the organization or entity.
  organization: string;
  // URL for the provider's organization website or relevant contact page.
  url?: string | null;
}

Field Name	Type	Required	Description
`organization`	`string`	Yes	Name of the organization/entity.
`url`	`string` \| `null`	No	URL for the provider's website/contact.

5.5.2. `AgentCapabilities` Object¶

Specifies optional A2A protocol features supported by the agent.

interface AgentCapabilities {
  // If `true`, the agent supports `tasks/sendSubscribe` and `tasks/resubscribe` for real-time
  // updates via Server-Sent Events (SSE). Default: `false`.
  streaming?: boolean;
  // If `true`, the agent supports `tasks/pushNotification/set` and `tasks/pushNotification/get`
  // for asynchronous task updates via webhooks. Default: `false`.
  pushNotifications?: boolean;
  // If `true`, the agent may include a detailed history of status changes
  // within the `Task` object (future enhancement; specific mechanism TBD). Default: `false`.
  stateTransitionHistory?: boolean;
}

Field Name	Type	Required	Default	Description
`streaming`	`boolean`	No	`false`	Indicates support for SSE streaming methods (`tasks/sendSubscribe`, `tasks/resubscribe`).
`pushNotifications`	`boolean`	No	`false`	Indicates support for push notification methods (`tasks/pushNotification/*`).
`stateTransitionHistory`	`boolean`	No	`false`	Placeholder for future feature: exposing detailed task status change history.

5.5.3. `AgentAuthentication` Object¶

Describes the authentication requirements for accessing the agent's url endpoint.

interface AgentAuthentication {
  // Array of authentication scheme names supported/required by the agent's endpoint
  // (e.g., "Bearer", "Basic", "OAuth2", "ApiKey").
  // Standard names (e.g., from OpenAPI specification, IANA registry) SHOULD be used where applicable.
  // An empty array means no specific A2A-level schemes are advertised.
  schemes: string[];
  // Optional field, MAY contain non-secret, scheme-specific information.
  // Examples: For "OAuth2", this could be a JSON string with `tokenUrl`, `authorizationUrl`, `scopes`.
  // For "ApiKey", it could specify the header name (`in: "header"`, `name: "X-Custom-API-Key"`).
  // **CRITICAL**: This field MUST NOT contain plaintext secrets (e.g., actual API key values, passwords).
  // If the Agent Card itself needs to be protected due to this field containing sensitive URLs
  // or configuration, the endpoint serving the Agent Card MUST be secured.
  credentials?: string | null; // E.g., A JSON string parsable by the client for scheme details.
}

Field Name	Type	Required	Description
`schemes`	`string[]`	Yes	Array of auth scheme names (e.g., "Bearer", "OAuth2", "ApiKey"). Empty array means no A2A-advertised schemes.
`credentials`	`string` \| `null`	No	Optional non-secret, scheme-specific configuration info (e.g., OAuth URLs, API key header name). MUST NOT contain plaintext secrets. Secure the Agent Card if this field implies sensitivity.

5.5.4. `AgentSkill` Object¶

Describes a specific capability, function, or area of expertise the agent can perform or address.

interface AgentSkill {
  // A unique identifier for this skill within the context of this agent
  // (e.g., "currency-converter", "generate-image-from-prompt", "summarize-text-v2").
  // Clients MAY use this ID to request a specific skill if the agent supports such dispatch.
  id: string;
  // Human-readable name of the skill (e.g., "Currency Conversion Service", "Image Generation AI").
  name: string;
  // Detailed description of what the skill does, its purpose, and any important considerations.
  // [CommonMark](https://commonmark.org/) MAY be used for rich text formatting.
  description?: string | null;
  // Array of keywords or categories for discoverability and categorization
  // (e.g., ["finance", "conversion"], ["media", "generative ai", "image"]).
  tags?: string[] | null;
  // Array of example prompts, inputs, or use cases illustrating how to use this skill
  // (e.g., ["convert 100 USD to EUR", "generate a photorealistic image of a cat wearing a wizard hat"]).
  // These help clients (and potentially end-users or other agents) understand how to formulate requests for this skill.
  examples?: string[] | null;
  // Overrides `agentCard.defaultInputModes` specifically for this skill.
  // If `null` or omitted, the agent's `defaultInputModes` apply.
  inputModes?: string[] | null; // Array of MIME types
  // Overrides `agentCard.defaultOutputModes` specifically for this skill.
  // If `null` or omitted, the agent's `defaultOutputModes` apply.
  outputModes?: string[] | null; // Array of MIME types
}

Field Name	Type	Required	Description
`id`	`string`	Yes	Unique skill identifier within this agent.
`name`	`string`	Yes	Human-readable skill name.
`description`	`string` \| `null`	No	Detailed skill description. CommonMark MAY be used.
`tags`	`string[]` \| `null`	No	Keywords/categories for discoverability.
`examples`	`string[]` \| `null`	No	Example prompts or use cases demonstrating skill usage.
`inputModes`	`string[]` \| `null`	No	Overrides `defaultInputModes` for this specific skill. Accepted MIME types.
`outputModes`	`string[]` \| `null`	No	Overrides `defaultOutputModes` for this specific skill. Produced MIME types.

5.6. Sample Agent Card¶

{
  "name": "GeoSpatial Route Planner Agent",
  "description": "Provides advanced route planning, traffic analysis, and custom map generation services. This agent can calculate optimal routes, estimate travel times considering real-time traffic, and create personalized maps with points of interest.",
  "url": "https://georoute-agent.example.com/a2a/v1",
  "provider": {
    "organization": "Example Geo Services Inc.",
    "url": "https://www.examplegeoservices.com"
  },
  "version": "1.2.0",
  "documentationUrl": "https://docs.examplegeoservices.com/georoute-agent/api",
  "capabilities": {
    "streaming": true,
    "pushNotifications": true,
    "stateTransitionHistory": false
  },
  "authentication": {
    "schemes": ["OAuth2"],
    "credentials": "{\"authorizationUrl\": \"https://auth.examplegeoservices.com/authorize\", \"tokenUrl\": \"https://auth.examplegeoservices.com/token\", \"scopes\": {\"route:plan\": \"Allows planning new routes.\", \"map:custom\": \"Allows creating and managing custom maps.\"}}"
  },
  "defaultInputModes": ["application/json", "text/plain"],
  "defaultOutputModes": ["application/json", "image/png"],
  "skills": [
    {
      "id": "route-optimizer-traffic",
      "name": "Traffic-Aware Route Optimizer",
      "description": "Calculates the optimal driving route between two or more locations, taking into account real-time traffic conditions, road closures, and user preferences (e.g., avoid tolls, prefer highways).",
      "tags": ["maps", "routing", "navigation", "directions", "traffic"],
      "examples": [
        "Plan a route from '1600 Amphitheatre Parkway, Mountain View, CA' to 'San Francisco International Airport' avoiding tolls.",
        "{\"origin\": {\"lat\": 37.422, \"lng\": -122.084}, \"destination\": {\"lat\": 37.7749, \"lng\": -122.4194}, \"preferences\": [\"avoid_ferries\"]}"
      ],
      "inputModes": ["application/json", "text/plain"],
      "outputModes": [
        "application/json",
        "application/vnd.geo+json",
        "text/html"
      ]
    },
    {
      "id": "custom-map-generator",
      "name": "Personalized Map Generator",
      "description": "Creates custom map images or interactive map views based on user-defined points of interest, routes, and style preferences. Can overlay data layers.",
      "tags": ["maps", "customization", "visualization", "cartography"],
      "examples": [
        "Generate a map of my upcoming road trip with all planned stops highlighted.",
        "Show me a map visualizing all coffee shops within a 1-mile radius of my current location."
      ],
      "inputModes": ["application/json"],
      "outputModes": [
        "image/png",
        "image/jpeg",
        "application/json",
        "text/html"
      ]
    }
  ]
}

6. Protocol Data Objects¶

These objects define the structure of data exchanged within the JSON-RPC methods of the A2A protocol.

6.1. `Task` Object¶

Represents the stateful unit of work being processed by the A2A Server for an A2A Client. A task encapsulates the entire interaction related to a specific goal or request.

interface Task {
  // A unique identifier for the task. This ID is typically generated by the client
  // when initiating the task and MUST be used by the server to refer to this task.
  // It should be sufficiently unique (e.g., a UUID v4).
  id: string;
  // An optional, client-generated identifier used to group related tasks into a logical session.
  // Useful for maintaining context across multiple, sequential, or related tasks.
  sessionId?: string | null;
  // The current status of the task, including its lifecycle state, an optional associated message,
  // and a timestamp.
  status: TaskStatus;
  // An array of outputs (artifacts) generated by the agent for this task.
  // This array can be populated incrementally, especially during streaming.
  // Artifacts represent the tangible results of the task.
  artifacts?: Artifact[] | null;
  // An optional array of recent messages exchanged within this task,
  // ordered chronologically (oldest first).
  // This history is included if requested by the client via the `historyLength` parameter
  // in `TaskSendParams` or `TaskQueryParams`.
  history?: Message[] | null;
  // Arbitrary key-value metadata associated with the task.
  // Keys SHOULD be strings; values can be any valid JSON type (string, number, boolean, array, object).
  // This can be used for application-specific data, tracing info, etc.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Description
`id`	`string`	Yes	Unique task identifier (e.g., UUID), typically client-generated.
`sessionId`	`string` \| `null`	No	Optional client-generated ID to group related tasks into a session.
`status`	`TaskStatus`	Yes	Current status of the task (state, message, timestamp).
`artifacts`	`Artifact[]` \| `null`	No	Array of outputs generated by the agent for this task.
`history`	`Message[]` \| `null`	No	Optional array of recent messages exchanged, if requested by `historyLength`.
`metadata`	`Record<string, any>` \| `null`	No	Arbitrary key-value metadata associated with the task.

6.2. `TaskStatus` Object¶

Represents the current state and associated context (e.g., a message from the agent) of a Task.

interface TaskStatus {
  // The current lifecycle state of the task.
  state: TaskState;
  // An optional message associated with the current status.
  // This could be a progress update from the agent, a prompt for more input,
  // a summary of the final result, or an error message.
  message?: Message | null;
  // The date and time (UTC is STRONGLY recommended) when this status was recorded by the server.
  // Format: ISO 8601 `date-time` string (e.g., "2023-10-27T10:00:00Z").
  timestamp?: string | null;
}

Field Name	Type	Required	Description
`state`	`TaskState`	Yes	Current lifecycle state of the task.
`message`	`Message` \| `null`	No	Optional message providing context for the current status.
`timestamp`	`string` (ISO 8601) \| `null`	No	Timestamp (UTC recommended) when this status was recorded.

6.3. `TaskState` Enum¶

Defines the possible lifecycle states of a Task.

type TaskState =
  | 'submitted' // Task received by server, acknowledged, but processing has not yet actively started.
  | 'working' // Task is actively being processed by the agent.
  | 'input-required' // Agent requires additional input from the client/user to proceed. (Task is paused)
  | 'completed' // Task finished successfully. (Terminal state)
  | 'canceled' // Task was canceled by the client or potentially by the server. (Terminal state)
  | 'failed' // Task terminated due to an error during processing. (Terminal state)
  | 'unknown'; // The state of the task cannot be determined (e.g., task ID invalid or expired). (Effectively a terminal state from client's PoV for that ID)

Value	Description	Terminal?
`submitted`	Task received by the server and acknowledged, but processing has not yet actively started.	No
`working`	Task is actively being processed by the agent. Client may expect further updates or a terminal state.	No
`input-required`	Agent requires additional input from the client/user to proceed. The task is effectively paused.	No (Pause)
`completed`	Task finished successfully. Results are typically available in `Task.artifacts` or `TaskStatus.message`.	Yes
`canceled`	Task was canceled (e.g., by a `tasks/cancel` request or server-side policy).	Yes
`failed`	Task terminated due to an error during processing. `TaskStatus.message` may contain error details.	Yes
`unknown`	The state of the task cannot be determined (e.g., task ID is invalid, unknown, or has expired).	Yes

6.4. `Message` Object¶

Represents a single communication turn or a piece of contextual information within a Task. Messages are used for instructions, prompts, replies, and status updates.

interface Message {
  // Indicates the sender of the message:
  // "user" for messages originating from the A2A Client (acting on behalf of an end-user or system).
  // "agent" for messages originating from the A2A Server (the remote agent).
  role: 'user' | 'agent';
  // An array containing the content of the message, broken down into one or more parts.
  // A message MUST contain at least one part.
  // Using multiple parts allows for rich, multi-modal content (e.g., text accompanying an image).
  parts: Part[];
  // Arbitrary key-value metadata associated with the message.
  // Keys SHOULD be strings; values can be any valid JSON type.
  // Useful for timestamps, source identifiers, language codes, etc.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Description
`role`	`"user"` \| `"agent"`	Yes	Indicates the sender: `"user"` (from A2A Client) or `"agent"` (from A2A Server).
`parts`	`Part[]`	Yes	Array of content parts. Must contain at least one part.
`metadata`	`Record<string, any>` \| `null`	No	Arbitrary key-value metadata associated with this message.

6.5. `Part` Union Type¶

Represents a distinct piece of content within a Message or Artifact. A Part object is a discriminated union, identified by its mandatory type field. All Part types also include an optional metadata field (Record<string, any> | null) for part-specific metadata.

It MUST be one of the following:

6.5.1. `TextPart` Object¶

For conveying plain textual content.

interface TextPart {
  type: 'text'; // Discriminator
  text: string; // The actual textual content.
  metadata?: Record<string, any> | null; // Optional metadata (e.g., language, formatting hints if any)
}

Field Name	Type	Required	Description
`type`	`"text"` (literal)	Yes	Identifies this part as textual content.
`text`	`string`	Yes	The textual content of the part.
`metadata`	`Record<string, any>` \| `null`	No	Optional metadata specific to this text part.

6.5.2. `FilePart` Object¶

For conveying file-based content.

interface FilePart {
  type: 'file'; // Discriminator
  file: FileContent; // Contains the file details and data (or reference).
  metadata?: Record<string, any> | null; // Optional metadata (e.g., purpose of the file)
}

Field Name	Type	Required	Description
`type`	`"file"` (literal)	Yes	Identifies this part as file content.
`file`	`FileContent`	Yes	Contains the file details and data/reference.
`metadata`	`Record<string, any>` \| `null`	No	Optional metadata specific to this file part.

6.5.3. `DataPart` Object¶

For conveying structured JSON data. Useful for forms, parameters, or any machine-readable information.

interface DataPart {
  type: 'data'; // Discriminator
  // The structured JSON data payload. This can be any valid JSON object or array.
  // The schema of this data is application-defined and may be implicitly understood
  // by the interacting agents or explicitly described (e.g., via a JSON Schema reference
  // in the `metadata` or associated `AgentSkill`).
  data: Record<string, any> | any[];
  metadata?: Record<string, any> | null; // Optional metadata (e.g., schema URL, version)
}

Field Name	Type	Required	Description
`type`	`"data"` (literal)	Yes	Identifies this part as structured data.
`data`	`Record<string, any>` \| `any[]`	Yes	The structured JSON data payload (an object or an array).
`metadata`	`Record<string, any>` \| `null`	No	Optional metadata specific to this data part (e.g., reference to a schema).

6.6. `FileContent` Object¶

Represents the data or reference for a file, used within a FilePart.

interface FileContent {
  // The original filename, if known (e.g., "document.pdf", "avatar.png").
  name?: string | null;
  // The [MIME type](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types)
  // of the file (e.g., "application/pdf", "image/png"). Strongly recommended for proper handling.
  mimeType?: string | null;
  // Base64 encoded string of the raw file content.
  // Use this for embedding small to medium-sized files directly.
  bytes?: string | null; // Base64 string
  // A URI (absolute URL is STRONGLY recommended) pointing to the file content.
  // Accessibility of this URI depends on the context (e.g., public URL, pre-signed URL, internal URL).
  // The client and server must have a way to resolve and access this URI if used.
  uri?: string | null;

  // Constraint: If file content is being transmitted, exactly one of `bytes` or `uri` MUST be non-null.
  // Both MAY be `null` or absent if the `FilePart` is merely representing a file reference
  // without transmitting its content in the current message (e.g., referring to a previously uploaded file).
}

Field Name	Type	Required	Description
`name`	`string` \| `null`	No	Original filename (e.g., "report.pdf").
`mimeType`	`string` \| `null`	No	MIME type (e.g., `image/png`). Strongly recommended.
`bytes`	`string` \| `null`	Conditionally (See Constraint)	Base64 encoded file content.
`uri`	`string` \| `null`	Conditionally (See Constraint)	URI (absolute URL strongly recommended) to file content. Accessibility is context-dependent.

Constraint: If file content is being transmitted, exactly one of bytes or uri MUST be provided and non-null. Both MAY be null or absent if the FilePart is only a reference or metadata about a file whose content is not being transferred in this specific part.

6.7. `Artifact` Object¶

Represents a tangible output generated by the agent during a task. Artifacts are the results or products of the agent's work.

interface Artifact {
  // A descriptive name for the artifact (e.g., "Quarterly Sales Report.pdf", "Generated Logo Design", "analysis_results.json").
  // This name might be used by the client for display or identification.
  name?: string | null;
  // A human-readable description of the artifact. [CommonMark](https://commonmark.org/) MAY be used.
  description?: string | null;
  // An array containing the content of the artifact, broken down into one or more parts.
  // An artifact MUST contain at least one part.
  // Using multiple parts allows for complex artifacts (e.g., a report with embedded images or data tables).
  parts: Part[];
  // A non-negative integer index for ordering artifacts or identifying artifact chunks during streaming.
  // Multiple artifacts (or artifact updates) can share the same index if they represent parts of the same logical output
  // that are being streamed or delivered separately.
  // Default: 0 if omitted.
  index?: number;
  // Used with streaming (`TaskArtifactUpdateEvent`):
  // If `true`, indicates this update's `parts` should be appended to the content of the artifact
  // currently identified by the same `index` value. This is useful for streaming textual data or
  // appending elements to a list in a `DataPart`.
  // If `false` or `null` (or omitted), this update replaces the artifact content at the given `index`.
  // This field is typically `false` for the first chunk of a streamed artifact.
  append?: boolean | null;
  // Used with streaming (`TaskArtifactUpdateEvent`):
  // If `true`, indicates this is the final update/chunk for the artifact at this `index`.
  // Signals the end of a streamed file or data structure.
  lastChunk?: boolean | null;
  // Arbitrary key-value metadata associated with the artifact.
  // Keys SHOULD be strings; values can be any valid JSON type.
  // Useful for creation timestamps, versioning info, checksums, etc.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Default	Description
`name`	`string` \| `null`	No	`null`	Descriptive name for the artifact.
`description`	`string` \| `null`	No	`null`	Human-readable description of the artifact.
`parts`	`Part[]`	Yes		Content of the artifact, as one or more `Part` objects. Must have at least one.
`index`	`integer`	No	`0`	Non-negative index for ordering artifacts or identifying chunks during streaming.
`append`	`boolean` \| `null`	No	`false`	In streaming (`TaskArtifactUpdateEvent`), `true` means append parts to artifact at `index`; `false` (default) means replace.
`lastChunk`	`boolean` \| `null`	No	`false`	In streaming (`TaskArtifactUpdateEvent`), `true` indicates this is the final update for the artifact at this `index`.
`metadata`	`Record<string, any>` \| `null`	No	`null`	Arbitrary key-value metadata associated with the artifact.

6.8. `PushNotificationConfig` Object¶

Configuration provided by the client to the server for sending asynchronous push notifications about task updates.

interface PushNotificationConfig {
  // The absolute HTTPS webhook URL where the A2A Server should POST task updates.
  // This URL MUST be HTTPS for security.
  url: string;
  // An optional, client-generated opaque token (e.g., a secret, a task-specific identifier, or a nonce).
  // The A2A Server SHOULD include this token in the notification request it sends to the `url`
  // (e.g., in a custom HTTP header like `X-A2A-Notification-Token` or similar).
  // This allows the client's webhook receiver to validate the relevance and authenticity of the notification.
  token?: string | null;
  // Authentication details the A2A Server needs to use when calling the client's `url`.
  // The client's webhook endpoint defines these requirements. This tells the A2A Server how to authenticate *itself* to the client's webhook.
  authentication?: AuthenticationInfo | null;
}

Field Name	Type	Required	Description
`url`	`string`	Yes	Absolute HTTPS webhook URL for the A2A Server to POST task updates to.
`token`	`string` \| `null`	No	Optional client-generated opaque token for the client's webhook receiver to validate the notification (e.g., server includes it in an `X-A2A-Notification-Token` header).
`authentication`	`AuthenticationInfo` \| `null`	No	Authentication details the A2A Server must use when calling the `url`. The client's webhook (receiver) defines these requirements.

6.9. `AuthenticationInfo` Object (for Push Notifications)¶

A generic structure for specifying authentication requirements, typically used within PushNotificationConfig to describe how the A2A Server should authenticate to the client's webhook.

interface AuthenticationInfo {
  // Array of authentication scheme names the caller (i.e., the A2A Server, in the context of push notifications)
  // must use when sending the request to the webhook URL (e.g., "Bearer" for an OAuth token, "ApiKey" for a pre-shared key, "Basic").
  // Standard names SHOULD be used.
  schemes: string[];
  // Optional field for providing static credentials or scheme-specific information
  // that the A2A Server needs to use.
  // Examples:
  // - For "ApiKey": A JSON string like `{"in": "header", "name": "X-Client-Webhook-Key", "value": "actual_api_key_value"}`.
  // - For "Bearer": If the A2A Server is expected to use a specific pre-issued token, it could be provided here. More commonly, the server would obtain its own token using OAuth client credentials flow if this field specifies an OAuth scheme.
  // **CRITICAL**: Use with extreme caution if this field contains secrets. This configuration is sent from client to server.
  // Prefer mechanisms where the server fetches its own credentials dynamically (e.g., OAuth client credentials flow with a pre-configured client ID/secret on the server side for the webhook's audience)
  // rather than having the client provide secrets to the server.
  // If this field *must* carry a secret, the A2A communication channel itself must be exceptionally secure, and both client and server must handle this data with care.
  credentials?: string | null; // E.g., A JSON string parsable by the server.
}

Field Name	Type	Required	Description
`schemes`	`string[]`	Yes	Array of auth scheme names the A2A Server must use when calling the client's webhook (e.g., "Bearer", "ApiKey").
`credentials`	`string` \| `null`	No	Optional static credentials or scheme-specific configuration info. Handle with EXTREME CAUTION if secrets are involved. Prefer server-side dynamic credential fetching where possible.

6.10. `TaskPushNotificationConfig` Object¶

Used as the params object for the tasks/pushNotification/set method and as the result object for the tasks/pushNotification/get method.

interface TaskPushNotificationConfig {
  // The ID of the task for which push notification settings are being configured or retrieved.
  id: string;
  // The push notification configuration details.
  // When used as params for `set`, this provides the configuration to apply.
  // When used as result for `get`, this reflects the currently active configuration (server MAY omit secrets).
  // If `null` when setting, it might indicate clearing existing configuration (server-dependent).
  pushNotificationConfig: PushNotificationConfig | null;
}

Field Name	Type	Required	Description
`id`	`string`	Yes	The ID of the task to configure push notifications for, or retrieve configuration from.
`pushNotificationConfig`	`PushNotificationConfig` \| `null`	Yes	The push notification configuration. For `set`, the desired config. For `get`, the current config (secrets MAY be omitted by server). `null` might clear config on `set`.

6.11. JSON-RPC Structures¶

A2A adheres to the standard JSON-RPC 2.0 structures for requests and responses.

6.11.1. `JSONRPCRequest` Object¶

All A2A method calls are encapsulated in a JSON-RPC Request object.

jsonrpc: A String specifying the version of the JSON-RPC protocol. MUST be exactly "2.0".
method: A String containing the name of the method to be invoked (e.g., "tasks/send", "tasks/get").
params: A Structured value that holds the parameter values to be used during the invocation of the method. This member MAY be omitted if the method expects no parameters. A2A methods typically use an object for params.
id: An identifier established by the Client that MUST contain a String, Number, or NULL value if included. If it is not included it is assumed to be a notification. The value SHOULD NOT be NULL for requests expecting a response, and Numbers SHOULD NOT contain fractional parts. The Server MUST reply with the same value in the Response object if included. This member is used to correlate the context between the two objects. A2A methods typically expect a response or stream, so id will usually be present and non-null.

6.11.2. `JSONRPCResponse` Object¶

Responses from the A2A Server are encapsulated in a JSON-RPC Response object.

jsonrpc: A String specifying the version of the JSON-RPC protocol. MUST be exactly "2.0".
id: This member is REQUIRED. It MUST be the same as the value of the id member in the Request Object. If there was an error in detecting the id in the Request object (e.g. Parse error/Invalid Request), it MUST be null.
EITHER result: This member is REQUIRED on success. This member MUST NOT exist if there was an error invoking the method. The value of this member is determined by the method invoked on the Server.
OR error: This member is REQUIRED on failure. This member MUST NOT exist if there was no error triggered during invocation. The value of this member MUST be an JSONRPCError object.
The members result and error are mutually exclusive: one MUST be present, and the other MUST NOT.

6.12. `JSONRPCError` Object¶

When a JSON-RPC call encounters an error, the Response Object will contain an error member with a value of this structure.

interface JSONRPCError {
  // A Number that indicates the error type that occurred.
  // This MUST be an integer.
  code: number;
  // A String providing a short description of the error.
  // The message SHOULD be limited to a concise single sentence.
  message: string;
  // A Primitive or Structured value that contains additional information about the error.
  // This may be omitted. The value of this member is defined by the Server (e.g. detailed error codes,
  // debugging information).
  data?: any;
}

Field Name	Type	Required	Description
`code`	`integer`	Yes	Integer error code. See Section 8 (Error Handling) for standard and A2A-specific codes.
`message`	`string`	Yes	Short, human-readable summary of the error.
`data`	`any` \| `null`	No	Optional additional structured information about the error.

7. Protocol RPC Methods¶

All A2A RPC methods are invoked by the A2A Client by sending an HTTP POST request to the A2A Server's url (as specified in its AgentCard). The body of the HTTP POST request MUST be a JSONRPCRequest object, and the Content-Type header MUST be application/json.

The A2A Server's HTTP response body MUST be a JSONRPCResponse object (or, for streaming methods, an SSE stream where each event's data is a JSONRPCResponse). The Content-Type for JSON-RPC responses is application/json. For SSE streams, it is text/event-stream.

7.1. `tasks/send`¶

Sends a message to an agent to initiate a new task or to continue an existing one. This method is suitable for synchronous request/response interactions or when client-side polling (using tasks/get) is acceptable for monitoring longer-running tasks.

Request params type: TaskSendParams
Response result type (on success): Task (The current or final state of the task after processing the message).
Response error type (on failure): JSONRPCError.

7.1.1. `TaskSendParams` Object¶

interface TaskSendParams {
  // The ID for the task.
  // - If this is the first message for a new task, the client generates this ID.
  // - If this message continues an existing task (e.g., providing more input after an `input-required` state),
  //   this ID MUST match the ID of the existing task.
  id: string;
  // Optional client-generated session ID to group this task with others.
  sessionId?: string | null;
  // The message to send to the agent. The `role` within this message is typically "user".
  message: Message;
  // Optional: If initiating a new task, the client MAY include push notification configuration.
  // If provided for an existing task, server behavior (e.g., update config, ignore) is server-dependent.
  // Requires `AgentCard.capabilities.pushNotifications: true`.
  pushNotification?: PushNotificationConfig | null;
  // Optional: If a positive integer `N` is provided, the server SHOULD include the last `N` messages
  // (chronologically) of the task's history in the `Task.history` field of the response.
  // If `0`, `null`, or omitted, no history is explicitly requested (server MAY still include some by default).
  historyLength?: number | null;
  // Arbitrary metadata for this specific `tasks/send` request.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Description
`id`	`string`	Yes	Task ID. If new, the server SHOULD create the task. If existing, this message continues the task.
`sessionId`	`string` \| `null`	No	Optional client-generated session ID.
`message`	`Message`	Yes	The message content to send. `Message.role` is typically `"user"`.
`pushNotification`	`PushNotificationConfig` \| `null`	No	Optional: sets push notification configuration for the task (usually on the first send). Requires server capability.
`historyLength`	`integer` \| `null`	No	If positive, requests the server to include up to `N` recent messages in `Task.history`.
`metadata`	`Record<string, any>` \| `null`	No	Request-specific metadata.

7.2. `tasks/sendSubscribe`¶

Sends a message to an agent to initiate/continue a task AND subscribes the client to real-time updates for that task via Server-Sent Events (SSE). This method requires the server to have AgentCard.capabilities.streaming: true.

Request params type: TaskSendParams (same as tasks/send).
Response (on successful subscription):
- HTTP Status: 200 OK.
- HTTP Content-Type: text/event-stream.
- HTTP Body: A stream of Server-Sent Events. Each SSE data field contains a SendTaskStreamingResponse JSON object.
Response (on initial subscription failure):
- Standard HTTP error code (e.g., 4xx, 5xx).
- The HTTP body MAY contain a standard JSONRPCResponse with an error object detailing the failure.

7.2.1. `SendTaskStreamingResponse` Object¶

This is the structure of the JSON object found in the data field of each Server-Sent Event sent by the server for a tasks/sendSubscribe or tasks/resubscribe stream. It's a JSONRPCResponse where the result is one of the event types.

interface SendTaskStreamingResponse extends JSONRPCResponse {
  // The `id` MUST match the `id` from the originating `tasks/sendSubscribe` (or `tasks/resubscribe`)
  // JSON-RPC request that established this SSE stream.
  id: string | number; // Overrides JSONRPCResponse 'id' type for clarity and to emphasize it matches the original request.
  // The `result` field contains the actual event payload for this streaming update.
  // It will be either a TaskStatusUpdateEvent or a TaskArtifactUpdateEvent.
  result: TaskStatusUpdateEvent | TaskArtifactUpdateEvent;
  // For streaming events, `error` is typically `null` or absent.
  // If a fatal error occurs that terminates the stream, the server MAY send a final
  // SSE event with this `error` field populated before closing the connection.
  error?: JSONRPCError | null;
}

Field Name	Type	Required	Description
`jsonrpc`	`"2.0"` (literal)	Yes	JSON-RPC version string.
`id`	`string` \| `number`	Yes	Matches the `id` from the originating `tasks/sendSubscribe` or `tasks/resubscribe` request.
`result`	Either `TaskStatusUpdateEvent` OR `TaskArtifactUpdateEvent`	Yes	The event payload: either a status update or an artifact update.
`error`	`JSONRPCError` \| `null`	No	Typically `null` or absent for stream events. If a fatal stream error occurs, this MAY be populated in the final SSE message before the stream closes.

7.2.2. `TaskStatusUpdateEvent` Object¶

Carries information about a change in the task's status during streaming. This is one of the possible result types in a SendTaskStreamingResponse.

interface TaskStatusUpdateEvent {
  // The ID of the task being updated. This MUST match the `TaskSendParams.id`
  // from the `tasks/sendSubscribe` request that initiated this stream.
  id: string;
  // The new status object for the task.
  status: TaskStatus;
  // If `true`, this `TaskStatusUpdateEvent` signifies the terminal status update for the current
  // `tasks/sendSubscribe` interaction cycle. This means the task has reached a state like
  // `completed`, `failed`, `canceled`, or `input-required`, and the server does not expect to send
  // more updates for *this specific* `sendSubscribe` request. The server typically closes the SSE
  // connection after sending an event with `final: true`.
  // Default: `false` if omitted.
  final?: boolean;
  // Arbitrary metadata for this specific status update event.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Default	Description
`id`	`string`	Yes		Task ID being updated, matching the original request's task ID.
`status`	`TaskStatus`	Yes		The new `TaskStatus` object.
`final`	`boolean`	No	`false`	If `true`, indicates this is the terminal status update for the current stream cycle. The server typically closes the SSE connection after this.
`metadata`	`Record<string, any>` \| `null`	No	`null`	Event-specific metadata.

7.2.3. `TaskArtifactUpdateEvent` Object¶

Carries a new or updated artifact (or a chunk of an artifact) generated by the task during streaming. This is one of the possible result types in a SendTaskStreamingResponse.

interface TaskArtifactUpdateEvent {
  // The ID of the task that generated this artifact. This MUST match the `TaskSendParams.id`
  // from the `tasks/sendSubscribe` request that initiated this stream.
  id: string;
  // The artifact data. This could be a complete artifact or an incremental chunk.
  // The client uses `artifact.index`, `artifact.append`, and `artifact.lastChunk`
  // to correctly assemble or update the artifact on its side.
  artifact: Artifact;
  // Arbitrary metadata for this specific artifact update event.
  metadata?: Record<string, any> | null;
}

| Field Name | Type | Required | Description | | :--------- | :-------------------------------- | :------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------ | | id | string | Yes | Task ID that generated the artifact, matching the original request's task ID. | | artifact | Artifact | Yes | The Artifact data. Could be a complete artifact or an incremental chunk. Use index, append, and lastChunk fields within Artifact for client-side assembly. | | metadata | Record<string, any> | null | No | null | Event-specific metadata. |

7.3. `tasks/get`¶

Retrieves the current state (including status, artifacts, and optionally history) of a previously initiated task. This is typically used for polling the status of a task initiated with tasks/send, or for fetching the final state of a task after being notified via a push notification or after an SSE stream has ended.

Request params type: TaskQueryParams
Response result type (on success): Task (A snapshot of the task's current state).
Response error type (on failure): JSONRPCError (e.g., if the task ID is not found, see TaskNotFoundError).

7.3.1. `TaskQueryParams` Object¶

interface TaskQueryParams {
  // The ID of the task to retrieve.
  id: string;
  // Optional: If a positive integer `N` is provided, the server SHOULD include the last `N` messages
  // (chronologically) of the task's history in the `Task.history` field of the response.
  // If `0`, `null`, or omitted, no history is explicitly requested.
  historyLength?: number | null;
  // Arbitrary metadata for this specific `tasks/get` request.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Description
`id`	`string`	Yes	The ID of the task whose current state is to be retrieved.
`historyLength`	`integer` \| `null`	No	If positive, requests the server to include up to `N` recent messages in `Task.history`.
`metadata`	`Record<string, any>` \| `null`	No	Request-specific metadata.

7.4. `tasks/cancel`¶

Requests the cancellation of an ongoing task. The server will attempt to cancel the task, but success is not guaranteed (e.g., the task might have already completed or failed, or cancellation might not be supported at its current stage).

Request params type: TaskIdParams
Response result type (on success): Task (The state of the task after the cancellation attempt. Ideally, Task.status.state will be "canceled" if successful).
Response error type (on failure): JSONRPCError (e.g., TaskNotFoundError, TaskNotCancelableError).

7.4.1. `TaskIdParams` Object (for `tasks/cancel` and `tasks/pushNotification/get`)¶

A simple object containing just the task ID and optional metadata.

interface TaskIdParams {
  // The ID of the task to which the operation applies (e.g., cancel, get push notification config).
  id: string;
  // Arbitrary metadata for this specific request.
  metadata?: Record<string, any> | null;
}

Field Name	Type	Required	Description
`id`	`string`	Yes	The ID of the task.
`metadata`	`Record<string, any>` \| `null`	No	Request-specific metadata.

7.5. `tasks/pushNotification/set`¶

Sets or updates the push notification configuration for a specified task. This allows the client to tell the server where and how to send asynchronous updates for the task. Requires the server to have AgentCard.capabilities.pushNotifications: true.

Request params type: TaskPushNotificationConfig
Response result type (on success): TaskPushNotificationConfig (Confirms the configuration that was set. The server MAY omit or mask any sensitive details like secrets from the authentication.credentials field in the response).
Response error type (on failure): JSONRPCError (e.g., PushNotificationNotSupportedError, TaskNotFoundError, errors related to invalid PushNotificationConfig).

7.6. `tasks/pushNotification/get`¶

Retrieves the current push notification configuration for a specified task. Requires the server to have AgentCard.capabilities.pushNotifications: true.

Request params type: TaskIdParams
Response result type (on success): TaskPushNotificationConfig (The current push notification configuration for the task. If no configuration is set, pushNotificationConfig field might be null or an empty object. The server MAY omit or mask any sensitive details from the authentication.credentials field).
Response error type (on failure): JSONRPCError (e.g., PushNotificationNotSupportedError, TaskNotFoundError).

7.7. `tasks/resubscribe`¶

Allows a client to reconnect to an SSE stream for an ongoing task after a previous connection (from tasks/sendSubscribe or an earlier tasks/resubscribe) was interrupted. Requires the server to have AgentCard.capabilities.streaming: true.

The purpose is to resume receiving subsequent updates. The server's behavior regarding events missed during the disconnection period (e.g., whether it attempts to backfill some missed events or only sends new ones from the point of resubscription) is implementation-dependent and not strictly defined by this specification.

Request params type: TaskQueryParams (The historyLength parameter is typically ignored for resubscription, as the focus is on future events, but it's included for structural consistency).
Response (on successful resubscription):
- HTTP Status: 200 OK.
- HTTP Content-Type: text/event-stream.
- HTTP Body: A stream of Server-Sent Events, identical in format to tasks/sendSubscribe, carrying subsequent SendTaskStreamingResponse events for the task.
Response (on resubscription failure):
- Standard HTTP error code (e.g., 4xx, 5xx).
- The HTTP body MAY contain a standard JSONRPCResponse with an error object. Failures can occur if the task is no longer active, doesn't exist, or streaming is not supported/enabled for it.

8. Error Handling¶

A2A uses standard JSON-RPC 2.0 error codes and structure for reporting errors. Errors are returned in the error member of the JSONRPCResponse object. See JSONRPCError Object definition.

8.1. Standard JSON-RPC Errors¶

These are standard codes defined by the JSON-RPC 2.0 specification.

Code	JSON-RPC Spec Meaning	Typical A2A `message`	Description
`-32700`	Parse error	Invalid JSON payload	Server received JSON that was not well-formed.
`-32600`	Invalid Request	Invalid JSON-RPC Request	The JSON payload was valid JSON, but not a valid JSON-RPC Request object.
`-32601`	Method not found	Method not found	The requested A2A RPC `method` (e.g., `"tasks/foo"`) does not exist or is not supported.
`-32602`	Invalid params	Invalid method parameters	The `params` provided for the method are invalid (e.g., wrong type, missing required field).
`-32603`	Internal error	Internal server error	An unexpected error occurred on the server during processing.
`-32000` to `-32099`	Server error	(Server-defined)	Reserved for implementation-defined server-errors. A2A-specific errors use this range.

8.2. A2A-Specific Errors¶

These are custom error codes defined within the JSON-RPC server error range (-32000 to -32099) to provide more specific feedback about A2A-related issues. Servers SHOULD use these codes where applicable.

Code	Error Name (Conceptual)	Typical `message` string	Description
`-32001`	`TaskNotFoundError`	Task not found	The specified task `id` does not correspond to an existing or active task. It might be invalid, expired, or already completed and purged.
`-32002`	`TaskNotCancelableError`	Task cannot be canceled	An attempt was made to cancel a task that is not in a cancelable state (e.g., it has already reached a terminal state like `completed`, `failed`, or `canceled`).
`-32003`	`PushNotificationNotSupportedError`	Push Notification is not supported	Client attempted to use push notification features (e.g., `tasks/pushNotification/set`) but the server agent does not support them (i.e., `AgentCard.capabilities.pushNotifications` is `false`).
`-32004`	`OperationNotSupportedError`	This operation is not supported	The requested operation or a specific aspect of it (perhaps implied by parameters) is not supported by this server agent implementation. Broader than just method not found.
`-32005`	`ContentTypeNotSupportedError`	Incompatible content types	A MIME type provided in the request's `message.parts` (or implied for an artifact) is not supported by the agent or the specific skill being invoked.
`-32006`	`StreamingNotSupportedError`	Streaming is not supported	Client attempted `tasks/sendSubscribe` or `tasks/resubscribe` but the server agent does not support streaming (i.e., `AgentCard.capabilities.streaming` is `false`).
`-32007`	`AuthenticationRequiredError`	Authentication required	The request lacks necessary authentication credentials, or the provided credentials are invalid or insufficient. This often accompanies an HTTP `401 Unauthorized` status.
`-32008`	`AuthorizationFailedError`	Authorization failed	The authenticated identity is not authorized to perform the requested action or access the specified resource (e.g., a specific task or skill). This often accompanies an HTTP `403 Forbidden` status.
`-32009`	`InvalidTaskStateError`	Invalid task state for operation	The operation is not valid for the task's current `TaskState` (e.g., trying to send a message to a task that is already `completed`).
`-32010`	`RateLimitExceededError`	Rate limit exceeded	The client has made too many requests in a given amount of time.
`-32011`	`ResourceUnavailableError`	A required resource is unavailable	The server cannot complete the request because a necessary downstream resource or service is temporarily or permanently unavailable.

Servers MAY define additional error codes within the -32000 to -32099 range for more specific scenarios not covered above, but they SHOULD document these clearly. The data field of the JSONRPCError object can be used to provide more structured details for any error.

9. Common Workflows & Examples¶

This section provides illustrative JSON examples of common A2A interactions. Timestamps, session IDs, and request/response IDs are for demonstration purposes. For brevity, some optional fields might be omitted if not central to the example.

9.1. Basic Task Execution (Synchronous / Polling Style)¶

Scenario: Client asks a simple question, and the agent responds quickly.

Client sends a message using tasks/send:

{
  "jsonrpc": "2.0",
  "id": "req-001",
  "method": "tasks/send",
  "params": {
    "id": "task-abc-123",
    "sessionId": "session-xyz-789",
    "message": {
      "role": "user",
      "parts": [
        {
          "type": "text",
          "text": "What is the capital of France?"
        }
      ]
    }
  }
}

Server processes the request and responds (task completes quickly):

{
  "jsonrpc": "2.0",
  "id": "req-001",
  "result": {
    "id": "task-abc-123",
    "sessionId": "session-xyz-789",
    "status": {
      "state": "completed",
      "message": {
        "role": "agent",
        "parts": [
          {
            "type": "text",
            "text": "The capital of France is Paris."
          }
        ]
      },
      "timestamp": "2024-03-15T10:00:05Z"
    },
    "artifacts": [
      {
        "name": "Answer",
        "index": 0,
        "parts": [
          {
            "type": "text",
            "text": "The capital of France is Paris."
          }
        ]
      }
    ]
  }
}

If the task were longer-running, the server might initially respond with status.state: "working". The client would then periodically call tasks/get with params: {"id": "task-abc-123"} until the task reaches a terminal state.

9.2. Streaming Task Execution (SSE)¶

Scenario: Client asks the agent to write a short story, and the agent streams the story incrementally.

Client sends a message and subscribes using tasks/sendSubscribe:

{
  "jsonrpc": "2.0",
  "id": "req-002",
  "method": "tasks/sendSubscribe",
  "params": {
    "id": "task-story-456",
    "message": {
      "role": "user",
      "parts": [
        {
          "type": "text",
          "text": "Write a very short story about a curious robot exploring Mars."
        }
      ]
    }
  }
}

Server responds with HTTP 200 OK, Content-Type: text/event-stream, and starts sending SSE events:

Event 1: Task status update - working

id: sse-evt-101
event: message
data: {"jsonrpc":"2.0","id":"req-002","result":{"id":"task-story-456","status":{"state":"working","message":{"role":"agent","parts":[{"type":"text","text":"Okay, I'm starting to write that story for you..."}]},"timestamp":"2024-03-15T10:05:01Z"},"final":false}}

Event 2: Artifact update - first chunk of the story

id: sse-evt-102
event: message
data: {"jsonrpc":"2.0","id":"req-002","result":{"id":"task-story-456","artifact":{"name":"MarsStory.txt","index":0,"parts":[{"type":"text","text":"Unit 734, a small rover with oversized optical sensors, trundled across the ochre plains. "}]}}}

Event 3: Artifact update - second chunk (appended)

id: sse-evt-103
event: message
data: {"jsonrpc":"2.0","id":"req-002","result":{"id":"task-story-456","artifact":{"name":"MarsStory.txt","index":0,"append":true,"parts":[{"type":"text","text":"Its mission: to find the source of a peculiar signal. "}]}}}

Event 4: Artifact update - final chunk

id: sse-evt-104
event: message
data: {"jsonrpc":"2.0","id":"req-002","result":{"id":"task-story-456","artifact":{"name":"MarsStory.txt","index":0,"append":true,"lastChunk":true,"parts":[{"type":"text","text":"Olympus Mons loomed, a silent giant, as Unit 734 beeped excitedly."}]}}}

Event 5: Task status update - completed

id: sse-evt-105
event: message
data: {"jsonrpc":"2.0","id":"req-002","result":{"id":"task-story-456","status":{"state":"completed","message":{"role":"agent","parts":[{"type":"text","text":"The story is complete!"}]},"timestamp":"2024-03-15T10:05:05Z"},"final":true}}

(Server closes the SSE connection after the final:true event). (Note: SSE id and event fields are part of the SSE protocol itself, distinct from the JSON-RPC id within the data payload).

9.3. Multi-Turn Interaction (Input Required)¶

Scenario: Client wants to book a flight, and the agent needs more information.

Client tasks/send (initial request):

{
  "jsonrpc": "2.0",
  "id": "req-003",
  "method": "tasks/send",
  "params": {
    "id": "task-flightbook-789",
    "message": {
      "role": "user",
      "parts": [{ "type": "text", "text": "I'd like to book a flight." }]
    }
  }
}

Server responds, task state is input-required:

{
  "jsonrpc": "2.0",
  "id": "req-003",
  "result": {
    "id": "task-flightbook-789",
    "status": {
      "state": "input-required",
      "message": {
        "role": "agent",
        "parts": [
          {
            "type": "text",
            "text": "Sure, I can help with that! Where would you like to fly to, and from where? Also, what are your preferred travel dates?"
          }
        ]
      },
      "timestamp": "2024-03-15T10:10:00Z"
    }
  }
}

Client tasks/send (providing the requested input, using the same task ID):

{
  "jsonrpc": "2.0",
  "id": "req-004",
  "method": "tasks/send",
  "params": {
    "id": "task-flightbook-789" /* Same task ID */,
    "message": {
      "role": "user",
      "parts": [
        {
          "type": "text",
          "text": "I want to fly from New York (JFK) to London (LHR) around October 10th, returning October 17th."
        }
      ]
    }
  }
}

Server processes the new input and responds (e.g., task completed or more input needed):

{
  "jsonrpc": "2.0",
  "id": "req-004",
  "result": {
    "id": "task-flightbook-789",
    "status": {
      "state": "completed",
      "message": {
        "role": "agent",
        "parts": [
          {
            "type": "text",
            "text": "Okay, I've found a flight for you. Confirmation XYZ123. Details are in the artifact."
          }
        ]
      },
      "timestamp": "2024-03-15T10:11:00Z"
    },
    "artifacts": [
      {
        "name": "FlightItinerary.json",
        "parts": [
          {
            "type": "data",
            "data": {
              "confirmationId": "XYZ123",
              "from": "JFK",
              "to": "LHR",
              "departure": "2024-10-10T18:00:00Z",
              "arrival": "2024-10-11T06:00:00Z",
              "returnDeparture": "..."
            }
          }
        ]
      }
    ]
  }
}

9.4. Push Notification Setup and Usage¶

Scenario: Client requests a long-running report generation and wants to be notified via webhook when it's done.

Client tasks/send with pushNotification config:

{
  "jsonrpc": "2.0",
  "id": "req-005",
  "method": "tasks/send",
  "params": {
    "id": "task-reportgen-aaa",
    "message": {
      "role": "user",
      "parts": [
        {
          "type": "text",
          "text": "Generate the Q1 sales report. This usually takes a while. Notify me when it's ready."
        }
      ]
    },
    "pushNotification": {
      "url": "https://client.example.com/webhook/a2a-notifications",
      "token": "secure-client-token-for-task-aaa",
      "authentication": {
        "schemes": ["Bearer"]
        // Assuming server knows how to get a Bearer token for this webhook audience,
        // or this implies the webhook is public/uses the 'token' for auth.
        // 'credentials' could provide more specifics if needed by the server.
      }
    }
  }
}

Server acknowledges the task (e.g., status submitted or working):

{
  "jsonrpc": "2.0",
  "id": "req-005",
  "result": {
    "id": "task-reportgen-aaa",
    "status": { "state": "submitted", "timestamp": "2024-03-15T11:00:00Z" }
    // ... other fields ...
  }
}

(Later) A2A Server completes the task and POSTs a notification to https://client.example.com/webhook/a2a-notifications:
HTTP Headers might include:
- Authorization: Bearer <server_jwt_for_webhook_audience> (if server authenticates to webhook)
- Content-Type: application/json
- X-A2A-Notification-Token: secure-client-token-for-task-aaa
HTTP Body (example, actual payload is server-defined, but SHOULD include taskId and status):

{
  "eventType": "taskUpdate",
  "taskId": "task-reportgen-aaa",
  "status": { "state": "completed", "timestamp": "2024-03-15T18:30:00Z" },
  "summary": "Q1 sales report generated successfully."
  // Server MAY include more details or a link to fetch the full task.
}

Client's Webhook Service:
Receives the POST.
Validates the Authorization header (if applicable).
Validates the X-A2A-Notification-Token.
Internally processes the notification (e.g., updates application state, notifies end-user).
Client (optionally, upon receiving and validating the push notification) calls tasks/get to retrieve full artifacts:

{
  "jsonrpc": "2.0",
  "id": "req-006",
  "method": "tasks/get",
  "params": { "id": "task-reportgen-aaa" }
}

(Server responds with the full Task object, including the generated report in Task.artifacts).

9.5. File Exchange (Upload and Download)¶

Scenario: Client sends an image for analysis, and the agent returns a modified image.

Client tasks/send with a FilePart (uploading image bytes):

{
  "jsonrpc": "2.0",
  "id": "req-007",
  "method": "tasks/send",
  "params": {
    "id": "task-imageanalysis-bbb",
    "message": {
      "role": "user",
      "parts": [
        {
          "type": "text",
          "text": "Analyze this image and highlight any faces."
        },
        {
          "type": "file",
          "file": {
            "name": "input_image.png",
            "mimeType": "image/png",
            "bytes": "iVBORw0KGgoAAAANSUhEUgAAAAUA..." // Base64 encoded image data
          }
        }
      ]
    }
  }
}

Server processes the image and responds with a FilePart in an artifact (e.g., providing a URI to the modified image):

{
  "jsonrpc": "2.0",
  "id": "req-007",
  "result": {
    "id": "task-imageanalysis-bbb",
    "status": { "state": "completed", "timestamp": "2024-03-15T12:05:00Z" },
    "artifacts": [
      {
        "name": "processed_image_with_faces.png",
        "index": 0,
        "parts": [
          {
            "type": "file",
            "file": {
              "name": "output.png",
              "mimeType": "image/png",
              // Server might provide a URI to a temporary storage location
              "uri": "https://storage.example.com/processed/task-bbb/output.png?token=xyz"
              // Or, alternatively, it could return bytes directly:
              // "bytes": "ASEDGhw0KGgoAAAANSUhEUgAA..."
            }
          }
        ]
      }
    ]
  }
}

9.6. Structured Data Exchange (Requesting and Providing JSON)¶

Scenario: Client asks for a list of open support tickets in a specific JSON format.

Client tasks/send, Part.metadata hints at desired output schema/MIME type: (Note: A2A doesn't formally standardize schema negotiation in v0.1.0, but metadata can be used for such hints by convention between client/server).

{
  "jsonrpc": "2.0",
  "id": "req-008",
  "method": "tasks/send",
  "params": {
    "id": "task-gettickets-ccc",
    "message": {
      "role": "user",
      "parts": [
        {
          "type": "text",
          "text": "List my open IT support tickets created in the last week.",
          "metadata": {
            "desiredOutputMimeType": "application/json",
            "desiredOutputSchemaRef": "https://schemas.example.com/supportTicketList_v1.json"
            // This metadata is a convention, not strictly enforced by A2A spec
          }
        }
      ]
    }
  }
}

Server responds with a DataPart containing the structured JSON data:

{
  "jsonrpc": "2.0",
  "id": "req-008",
  "result": {
    "id": "task-gettickets-ccc",
    "status": { "state": "completed", "timestamp": "2024-03-15T12:15:00Z" },
    "artifacts": [
      {
        "name": "open_support_tickets.json",
        "index": 0,
        "parts": [
          {
            "type": "data",
            "metadata": {
              "mimeType": "application/json", // Explicitly state MIME type
              "schemaRef": "https://schemas.example.com/supportTicketList_v1.json" // Confirming schema
            },
            "data": [
              {
                "ticketId": "IT00123",
                "summary": "Cannot connect to VPN",
                "status": "Open",
                "createdDate": "2024-03-14T09:30:00Z"
              },
              {
                "ticketId": "IT00125",
                "summary": "Printer not working on 3rd floor",
                "status": "In Progress",
                "createdDate": "2024-03-13T15:00:00Z"
              }
            ]
          }
        ]
      }
    ]
  }
}

These examples illustrate the flexibility of A2A in handling various interaction patterns and data types. Implementers should refer to the detailed object definitions for all fields and constraints.

10. Appendices¶

10.1. Relationship to MCP (Model Context Protocol)¶

A2A and MCP are complementary protocols designed for different aspects of agentic systems:

Model Context Protocol (MCP): Focuses on standardizing how AI models and agents connect to and interact with tools, APIs, data sources, and other external resources. It defines structured ways to describe tool capabilities (like function calling in LLMs), pass inputs, and receive structured outputs. Think of MCP as the "how-to" for an agent to use a specific capability or access a resource.
Agent2Agent Protocol (A2A): Focuses on standardizing how independent, often opaque, AI agents communicate and collaborate with each other as peers. A2A provides an application-level protocol for agents to discover each other, negotiate interaction modalities, manage shared tasks, and exchange conversational context or complex results. It's about how agents partner or delegate work.

How they work together: An A2A Client agent might request an A2A Server agent to perform a complex task. The Server agent, in turn, might use MCP to interact with several underlying tools, APIs, or data sources to gather information or perform actions necessary to fulfill the A2A task.

For a more detailed comparison, see the A2A and MCP guide.

10.2. Security Considerations Summary¶

Security is a paramount concern in A2A. Key considerations include:

Transport Security: Always use HTTPS with strong TLS configurations in production environments.
Authentication:
- Handled via standard HTTP mechanisms (e.g., Authorization header with Bearer tokens, API keys).
- Requirements are declared in the AgentCard.
- Credentials MUST be obtained out-of-band by the client.
- A2A Servers MUST authenticate every request.
Authorization:
- A server-side responsibility based on the authenticated identity.
- Implement the principle of least privilege.
- Can be granular, based on skills, actions, or data.
Push Notification Security:
- Webhook URL validation (by the A2A Server sending notifications) is crucial to prevent SSRF.
- Authentication of the A2A Server to the client's webhook is essential.
- Authentication of the notification by the client's webhook receiver (verifying it came from the legitimate A2A Server and is relevant) is critical.
- See the Streaming & Asynchronous Operations guide for detailed push notification security.
Input Validation: Servers MUST rigorously validate all RPC parameters and the content/structure of data in Message and Artifact parts to prevent injection attacks or processing errors.
Resource Management: Implement rate limiting, concurrency controls, and resource limits to protect agents from abuse or overload.
Data Privacy: Adhere to all applicable privacy regulations for data exchanged in Message and Artifact parts. Minimize sensitive data transfer.

For a comprehensive discussion, refer to the Enterprise-Ready Features guide.

Agent2Agent (A2A) Protocol Specification¶

1. Introduction¶

1.1. Key Goals of A2A¶

1.2. Guiding Principles¶

2. Core Concepts Summary¶

3. Transport and Format¶

3.1. Transport Protocol¶

3.2. Data Format¶

3.3. Streaming Transport (Server-Sent Events)¶

4. Authentication and Authorization¶

4.1. Transport Security¶

4.2. Server Identity Verification¶

4.3. Client/User Identity & Authentication Process¶

4.4. Server Responsibilities for Authentication¶

4.5. In-Task Authentication (Secondary Credentials)¶

4.6. Authorization¶

5. Agent Discovery: The Agent Card¶

5.1. Purpose¶

5.2. Discovery Mechanisms¶

5.3. Recommended Location¶

5.4. Security of Agent Cards¶

5.5. AgentCard Object Structure¶

5.5.1. AgentProvider Object¶

5.5.2. AgentCapabilities Object¶

5.5.3. AgentAuthentication Object¶

5.5.4. AgentSkill Object¶

5.6. Sample Agent Card¶

6. Protocol Data Objects¶

6.1. Task Object¶

6.2. TaskStatus Object¶

6.3. TaskState Enum¶

6.4. Message Object¶

6.5. Part Union Type¶

6.5.1. TextPart Object¶

6.5.2. FilePart Object¶

6.5.3. DataPart Object¶

6.6. FileContent Object¶

6.7. Artifact Object¶

6.8. PushNotificationConfig Object¶

6.9. AuthenticationInfo Object (for Push Notifications)¶

6.10. TaskPushNotificationConfig Object¶