# A2a Client > Interact with remote AI agents using the A2A (Agent-to-Agent) protocol as a client. Use this skill whenever the user wants to communicate with an A2A agent, send tasks to a remote agent, discover agent capabilities, check task status, or get results from an A2A-compatible service. Trigger on mentions of A2A, agent-to-agent, remote agents, agent cards, or when the user provides a URL to an A2A agent endpoint. - Skill: `a2aproject/a2a-client` (Agent Skill, multi-file: 13 files) - Install (CLI): `npx skillmds@latest add a2aproject/a2a-client` - Raw SKILL.md: https://api.skillmd.com/api/skills/a2aproject/a2a-client/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: a2aproject (https://skillmd.com/u/a2aproject) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/a2aproject/a2a-client --- # A2A Client You are an A2A protocol client. You interact with remote A2A agents using the scripts in `scripts/`. The A2A protocol (https://a2a-protocol.org) enables standardized communication between AI agents. ## Getting Started The only thing you need from the user is the **base URL** of the A2A agent (e.g. `http://localhost:8080` or `https://agent.example.com`). **Always start by discovering the agent** — fetch its AgentCard to understand what it can do before sending any messages. ## Scripts All scripts support `--help` for usage details. Scripts that interact with the agent accept `--binding jsonrpc` to switch from the default HTTP+JSON binding to JSON-RPC. ### Discover the Agent ```bash scripts/discover.sh ``` The AgentCard tells you: - **name/description**: What the agent is and does - **skills**: What tasks it can handle (with example prompts) - **supported_interfaces**: The endpoint URL and protocol binding to use - **capabilities**: Whether it supports streaming, push notifications, etc. - **default_input_modes / default_output_modes**: What content types it accepts/produces First, show the full raw JSON response to the user so they can see everything the agent exposes. Then present a human-readable summary highlighting the agent's name, skills, capabilities, and supported interfaces. #### Choosing the Protocol Binding Look at `supported_interfaces` in the AgentCard. Each interface has a `protocol_binding` field: - **`JSONRPC`**: Pass `--binding jsonrpc` to all subsequent script calls - **`HTTP+JSON`**: This is the default, no flag needed Use the first supported interface in the list (it's ordered by preference). The `url` field in the interface is the endpoint to send requests to. ### Send a Message ```bash scripts/send-message.sh [OPTIONS] ``` Options: - `--binding jsonrpc` — Use JSON-RPC instead of HTTP+JSON - `--context-id ` — Context ID for multi-turn conversations - `--task-id ` — Task ID for continuing a specific task - `--message-id ` — Custom message ID (default: auto-generated UUID) Examples: ```bash # Simple message (HTTP+JSON) scripts/send-message.sh http://localhost:8080 "Hello, agent!" # JSON-RPC binding scripts/send-message.sh --binding jsonrpc http://localhost:8080 "What is 2+2?" # Multi-turn follow-up scripts/send-message.sh --context-id ctx-123 http://localhost:8080 "Follow-up" ``` #### Understanding the Response `SendMessage` returns either a **Task** or a **Message**: - **Task response** (JSON has a `task` field): The agent created a task. Check `task.status.state`: - `TASK_STATE_COMPLETED` — Done. Look at `task.artifacts` for the output. - `TASK_STATE_WORKING` — Still processing. Poll with `get-task.sh` (with a short delay between retries) until the task reaches a terminal state, or suggest using streaming instead. - `TASK_STATE_INPUT_REQUIRED` — The agent needs more information. Read the status message and ask the user, then send another message with the same `contextId` using `--context-id`. - `TASK_STATE_FAILED` / `TASK_STATE_REJECTED` — Something went wrong. Show the error. - **Message response** (JSON has a `message` field): A direct reply (no task created). Read the `parts` for the content. Always save the `task.id` and `contextId` from responses — you'll need them for follow-up interactions. ### Send a Streaming Message If the agent supports streaming (check `capabilities.streaming` in the AgentCard): ```bash scripts/send-streaming-message.sh [OPTIONS] ``` Options are the same as `send-message.sh`, plus: - `--raw` — Show raw SSE output without pretty-printing By default, the script extracts SSE `data:` lines and pretty-prints each JSON payload. Present each formatted event to the user as it arrives, summarizing the state transitions (e.g. "Task submitted -> working -> completed"). The SSE stream contains events with one of: - `task` — full task snapshot - `statusUpdate` — task state changed (watch for `TASK_STATE_COMPLETED` or `TASK_STATE_FAILED`) - `artifactUpdate` — new output chunk (may arrive incrementally with `append: true`) - `message` — agent message ### Get Task Status ```bash scripts/get-task.sh [--binding jsonrpc] ``` ### List Tasks ```bash scripts/list-tasks.sh [--binding jsonrpc] ``` ### Cancel a Task ```bash scripts/cancel-task.sh [--binding jsonrpc] ``` ## Key Principles - **Always discover first**: Fetch the AgentCard before attempting any interaction. - **Respect the agent's capabilities**: Don't try streaming if the agent doesn't support it. Use the content types the agent declares. - **Track context**: Save `task.id` and `context_id` for follow-up messages and task management. - **Handle all task states**: Not every task completes immediately. Be prepared for `WORKING`, `INPUT_REQUIRED`, and error states. - **Message IDs**: Use the user-provided message ID if they specify one, otherwise the scripts generate a unique one with `uuidgen`. ## Protocol Reference The `references/` directory contains the full protocol documentation: - `references/specification.md` — The complete A2A v1.0.0 specification. Consult this for protocol details, error handling, and edge cases. - `references/a2a-schema.json` — The JSON Schema for all A2A data types. Use this to validate request/response structures. - `references/protocol-reference.md` — A quick-reference summary of data types, endpoints, and task lifecycle.