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663//! A2A (Agent-to-Agent) Protocol — cross-framework agent interoperability.
//!
//! Google's A2A protocol enables cross-framework agent interoperability via
//! **Agent Cards** (JSON capability manifests) and **Task-based coordination**.
//!
//! This module provides:
//! - `AgentCard` — describes an agent's capabilities to external systems
//! - `A2aTask` — unit of work exchanged between agents
//! - `build_agent_card` — expose OpenFang agents via A2A
//! - `A2aClient` — discover and interact with external A2A agents
use openfang_types::agent::AgentManifest;
use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use std::sync::Mutex;
use tracing::{debug, info, warn};
// ---------------------------------------------------------------------------
// A2A Agent Card
// ---------------------------------------------------------------------------
/// A2A Agent Card — describes an agent's capabilities to external systems.
///
/// Served at `/.well-known/agent.json` per the A2A specification.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct AgentCard {
/// Agent display name.
pub name: String,
/// Human-readable description.
pub description: String,
/// Agent endpoint URL.
pub url: String,
/// Protocol version.
pub version: String,
/// Agent capabilities.
pub capabilities: AgentCapabilities,
/// Skills this agent can perform (A2A skill descriptors, not OpenFang skills).
pub skills: Vec<AgentSkill>,
/// Supported input content types.
#[serde(default)]
pub default_input_modes: Vec<String>,
/// Supported output content types.
#[serde(default)]
pub default_output_modes: Vec<String>,
}
/// A2A agent capabilities.
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct AgentCapabilities {
/// Whether this agent supports streaming responses.
pub streaming: bool,
/// Whether this agent supports push notifications.
pub push_notifications: bool,
/// Whether task status history is available.
pub state_transition_history: bool,
}
/// A2A skill descriptor (not an OpenFang skill — describes a capability).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AgentSkill {
/// Unique skill identifier.
pub id: String,
/// Display name.
pub name: String,
/// Description of what this skill does.
pub description: String,
/// Tags for discovery.
#[serde(default)]
pub tags: Vec<String>,
/// Example prompts that trigger this skill.
#[serde(default)]
pub examples: Vec<String>,
}
// ---------------------------------------------------------------------------
// A2A Task
// ---------------------------------------------------------------------------
/// A2A Task — unit of work exchanged between agents.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct A2aTask {
/// Unique task identifier.
pub id: String,
/// Optional session identifier for conversation continuity.
#[serde(default)]
pub session_id: Option<String>,
/// Current task status.
pub status: A2aTaskStatus,
/// Messages exchanged during the task.
#[serde(default)]
pub messages: Vec<A2aMessage>,
/// Artifacts produced by the task.
#[serde(default)]
pub artifacts: Vec<A2aArtifact>,
}
/// A2A task status.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub enum A2aTaskStatus {
/// Task has been received but not started.
Submitted,
/// Task is being processed.
Working,
/// Agent needs more input from the caller.
InputRequired,
/// Task completed successfully.
Completed,
/// Task was cancelled.
Cancelled,
/// Task failed.
Failed,
}
/// A2A message in a task conversation.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct A2aMessage {
/// Message role ("user" or "agent").
pub role: String,
/// Message content parts.
pub parts: Vec<A2aPart>,
}
/// A2A message content part.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "type", rename_all = "camelCase")]
pub enum A2aPart {
/// Text content.
Text { text: String },
/// File content (base64-encoded).
File {
name: String,
mime_type: String,
data: String,
},
/// Structured data.
Data {
mime_type: String,
data: serde_json::Value,
},
}
/// A2A artifact produced by a task.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct A2aArtifact {
/// Artifact name.
pub name: String,
/// Artifact content parts.
pub parts: Vec<A2aPart>,
}
// ---------------------------------------------------------------------------
// A2A Task Store — tracks task lifecycle
// ---------------------------------------------------------------------------
/// In-memory store for tracking A2A task lifecycle.
///
/// Tasks are created by `tasks/send`, polled by `tasks/get`, and cancelled
/// by `tasks/cancel`. The store is bounded to prevent memory exhaustion.
#[derive(Debug)]
pub struct A2aTaskStore {
tasks: Mutex<HashMap<String, A2aTask>>,
/// Maximum number of tasks to retain (FIFO eviction).
max_tasks: usize,
}
impl A2aTaskStore {
/// Create a new task store with a capacity limit.
pub fn new(max_tasks: usize) -> Self {
Self {
tasks: Mutex::new(HashMap::new()),
max_tasks,
}
}
/// Insert a task. If the store is at capacity, the oldest task is evicted.
pub fn insert(&self, task: A2aTask) {
let mut tasks = self.tasks.lock().unwrap_or_else(|e| e.into_inner());
// Evict oldest completed/failed/cancelled tasks if at capacity
if tasks.len() >= self.max_tasks {
let evict_key = tasks
.iter()
.filter(|(_, t)| {
matches!(
t.status,
A2aTaskStatus::Completed | A2aTaskStatus::Failed | A2aTaskStatus::Cancelled
)
})
.map(|(k, _)| k.clone())
.next();
if let Some(key) = evict_key {
tasks.remove(&key);
}
}
tasks.insert(task.id.clone(), task);
}
/// Get a task by ID.
pub fn get(&self, task_id: &str) -> Option<A2aTask> {
self.tasks
.lock()
.unwrap_or_else(|e| e.into_inner())
.get(task_id)
.cloned()
}
/// Update a task's status and optionally add messages/artifacts.
pub fn update_status(&self, task_id: &str, status: A2aTaskStatus) -> bool {
let mut tasks = self.tasks.lock().unwrap_or_else(|e| e.into_inner());
if let Some(task) = tasks.get_mut(task_id) {
task.status = status;
true
} else {
false
}
}
/// Complete a task with a response message and optional artifacts.
pub fn complete(&self, task_id: &str, response: A2aMessage, artifacts: Vec<A2aArtifact>) {
let mut tasks = self.tasks.lock().unwrap_or_else(|e| e.into_inner());
if let Some(task) = tasks.get_mut(task_id) {
task.messages.push(response);
task.artifacts.extend(artifacts);
task.status = A2aTaskStatus::Completed;
}
}
/// Fail a task with an error message.
pub fn fail(&self, task_id: &str, error_message: A2aMessage) {
let mut tasks = self.tasks.lock().unwrap_or_else(|e| e.into_inner());
if let Some(task) = tasks.get_mut(task_id) {
task.messages.push(error_message);
task.status = A2aTaskStatus::Failed;
}
}
/// Cancel a task.
pub fn cancel(&self, task_id: &str) -> bool {
self.update_status(task_id, A2aTaskStatus::Cancelled)
}
/// Count of tracked tasks.
pub fn len(&self) -> usize {
self.tasks.lock().unwrap_or_else(|e| e.into_inner()).len()
}
/// Whether the store is empty.
pub fn is_empty(&self) -> bool {
self.len() == 0
}
}
impl Default for A2aTaskStore {
fn default() -> Self {
Self::new(1000)
}
}
// ---------------------------------------------------------------------------
// A2A Discovery — auto-discover external agents at boot
// ---------------------------------------------------------------------------
/// Discover all configured external A2A agents and return their cards.
///
/// Called during kernel boot to populate the list of known external agents.
pub async fn discover_external_agents(
agents: &[openfang_types::config::ExternalAgent],
) -> Vec<(String, AgentCard)> {
let client = A2aClient::new();
let mut discovered = Vec::new();
for agent in agents {
match client.discover(&agent.url).await {
Ok(card) => {
info!(
name = %agent.name,
url = %agent.url,
skills = card.skills.len(),
"Discovered external A2A agent"
);
discovered.push((agent.name.clone(), card));
}
Err(e) => {
warn!(
name = %agent.name,
url = %agent.url,
error = %e,
"Failed to discover external A2A agent"
);
}
}
}
if !discovered.is_empty() {
info!("A2A: discovered {} external agent(s)", discovered.len());
}
discovered
}
// ---------------------------------------------------------------------------
// A2A Server — expose OpenFang agents via A2A
// ---------------------------------------------------------------------------
/// Build an A2A Agent Card from an OpenFang agent manifest.
pub fn build_agent_card(manifest: &AgentManifest, base_url: &str) -> AgentCard {
let tools: Vec<String> = manifest.capabilities.tools.clone();
// Convert tool names to A2A skill descriptors
let skills: Vec<AgentSkill> = tools
.iter()
.map(|tool| AgentSkill {
id: tool.clone(),
name: tool.replace('_', " "),
description: format!("Can use the {tool} tool"),
tags: vec!["tool".to_string()],
examples: vec![],
})
.collect();
AgentCard {
name: manifest.name.clone(),
description: manifest.description.clone(),
url: format!("{base_url}/a2a"),
version: "0.1.0".to_string(),
capabilities: AgentCapabilities {
streaming: true,
push_notifications: false,
state_transition_history: true,
},
skills,
default_input_modes: vec!["text".to_string()],
default_output_modes: vec!["text".to_string()],
}
}
// ---------------------------------------------------------------------------
// A2A Client — discover and interact with external A2A agents
// ---------------------------------------------------------------------------
/// Client for discovering and interacting with external A2A agents.
pub struct A2aClient {
client: reqwest::Client,
}
impl A2aClient {
/// Create a new A2A client.
pub fn new() -> Self {
Self {
client: reqwest::Client::builder()
.timeout(std::time::Duration::from_secs(30))
.build()
.unwrap_or_default(),
}
}
/// Discover an external agent by fetching its Agent Card.
pub async fn discover(&self, url: &str) -> Result<AgentCard, String> {
let agent_json_url = format!("{}/.well-known/agent.json", url.trim_end_matches('/'));
debug!(url = %agent_json_url, "Discovering A2A agent");
let response = self
.client
.get(&agent_json_url)
.header("User-Agent", "OpenFang/0.1 A2A")
.send()
.await
.map_err(|e| format!("A2A discovery failed: {e}"))?;
if !response.status().is_success() {
return Err(format!("A2A discovery returned {}", response.status()));
}
let card: AgentCard = response
.json()
.await
.map_err(|e| format!("Invalid Agent Card: {e}"))?;
info!(agent = %card.name, skills = card.skills.len(), "Discovered A2A agent");
Ok(card)
}
/// Send a task to an external A2A agent.
pub async fn send_task(
&self,
url: &str,
message: &str,
session_id: Option<&str>,
) -> Result<A2aTask, String> {
let request = serde_json::json!({
"jsonrpc": "2.0",
"id": 1,
"method": "tasks/send",
"params": {
"message": {
"role": "user",
"parts": [{"type": "text", "text": message}]
},
"sessionId": session_id,
}
});
let response = self
.client
.post(url)
.json(&request)
.send()
.await
.map_err(|e| format!("A2A send_task failed: {e}"))?;
let body: serde_json::Value = response
.json()
.await
.map_err(|e| format!("Invalid A2A response: {e}"))?;
if let Some(result) = body.get("result") {
serde_json::from_value(result.clone())
.map_err(|e| format!("Invalid A2A task response: {e}"))
} else if let Some(error) = body.get("error") {
Err(format!("A2A error: {}", error))
} else {
Err("Empty A2A response".to_string())
}
}
/// Get the status of a task from an external A2A agent.
pub async fn get_task(&self, url: &str, task_id: &str) -> Result<A2aTask, String> {
let request = serde_json::json!({
"jsonrpc": "2.0",
"id": 1,
"method": "tasks/get",
"params": {
"id": task_id,
}
});
let response = self
.client
.post(url)
.json(&request)
.send()
.await
.map_err(|e| format!("A2A get_task failed: {e}"))?;
let body: serde_json::Value = response
.json()
.await
.map_err(|e| format!("Invalid A2A response: {e}"))?;
if let Some(result) = body.get("result") {
serde_json::from_value(result.clone()).map_err(|e| format!("Invalid A2A task: {e}"))
} else {
Err("Empty A2A response".to_string())
}
}
}
impl Default for A2aClient {
fn default() -> Self {
Self::new()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_agent_card_from_manifest() {
let manifest = AgentManifest {
name: "test-agent".to_string(),
description: "A test agent".to_string(),
..Default::default()
};
let card = build_agent_card(&manifest, "https://example.com");
assert_eq!(card.name, "test-agent");
assert_eq!(card.description, "A test agent");
assert!(card.url.contains("/a2a"));
assert!(card.capabilities.streaming);
assert_eq!(card.default_input_modes, vec!["text"]);
}
#[test]
fn test_a2a_task_status_transitions() {
let task = A2aTask {
id: "task-1".to_string(),
session_id: None,
status: A2aTaskStatus::Submitted,
messages: vec![],
artifacts: vec![],
};
assert_eq!(task.status, A2aTaskStatus::Submitted);
// Simulate progression
let working = A2aTask {
status: A2aTaskStatus::Working,
..task.clone()
};
assert_eq!(working.status, A2aTaskStatus::Working);
let completed = A2aTask {
status: A2aTaskStatus::Completed,
..task.clone()
};
assert_eq!(completed.status, A2aTaskStatus::Completed);
let cancelled = A2aTask {
status: A2aTaskStatus::Cancelled,
..task.clone()
};
assert_eq!(cancelled.status, A2aTaskStatus::Cancelled);
let failed = A2aTask {
status: A2aTaskStatus::Failed,
..task
};
assert_eq!(failed.status, A2aTaskStatus::Failed);
}
#[test]
fn test_a2a_message_serde() {
let msg = A2aMessage {
role: "user".to_string(),
parts: vec![
A2aPart::Text {
text: "Hello".to_string(),
},
A2aPart::Data {
mime_type: "application/json".to_string(),
data: serde_json::json!({"key": "value"}),
},
],
};
let json = serde_json::to_string(&msg).unwrap();
let back: A2aMessage = serde_json::from_str(&json).unwrap();
assert_eq!(back.role, "user");
assert_eq!(back.parts.len(), 2);
match &back.parts[0] {
A2aPart::Text { text } => assert_eq!(text, "Hello"),
_ => panic!("Expected Text part"),
}
}
#[test]
fn test_task_store_insert_and_get() {
let store = A2aTaskStore::new(10);
let task = A2aTask {
id: "t-1".to_string(),
session_id: None,
status: A2aTaskStatus::Working,
messages: vec![],
artifacts: vec![],
};
store.insert(task);
assert_eq!(store.len(), 1);
let got = store.get("t-1").unwrap();
assert_eq!(got.status, A2aTaskStatus::Working);
}
#[test]
fn test_task_store_complete_and_fail() {
let store = A2aTaskStore::new(10);
let task = A2aTask {
id: "t-2".to_string(),
session_id: None,
status: A2aTaskStatus::Working,
messages: vec![],
artifacts: vec![],
};
store.insert(task);
store.complete(
"t-2",
A2aMessage {
role: "agent".to_string(),
parts: vec![A2aPart::Text {
text: "Done".to_string(),
}],
},
vec![],
);
let completed = store.get("t-2").unwrap();
assert_eq!(completed.status, A2aTaskStatus::Completed);
assert_eq!(completed.messages.len(), 1);
}
#[test]
fn test_task_store_cancel() {
let store = A2aTaskStore::new(10);
let task = A2aTask {
id: "t-3".to_string(),
session_id: None,
status: A2aTaskStatus::Working,
messages: vec![],
artifacts: vec![],
};
store.insert(task);
assert!(store.cancel("t-3"));
assert_eq!(store.get("t-3").unwrap().status, A2aTaskStatus::Cancelled);
// Cancel a nonexistent task returns false
assert!(!store.cancel("t-999"));
}
#[test]
fn test_task_store_eviction() {
let store = A2aTaskStore::new(2);
// Insert 2 tasks
for i in 0..2 {
let task = A2aTask {
id: format!("t-{i}"),
session_id: None,
status: A2aTaskStatus::Completed,
messages: vec![],
artifacts: vec![],
};
store.insert(task);
}
assert_eq!(store.len(), 2);
// Insert a 3rd — one completed task should be evicted
let task = A2aTask {
id: "t-2".to_string(),
session_id: None,
status: A2aTaskStatus::Working,
messages: vec![],
artifacts: vec![],
};
store.insert(task);
// One was evicted, plus the new one
assert!(store.len() <= 2);
}
#[test]
fn test_a2a_config_serde() {
use openfang_types::config::{A2aConfig, ExternalAgent};
let config = A2aConfig {
enabled: true,
listen_path: "/a2a".to_string(),
external_agents: vec![ExternalAgent {
name: "other-agent".to_string(),
url: "https://other.example.com".to_string(),
}],
};
let json = serde_json::to_string(&config).unwrap();
let back: A2aConfig = serde_json::from_str(&json).unwrap();
assert!(back.enabled);
assert_eq!(back.listen_path, "/a2a");
assert_eq!(back.external_agents.len(), 1);
assert_eq!(back.external_agents[0].name, "other-agent");
}
}