a2a_http_client

Crate: a2a_http_client · Path: crates/a2a_http_client
Description (Cargo.toml): A2A HTTP client - WASM default, native testing support

Role

Target-adaptive A2A HTTP client: uses Spin SDK on WASM and Reqwest on native, with identical public APIs across both targets. Includes activation-aware retry logic for KEDA scale-to-zero cold starts with exponential backoff and idempotency keys.

Feature flags

From crates/a2a_http_client/Cargo.toml:

Feature	Purpose
`default`	No features enabled by default
`observability`	Emit OTEL spans/metrics + W3C trace propagation (via `observability` crate)
`streaming`	SSE streaming support (native only: `futures-util`, `async-stream`, `a2a_protocol_core/event-stream`)

Public API (from `src/lib.rs`)

Modules: activation (always), wasm_client / native_client (target-selected)

Core types:

Client — A2A HTTP client (target-adaptive: Spin SDK or Reqwest)
- Client::external(url) — create client for a remote endpoint
- client.call(method, params) — send a JSON-RPC request
- client.call_with_activation(method, params, config, request_id) — call with cold-start retry
- client.with_header(key, value) — add custom header
ClientError / RpcError — error types
ActivationConfig — retry configuration (max retries, backoff, cold-start timeout)
activation_delay, idempotency_key, retry_with_activation — retry helpers
check_connectivity — probe server reachability

Re-exports:

From a2a_protocol_core: Message, Task, A2A_PROTOCOL_VERSION
From protocol_transport_core: JsonRpcError, JsonRpcId, JsonRpcRequest, JsonRpcResponse, StreamingPolicy, JSONRPC_VERSION

Example sketch

use a2a_http_client::{Client, ActivationConfig};
use serde_json::json;

let client = Client::external("http://my-agent.default.agentmesh/a2a")
    .with_header("Authorization".into(), "Bearer token".into());

// Simple call
let result = client.call("message/send", json!({
    "message": { "role": "user", "parts": [{"text": "Hello"}] }
})).await?;

// With KEDA activation-aware retries
let config = ActivationConfig::default();
let result = client.call_with_activation(
    "message/send", params, &config, "req-123"
).await?;

Full API reference: cargo doc -p a2a_http_client --no-deps