Changelog

All notable changes to the A2E protocol and SDK are documented here.

The format follows Keep a Changelog, and this project adheres to Semantic Versioning.

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[0.1.2] — 2026-06-01

Removed

• Experimental subprocess transport prototype (a2e/experimental/core/transports/subprocess.py) — fully replaced by the core a2e/core/transports/subprocess.py since v0.1.1; the experimental directory is now removed

[0.1.1] — 2026-06-01

Added

Transport

• SubprocessTransport promoted from experimental to core (a2e/core/transports/subprocess.py) — spawns host as subprocess, communicates via stdin/stdout NDJSON
• SubprocessTransportConfig.env and cwd fields for environment/working dir control
• SubprocessTransport and DirectTransportConfig/SubprocessTransportConfig exported in __all__

Cookbook

• cookbook/agents/direct_agent.py — agent connecting via DirectTransport pair with --self-contained mode
• cookbook/agents/subprocess_agent.py — agent that launches host as subprocess over SubprocessTransport
• cookbook/servers/a2e_subprocess_host.py — host that reads stdin via DirectTransport bridge
• cookbook/servers/config_subprocess.yaml — standalone subprocess transport config example

Fixed

• DirectTransport.send(): Now falls back to _out_queue.put(msg) when _out_handler is None, making connect() wiring actually work. Previously all outgoing messages were silently dropped when no _out_handler was set (the server direct mode case).
• Client rpc() event loop: Added missing continue after processing A2EEvent messages — previously events fell through and were returned as the final RPC response instead of waiting for the actual response.
• Executor _build_registry(): Added type_to_plugins.clear() before rebuild to prevent duplicate plugin registration entries when _build_registry() is called multiple times.
• Capability enum: Added TEST = "test" to A2ECapability for generic test capability.

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[0.1.0] — 2026-05-18

Added

Protocol

• NDJSON-based message protocol with A2EMessage base model (type, id, a2e, ts)
• Handshake flow: handshake/req / handshake/resp with authentication and capability negotiation
• Capability negotiation: agent_caps → accepted_caps matching against loaded plugins
• A2EError message format with code, message, detail, retryable, capability_name
• Core error codes: parse_error, runtime_error, invalid_message, version_mismatch, unauthorized, schema_violation, timeout, out_of_memory, sandbox_crash
• Protocol version 1.0

Capabilities — 9 Namespaces

• Tools (tools): tool/list, tool/call, tool/event — named functions with JSON Schema I/O, streaming events
• Memory (memory): memory/store, memory/recall, memory/forget, memory/search — 3-tier storage (working, episodic, semantic) with TTL, tags, and full-text search
• Environment (env): env/reset, env/step, env/observe, env/reward — RL environment primitives with action space validation
• Processes (proc): proc/spawn, proc/write, proc/read, proc/signal, proc/kill — long-running subprocess management with command allowlist, shell=False, and process quotas
• Learning (learn): learn/feedback, learn/experience, learn/adapt, learn/stats — feedback (POSITIVE/NEGATIVE/NEUTRAL/CORRECTIVE), experience tuples, UCB1 adaptation
• Skills (skills): skill/discover, skill/call, skill/list — named, versioned execution units with input_schema, output_schema, and optional Docker sandboxing
• Toolkits (toolkits): toolkit/list, toolkit/install, toolkit/configure, toolkit/uninstall — tool bundles with shared JSON Schema configuration and lifecycle management
• Chains (chains): chain/execute, chain/validate — DAG pipelines with action, branch, map, gather, reduce node kinds, cycle detection, and timeout enforcement
• MCP (mcp): mcp/connect, mcp/disconnect, mcp/call_tool, mcp/read_resource, mcp/get_prompt, mcp/list_tools — Model Context Protocol bridge proxying external MCP servers

Subagents

• subagent/spawn, subagent/cancel, subagent/list — child agent spawning with shared, restricted, isolated, and snapshot memory scopes
• Depth limiting, step limiting, timeout enforcement, and cancellation propagation

Runtime

• A2EServer — FastAPI-based server with session-per-connection model
• A2EClient — RPC client with queue-based request correlation and event streaming via event_callback
• A2EServerRuntimeExecutor — dispatch core routing messages to plugins via type registry
• PluginRegistry — maps message types to plugin handlers, supports priority-based and exclusive dispatch
• SessionManager — creates and tracks isolated sessions in HTTP mode

Transport

• HTTPTransport — POST /send + GET /stream (SSE) with session management and reconnection
• DirectTransport — in-process cross-wired queues for testing and RL step loops
• BaseTransport ABC with start(), send(), deliver(), handler slots

Audit

• AuditLog — thread-safe structured audit logging for all plugin handler invocations
• AuditEntry — records ts, session_id, req_id, success, duration_ms, error_code, input_bytes, output_bytes
• RotatingFileHandler — 10 MB max file size, 5 backup files, JSONL format
• build_audit_log() and build_session_id() factory functions

Persistence

• File-based and SQLite storage backends for session snapshot/restore
• SnapshotStore interface for pluggable persistence

Experimental

• SubprocessTransport — stdin/stdout transport for host subprocess communication

Configuration

• A2EHostConfig — Pydantic v2 configuration model loaded from YAML
• Plugin configuration with name, type, cls, metadata
• Audit configuration: enabled, path, rotate, session_id_source
• Transport configuration: type (http / direct)

Cookbook

• Writing a Plugin — step-by-step plugin development guide
• Environment Agent Loop — RL step loop patterns with DirectTransport
• Chain Pipeline — DAG pipeline construction and execution
• MCP Integration — connecting external MCP servers
• MCP Bridge (Plugin & Client) — full WSMCPConnection transport adapter implementation
• Memory (Plugin & Client) — SQLiteMemoryPlugin with TTL, tags, and agent loop pattern
• Custom Tools (Plugin & Client) — HTTPToolsPlugin with streaming events and correlation IDs
• Toolkit Builder (Plugin & Client) — DatabaseToolkitPlugin with PostgreSQL toolkit and JSON Schema config

Documentation

• VitePress documentation site (codewiki/) with 7 sections: Getting Started, Architecture, SDK Reference, Capabilities, Protocol Specification, Cookbook, Resources
• Architecture Overview with three-layer design, system diagram, and key patterns
• Protocol Specification: Message Format, Handshake, Capability Negotiation, Message Types, Error Codes
• Capability-specific protocol specs for all 9 namespaces + subagents
• SDK Reference: Client API, Server API, Configuration, Audit System, Persistence

Resources

• Glossary — A-Z reference of 40+ A2E terms and concepts
• FAQ — 30+ frequently asked questions across 10 categories
• Security & Trust — trust model, authentication, transport security, per-capability security tables, threat model, plugin security checklist
• Support — troubleshooting guide, debugging techniques, contribution guidelines

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[0.1.0-dev] — 2026-05-13

Added

Core Runtime

• A2EEvent message model replacing InvokeEvent for progressive streaming
• Event streaming support in client rpc() with event_callback
• Audit logging system: AuditLog, AuditEntry, RotatingFileHandler
• Audit configuration in A2EHostConfig

Capabilities

• Memory plugin with 3-tier storage (working, episodic, semantic)
• Learning plugin with feedback, experience, and UCB1 adaptation
• Proc plugin with subprocess management and command allowlist
• Skills plugin with sandboxed execution and input_schema validation
• Toolkits plugin with JSON Schema configuration and lifecycle management
• MCP plugin with tool/resource/prompt proxying

Transport

• HTTPTransport with POST /send and GET /stream (SSE)
• DirectTransport for in-process testing

Experimental

• SubprocessTransport for host subprocess communication

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[0.1.0-alpha] — 2026-04-30

Added

Initial Release

• NDJSON-based message protocol with A2EMessage base model
• Handshake flow with auth_token authentication
• Capability negotiation between agent and host
• 9 capability namespaces: tools, memory, env, proc, learn, skills, toolkits, chains, mcp
• Plugin system: A2EPlugin ABC with setup(), handle(), supported_messages()
• PluginRegistry with priority-based and exclusive dispatch
• A2EServer with FastAPI backend and session-per-connection model
• A2EClient with RPC and queue-based request correlation
• BaseTransport ABC with HTTP and Direct implementations
• Session management with isolated executors per connection
• File-based and SQLite persistence for snapshot/restore
• A2EHostConfig loaded from YAML with plugin, transport, and audit sections

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Release Notes Format

Each release entry contains the following sections where applicable:

Section	Description
Added	New features, capabilities, and APIs
Changed	Changes to existing functionality
Deprecated	Features to be removed in future releases
Removed	Features removed in this release
Fixed	Bug fixes
Security	Security-related changes and fixes

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Version Compatibility

A2E Version	Protocol	Python	Pydantic	FastAPI	MCP
0.1.x	1.0	3.10+	v2 (2.12+)	0.135+	1.27+

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Upgrade Guide

Upgrading from alpha to 0.1.0

1. Event model renamed: InvokeEvent → A2EEvent. Update any client code that references the old model name.
2. Audit logging: Audit is now enabled by default. Add audit section to your config.yaml or explicitly set enabled: false.
3. Client API paths: Some internal module paths were reorganized. Update imports:
   • a2e.agent → a2e.core.client.client
   • Cookbook modules moved from cookbook/ to cookbook/servers/
4. Event streaming: The rpc() method now supports event_callback for progressive streaming. Existing code without callbacks continues to work.
5. Proc plugin: Command allowlist is now enforced. Previously all commands were allowed. Add allowed commands to your plugin config.

General Upgrade Steps

1. Update the package: pip install --upgrade a2e
2. Check the changelog for breaking changes
3. Update config.yaml if new configuration fields are required
4. Ensure client and server use the same protocol version
5. Run the test suite to verify compatibility