axc-agent-engine 2.3.1

Agent execution engine with Plan-Observe-Replan (POR) and plugin architecture

AxcAgentEngine

Agent execution engine with Plan-Observe-Replan, tool calling, and a plugin system

Quick Start · Features · Architecture · Plugins · Examples

🇨🇳 Chinese

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Most agent frameworks rely on a ReAct loop: think, call a tool, observe, repeat. As tasks grow more complex, plain ReAct tends to drift.

AxcAgentEngine adds POR (Plan-Observe-Replan) on top of ReAct: the agent produces a structured plan, schedules steps by dependency, observes outcomes, and replans when needed.

🚀 Quick Start

pip install axc-agent-engine

# Pin the current 2.3 release
pip install axc-agent-engine==2.3.1

# Optional extras
pip install "axc-agent-engine[api]"
pip install "axc-agent-engine[workflow]"
pip install "axc-agent-engine[api,knowledge,workflow]"

Requires Python 3.11 or newer.

from axc_agent_engine import AgentModels, Engine, LLMConfig, PluginRegistry
from axc_agent_engine.llm.client import OpenAIClient
from axc_agent_engine.plugins.builtin import BuiltinToolsPlugin

registry = PluginRegistry()
registry.register(BuiltinToolsPlugin)

engine = Engine(plugin_registry=registry)
models = AgentModels(default=OpenAIClient(LLMConfig(
    base_url="https://api.openai.com/v1",
    api_key="sk-xxx",
    model="gpt-4o",
)))
agent = engine.load_agent_template("./agents/my_agent.yaml").instantiate(models=models)

# Non-streaming
result = await agent.chat("Analyze last month's sales data")

# Streaming
async for event in agent.stream("Build a REST API for user management"):
    if event.type == "stream_delta":
        print(event.content, end="")
    elif event.type == "tool_call":
        print(f"\n[Tool: {event.tool_name}]")
    elif event.type == "plan_created":
        print(f"\n[Plan: {event.content}, {len(event.steps)} steps]")

✨ Features

• POR planning - structured plans, dependency scheduling, replanning; routes via auto / react_only / por_first
• ReAct executor - standard think / call / observe loop
• Plugin system - built-in spec registry, YAML-driven loading; optional capabilities live in plugins
• Tool protocol - every tool returns ToolOutput; read-only runs concurrent, write serial; safe function-name mapping
• Durable workflow - WorkflowRuntime + CheckpointStore + Agent resume API; Burr is optional via axc-agent-engine[workflow]
• OpenAI compatible - provider protocol + OpenAI-compatible HTTP client and API subset
• Memory & knowledge - four-layer memory (KV, dedup, decay, graph hooks) + plugin-owned hybrid retrieval
• Runtime binding - models, mounted resources, request metadata, and YAML overrides stay in separate channels
• MCP - stdio, JSON-RPC HTTP, official SDK transports
• Human-in-the-loop - approval queue and ask_human tool
• Sidecar suite - multi-agent, simulation, eval, cost, failure mining, trace distillation

Full capability matrix

Capability	Implementation
ReAct loop	Executor
POR planning	auto / react_only / por_first
Durable workflow	MemoryWorkflowRuntime by default; optional BurrWorkflowRuntime via axc-agent-engine[workflow]
Plugin system	spec registry + YAML-driven loading
LLM provider	provider protocol + OpenAI-compatible HTTP
Parallel tools	read concurrent, write serial
Tool output	enforced ToolOutput
Tool name mapping	provider-side model-safe mapping
Context compression	built-in compress plugin
Memory	four layers + KV persistence + dedup + decay
Knowledge	plugin-owned retrieval over local sources or mounted indexes/resources
MCP	stdio / JSON-RPC HTTP / official SDK
Human approval	approval queue + ask_human
Sidecar	multi-agent / simulation / eval / cost / failure mining / distillation
API server	OpenAI Chat Completions compatible subset

📦 Docs

Architecture	Engine and plugin boundaries
API	HTTP API subset notes
Plugin development	Build your own plugin
Security model	Capabilities, risk, workspace
Examples	7 end-to-end demos
Contributing / Security / LICENSE	Apache-2.0

Agent YAML

name: "data-analyst"
description: "Data analysis assistant"

runtime:
  max_rounds: 50
  thinking: "auto"
  workspace: "/tmp/agent-workspace"
  allowed_capabilities:
    - "file_read"
    - "file_write"
    - "http_request"

system_prompt: |
  You are a data analysis assistant...

plugins:
  builtin_tools:
    enabled: true
    load: ["get_time", "file_read", "file_write", "http_request", "result_read"]
    defer: ["file_write", "http_request"]

  knowledge:
    enabled: true
    sources: ["./docs"]
    namespace: "default"

  memory:
    enabled: true
    namespace: "default"
    scope_keys: ["tenant_id", "user_id", "agent_name"]
    sensitive_policy: "redact"

  compress:
    enabled: true
    summary:
      after_rounds: 8
    durable_tools:
      names: ["agent_call", "knowledge_search"]
      keep: 12

  risk_guard:
    enabled: true

Notes:

• Plugin registration is explicit host code via PluginRegistry. Agent YAML only enables and configures already-registered plugins.
• builtin_tools loads only get_time when load is omitted; other built-ins must be explicitly enabled.
• Tools with a non-empty capability are denied by default; list them in runtime.allowed_capabilities.
• File and command tools require runtime.workspace by default.
• LLM configuration is provided in code, not in Agent YAML.
• Official builtin plugin YAML is for behavior parameters only. External endpoints, API keys, client objects, indexes, stores, and catalogs are mounted in code.

Provider Configuration

AgentModels accepts model provider objects. OpenAIClient(LLMConfig(...)) is the built-in OpenAI-compatible provider; custom providers implement LLMProvider (model, tool_name_mapping, chat, stream, ask, close).

from axc_agent_engine import AgentModels, ConcurrencyConfig, Engine, LLMConfig
from axc_agent_engine.llm.client import OpenAIClient
from axc_agent_engine.tools.name_mapping import ToolNameMappingConfig

main_model_config = LLMConfig(
    base_url="https://api.openai.com/v1",
    api_key="sk-xxx",
    model="gpt-4o",
    timeout=120,
    max_concurrent_requests=32,
    requests_per_minute=0,
    rate_limit_queue_timeout=10,
    tool_name_mapping=ToolNameMappingConfig(),
)

engine = Engine(
    concurrency=ConcurrencyConfig(
        max_engine_concurrent_runs=128,
        queue_timeout=30,
    ),
)
agent = engine.load_agent_template("./agents/my_agent.yaml").instantiate(
    models=AgentModels(default=OpenAIClient(main_model_config)),
)

Tool-name mapping is the provider's job. Internal tool names are encoded to model-safe function names before the LLM call and decoded before hooks and tool execution.

Runtime Binding

Agent YAML describes stable behavior. Runtime objects are bound in code when the template is instantiated:

agent = template.instantiate(
    models=AgentModels(
        default=gpt5_provider,
        utility=fast_provider,
        fallback=backup_provider,
    ),
    mounts={
        "knowledge.index": tenant_knowledge_index,
        "graph.store": tenant_graph_store,
        "skill.catalog": tenant_skill_catalog,
    },
    metadata={
        "tenant_id": "t_001",
    },
    overrides={
        "plugins.knowledge.namespace": "tenant:t_001",
        "plugins.skill.timeout": 30,
    },
)

• models binds provider objects for this Agent instance; the Engine does not own model configuration.
• mounts injects host-owned runtime resources for this instantiation and overrides Engine-level resources with the same name.
• metadata attaches instance metadata; request metadata can still override it inside a single chat/stream call.
• overrides patches validated Agent YAML fields before plugins are initialized. It only accepts YAML-serializable values and cannot bind runtime resources.

Preferred official resource slots are knowledge.index, graph.store, skill.catalog, and tracing.exporter. Advanced knowledge slots such as knowledge.documents, knowledge.embedding, knowledge.vector_store, and knowledge.reranker are also supported by the current plugin, but they are still runtime mounts, not YAML or overrides values.

Request Metadata and Tracing

Run-level metadata is passed through the public Agent entry points and reaches ExecutionContext.state.metadata. The tracing plugin copies this metadata into every span, while keeping top-level run_id, session_id, span_id, and parent_span_id.

async for event in agent.stream_with_messages(
    messages,
    session_id="123",
    llm_options={"temperature": 0.2},
    run_options={"run_id": "run_abc"},
    metadata={
        "external_trace_id": "trace_1001",
        "conversation_id": 123,
        "agent_profile_id": 9,
    },
):
    ...

Use this for host-side trace or audit correlation. Do not encode external correlation IDs in prompts, tool arguments, or fake tool messages.

Cancellation and Usage

Every run receives a run_id through run_options.run_id, metadata.run_id, or engine generation. Hosts should cancel a run through the Engine so nested agent_call, swarm, sidecar-owned Agent runs, and active tools share the same cancellation signal:

engine.cancel_run("run_abc", reason="user_cancelled")

Streaming callers receive a terminal cancelled event. Non-streaming callers receive CancelledError. Terminal done and cancelled events include aggregated usage in event.metadata["usage"]:

{
    "input_tokens": 1200,
    "output_tokens": 380,
    "total_tokens": 1580,
}

Hosts should read this aggregate instead of summing scattered LLM or sub-Agent events.

Multimodal Messages

The host owns uploads, authorization, OCR, image recognition, media URLs, and base64 generation. The engine only accepts normalized message content parts and passes them through the input/provider boundary:

messages = [{
    "role": "user",
    "content": [
        {"type": "text", "text": "Explain this screenshot."},
        {"type": "image_url", "image_url": {"url": "https://example.com/screen.png"}},
        {"type": "image_base64", "media_type": "image/png", "data": "..."},
        {"type": "file_ref", "ref": "artifact:123", "metadata": {"name": "report.pdf"}},
    ],
}]

Unsupported part types fail immediately. Official plugins do not fetch host media services or perform deployment-specific multimodal preprocessing.

Tool Output Views

ToolOutput separates the LLM context view from the UI display view:

• context_view(max_chars=2000) is written into MessageStore for the next LLM round. It prefers durable_summary, then summary, then compact content, and preserves artifact refs.
• display_view(max_chars=0) is used by tool_result events for host/UI display. It can expose full content or artifact refs without polluting LLM context.
• compact_view() is not a UI contract. New code should call context_view() or display_view() explicitly.

Hosts should not monkey patch ToolOutput to change LLM context behavior.

Durable Tool Results and Sub-Agent Events

The compress plugin preserves assistant tool calls and their matching tool results as atomic groups. Durable tool results are also saved into the compression boundary and reinjected after context packing. By default, agent_call and knowledge_search are durable; domain tools can opt in with compress.durable_tools.names, compress.durable_tools.capabilities, or ToolOutput.with_metadata({"durable": True, "durable_summary": "..."}).

collaboration.agent_call streams child Agent activity back to the parent run as standard events:

• sub_agent_start
• sub_agent_step
• sub_agent_complete

Stable fields include parent_tool_call_id, sub_run_id, agent_name, agent_id, step.type, tool_call_id, tool_name, content, artifacts, error, and duration_ms. Frontends should render these events directly. They should not parse agent_call tool results to invent a child execution timeline.

API

The HTTP API is an OpenAI Chat Completions compatible subset.

• POST /v1/chat/completions
• GET /v1/agents
• GET /v1/capabilities

Request-level tools and tool_choice are intentionally unsupported. Tools come from Agent YAML and plugins so the engine can enforce capabilities, risk metadata, plugin hooks, workspace policy, and audit events.

Clients should not assume full OpenAI parity; call /v1/capabilities first. See docs/API.md.

Built-in Plugins

Capabilities not required by a basic agent live in plugins. The default Engine.plugin_registry is empty; both built-in and custom plugins must be registered explicitly.

from axc_agent_engine import AgentModels, Engine, PluginRegistry
from axc_agent_engine.plugins.builtin import BuiltinToolsPlugin, MemoryPlugin
from my_project.plugins import MyCustomPlugin

registry = PluginRegistry()
registry.register_many([BuiltinToolsPlugin, MemoryPlugin, MyCustomPlugin])
engine = Engine(plugin_registry=registry)
agent = engine.load_agent_template("./agents/my_agent.yaml").instantiate(models=AgentModels(default=llm))

Plugin	Purpose
builtin_tools	Basic tools and artifact paging
knowledge	Ingestion, semantic chunking, hybrid retrieval, citations, rerank
memory	Memory, governance tools, sensitive-data policy, decay, TTL
output_format	Final output contracts, validation, repair, audit
graph	Entity/relation graph search and CRUD
skill	Load skills, run scripts through sandbox
mcp	MCP server tool loading and guarding
hooks	Declarative LLM/tool hook rules
compress	Context window management, summaries, recall, file restore
human_in_the_loop	Human approval and ask_human
risk_guard	Dynamic tool risk classification
safety	Input sanitization, prompt-injection checks, PII masking
tracing	Trace/span collection, audit mode, query tools
reflexion	End-of-round and end-of-run self-reflection
repetition_guard	Repeated tool / response / result detection
cost_statistics	Token and tool-call accounting
collaboration	Agent-to-agent calls and host orchestration entry
swarm	Lightweight parallel fan-out

Sidecar Capabilities

Sidecars live under axc_agent_engine.sidecar and are invoked explicitly by the host, not part of the agent's core execution path. See axc_agent_engine/sidecar/README.md.

Package	Purpose
sidecar.multi_agent	Multi-agent sessions, schedulers, stop conditions, shared context
sidecar.simulation	Structured simulation kernel
sidecar.eval	Evaluation cases, annotation stores, matcher, runner, reports
sidecar.agent_selector	Host-side agent routing and candidate scoring
sidecar.distiller	Distill rules, tool preferences, and skill candidates from traces
sidecar.failure_miner	Cluster failures and suggest remediation / eval coverage
sidecar.cost_optimizer	Cost estimation and optimization findings

from axc_agent_engine import AgentModels, Engine
from axc_agent_engine.sidecar import OrchestrationTaskService
from axc_agent_engine.storage.in_memory import InMemoryMessageBus

engine = Engine(message_bus=InMemoryMessageBus())
models = AgentModels(default=main_model, utility=utility_model)
red = engine.load_agent_template("./agents/red.yaml").instantiate(models=models)
blue = engine.load_agent_template("./agents/blue.yaml").instantiate(models=models)

service = OrchestrationTaskService(
    agent_getter=engine.get_agent,
    agent_lister=engine.list_agents,
    dispatcher=engine._dispatcher,
    utility_model=utility_model,
)

task = await service.run_task(
    agent_names=[red.name, blue.name],
    mode="redblue",
    topic="Plugin marketplace security tabletop",
    max_rounds=3,
)

Sidecar components that create internal Agent runs accept standard run context:

• EvalRunner.run_cases(..., run_options, metadata, case_run_options, case_metadata)
• MultiAgentSession.run/stream(..., run_options, metadata, agent_run_options, agent_metadata)
• SimulationRunner.run/stream(..., run_options, metadata, actor_run_options, actor_metadata)

Factory metadata supplements the base metadata for each case, agent, or actor. EvalCase.metadata remains evaluation/scoring metadata and is not request metadata.

Runtime Flow

Load time

flowchart TD
    A["Application creates Engine"] --> B["Inject shared services and PluginRegistry"]
    B --> C["Engine.load_agent_template(agent.yaml)"]
    C --> D["Validate Agent YAML"]
    D --> E["AgentTemplate.instantiate(models, mounts, metadata, overrides)"]
    E --> F["Apply YAML overrides"]
    F --> G["Merge Engine resources + mounts"]
    G --> H["Build PluginContext"]
    H --> I["Load enabled plugins from registry"]
    I --> J["Plugin.initialize(config, ctx)"]
    J --> K["Plugin.get_tools()"]
    K --> L["Register ToolDefinition"]
    L --> M["Create Agent and dispatcher consumer"]

One agent run

flowchart TD
    A["Agent.chat / stream / *_with_messages / resume"] --> B["RunRequest.create"]
    B --> C["Merge run_options.run_id and request metadata"]
    C --> D["Create ExecutionContext"]
    D --> E["ExecutionRunLifecycle.on_execution_start"]
    E --> F["MessageStore initializes system/user/plugin context"]
    F --> G{"Routing mode"}
    G -->|react_only or auto ReAct| H["ReActKernel round"]
    G -->|por_first or POR handoff| I["PORRunner / PORGraphRuntime"]
    H --> J["Plugin.transform_messages"]
    J --> K["compress: recent window + durable results + atomic tool groups"]
    K --> L["LLMCaller"]
    L --> M["Plugin.pre_llm_call"]
    M --> N["Strip engine-only message fields"]
    N --> O["LLM provider chat/stream"]
    O --> P{"LLM response"}
    P -->|final answer| Q["on_round_end -> on_execution_complete -> done"]
    P -->|tool_calls| R["Tool orchestrator"]
    R --> S["pre_tool_call / execute / post_tool_call"]
    S --> T["ToolOutput -> MessageStore role=tool"]
    T --> U["tool_result event"]
    U --> H
    S -->|agent_call / swarm| V["Child Agent dispatcher"]
    V --> W["sub_agent_start / sub_agent_step / sub_agent_complete"]
    W --> H
    I --> Q
    Q --> X["done/cancelled event with usage"]
    X --> Y["tracing spans saved with metadata and parent_span_id"]

Plugin Development

from axc_agent_engine import BasePlugin, ToolDefinition, ToolOutput
from axc_agent_engine.plugins.config_schema import config_field, config_schema

class MyPlugin(BasePlugin):
    name = "my_plugin"
    display_name = "My Plugin"
    priority = 30
    phase = "core"
    config_schema = config_schema(
        "my_plugin",
        "My Plugin",
        "Configuration for MyPlugin.",
        [
            config_field(
                "api_url",
                "接口地址",
                "string",
				"插件调用的后端接口地址。",
				label_en="API URL",
				default="http://localhost:5000",
			),
		],
        display_name_en="My Plugin",
    )

    def initialize(self, config: dict, ctx) -> None:
        super().initialize(config, ctx)
        self.api_url = config["api_url"]

    def get_tools(self) -> list[ToolDefinition]:
        return [ToolDefinition(
            name="my_tool",
            description="Does something useful",
            parameters={"type": "object", "properties": {"query": {"type": "string"}}},
            execute=self._execute,
        )]

    async def pre_tool_call(self, exec_ctx, tool_name, arguments):
        return True, arguments

    async def _execute(self, args: dict, context: dict) -> ToolOutput:
        return ToolOutput.text(f"Result for {args['query']}")

Plugins must inherit BasePlugin, declare config_schema, and return tools as ToolDefinition instances. ToolRegistry does not accept dicts.

Hosts can inspect registered plugin schemas through registry.list_plugin_config_schemas() or registry.get_plugin_config_schema("my_plugin"). The schema is for UI, templates, default display, and optional validation; YAML extra keys are still passed to initialize(config, ctx).

plugins:
  my_plugin:
    enabled: true
    api_url: "http://localhost:5000"

CLI

export AXC_LLM_BASE_URL="https://api.openai.com/v1"
export AXC_LLM_API_KEY="sk-xxx"
export AXC_LLM_MODEL="gpt-4o"

axc chat --agent ./agents/my_agent.yaml
axc serve --agent ./agents/my_agent.yaml --port 8000
axc --log-level DEBUG --json-logs chat --agent ./agents/my_agent.yaml

CLI logging flags are global and must be placed before the subcommand.

Design Decisions

• Engine core = Executor + ReActKernel + LLMCaller. Reads Agent YAML, calls LLM providers, runs the ReAct loop, emits events/results.
• POR state transitions live in pydantic-graph. PORGraphRuntime owns the plan/step/observe/replan loop; services execute work and persist checkpoints.
• Workflow resume is a runtime boundary. MemoryWorkflowRuntime is the default; BurrWorkflowRuntime is selected when the optional workflow dependency is installed.
• Plugins are the runtime extension boundary. Knowledge, memory, graph, MCP, output repair, skills belong in plugins.
• Orchestration is sidecar. Multi-agent sessions, simulation, mode adapters are host-driven.
• Evaluation is sidecar. EvalRunner, stores, matchers, reports are host-driven test framework pieces.
• Registration is not loading. The spec registry is the full plugin table; agents load only YAML-enabled plugins.
• Plugin schema is mandatory. A plugin without config_schema cannot be registered.
• Tools come from plugins. The engine core embeds no business tools.
• Tools must return ToolOutput. Non-ToolOutput returns are rejected.
• Tool definitions must be ToolDefinition. No dicts.
• Deployment-specific protocols stay out. External APIs, deployment databases, auth flows, and service discovery belong in host plugins.
• LLM config lives in code. Agent YAML describes runtime limits, capabilities, and plugins.
• Fatal errors are not hidden by fallback logic. Invalid configuration, invalid runtime resources, broken tool contracts, and flow errors must fail clearly.
• Official plugins stay lean. They may strengthen Agent behavior, but they do not own host network clients, deployment API keys, external services, or deployment-specific protocols.
• Runtime resources use mounts. Do not pass resources through YAML or overrides.
• Request correlation uses metadata. Hosts pass external trace identifiers through metadata; tracing spans persist them directly.
• The API is a subset. Request-level tools, tool_choice, n > 1 are rejected.

Tests

python3 -m pytest -q
python3 -m pytest --cov --cov-report=term-missing:skip-covered -q

The release gate requires at least 95% total coverage.