ratel-ai 0.2.1rc1

Python SDK for Ratel — context engineering platform for AI agents. BM25 tool retrieval, MCP ingestion, framework-neutral gateway tools.

ratel-ai

Python SDK for Ratel — drop context engineering into any Python agent with one dependency.

Docs • Discord

As an agent runs, its context window fills with tool definitions it will never call this turn. A model staring at 100 tools picks the wrong one, burns tokens on schemas it ignores, and drifts. Ratel keeps the full toolset out of the prompt and surfaces only the handful that matter for the current turn.

ratel-ai is the Python surface of Ratel. It bundles the Rust core (ratel-ai-core) via PyO3, so a Python agent gets ranked tool selection by adding one dependency, in-process, no API key, no service to deploy, no Rust toolchain. Retrieval is BM25 over a schema-aware text projection of each tool: deterministic, with no embeddings, no vector DB, and no inference cost on the retrieval path. The API mirrors the TypeScript SDK one-to-one; the binding strategy is locked in ADR 0011.

Install

pip install ratel-ai
# upstream MCP ingestion (register_mcp_server) needs the extra:
pip install 'ratel-ai[mcp]'

Prebuilt abi3 wheels ship for darwin-arm64, darwin-x64, linux-x64-gnu, linux-arm64-gnu, and win32-x64-msvc, so there is nothing to compile. The base SDK runs on CPython ≥ 3.9; the mcp extra requires ≥ 3.10.

How it works

Everything starts with a ToolCatalog: register each of your tools once, pairing its metadata (id, description, JSON schemas) with the handler that runs it. From there you reach the model in one of two ways, and most agents use both at once:

• Pre-filter (top-K). Before each model call, ask the catalog for the few tools most relevant to the user's message and put those in the tool list. The full catalog never enters the prompt. This is Ratel's replace-by-default tool injection (ADR 0003).
• Dynamic gateway. Give the agent two always-present tools, search_capabilities (find more tools by description) and invoke_tool (run one by id), so it can reach the rest of the catalog on its own when the pre-filtered set is not enough.

The two compose: the pre-filter covers the common case in the prompt, and the gateway is the escape hatch for everything else. Tools can be local functions, an upstream MCP server's tools (via register_mcp_server), or both. The model sees one unified, ranked surface.

Quickstart

Register a catalog, then build each turn's tool list from the gateway plus the top-K hits for the user's message.

from ratel_ai import (
    ToolCatalog,
    ExecutableTool,
    search_capabilities_tool,
    invoke_tool_tool,
)

catalog = ToolCatalog()
catalog.register(
    ExecutableTool(
        id="read_file",
        name="read_file",
        description="Read a file from local disk and return its textual contents.",
        input_schema={
            "type": "object",
            "properties": {"path": {"type": "string", "description": "absolute path to the file"}},
            "required": ["path"],
        },
        output_schema={"type": "object", "properties": {"contents": {"type": "string"}}},
        execute=lambda args: {"contents": open(args["path"]).read()},
    )
)
# ...register the rest of your tools the same way.


# Each turn, assemble the tools the model is allowed to see:
def tools_for_turn(user_message: str) -> list[ExecutableTool]:
    gateway = [search_capabilities_tool(catalog), invoke_tool_tool(catalog)]
    top_k = [
        executable
        for hit in catalog.search(user_message, 3)  # BM25: the 3 most relevant tools
        if (executable := catalog.get_executable(hit.tool_id)) is not None
    ]
    return [*gateway, *top_k]

search_capabilities_tool and invoke_tool_tool return plain ExecutableTool objects (id, name, description, input_schema, output_schema, execute). Wrap each one in your framework's tool type and run your normal loop. There are two dispatch paths, and getting them right matters because a registered executor may be sync or async:

• Catalog tools (your top-K hits): dispatch through catalog.invoke(tool_id, args). It awaits coroutines only when needed and records the invoke_* trace events. Never await a raw execute yourself, or a sync handler raises.
• The two gateway tools (search_capabilities / invoke_tool): they are not catalog entries, and they own async handlers, so await t.execute(args) directly.

With Pydantic AI, the catalog-tool wrapper is:

from pydantic_ai import Tool

def catalog_tool(catalog: ToolCatalog, t: ExecutableTool) -> Tool:
    async def fn(**kwargs):
        return await catalog.invoke(t.id, kwargs)  # handles sync/async + trace events
    fn.__name__ = t.id
    # Tool.from_schema builds a tool from a JSON schema directly — exactly what a dynamic catalog needs.
    return Tool.from_schema(fn, name=t.id, description=t.description, json_schema=t.input_schema)

A complete runnable agent that wires both paths into a Pydantic AI Agent lives in examples/pydantic-ai/.

ToolCatalog

The catalog is the registry plus an executor per tool. The methods you will use:

catalog = ToolCatalog()

catalog.register(tool)                      # ExecutableTool: metadata + execute
catalog.search(query, top_k)                # → list[SearchHit]  (.tool_id, .score), BM25-ranked
catalog.has(tool_id)                        # → bool
catalog.get(tool_id)                        # → Tool | None            (metadata only)
catalog.get_executable(tool_id)             # → ExecutableTool | None  (metadata + execute)
await catalog.invoke(tool_id, args)         # run the handler, return its result

invoke calls the handler, awaits it only if it returned a coroutine (so sync and async executors both work), and re-raises whatever it throws after recording an invoke_error trace event. search defaults to origin="direct"; pass catalog.search(query, k, "agent") to tag a search as model-initiated in telemetry.

search_capabilities_tool / invoke_tool_tool: the agent gateway

These wrap a catalog into two tools an agent can call itself. Hand them to your loop and the model gets self-service access to the whole catalog without it living in the prompt.

search = search_capabilities_tool(catalog)  # id == "search_capabilities"
invoke = invoke_tool_tool(catalog)          # id == "invoke_tool"

search_capabilities({query, topKTools?, topKSkills?}) returns two independently-ranked buckets, so a relevant skill is never crowded out by matching tools (result keys are camelCase, a wire contract shared with the TypeScript SDK and MCP):

{
  "tools": {
    "groups": [
      {
        "server": {"name": "fs"},                    // grouped by server (the id prefix before "__")
        "hits": [
          {"toolId": "fs__read_file", "score": 1.42, "description": "...", "inputSchema": {}}
        ]
      }
    ]
  },
  "skills": [{"skillId": "deploy-vercel", "score": 0.9, "description": "..."}]
}

topKTools defaults to 5 and topKSkills to 3, each clamped to [1, 50]. The skills bucket is always present and stays empty until you pass a SkillCatalog.

invoke_tool({toolId, args}) runs catalog.invoke(tool_id, args) and returns the tool's result. Arguments go nested under args. On a bad call it returns a structured {"error": ..., "isError": True} instead of raising, so a model mistake (unknown id, malformed args, a handler that throws) stays recoverable inside the loop rather than crashing the host.

Upgrading from 0.1.x? search_tools_tool (id search_tools) is still exported as a deprecated, tools-only shim that keeps its original {"groups": ...} result shape. Migrate to search_capabilities_tool; see ratel_ai/gateway_compat.py.

ToolRegistry: ranking without execution

Need only the ranking, and you will dispatch tool calls yourself? ToolRegistry is the metadata-only BM25 index underneath ToolCatalog, with no executors and no gateway. It takes positional metadata rather than a dataclass:

from ratel_ai import ToolRegistry

registry = ToolRegistry()
registry.register(
    "read_file",
    "read_file",
    "Read a file from local disk and return its textual contents.",
    {"properties": {"path": {"type": "string"}}},
    {"properties": {"contents": {"type": "string"}}},
)

registry.search("read a text file", 5)
# → [SearchHit(tool_id="read_file", score=1.42), ...]

SkillCatalog: reusable playbooks, on demand

Skills are Markdown playbooks (a deploy runbook, a debugging checklist) ranked by a separate BM25 corpus from tools. Pass a SkillCatalog as the second argument to search_capabilities_tool and search returns the skills bucket alongside tools, each with its own result budget so a relevant skill is never starved by matching tools. The agent pulls a skill's full body into context on demand via get_skill_content_tool (id get_skill_content).

A skill can also declare the tools its instructions call: when the skill matches a query, those tools are pulled into the tools bucket (additively, deduped) so the agent gets the playbook and the tools it needs in one turn instead of a second search.

from ratel_ai import Skill, SkillCatalog, get_skill_content_tool, search_capabilities_tool

skills = SkillCatalog()
skills.register(
    Skill(
        id="vercel-deploy",
        name="vercel-deploy",
        description="How to deploy to Vercel: env vars, preview vs production, rollbacks.",
        tags=["deploy", "ship to production"],      # indexed for ranking
        tools=["vercel__deploy", "fs__read_file"],  # surfaced alongside the skill when it matches
        metadata={"stacks": ["next", "vercel"]},    # non-indexed context for higher-layer ranking
        body="## Deploying to Vercel\n1. ...",       # returned by get_skill_content_tool
    )
)

search = search_capabilities_tool(catalog, skills)  # 2nd arg → result gains a populated `skills` bucket
load = get_skill_content_tool(skills)               # id == "get_skill_content"

Only id, name, and description are required; tags, tools, metadata, and body are optional. get_skill_content({skillId}) returns {"body": ...}, or {"error": ..., "isError": True} for an unknown id.

register_mcp_server: ingest an MCP server

Requires the mcp extra. The caller owns the ClientSession lifecycle (set it up with async with) and passes the initialized session in. register_mcp_server lists the upstream's tools, registers each under a namespaced id (<name>__<tool>), and wires its executor to the upstream call_tool.

from ratel_ai import register_mcp_server

handle = await register_mcp_server(
    catalog,
    name="github",
    session=session,            # an initialized mcp.ClientSession you own
    transport_label="stdio",    # recorded on the upstream_register trace event
)
# handle.tool_ids           → ["github__create_issue", ...]
# handle.server_instructions → whatever you passed as `instructions`
# catalog.search / catalog.invoke now rank and run the upstream tools alongside local ones.

The caller-owns-the-session split is the one deliberate divergence from the TypeScript SDK, whose registerMcpServer reads the server instructions and transport label from the live handshake itself. Pass instructions= and transport_label= from your await session.initialize() result to make the emitted upstream_register event byte-identical across the two SDKs.

Telemetry

Pass trace to ToolCatalog to capture every search / invoke / gateway / upstream / auth event into a sink owned by the Rust core (ADR 0009). The default is no-op: nothing is captured unless you opt in.

from ratel_ai import ToolCatalog, TraceSinkConfig

catalog = ToolCatalog(
    trace=TraceSinkConfig(kind="jsonl", session_id="session-1", path="/tmp/ratel.jsonl"),
)
# every catalog.invoke, search_capabilities_tool, and register_mcp_server call now writes
# one JSON line per event to /tmp/ratel.jsonl.

Sink kinds:

• kind="noop", drop everything (default).
• kind="memory", session_id, keep events in memory; drain via catalog.drain_trace_events(). Useful in tests.
• kind="jsonl", session_id, path, append one JSON line per event to path (mode 0600 on Unix). Best-effort, lossy on backpressure; see ADR 0009 for the reliability profile.

search_capabilities_tool tags its emitted search event with origin="agent"; direct callers (catalog.search(query, k)) default to "direct". Override per call via catalog.search(query, k, "agent").

Develop

This package is part of the Cargo workspace at the repo root and builds into a local venv. From src/sdk/python/:

uv venv --python 3.11 .venv
uv pip install --python .venv maturin pytest pytest-asyncio ruff mypy
.venv/bin/maturin develop        # build the native extension into the venv
.venv/bin/pytest                 # run tests
.venv/bin/ruff check .           # lint
.venv/bin/mypy ratel_ai          # type-check

Layout

native/         PyO3 binding to ratel-ai-core (cdylib Cargo workspace member)
ratel_ai/       pure-Python SDK: catalog, gateway tools, skills, MCP ingestion
tests/          pytest suite
pyproject.toml  maturin build backend + tooling config

The native crate is a member of the top-level Cargo workspace, so cargo build --workspace picks it up automatically.