fast-rlm 0.1.17

Light-dependency, Feature-Rich Implementation of Recursive Language Models

fast-rlm

A minimal implementation of Recursive Language Models (RLMs) using Deno and Pyodide.

GitHub | Documentation | PyPI

Watch the full video on YouTube RLM Tutorial

What are RLMs

RLMs are an inference technique where an LLM interacts with arbitrarily long prompts through an external REPL. The LLM can write code to explore, decompose, and transform the prompt. It can recursively invoke sub-agents to complete smaller subtasks. Crucially, sub-agent responses are not automatically loaded into the parent agent's context — they are returned as symbols or variables inside the parent's REPL.

Support

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Demo

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Install

pip install fast-rlm

Requirements

• Python 3.10+
• Deno 2+
  • macOS/Linux: curl -fsSL https://deno.land/install.sh | sh
  • Windows (npm): npm install -g deno
• (Optional) Bun — only needed for the TUI log viewer

Environment Variables

Set your LLM API key before running:

export RLM_MODEL_API_KEY=sk-or-...

Variable	Description	Default
RLM_MODEL_API_KEY	API key for your LLM provider	—
RLM_MODEL_BASE_URL	OpenAI-compatible base URL	https://openrouter.ai/api/v1

By default, fast-rlm uses OpenRouter. You can point it at any OpenAI-compatible API by setting RLM_MODEL_BASE_URL.

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Quick Start

import fast_rlm

result = fast_rlm.run("Generate 50 fruits and count number of r")
print(result["results"])
print(result["usage"])

Arbitrarily Long Context

The key idea behind RLMs is that the prompt can be arbitrarily long — far beyond any model's context window. The agent explores it programmatically through the REPL rather than trying to fit it all into a single call.

import fast_rlm

transcripts = open("lex_fridman_all_transcripts.txt").read()  # millions of tokens

result = fast_rlm.run(
    "Here are the transcripts of all Lex Fridman podcasts. "
    "Summarize what the first 5 Machine Learning guests had to say about AGI.\n\n"
    + transcripts
)
print(result["results"])

The agent will write code to search, filter, and chunk the transcripts on its own — no manual splitting required.

Structured Input & Output

Instead of squeezing your data into a string, you can pass a dict as the query and ask for a typed result back via output_schema. The agent receives the dict as a real Python dict (no parsing on its first turn), and its FINAL value is validated against the schema before being returned.

import fast_rlm
from pydantic import BaseModel

class Verdict(BaseModel):
    movie: str
    average_score: float
    consensus: str

result = fast_rlm.run(
    {
        "task": "Aggregate the reviews into a single verdict.",
        "movie": "The Trail of Pixels",
        "reviews": [
            {"name": "Asha", "score": 8, "text": "Tight pacing..."},
            {"name": "Bo",   "score": 6, "text": "Beautiful but thin..."},
            {"name": "Cy",   "score": 9, "text": "Instant favorite..."},
        ],
    },
    output_schema=Verdict,
)

verdict = Verdict.model_validate(result["results"])

Structured input. When query is a dict, the agent's initial probe prints a flat top-level schema (keys + type + length + truncated preview) so it can index context["reviews"] directly instead of stringifying.

Structured output. output_schema accepts:

Form	Example
Pydantic model class	output_schema=MyModel
Pydantic generic	output_schema=list[MyModel]
Python primitive	output_schema=int (also str, float, bool, list, dict)
Raw JSON Schema dict	output_schema={"type": "array", "items": {"type": "string"}}

The schema is shown to the agent at step 0 (Required output schema for FINAL (JSON Schema):). After every FINAL(...) call the value is validated; on failure the agent receives the schema and the specific validation errors (path + message) and may retry within its remaining call budget. Pydantic is an optional dependency — only required if you pass a Pydantic class or generic.

Schemas for subagents. Inside the REPL the agent can require a subagent's output shape by passing a JSON Schema dict as the second argument to llm_query:

schema = {"type": "array", "items": {"type": "string"}}
fruits = await llm_query("Generate 25 fruit names.", schema)

The child subagent enforces the schema the same way. See examples/structured_io.py and examples/parallel_r_count.py for end-to-end demos.

Tools

Inside the REPL the agent has two built-in tools and may also receive user-defined tools as ordinary Python functions. There is no separate tool-calling API — tools are just callables in the REPL namespace.

Pass Python functions to fast_rlm.run(..., tools=[my_fn]) and they will be pre-loaded into the root agent's REPL. The RLM is shown the function name, input names, and docstring as description. They are not shown the full internal code of the tool (although they can choose to inspect it if the task requires them to). The agent calls them like any normal function inside the REPL.

def filter_short(items: list[str], max_len: int = 20) -> list[str]:
    """Return only items shorter than max_len."""
    return [x for x in items if len(x) < max_len]

result = fast_rlm.run("Pick the short titles from the list.", tools=[filter_short])

Two rules apply to any tool that may be handed to a sub-agent:

• Sub-agents do NOT inherit tools automatically. To give a child a tool, the main agent must pass it explicitly in the REPL: await llm_query("...", tools=[filter_short]).
• Tools must be self-contained. Do imports inside the function body and don't close over REPL-level variables - the child runs in a fresh REPL where outer state does not exist.

The agent can also def new functions inside the REPL at any time and pass them down the same way.

Currently all tools are expected to be Python functions. These functions are available inside the REPL. They are NOT available when the LLM produces code or generates reasoning steps.

Passing environment variables inside the REPL

Tools often need credentials or configuration (API keys, base URLs, account IDs). Pass them through the env_variables kwarg on fast_rlm.run(...):

import os
import fast_rlm

def search_web(query: str, top_k: int = 5) -> list[dict]:
    """Search the web via Tavily and return the top results."""
    import os, urllib.request, json
    req = urllib.request.Request(
        "https://api.tavily.com/search",
        data=json.dumps({"query": query, "max_results": top_k}).encode(),
        headers={
            "Authorization": f"Bearer {os.environ['TAVILY_API_KEY']}",
            "Content-Type": "application/json",
        },
    )
    return json.loads(urllib.request.urlopen(req).read())["results"]

result = fast_rlm.run(
    "Find three recent papers on recursive language models.",
    tools=[search_web],
    env_variables={"TAVILY_API_KEY": os.environ["TAVILY_API_KEY"]},
)

Behavior:

• env_variables must be a dict[str, str].
• Each entry is injected into os.environ inside every Pyodide REPL spawned by the run — the root agent and all sub-agents.
• They are not set on the host Deno process and never appear in prompts, logs, or model context. The model only ever sees a tool's signature + docstring, so the key stays hidden as long as your tool doesn't print or return it.
• Tools read them with the normal os.environ["..."] (do the import os inside the tool body — see the self-containment rule above).

MCP servers

fast-rlm can connect to Model Context Protocol servers and expose their tools and resources inside the REPL. The agent calls them with await mcp_call(server, tool, **kwargs) and reads resources with await mcp_read_resource(uri) — just like any other REPL function.

Nothing extra to install for fast-rlm. MCP support is optional and lazy: the MCP client lives in the Deno engine, and Deno auto-downloads it on first use. There is no pip install fast-rlm[mcp] — runs that don't use MCP never load it. You only install the MCP servers you actually want to connect to (each per its own docs).

Pass servers to run(..., mcp_servers={...}), keyed by name. Transport is chosen by the config shape:

import fast_rlm

result = fast_rlm.run(
    "Read /data/report.md and summarize it in three bullets.",
    mcp_servers={
        # stdio: fast-rlm SPAWNS the server (and kills it on exit) — you don't run it.
        "fs":   {"command": "npx", "args": ["-y", "@modelcontextprotocol/server-filesystem", "/data"]},
        # http: the server must already be running; you point at its URL.
        "web":  {"url": "http://localhost:3333/mcp", "headers": {"Authorization": "Bearer ..."}},
    },
)

Install a server the usual way before pointing fast-rlm at it, e.g.:

# stdio servers are launched on demand via their command (npx/uvx/node/...)
npx -y @modelcontextprotocol/server-filesystem /data    # Node-based
uvx mcp-server-fetch                                     # Python-based

Config key	Transport	Who runs the server?	Notes
command (+ args, cwd, env)	stdio	fast-rlm spawns it	grants Deno --allow-run; a shell/filesystem server is full host access, not sandboxed
url (+ headers)	HTTP	you (must be listening)

Inside the REPL the agent gets a small, lazy discovery API (the step-0 probe only shows counts, never full schemas):

• mcp_list_tools(server=None) / mcp_tool_schema("server.tool") / await mcp_call(server, tool, **kwargs)
• mcp_list_resources() / mcp_list_resource_templates() / await mcp_read_resource(uri, server=None)

Configuration

from fast_rlm import run, RLMConfig

config = RLMConfig.default()
config.primary_agent = "minimax/minimax-m2.5"
config.sub_agent = "minimax/minimax-m2.5"
config.max_depth = 5
config.max_money_spent = 2.0

result = run(
    "Count the r's in 50 fruit names",
    prefix="r_count",
    config=config,
)

All config fields:

Field	Type	Default	Description
primary_agent	str	z-ai/glm-5	Model for the root agent
sub_agent	str	minimax/minimax-m2.5	Model for child subagents
max_depth	int	3	Max recursive subagent depth
max_calls_per_subagent	int	20	Max LLM calls per subagent
truncate_len	int	2000	Output chars shown to the LLM per step
max_money_spent	float	1.0	Hard budget cap in USD
max_completion_tokens	int	50000	Max total completion tokens across all subagents
max_prompt_tokens	int	200000	Max total prompt tokens across all subagents

Best Practices & Troubleshooting

• Place your task at the top or bottom of the prompt — the REPL restricts how much context the LLM sees, so don't bury the task in the middle.
• Mark structured data with backtick blocks — wrap JSON, CSV, etc. in fenced code blocks and name the format in the prompt.
• Use strong coding models — agents write and execute Python, so coding benchmarks matter. See recommended models.
• Inject domain docs when needed — for obscure domains, add reference material and tell the agent how it's organized (e.g. with ## headers).
• Check logs and start with strict limits — review what the agent is doing before scaling up. Prompt changes usually help more than bigger budgets.

For the full guide, see the Best Practices & Troubleshooting docs page.

Log Viewer

Every run saves a .jsonl log file to logs/.

# Print stats (no extra dependencies)
fast-rlm-log logs/run_xxx.jsonl

# Interactive TUI viewer (requires bun)
fast-rlm-log logs/run_xxx.jsonl --tui

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Development (from source)

1. Install Deno

Windows (npm):

npm install -g deno

macOS / Linux:

curl -fsSL https://deno.land/install.sh | sh

Then add Deno to your PATH:

export DENO_INSTALL="$HOME/.deno"
export PATH="$DENO_INSTALL/bin:$PATH"

2. Install Bun (for the log viewer)

curl -fsSL https://bun.sh/install | bash
cd tui_log_viewer && bun install

3. API Key Setup

Set your key in .env or .envrc:

export RLM_MODEL_API_KEY=sk-or-...

4. Configuration

Edit rlm_config.yaml at the project root:

max_calls_per_subagent: 20
max_depth: 3
truncate_len: 2000
primary_agent: "z-ai/glm-5"
sub_agent: "minimax/minimax-m2.5"
max_money_spent: 1.0
max_completion_tokens: 50000
max_prompt_tokens: 200000

5. Running

# Run the example
deno task test_counting_r

# Run the subagent directly
echo "What is 2+2?" | deno task subagent

# View logs
./viewlog logs/<logfile>.jsonl

6. Benchmarks

uv sync --extra benchmarks
uv run benchmarks/oolong_synth_benchmark.py
uv run benchmarks/longbench_benchmark.py

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Contributing

• Small PRs only — keep changes focused and minimal. Large PRs will not be accepted.
• No LLM-generated slop — AI-assisted code is fine, but bulk-generated boilerplate with no thought behind it will be rejected.
• Minor features welcome — small, well-scoped PRs that add useful functionality will be considered.
• Large feature requests — open an issue first to discuss the design before writing any code.

License

MIT License. See LICENSE.