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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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.

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.