dspy.RLM — Recursive Language Model

dspy.RLM runs the LLM in a sandboxed Python REPL (Pyodide/WASM via Deno) with access to the full context as variables. The LLM writes code to slice, grep, summarize, and recursively sub-query the data, iterating until it can answer. Use it when the context is too large to cram into a single prompt.

Prerequisites

• Deno installed (for the default PythonInterpreter): brew install deno or see https://deno.land. The interpreter is a Pyodide-in-WASM sandbox spawned by Deno.
• A sub-LM for inner calls — usually a cheaper model than the outer LM. Defaults to dspy.settings.lm.

Canonical usage

import dspy

dspy.configure(lm=dspy.LM("openai/gpt-4o"))
sub_lm = dspy.LM("openai/gpt-4o-mini")    # cheap inner model

rlm = dspy.RLM(
    "context, query -> answer",
    max_iterations=20,
    max_llm_calls=50,
    max_output_chars=10_000,
    sub_lm=sub_lm,
    tools=[],
    verbose=False,
)

result = rlm(
    context=open("huge_log.txt").read(),   # can be 500k+ tokens
    query="Summarize every unique error class and how many times each appeared.",
)
print(result.answer)

Full constructor

dspy.RLM(
    signature: type[Signature] | str,
    max_iterations: int = 20,       # REPL loop cap
    max_llm_calls: int = 50,        # sub-LM call cap (stops runaway recursion)
    max_output_chars: int = 10_000, # truncate REPL stdout per step
    verbose: bool = False,          # print the REPL trace
    tools: list[Callable] | None = None,
    sub_lm: dspy.LM | None = None,
    interpreter: CodeInterpreter | None = None,  # custom sandbox
)

When to reach for RLM vs. alternatives

Situation	Use
Context <100k, answer fits one LM call	dspy.Predict / dspy.ChainOfThought
Need external tools (web, db)	dspy.ReAct(tools=[...])
Math/code that must run	dspy.ProgramOfThought
Huge context, recursive chunking, or data-exploration loop	dspy.RLM
Entire-codebase reasoning where the LM should grep/read files	dspy.RLM with file-reading tools=[...]

Composition — RLM as a module inside a larger program

Wrap the RLM in your own dspy.Module and optimize the enclosing program with GEPA. GEPA can tune both the RLM's outer signature instruction and the surrounding predictors.

class RepoAuditor(dspy.Module):
    def __init__(self):
        super().__init__()
        self.explore = dspy.RLM("repo_tree, question -> findings",
                                max_iterations=30, sub_lm=dspy.LM("openai/gpt-4o-mini"))
        self.synth = dspy.ChainOfThought("findings, question -> report")

    def forward(self, repo_tree, question):
        f = self.explore(repo_tree=repo_tree, question=question).findings
        return self.synth(findings=f, question=question)

Then: dspy.GEPA(metric=..., ...).compile(student=RepoAuditor(), trainset=..., valset=...).

Practical tips

• Budget carefully. A single RLM call can issue dozens of sub-LM calls. Keep max_llm_calls tight (20–50) in production; raise for research.
• The default stdout cap is smaller in DSPy 3.2.x. max_output_chars now defaults to 10_000; raise it deliberately if your REPL tools print large tables or document slices.
• Use a cheap sub_lm. The outer LM orchestrates; inner calls (summarize, filter, score) don't need the flagship model.
• Pass data as kwargs, not in the instruction. rlm(context=huge_string, query="...") lets the REPL treat context as a Python variable. Avoid concatenating it into the prompt.
• verbose=True while debugging. Prints every REPL step — invaluable when the RLM appears to hang or loop.
• Custom tools are regular Python callables passed via tools=[...]; they are exposed inside the sandbox. Useful for read_file, grep, vector_search, etc. In DSPy 3.2.x they are invoked by keyword, so give them named, typed parameters rather than positional-only signatures.
• Deno install is required. Missing Deno is the #1 RLM error. Check which deno before reporting bugs.

Security note

The default interpreter is a Deno-sandboxed Pyodide WASM runtime — no filesystem, network, or subprocess access by default. If you pass custom tools that do I/O, your tools' security posture is yours. Never hand raw subprocess.run to the RLM.

Anti-patterns

• Using RLM when a 32k-token prompt would fit — overhead is not worth it.
• Missing Deno → hard-to-diagnose failures. Install it.
• max_llm_calls left at default in a production path — runaway cost.
• Passing secrets in the context string — they get echoed into REPL state.

Next

• Wrap-and-optimize with GEPA → dspy-gepa-optimizer.
• Full reference → reference.md.