Building C Compiler

Building a C compiler with a team of parallel Claudes \ Anthropic Engineering at Anthropic Building a C compiler with a team of parallel Claudes

Published Feb 05, 2026 We tasked Opus 4.6 using agent teams to build a C Compiler, and then (mostly) walked away. Here's what it taught us about the future of autonomous software development.

Written by Nicholas Carlini, a researcher on our Safeguards team.

I've been experimenting with a new approach to supervising language models that we’re calling "agent teams." With agent teams, multiple Claude instances work in parallel on a shared codebase without active human intervention. This approach dramatically expands the scope of what's achievable with LLM agents. To stress test it, I tasked 16 agents with writing a Rust-based C compiler, from scratch, capable of compiling the Linux kernel. Over nearly 2,000 Claude Code sessions and $20,000 in API costs, the agent team produced a 100,000-line compiler that can build Linux 6.9 on x86, ARM, and RISC-V.

The compiler is an interesting artifact on its own, but I focus here on what I learned about designing harnesses for long-running autonomous agent teams: how to write tests that keep agents on track without human oversight, how to structure work so multiple agents can make progress in parallel, and where this approach hits its ceiling. Enabling long-running Claudes Existing agent scaffolds like Claude Code require an operator to be online and available to work jointly. If you ask for a solution to a long and complex problem, the model may solve part of it, but eventually it will stop and wait for continued input—a question, a status update, or a request for clarification.

To elicit sustained, autonomous progress, I built a harness that sticks Claude in a simple loop (if you’ve seen Ralph-loop, this should look familiar). When it finishes one task, it immediately picks up the next. (Run this in a container, not your actual machine). #!/bin/bash

while true; do COMMIT=$(git rev-parse --short=6 HEAD) LOGFILE="agent_logs/agent_${COMMIT}.log"

claude --dangerously-skip-permissions \ -p "$(cat AGENT_PROMPT.md)" \ --model claude-opus-X-Y &> "$LOGFILE" done Copy

In the agent prompt, I tell Claude what problem to solve and ask it to approach the problem by breaking it into small pieces, tracking what it’s working on, figuring out what to work on next, and to effectively keep going until it’s perfect. (On this last point, Claude has no choice. The loop runs forever—although in one instance, I did see Claude pkill -9 bash on accident, thus killing itself and ending the loop. Whoops!).

Running Claude in parallel Running multiple instances in parallel can address two weaknesses of a single-agent harness: One Claude Code session can only do one thing at a time. Especially as the scope of a project expands, debugging multiple issues in parallel is far more efficient. Running multiple Claude agents allows for specialization. While a few agents are tasked to solve the actual problem at hand, other specialized agents can be invoked to (for example) maintain documentation, keep an eye on code quality, or solve specialized sub-tasks.

My implementation of parallel Claude is bare-bones. A new bare git repo is created, and for each agent, a Docker container is spun up with the repo mounted to /upstream . Each agent clones a local copy to /workspace , and when it's done, pushes from its own local container to upstream. To prevent two agents from trying to solve the same problem at the same time, the harness uses a simple synchronization algorithm: Claude takes a "lock" on a task by writing a text file to current_tasks/ (e.g., one agent might lock current_tasks/parse_if_statement.txt, while another locks current_tasks/codegen_function_definition.txt). If two agents try to claim the same task, git's synchronization forces the second agent to pick a different one. Claude works on the task, then pulls from upstream, merges changes from other agents, pushes its changes, and removes the lock. Merge conflicts are frequent, but Claude is smart enough to figure that out. The infinite agent-generation-loop spawns a new Claude Code session in a fresh container, and the cycle repeats.

This is a very early research prototype. I haven’t yet implemented any other method for communication between agents, nor do I enforce any process for managing high-level goals. I don’t use an orchestration agent.

Instead, I leave it up to each Claude agent to decide how to act. In most cases, Claude picks up the “next most obvious” problem. When stuck on a bug, Claude will often maintain a running doc of failed approaches and remaining tasks. In the git repository of the project, you can read through the history and watch it take out locks on various tasks.

Lessons from programming with Claude agent teams

The scaffolding runs Claude in a loop, but that loop is only useful if Claude can tell how to make progress. Most of my effort went into designing the environment around Claude—the tests, the environment, the feedback—so that it could orient itself without me. These are the approaches I’ve found most helpful when orchestrating multiple Claude instances. Write extremely high-quality tests Claude will work autonomously to solve whatever problem I give it. So it’s important that the task verifier is nearly perfect, otherwise Claude will solve the wrong problem. Improving the testing harness required finding high-quality compiler test suites, writing verifiers and build scripts for open-source software packages, and watching for mistakes Claude was making, then designing new tests as I identified those failure modes.

For example, near the end of the project, Claude started to frequently break existing functionality each time it implemented a new feature. To address this, I built a continuous integration pipeline and implemented stricter enforcement that allowed Claude to better test its work so that new commits can’t break existing code. Put yourself in Claude’s shoes I had to constantly remind myself that I was writing this test harness for Claude and not for myself, which meant rethinking many of my assumptions about how tests should communicate results. For example, each agent is dropped into a fresh container with no context and will spend significant time orienting itself, especially on large projects. Before we even reach the tests, to help Claude help itself, I included instructions to maintain extensive…