Defending Code Reference Harness

A reference implementation for autonomous vulnerability discovery and remediation with Claude, based on our learnings from partnering with security teams at several organizations since launching Claude Mythos Preview. For a write up of these learnings along with best practices, see the accompanying blog post (also available in blog-post.md). For a lightweight SDK-only walkthrough of the same recon → find → triage → report → patch loop, see the companion cookbook.

This repo is not maintained and is not accepting contributions.

🔒 Want a managed option? Anthropic offers Claude Security, a hosted product that finds and fixes vulnerabilities in your source code across multiple projects. Claude Security scans your repository for vulnerabilities, applies a multi-stage verification pipeline to reduce false positives, and lets you manage findings through their lifecycle: triage, fix validation, and rapid fix generation.

This repository is an open-source reference implementation based on general best practices for finding vulnerabilities using Claude. You can use it to build your own vulnerability finding pipeline, customize the logic, and it can be used with whatever access you have to Claude APIs (including Bedrock, Vertex, or Azure).

Contents

• Claude Code skills: /quickstart, /threat-model, /vuln-scan, /triage, /patch, /customize: interactive scoping, scanning, triage, and patching. Open this repo in Claude Code and run /quickstart to get oriented.
• harness/: the autonomous reference pipeline (recon → find → verify → report → patch), configured for finding C/C++ memory vulnerabilities using Docker and ASAN. This harness is a reference, not a product. The general shape, prompts, and sandboxing are reusable, but the harness will not work on every codebase out of the box. Run /customize to port it to your language, detector, or vuln class.

⚠️ Security: /quickstart, /threat-model, /vuln-scan, and /triage only read and write files. Running /patch on static findings (TRIAGE.json or VULN-FINDINGS.json) is likewise read- and write-only. /customize edits the harness code and runs validation commands. Any of these skills are safe to run unsandboxed, as long as you review and approve each tool use in Claude Code. The autonomous reference pipeline (including /patch on pipeline results) executes target code, so it refuses to run outside of a gVisor sandbox unless explicitly overridden. To get set up, run scripts/setup_sandbox.sh once, then invoke the pipeline via bin/vp-sandboxed. See docs/security.md and docs/agent-sandbox.md for more details.

Getting Started

git clone https://github.com/anthropics/defending-code-reference-harness
cd defending-code-reference-harness
claude

# 30-sec intro + guided first run on the canary target
> /quickstart

> /quickstart how do I port the pipeline to Java?
> /quickstart how do I triage all these bugs?

Further Reading

• Blog Post · The accompanying blog post with learnings + best practices
• Pipeline · How it works: diagram, stages, CLI flags
• Security · Sandboxing, what not to mount
• Agent sandbox · gVisor isolation + egress allowlist for every agent
• Customize · Port to my stack; which files change and why
• Patching · Generate and verify fixes for verified crashes
• Troubleshooting · Duplicates, rate limits, subagent model pinning
• Safeguards · Block for dangerous cyber work

Ramp Up

The most successful security teams we've partnered with are those that have gotten hands-on the fastest. Though it's tempting to spend months designing the perfect pipeline, we recommend starting small on Day 1 and building from there as learnings come. The steps below follow that pattern and set an ambitious (but reasonable) pace based on what we've seen.

Step 1 (Day 1): Build a threat model and run your first static scan + triage

Day 1 is focused on seeing the whole loop end-to-end. Using only the interactive skills, you'll build a threat model, run a static scan scoped by it, triage what comes back, and draft candidate fixes. You'll finish the day with a threat model, a ranked list of static findings, and candidate patches.

The relevant skills only read and write files in your repo. As long as you run Claude Code interactively and approve each tool use, no sandbox is needed.

# Pin every subagent to the model you want
export CLAUDE_CODE_SUBAGENT_MODEL=<model-id>
claude

# 0. intro + guided first run
> /quickstart

# 1. Build a threat model (aim before you shoot)
> /threat-model bootstrap targets/canary

# 2. Run a static scan, scoped by that threat model
> /vuln-scan targets/canary

# 3. Verify, dedupe, and rank what came back
> /triage targets/canary/VULN-FINDINGS.json

# 4. Generate candidate fixes for the verified findings
> /patch ./TRIAGE.json --repo targets/canary

This flow produces THREAT_MODEL.md, VULN-FINDINGS.{json,md}, TRIAGE.{json,md}, and PATCHES/.

The vulnerability candidates produced in Step 1 come from Claude's static review of the source (nothing is built or run), so expect more false positives on any non-canary targets. In Step 2, you'll produce execution-verified findings.

Note: on the canary target, /triage may dismiss the scan's findings as false positives. entry.c announces itself as deliberately vulnerable demo code, and /triage correctly excludes bugs in test / fixture code. To see the full confirm / dedupe / false positive flow, run it on the curated fixture instead (/triage .claude/skills/triage/fixtures/canary-findings.json --repo targets/canary) or point the Step 1 skills at your own code.

Step 2 (Day 2): Run the reference pipeline on a C/C++ library

On Day 2, you'll move from interactive skills to your first autonomous run using the reference pipeline. You'll run the full recon → find → verify → report loop in your environment on a known-vulnerable open-source library, then generate a candidate patch for what it finds. You'll finish with a set of reproducible crashes, exploitability reports, and candidate patches, along with a feel for how the pipeline works.

Running the pipeline is simple:

# One-time setup
python3 -m venv .venv && .venv/bin/pip install -e .
./scripts/setup_sandbox.sh   # installs gVisor, builds the agent images, and verifies isolation; note: requires Docker
export ANTHROPIC_API_KEY=sk-ant-...   # or CLAUDE_CODE_OAUTH_TOKEN; the pipeline requires one in env

# Run the recon → find → verify → report loop
bin/vp-sandboxed run drlibs --model <model-id> --runs 3 --parallel --stream --auto-focus
# Generate a candidate patch for each finding
bin/vp-sandboxed patch results/drlibs/<timestamp>/ --model <model-id>

# Or, ask Claude Code to launch the pipeline and watch the run for you
claude
> run the pipeline on drlibs and explain findings as they come

Results from the loop land in a results/drlibs/<timestamp>/ directory. With the --stream flag, the first report will appear in minutes under reports/bug_NN/.

⚠️ run spawns autonomous agents. The pipeline runs each agent inside a gVisor container with egress restricted to the Claude API. Agent-spawning subcommands refuse to start outside it unless explicitly overridden. For more information, see docs/security.md and docs/agent-sandbox.md.

Under the hood, the pipeline walks through seven stages:

1. Build: Compiles the target into a Docker image with ASAN (the memory error detector for C and C++). The pipeline builds this image automatically on first run using the target's Dockerfile.
2. Recon: A lightweight agent reads the source inside a network-isolated container and proposes a partition, i.e., "here are N distinct input-parsing subsystems worth attacking separately", so that parallel find agents explore different areas instead of converging on the same bug. Without the --auto-focus flag, the pipeline uses the focus_areas list from the target's config.yaml.
3. Find: N agents run in parallel, each in its own isolated container. Each agent reads the source, crafts malformed inputs, and runs the ASAN binary until a given input produces a crash 3 out of 3 times.
4. Verify: A separate grader agent reproduces each crash in a fresh container that the find agent hasn't touched. The only thing that crosses over from the find agent to the grader is the proof of concept it produced.
5. Dedupe: A judge agent compares verified crashes against bugs already reported and decides whether each is a new bug, a better example of a known bug, or a duplicate to skip.
6. Report: A report agent writes a structured exploitability analysis per unique bug, including details on primitive class, reachability, escalation path, and severity.
7. Patch (the separate patch command above): A patch agent writes a proposed fix, and a grader agent confirms that the new code builds, that the original proof of concept input no longer crashes, that the target's test suite still passes, and that a fresh find agent can't find a way around the fix.

For more details, see docs/pipeline.md.

Step 3 (Days 3-5): Customize the pipeline for your target

On Days 3-5, you'll customize the harness for your own target. First, you'll point the Step 1 skills at your code, then you'll use /customize to port the pipeline to your stack. By the end of the week, you'll have a targets/<your-service>/ directory that the pipeline can run against, validated with a single smoke run of the pipeline, and ready to scale up in Step 4.

While the reference pipeline is designed for finding memory vulnerabilities in C and C++ code, its shape is generic. Porting it to a new vuln class or language just means answering the following questions for your target stack:

Before customizing, point the Step 1 skills at your own code. As a reminder, they're read- and write-only, so they can run unsandboxed.

claude

> /quickstart how do I customize this for ~/code/my-service?

> /threat-model bootstrap-then-interview ~/code/my-service
> /vuln-scan ~/code/my-service
> /triage ~/code/my-service/VULN-FINDINGS.json --repo ~/code/my-service

Then, use the artifacts produced by those skills in the /customize skill, which modifies the harness for your codebase.

> /customize use ~/code/my-service/{THREAT_MODEL.md,VULN-FINDINGS.json} and ./TRIAGE.md

When /customize is done, you'll have a targets/my-service/ directory set up. Validate it with a smoke run of the pipeline before scaling up.

bin/vp-sandboxed run my-service --model <model-id> --runs 1

For more details, see docs/customizing.md.

Step 4 (Week 2): Start autonomous scanning, triage, and patching

In Week 2, you'll use the pipeline you customized in Step 3 on your own targets, adding an outer loop to the inner pipeline loop - run multiple pipeline scans, triage the findings from across those runs, patch based on prioritization, and repeat.

# Scan - run a wave of parallel runs against your target
bin/vp-sandboxed run my-service --model <model-id> --runs 5 --parallel --stream --auto-focus

# Triage - dedupe and rank every finding across all waves using your threat model
> /triage results/my-service/ --repo ~/code/my-service --auto --votes 5

# Patch - generate and validate fixes, starting with what triage ranked the highest
> /patch results/my-service/<timestamp>/ --model <model-id>

⚠️ Follow the same sandboxing guidelines as in Step 2

A given pipeline run already verifies and deduplicates its own findings. /triage works across many pipeline runs. When pointed at the results/ directory, it collapses duplicates across all runs (and any static findings from /vuln-scan if present), recalibrates severity ratings against your threat model, and attempts to route every finding to the component owner.

When possible, patching findings quickly helps keep the outer loop as productive as possible. When findings are fixed, the model can't re-find them, and instead will surface net new, typically deeper issues. As you run more pipeline waves, the number of findings will likely go down, but the complexity will likely also go up. If quick patching isn't possible, even just recording prior findings in the target's known_bugs can help steer future runs toward newer bugs.

Autonomous triage and patching are still open issues, and this reference harness doesn't fully solve them. The verification strategies in /patch help raise the bar, but severity and prioritization are ultimately judgments about your environment, and verified patches are not always upstreamable. Many partners have reported these steps as their current bottlenecks, and you should budget real engineering time for them.

For more details, see docs/triage.md and docs/patching.md.

Looking Forward

After the initial ramp up, the teams we've worked with have tended to invest in a few directions:

1. Reviewing all their internal repos and key open-source dependencies, ranking which are the most important to scan (e.g., based on their exposure, history of CVEs, business-criticality), then working through scanning the list in priority order.
2. Setting up bespoke infrastructure for scanning to move scans off of laptops or one-off VMs. The most successful teams resist the urge to build the perfect scanning platform before scaling up.
3. Incorporating scans into their SDLC. Some teams have set up recurring scans (e.g., daily, weekly) or have added scanning into their CI pipelines.
4. Testing and experimenting with the models to find what works best for them.