Benchmarking Claude Opus 4.6 Vulnerability Detection

Benchmarking Claude Opus 4.6's ability to detect real-world C/C++ vulnerabilities across four prompting and agent strategies. We evaluate on the PrimeVul paired test set (435 vulnerability/fix pairs from open-source projects), measuring precision, recall, and CVE-correctness to understand how structured reasoning, justification depth, and verification agents affect detection quality.

Check out our blog post for a more in-depth discussion of these results.

Key Finding

Requiring the model to produce increasingly rigorous justifications (execution traces, state proofs) improves pair-correct precision (P-C) from 13.6% to 20.3%, with rigorous precision nearly doubling from 8.7% to 15.8%. Adding a verification agent pushes P-C to 23.3% and CVE recall to 28.9%.

Experiments

Each experiment uses Claude Opus 4.6 as the analyzer and runs 3 times for consistency. All experiments share the same three-phase pipeline but differ in the structured output the model must produce.

#	Experiment	P-C	P-C Rigorous	P-C Flexible	CVE Recall	Vuln Findings	Benign Findings	Benign-Only Findings
1	No Justification	13.6%	8.7%	13.6%	27.6%	63.4%	52.4%	27.4%
2	Limited Justification	19.3%	14.5%	17.7%	25.5%	52.2%	36.6%	15.6%
3	Extensive Justification	20.3%	15.8%	17.7%	28.5%	54.6%	37.7%	18.6%
4	Verification Agent	23.3%	16.2%	18.5%	28.9%	57.2%	43.2%	24.7%

All values are medians across 3 runs. Reference baseline: GPT-4 CoT = 12.94% P-C.

Cross-Run Consistency

Each experiment runs 3 times. "All 3" means the result held in every run; "Any" means it held in at least one. This captures how stable the results were across runs.

#	Experiment	CVE Recall All 3	CVE Recall Any	P-C Rigorous All 3	P-C Rigorous Any	P-C Flexible All 3	P-C Flexible Any
1	No Justification	22.0%	33.1%	5.9%	11.1%	7.6%	19.6%
2	Limited Justification	21.7%	30.0%	12.5%	17.5%	14.2%	22.2%
3	Extensive Justification	23.6%	33.3%	11.6%	20.1%	12.8%	24.3%
4	Verification Agent	19.4%	37.6%	9.9%	23.4%	10.6%	27.2%

1. No Justification

Simple vulnerability analysis. The model reports CWEs, code snippets, and descriptions with no structured reasoning required.

2. Limited Justification

Requires a Justification with an UndesiredOperation (code + CWEs) and step_by_step_execution tracking variable state through ProgramSteps. The model must demonstrate a concrete execution path from function entry to the undesired operation.

3. Extensive Justification

Full proof of reachability. The model must provide:

• UndesiredOperation: description, code, CWEs, impact, and the variable states required to trigger it
• Justification: initial variable state at function entry, then a trace of DataTransformation steps (in_state -> out_state) and ConditionalStep steps (prove each branch is taken given current state)

4. Verification Agent

Same structured reasoning as experiment 3, plus a Claude Sonnet 4.6 verifier agent that checks each finding before inclusion. The verifier validates: is the undesired operation real, is the initial state correct, do steps follow logically, are conditionals justified, and does the final state match the preconditions. Findings get up to 2 verification attempts; unverified findings are discarded.

Pipeline

Each experiment follows the same three-phase pipeline:

analyze.py -> diff_judge.py -> judge.py

1. Analyze (analyze.py): Claude Opus 4.6 analyzes each of the 870 functions independently, producing structured vulnerability findings.
2. Diff Judge (diff_judge.py): For each commit pair (vulnerable + fixed), matches findings across versions and categorizes them as vuln_only, benign_only, or shared.
3. Judge (judge.py): Evaluates vuln_only findings against ground-truth CVE data to determine correctness.

Metrics

All metrics are computed over the 435 vulnerability/fix pairs. After analysis, the diff judge categorizes each finding as vuln_only (unique to vulnerable version), benign_only (unique to fixed version), or shared (present in both). The judge then evaluates whether each vuln_only finding correctly identifies the ground-truth CVE.

Metric	Definition
P-C	% of pairs where the vulnerable side has at least one finding and the benign side has zero findings (no benign_only, no shared). Measures raw discrimination: can the model tell vulnerable code from fixed code?
P-C Rigorous	P-C with the additional requirement that allvuln_only findings are judged as related to the ground-truth CVE. The strictest metric — the model must flag only the real vulnerability and nothing else, with a clean benign side.
P-C Flexible	% of pairs where all vuln_only findings are CVE-correct (at least one exists) and there are no benign_only findings. shared findings are permitted — these represent underlying issues not addressed by the patch. Every benign-side finding must have a corresponding linked vulnerable-side finding.
CVE Recall	% of pairs where the vulnerable side has at least one finding judged as related to the ground-truth CVE, regardless of what appears on the benign side. Measures the model's ability to detect the actual vulnerability.
Vuln Findings	% of vulnerable functions that have at least one finding (any category).
Benign Findings	% of benign functions that have at least one finding (any category).
Benign-Only Findings	% of benign functions that have at least one finding not also found on the vulnerable side (i.e., a benign_only finding with no linked shared counterpart).

Dataset

The PrimeVul paired test set contains 435 pairs (870 functions) from real security fixes across open-source C/C++ projects including Linux, TensorFlow, ImageMagick, FFmpeg, OpenSSL, mruby, and others. Each pair consists of:

• A vulnerable function (before the fix, target=0)
• A benign function (after the fix, target=1)

Ground truth includes CVE ID, CWE classification, NVD URL, and commit message.

Project Structure

src/
experiments/
no-justification/ # Experiment 1
limited-justification/ # Experiment 2
extensive-justification/ # Experiment 3
verification-agent/ # Experiment 4
common/
primevul.duckdb # Dataset in DuckDB format
data/
experiments/
*/experiment.json # Experiment metadata
*/runs/{1,2,3}/ # Per-run outputs (analysis, diffed, judged, stats)
experiment_comparison.json # Cross-experiment metrics comparison

Setup

Requires Python 3.12+ and uv.

uv sync

Set your Anthropic API key:

export ANTHROPIC_API_KEY=sk-...

Running an experiment

Each experiment has a run_experiment.sh script:

cd src/experiments/extensive-justification
bash run_experiment.sh 1 # run number

Dependencies

• pydantic-ai - Claude agent framework with structured outputs
• duckdb - Dataset storage and querying
• datasets - HuggingFace dataset loading