Since 2026-09-21 CircleCI rejects version 2.0 configs, branch filters such as /^(?!main).*/ and parameters that are used but never declared, so the pipeline fails before any job starts. A Sentry token pasted under environment: is readable by everyone with access to the project, and a deploy job with an aws-production context that runs on every branch hands those credentials to anyone who can push one. This skill finds those and the other CircleCI mistakes below, and tells you how to fix each one.

Your agent runs a scanner on every YAML file under .circleci/ (config.yml, continuation configs for setup workflows and dynamic config, packed config fragments) or on a config file you name. It understands orbs (registry, URL and inline), executors, jobs, commands, workflows, contexts, approval jobs, filters, pipeline parameters and << >> values, YAML anchors, and maps run: | blocks back to their real line numbers. It knows $NAME references, env_var_name orb parameters, test database passwords on service containers (also when a command reuses them) and settings such as OIDC_TOKEN_AUDIENCE: sts.amazonaws.com, so they are not reported as leaks. Rules with a CircleCI deadline compare with today's date. It never runs a job, the CircleCI CLI or the network. It returns BLOCK, REVIEW, or NOT FLAGGED, with the file, line, reason, and a safer approach, and links the CircleCI page behind each rule that has one. Secrets are never printed.

What it checks

BLOCK (fix before shipping)

• C01: passwords, tokens and keys written in the config (environment values, docker auth, orb and job parameters, parameter defaults, curl -u, Authorization headers, docker login -p, a literal echoed into docker login --password-stdin, npm _authToken, passwords in URLs, GitHub, AWS, CircleCI, npm, Slack and other token shapes, private keys)
• C02: scripts or binaries downloaded over http:// or with TLS checks off and then run without a checksum
• C03: orbs referenced as @volatile or with a dev: version (except the orb an orb repository publishes, in its own test pipeline)
• C04: config CircleCI no longer compiles: version other than 2.1, regex filters with negative lookarounds, possessive quantifiers or backreferences, undeclared parameters, pipeline parameters in inline orbs, job names with colons or parentheses, branches on a job, resource_class inside machine, an empty config.yml, and from 2026-12-14 the C19 validator warnings
• C05: removed executors: Ubuntu 14.04/16.04 and removed dated 20.04/22.04 machine images, removed Xcode versions (16.2.0 and 26.2.0 from 2026-12-07), Intel macOS and M1/M2 resource classes

REVIEW (check before release)

• C11: partner and community orbs pinned only to a major or minor version, and URL orbs on a branch instead of a commit
• C12: Docker images with no tag or latest, machine images on edge
• C13: branch names, tags, commit messages, pull request titles, webhook bodies and string pipeline parameters pasted into run commands (also through a job parameter)
• C14: add_ssh_keys without fingerprints
• C15: legacy circleci/ images, circleci/classic and Ubuntu 20.04 machine images, Xcode 16.2.0 and 26.2.0 before their removal on 2026-12-07, machine: true with no image
• C16: curl | bash, bash <(curl ...) and download-then-run over HTTPS without a checksum or signature check
• C17: deploy, release, publish or signing jobs with a context that run on every branch, with no filter, approval job or workflow condition (non-prod, preview, staging and dev deploys are not counted)
• C18: retired pipeline values: pipeline.trigger_parameters.circleci.*, pipeline.trigger_parameters.github_app.*, pipeline.trigger_source, $CIRCLE_COMPARE_URL
• C19: config validator warnings that block pipelines from 2026-12-14 (enum default not in its enum, run step without a command, when beside a step, workflow with both when and unless, invalid command name, empty requires, bare docker: or steps:, bad parallelism or checkout method, undeclared job or command arguments, templated job type); reported as C04 from that day

What's included

• The scanner (Python 3.9+, standard library only)
• A rules guide with the reason, safer approach and official CircleCI source for each rule (C13 has none: no CircleCI page covers it)
• A risky and a safe example (an online shop with web, iOS and AWS deploy jobs), and the real report the scanner produces on the risky one

Limits

It reads each file as written. Settings kept outside the repository are not seen: project environment variables, context restrictions, passing secrets to forked pull requests, orb allow-lists, config policies and branch protection. Orb source is not fetched, so what an orb does with its parameters is not followed. The removed image, Xcode and resource-class lists follow CircleCI's changelog up to November 2026. A few official documentation snippets are reported on purpose (for example echo of << pipeline.git.branch >> or of a string pipeline parameter in a run step); C13 rests on how CircleCI writes << >> values into the config, as no CircleCI page warns about it. The risky example has no C19 finding, because C19 becomes C04 on 2026-12-14. NOT FLAGGED is not a guarantee.

Contents

One ZIP file with 11 files: SKILL.md, the scanner in scripts/ (6 Python files), references/rules.md (every rule with its reason and safer approach), and examples/ (2 example files plus report.md, the real scanner output on the risky example).

How to use: This is an agent skill: a folder with a SKILL.md file and Python scripts that an AI coding agent such as Claude Code, Cursor, or Codex CLI can run. Unzip it into your agent's skills folder (for Claude Code: ~/.claude/skills/), then ask your agent to check your project. The scripts also run on their own with Python 3.9 or newer, with nothing to install. They read your files only: no network access, and nothing is sent anywhere.