cosign

cosign is a skill for Claude Code, Codex from fmind/dot. It costs 40 tokens per session (670 once invoked), scanned A, original, MIT.

A security workflow for signing and checking container images with Sigstore Cosign. A container image is the packaged application that a container runtime starts.

In plain words
What is it for?
Sign and verify images using an OIDC identity, and create or verify CycloneDX SBOM attestations for release checks.
Why use it?
It lets teams verify who produced an image and attach an SBOM, a list of software components inside it, without managing signing keys.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/fmind/dot/cosign
Any agent
npx skills add fmind/dot --skill cosign
Clone the repo
git clone --depth 1 https://github.com/fmind/dot

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for cosign

README.md
[![agentmods](https://agentmods.dev/badge/skills/fmind/dot/cosign.svg)](https://agentmods.dev/skills/fmind/dot/cosign)
Your own site
<a href="https://agentmods.dev/skills/fmind/dot/cosign"><img src="https://agentmods.dev/badge/skills/fmind/dot/cosign.svg" alt="Measured on agentmods" height="20"></a>
Per session 40 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 670 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00040 $0.00670
Opus 5 $0.00020 $0.00335
Sonnet 5 $0.00008 $0.00134
Haiku 4.5 $0.00004 $0.00067

Measured yesterday against content hash 30de240fab52, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

cosign scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/cosign/SKILL.md · 51 lines

How it starts

The opening of the file, as written. The whole thing — 51 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Cosign

Keyless signing with Sigstore: an OIDC identity (a GitHub Actions workflow or a developer's browser login) signs the image digest and anyone verifies it without managing keys; containerize builds the image and github-actions wires the CD job.

Commands

cosign sign --yes <registry>/<slug>@<digest>              # --yes is mandatory: cosign prompts otherwise and hangs agents and CI
cosign verify \
  --certificate-identity 'https://github.com/<owner>/<repo>/.github/workflows/cd.yml@refs/tags/<tag>' \
  --certificate-oidc-issuer 'https://token.actions.githubusercontent.com' \
  <registry>/<slug>@<digest>
cosign attest --yes --type cyclonedx --predicate sbom.json <registry>/<slug>@<digest>
cosign verify-attestation --type cyclonedx \
  --certificate-identity-regexp 'https://github.com/<owner>/<repo>/' \
  --certificate-oidc-issuer 'https://token.actions.githubusercontent.com' \
  <registry>/<slug>@<digest>

GitHub Actions

Pin cosign in mise.toml [tools] so mise-action installs it with the rest of the toolchain; the signing job needs permissions: id-token: write plus packages: write (or the registry's equivalent), cache: false on mise-action, and signs the digest the build step recorded (ko build --image-refs). The github-actions cd.yml template implements this wiring.

[tools]
cosign = "latest"

Gotchas

  • Sign digests, never tags: a tag can be repointed after signing.
  • Verification pins identity and issuer: a valid signature from an unexpected workflow is not provenance; record the expected identity in the release documentation.
  • Signing writes to the registry: sign and attest push signatures next to the image, so a local packaging request does not authorize them.
  • SBOM first, then attest: generate the SBOM per trivy and attach it as an attestation so consumers verify inventory and signature together.

Read the full file on GitHub · 51 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 51 lines · 40 tokens per session scan A 30de240fab52

Subscribe to this mod's changes

cosign is a skill published in the GitHub repository fmind/dot (4 stars, last pushed today), licensed MIT. It adds 40 tokens to every session and 670 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.