CI gates for AI-generated PRs need re-derivable evidence

When a CI gate flags an AI-generated PR, the important question is not only "what did it flag?"

It is also:

"Could someone else come back later and re-derive why this finding fired?"

That is the reason I added evidence snapshots to Agent Gate v0.2.1.

What Agent Gate is

Agent Gate is a GitHub Action for AI-generated pull requests.

It does not review code with an LLM. It checks deterministic merge evidence in CI:

PR scope escapes
GitHub Actions permission escalation
AGENTS.md / .mcp.json drift
missing test-file evidence
high-risk path changes

The Action does not checkout PR code, call LLMs at runtime, or execute repository scripts.

Why finding IDs were not enough

In v0.2.0, Agent Gate added stable finding IDs.

That gave every finding a short audit handle, for example:

agf_987ab9ddb8c1b299

That is useful for references, comments, future override workflows, and log-based debugging.

But an ID by itself is not proof. If someone sees the ID later, they still need to know what recorded material produced it.

What v0.2.1 adds

v0.2.1 adds evidenceSnapshot to public findings.

The split is:

findingId = short audit handle
evidenceSnapshot = canonical material used to derive that handle

The snapshot is intentionally boring. It contains stable rule material such as:

rule id
severity
path or line when present
normalized evidence label/value pairs

It does not include timestamps, report order, risk score, version, commit SHA, or mutable display text.

A real report shape

Example compact log output:

Agent Gate: NEEDS HUMAN DECISION
Decision: warn
Risk score: 49 / 100
Why: Agent-generated PRs must include an agent-gate contract.
Recommended next step: Add a PR contract before relying on scope checks.
Policy status: warning today; eligible to become a merge gate after tuning.

Findings:
- error agf_be0c2c2a66312aff contract/missing
- error agf_987ab9ddb8c1b299 risk/high-risk-path .github/workflows/agent-gate.yml
- warn agf_6016e753491255d7 workflow/dangerous-pattern .github/workflows/agent-gate.yml

The compact log stays short, but the JSON and Markdown reports carry the fuller evidence.

Example JSON shape:

{
  "findingId": "agf_987ab9ddb8c1b299",
  "ruleId": "risk/high-risk-path",
  "severity": "error",
  "path": ".github/workflows/agent-gate.yml",
  "evidenceSnapshot": {
    "ruleId": "risk/high-risk-path",
    "severity": "error",
    "path": ".github/workflows/agent-gate.yml",
    "evidence": [
      {
        "label": "changed_file",
        "value": ".github/workflows/agent-gate.yml"
      }
    ]
  }
}

Why this matters

For me, the bar for promoting a finding from warning to blocking is:

A third party should be able to re-derive the finding from recorded evidence.

That does not mean the check is magically correct.

It means the failure mode is visible, reproducible, and tunable.

A repo can start in warn mode, observe which findings are useful, and only later promote low-noise findings into merge gates.

What this does not solve yet

Agent Gate still does not prove semantic correctness.

Matching test-file evidence is not proof that the tests cover the behavior. It is change evidence / self-consistency evidence.

Maintainer override storage is also not implemented yet. That is probably the next hard design question: if someone bypasses a finding, where should that override live so it is durable enough to inspect later?

CODEOWNERS / reviewer evidence and package dependency drift are also future work.