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.
npx skills add OutlineDriven/odin-gemini-cli-extension --skill design-by-contractgit clone --depth 1 https://github.com/OutlineDriven/odin-gemini-cli-extensionWrote 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.
[](https://agentmods.dev/skills/outlinedriven/odin-gemini-cli-extension/design-by-contract)<a href="https://agentmods.dev/skills/outlinedriven/odin-gemini-cli-extension/design-by-contract"><img src="https://agentmods.dev/badge/skills/outlinedriven/odin-gemini-cli-extension/design-by-contract/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/outlinedriven/odin-gemini-cli-extension/design-by-contract"><img src="https://agentmods.dev/badge/skills/outlinedriven/odin-gemini-cli-extension/design-by-contract.svg" alt="Reviewed on agentmods" width="80" height="20"></a>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.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00032 | $0.01177 |
| Opus 5 | $0.00016 | $0.00589 |
| Sonnet 5 | $0.00006 | $0.00235 |
| Haiku 4.5 | $0.00003 | $0.00118 |
Grade A, and why
design-by-contract 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 10d ago.
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.
This is a copy
92% identical to contract-driven — 153 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 132 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Design-by-Contract development
Contracts (PRE/POST/INV) define behavioral specification -- design from requirements before code exists. Formalized as Hoare Triples: {P} C {Q} where P=precondition, C=code, Q=postcondition.
Modern insight (2025): DbC complements LLM-generated code by serving as safety guardrails -- contracts clarify intent and prevent AI from breaking integrations. Spec-driven development (2025) positions contracts as "executable specifications."
See libraries for language-specific contract tools. See examples for brief contract patterns per language.
Verification Hierarchy
Use compile-time verification before runtime contracts. If a property can be verified statically, do NOT add a runtime contract.
Static Assertions (compile-time) > Test/Debug Contracts > Runtime Contracts
| Property | Static | Test | Debug | Runtime |
|---|---|---|---|---|
| Type size/alignment | static_assert |
- | - | - |
| Null/type safety | Type checker | - | - | - |
| Exhaustiveness | Pattern match | - | - | - |
| Expensive O(n)+ | - | test_ensures | - | - |
| Internal invariants | - | - | debug_invariant | - |
| Public API input | - | - | - | requires |
| External/untrusted | - | - | - | Always required |
When to Apply
- Public API boundaries -- callers need clear contracts
- Complex state invariants -- balance >= 0, capacity limits
- Financial/business rule enforcement -- regulatory compliance
- Untrusted external data -- always validate at boundaries
- Multi-component integration points -- service contracts
- AI-generated code -- contracts serve as guardrails for LLM output
When NOT to Apply
- Internal helpers with obvious behavior
- Simple getters/setters, trivial pure functions
- Performance-critical hot paths (runtime contract overhead)
- Prototyping -- contracts add ceremony
- When the type system already enforces the property (prefer static)
What ships with it
2 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 10d ago First seen · 132 lines · 32 tokens per session scan A 921ca83378d7
design-by-contract is a skill published in the GitHub repository OutlineDriven/odin-gemini-cli-extension (5 stars, last pushed 2mo ago), licensed Apache-2.0. It adds 32 tokens to every session and 1,177 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 92% identical to contract-driven, differing in 153 lines, and is treated as a copy.
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