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-codex-plugin --skill validation-firstgit clone --depth 1 https://github.com/OutlineDriven/odin-codex-pluginWrote 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-codex-plugin/validation-first)<a href="https://agentmods.dev/skills/outlinedriven/odin-codex-plugin/validation-first"><img src="https://agentmods.dev/badge/skills/outlinedriven/odin-codex-plugin/validation-first.svg" alt="Measured on agentmods" 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 | $0.00027 | $0.01106 |
| Opus 5 | $0.00014 | $0.00553 |
| Sonnet 5 | $0.00005 | $0.00221 |
| Haiku 4.5 | $0.00003 | $0.00111 |
Grade A, and why
validation-first 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 4d 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.
Copies of this mod
1 near-identical copy found in the catalogue:
- validation-first — 100% identical, 0 lines differ
How it starts
The opening of the file, as written. The whole thing — 124 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Validation-first development
Define state machines from requirements before implementation. Specifications say what the system MUST do. Encode compile-time properties in types first, then layer state machine modeling for properties types cannot express.
Modern insight (2025): State machines exist on a spectrum from runtime enums to compile-time typestates. Use the strongest mechanism available. XState v5 introduces actor model semantics -- state machines are now first-class concurrent entities, not just enum switches.
See approaches for language-specific state machine mechanisms. See examples for brief state machine patterns per language. See formal-tools for specification and model checking tools.
State Machine Taxonomy (decision guidance)
| Level | Mechanism | Strength | Use When |
|---|---|---|---|
| Typestate (compile-time) | Generic type params, phantom data | Invalid transitions unrepresentable | Protocol APIs, builder patterns, Rust FFI |
| Statecharts (hierarchical) | Nested states, parallel regions | Complex workflows, entry/exit | Game state, multi-modal UI, XState |
| Flat FSM (runtime) | Enum + match/switch | Simple, auditable | Order lifecycle, connection mgmt |
| Actor model | Independent entities, message passing | Concurrent state | Distributed systems, Erlang/Elixir, XState v5 |
Default choice: Use the strongest mechanism the language supports. Typestate in Rust, sealed classes in Kotlin, discriminated unions in TypeScript.
Validation Levels
Type system (strongest) > State machine > Contract > Runtime check (weakest)
When to Apply
- Protocol implementations (network, API, auth flows)
- Workflow engines (approval chains, CI/CD pipelines)
- Concurrent/distributed systems (coordination state)
- Order lifecycle (e-commerce, payments, shipping)
- Connection/session management
- Actor systems with message-driven state
- Event sourcing aggregates (command validation against current state)
What ships with it
3 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.
- 4d ago First seen · 124 lines · 27 tokens per session scan A 5d55c886ea85
validation-first is a skill published in the GitHub repository OutlineDriven/odin-codex-plugin (15 stars, last pushed 2mo ago), licensed Apache-2.0. It adds 27 tokens to every session and 1,106 once invoked, about $0.0001 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.
Other skills, from other repositories
type-driven
Type-driven development. Use when developing with refined types, state machines encoded in types, or proof-carrying types; enforces totality and exhaustive pattern matching.
proof-driven
Proof-driven development. Use when implementing with formal verification using property-based testing, theorem proving, or proof tactics; zero unproven property policy enforced.
validation-first
Validation-first development. Use when developing with formal state machine specifications, invariants, and temporal properties before writing implementation code.
triage-issue
Investigate a reported bug to root cause, then emit a TDD-shaped fix plan as an issue artifact. Trigger when the user reports a bug, says "triage", asks for issue investigation, or wants a fix plan before code changes.
tests-purge-unneeded
Delete tests that don't catch real bugs — the inverse of TDD. Use when reviewing legacy test suites, slow CI investigations, refactor-driven test sweeps, or evaluating whether a test the type system already covers should stay. Thesis — a test exists only if removing it would let a real bug reach production.
tests-adversarial
Write adversarial tests that intentionally stress failure paths. Use when hardening error handling, stress-testing assumptions, validating boundary behavior, or hunting silent failures.