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 itseffi/agentic-os --skill tddgit clone --depth 1 https://github.com/itseffi/agentic-osWrote 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/itseffi/agentic-os/tdd)<a href="https://agentmods.dev/skills/itseffi/agentic-os/tdd"><img src="https://agentmods.dev/badge/skills/itseffi/agentic-os/tdd/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/itseffi/agentic-os/tdd"><img src="https://agentmods.dev/badge/skills/itseffi/agentic-os/tdd.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 | $0.00025 | $0.00684 |
| Opus 5.5 | $0.00010 | $0.00274 |
| Sonnet 5.5 | $0.00005 | $0.00137 |
| Haiku 4.5 | $0.00003 | $0.00068 |
Grade A, and why
tdd 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 1mo 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.
How it starts
The opening of the file, as written. The whole thing — 106 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Test-Driven Development (TDD)
Use when implementing any feature or bugfix, before writing implementation code.
The Iron Law
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
Write code before the test? Delete it. Start over.
No exceptions:
- Don't keep it as "reference"
- Don't "adapt" it while writing tests
- Delete means delete
Red-Green-Refactor
RED - Write Failing Test
Write one minimal test showing what should happen.
- One behavior
- Clear name
- Real code (no mocks unless unavoidable)
Verify RED - Watch It Fail
MANDATORY. Never skip.
- Run the test, confirm it fails (not errors)
- Failure message is expected
- Test passes? You're testing existing behavior. Fix test.
GREEN - Minimal Code
Write simplest code to pass the test.
- Just enough to pass
- No extra features
- Don't add behavior beyond the test
Verify GREEN - Watch It Pass
MANDATORY.
- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)
REFACTOR - Clean Up
After green only:
- Remove duplication
- Improve names
- Extract helpers Keep tests green. Don't add behavior.
Why Order Matters
"I'll write tests after" Tests written after pass immediately. Passing immediately proves nothing. Test-first forces you to see the test fail, proving it actually tests something.
"Tests after achieve same goals" No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"
"Deleting X hours of work is wasteful" Sunk cost fallacy. The time is gone. The waste is keeping code you can't trust.
Red Flags - STOP and Start Over
- Code before test
- Test after implementation
- Test passes immediately
- "I already manually tested it"
- "Tests after achieve the same purpose"
- "This is different because..."
All mean: Delete code. Start over with TDD.
Common Rationalizations
| Excuse | Reality |
|---|---|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests passing immediately prove nothing. |
| "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. |
| "TDD will slow me down" | TDD faster than debugging later. |
What ships with it
1 file 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.
- 1mo ago First seen · 106 lines · 25 tokens per session scan A 9b3a7c4429db
tdd is a skill published in the GitHub repository itseffi/agentic-os (117 stars, last pushed 14d ago), licensed MIT. It adds 25 tokens to every session and 684 once invoked, about $0.0001 per session on Opus 5.5. 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 2026-08-30.
Other skills, from other repositories
test-driven-development
Drives development with tests using the red-green-refactor loop. Use when implementing any logic, fixing any bug, or changing any behavior. Use when you need to prove that code works, when a bug report arrives, or when you're about to modify existing functionality. Use when asked to add tests to a function, fix a bug…
workflow-patterns
Use this skill when implementing tasks according to Conductor's TDD workflow, handling phase checkpoints, managing git commits for tasks, or understanding the verification protocol.
writing-skills
A guide for creating and testing reusable instructions for AI agents, called skills. It applies test-driven development, or TDD—the practice of writing tests before implementation—to instruction documents.
test-driven-development
Test-driven development, or TDD, is a way to build software by writing a test that fails, adding the smallest code that makes it pass, and then cleaning up the code. These instructions require that process for features, bug fixes, refactors, and behavior changes.
kiro-impl
Implement approved tasks using TDD with subagent dispatch. Runs all pending tasks autonomously or selected tasks manually.
ijfw-tdd
RED-GREEN-REFACTOR enforcement. Use when implementing a feature or bugfix before writing implementation code. Trigger: tdd, test first, red green refactor, /ijfw-tdd.