.NET/skills is a repository of reusable instructions and custom agents that help AI coding agents work with .NET and C#. It supports tasks such as C# language-server integration, data access, diagnostics, builds, packages, upgrades, MAUI, templates, and AI development. The catalogue entries are the repository's skills, agents, plugins, and instructions.
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.
git clone --depth 1 https://github.com/dotnet/skillsWrote 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/agents/dotnet/skills/testability-migration)<a href="https://agentmods.dev/agents/dotnet/skills/testability-migration"><img src="https://agentmods.dev/badge/agents/dotnet/skills/testability-migration/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/agents/dotnet/skills/testability-migration"><img src="https://agentmods.dev/badge/agents/dotnet/skills/testability-migration.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 | $0.00066 | $0.02522 |
| Opus 5.5 | $0.00026 | $0.01009 |
| Sonnet 5.5 | $0.00013 | $0.00504 |
| Haiku 4.5 | $0.00007 | $0.00252 |
Grade A, and why
testability-migration 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 12d 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 — 251 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Testability Migration Agent
You are a testability migration agent for .NET codebases. Your mission is to help developers incrementally replace hard-to-test static dependencies with injectable abstractions, making their code unit-testable without requiring a risky big-bang rewrite.
Pipeline Overview
For a focused TimeProvider migration in an application, keep two deliverables
on the checklist before editing: the injected class and its production
construction/DI path. If the project has an existing WebApplication builder,
the ordinary missing-clock registration is:
builder.Services.AddSingleton<TimeProvider>(TimeProvider.System);
Preserve an existing registration instead of duplicating it. A provider field and constructor alone do not finish the migration. Include the composition file in the changed-file review before running validation.
Choose one of three paths:
- A named available seam wins routing. A request explicitly naming
TimeProvider,IFileSystem, or an existing wrapper follows Focused migration, even when it also requests tests. Complete its production wiring and write tests inline; do not switch totestability-obstacleafter choosing the seam. - Migration pipeline: Detect → Generate → Migrate → Test for a broad or
multi-call-site migration. After migration, the seam exists; write the
deterministic tests inline. Do not invoke
code-testingortest-engineerfrom this internal specialist. - Focused migration: for an inventory-only request, invoke
detect-static-dependenciesand stop. For one named dependency, invokemigrate-static-to-wrapper; stop after migration only when tests were not requested, otherwise continue to the Test phase. - Targeted obstacle: use
testability-obstacledirectly when one bounded behavior needs a missing seam and deterministic tests. This path skips Detect/Generate/Migrate rather than running after them.
If the request maps to one specialist skill, invoke it once and return its focused result unless the request also requires deterministic tests. In that case, reuse the migrated seam and continue directly to the Test phase without running unrelated detection or generation phases. For broader work, invoke each applicable skill once for its phase and do not ask subagents to rescan the same scope.
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.
- 12d ago Changed · +56 lines 7d52574c23ed
- 13d ago Changed · -8 lines · -36 tokens per session 13c9f6db08d2
- 26d ago Changed · +25 lines · +12 tokens per session 499acf3253b3
- 1mo ago Changed · +2 lines e31c61615f4c
- 1mo ago First seen · 176 lines · 90 tokens per session scan A d1a5d3686e3c
testability-migration is an agent published in the GitHub repository dotnet/skills (5,696 stars, last pushed today), licensed MIT. It adds 66 tokens to every session and 2,522 once invoked, about $0.0003 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 2026-09-03.
Other agents, from other repositories
Expert .NET software engineer mode instructions
Provide expert .NET software engineering guidance using modern software design patterns.
dnp-tdd-developer-hard
🔬 Deep TDD for complex .NET tasks: architectural decisions, ambiguous edge cases, high-risk refactoring. Writes both tests and production code with rigorous RED-GREEN-REFACTOR.
dnp-tdd-developer-easy
⚡ Fast TDD for routine .NET tasks: clear requirements, low-risk changes, well-defined scope. Writes both tests and production code following RED-GREEN-REFACTOR.
testability-migration
Orchestrates end-to-end testability migration for .NET codebases: detects untestable static dependencies, generates wrapper abstractions or guides built-in adoption, and performs mechanical bulk migration of call sites. Use when asked to make code testable, remove static coupling, migrate to TimeProvider, adopt…
test-engineer
.NET/C# QA engineer specialized in test strategy, test writing, and coverage analysis — xUnit v3 (or v2) or MSTest with native Assert.X, WebApplicationFactory, Testcontainers, Microsoft.Playwright, and Avalonia.Headless. Use for designing test suites, writing tests for existing code, or evaluating test quality.
test-writer
Generates xUnit test suites for .NET code. Use for mechanical test generation, filling coverage gaps, or writing tests for a newly implemented feature. Runs on Haiku for cost efficiency.