runtime

Guidance for writing and reviewing SIMD / hardware-intrinsics code in dotnet/runtime. USE FOR: vectorizing a scalar algorithm, writing or reviewing code that uses Vector128/Vector256/Vector512, Vector<T>, or the platform intrinsics in System.Runtime.Intrinsics.X86/Arm/Wasm, and validating remainder handling, load/store safety, and hardware-acceleration fallbacks. DO NOT USE FOR: general performance work unrelated to SIMD (use performance-benchmark), or non-vectorized code review (use code-review).

Update OS version references in Helix queue definitions to add new versions, replace EOL versions, or audit coverage against the supported-os matrix. USE FOR: adding new OS versions to Helix queues, replacing EOL OS versions, upgrading "oldest" or "latest" version references, auditing Helix coverage. DO NOT USE FOR: creating new container images (that's dotnet-buildtools-prereqs-docker), updating supported-os.json (that's the update-supported-os skill in dotnet/core).

Guidance for writing and modifying System.Net networking code in dotnet/runtime. Covers resource lifecycle, connection pooling, cross-platform interop, protocol compliance, async patterns, and security posture. For full code review, delegates to the @system-net-review agent. Trigger words: system.net, networking, http client, sockets, ssl, tls, quic, http2, http3, socketshttphandler, sslstream, connection pool.

Analyze a dotnet/runtime PR's CI failures, skip failures already known to Build Analysis, find matching Known Build Errors, and create or draft new KBEs for the remaining failures. Supports dry-run output to local markdown files instead of creating GitHub issues.

Generate and run ad hoc performance benchmarks to validate code changes. Use this when asked to benchmark, profile, or validate the performance impact of a code change in dotnet/runtime.

Domain knowledge for triaging and fixing .NET failures on Apple mobile (iOS, tvOS, MacCatalyst) and Android. Use when runtime-extra-platforms or mobile CI is failing, when investigating iOS, tvOS, MacCatalyst, iossimulator, tvossimulator, or Android build/test failures, or when a change touches mobile pipeline YAML, AppleAppBuilder/AndroidAppBuilder, code signing, provisioning, simulator/emulator startup, platform conditionals, or NativeAOT-on-mobile behavior. Covers failure triage (infrastructure vs code), CI pipeline structure, platform-specific code paths, and NativeAOT compilation on mobile.

Extract a standalone JIT regression test case from a given GitHub issue and save it under the JitBlue folder. USE FOR: creating JIT regression tests, extracting repro code from dotnet/runtime issues, "write a test for this JIT bug", "create a regression test for issue #NNNNN", converting issue repro to xunit test. DO NOT USE FOR: non-JIT tests (use standard test patterns), debugging JIT issues without a known repro, performance benchmarks (use performance-benchmark skill).

Triage a dotnet/runtime GitHub issue with duplicate search, label check, reproduction, and ecosystem research, then recommend KEEP/CLOSE/NEEDS INFO. Use when asked to triage, evaluate, assess, or check a specific GitHub issue. Also use when asked "is this a duplicate", "should we close this", "check this issue", "what do you think about this issue", or when given a dotnet/runtime issue URL or number and asked for an opinion. Handles bug reports, API proposals, enhancements, performance regressions, and questions.

Triage Fuzzlyn CI runs.

Guidance for writing and modifying Microsoft.Extensions.* and System.IO.Compression code in dotnet/runtime. Covers DI lifetime management, configuration binding, options validation, logging provider patterns, caching semantics, compression format compliance, and host lifecycle. For full code review, delegates to the @extensions-reviewer agent. Trigger words: Microsoft.Extensions, IServiceCollection, IConfiguration, ILogger, IHost, IMemoryCache, IOptions, ZipArchive, HttpClientFactory, IFileProvider, IChangeToken.

Review code changes in dotnet/runtime for correctness, performance, and consistency with project conventions. Use when reviewing PRs or code changes.

Monitors .NET runtime CI test pipelines on Azure DevOps. Use this skill when asked to monitor CI pipeline test results, triage CI test failures across ADO pipelines, or generate CI test monitoring reports.

Generate breaking change documentation for merged dotnet/runtime PRs. USE FOR: creating breaking change docs, "document this breaking change", "write breaking change issue for PR #NNNNN", processing PRs labeled needs-breaking-change-doc-created. DO NOT USE FOR: general code review (use code-review skill), bug fixes, API proposals (use api-proposal skill).

Create prototype-backed API proposals for dotnet/runtime. Use when asked to draft an API proposal, write an api-suggestion issue, refine a vague API idea into a complete proposal, or improve a proposal marked api-needs-work. Covers the full pipeline from research through prototyping, ref source generation, and publishing. DO NOT USE FOR bug fixes, code review, performance benchmarking, or internal API changes that don't affect public surface area.

Add a new API to the JIT-VM (aka JIT-EE) interface in the codebase.