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Your agent's tool call fails with Pydantic's 'Extra inputs are not permitted'. The model added a field your structured argument forbids. Turn on strict tool use so the grammar blocks it, relax the validator, or recover in the loop.

Never new up HttpClient per request. Use IHttpClientFactory to manage lifetime, and add Refit on top when you want a typed interface instead of hand-written request code. Raw singleton HttpClient is fine only for the simplest cases.

For new code, plain service classes are the better default. MediatR's July 2025 license change matters only if you sit above the $5M Community threshold or refuse the RPL-1.5 copyleft. Keep MediatR when pipeline behaviors are load-bearing.

C# 16 redesigns the unsafe keyword so it propagates a caller obligation instead of silently opening an unsafe context, with inner unsafe blocks now mandatory.

ECONNREFUSED from an MCP client means nothing was listening on that host:port yet. Fix the startup race, the localhost IPv4/IPv6 trap, and wrong-port mistakes for HTTP MCP servers.

Plain JIT with Dynamic PGO wins steady-state throughput, ReadyToRun cuts startup with zero code changes, and Native AOT gives the smallest, fastest-starting binary at the cost of reflection and dynamic code. Pick by deployment shape, not raw benchmarks.

Pick System.Text.Json for new .NET 11 code: it ships in-box, is roughly 2x faster, and is the only one that works with Native AOT. Reach for Newtonsoft.Json only for JSONPath, TypeNameHandling, or genuinely lenient JSON.

ConfigureAwait(false) is still mandatory in library code that may run under a SynchronizationContext (WinForms, WPF, MAUI). In application code on ASP.NET Core, a console app, or a worker service running on .NET 11, it is a no-op.

Cursor 3.4 (May 13, 2026) lets one cloud agent environment include multiple repositories, adds Dockerfile build secrets, layer-cached rebuilds that run 70% faster, and an agent-led setup step that validates credentials before the first run.

Copilot Chat reads your .github/copilot-instructions.md fine, but Agent mode and *.instructions.md files often look ignored. Here is the actual checklist: settings, file locations, applyTo globs, and how to prove the file made it into context.