Dotnet isolated functions

Skill ericsche/CodingSkills/plugins/codingskills/skills/dotnet-isolated-functions

Custom agent skills for GitHub Copilot & Claude Code, from Microsoft-ecosystem dev experience (Azure, .NET, M365, Graph).

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npx -y skills add ericsche/CodingSkills --skill dotnet-isolated-functions

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Run and debug .NET-isolated Azure Functions locally. Use when a cold first request throws "A function with the id ... already exists", when function extensions fail to load under func start, or when choosing between func start and dotnet run.

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SKILL.md

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.NET-isolated Azure Functions: local dev

Practical guidance for running .NET-isolated Azure Functions on your machine.

The cold-start double-load error

On a cold first request, func start can double-load functions and throw:

System.InvalidOperationException: Unable to load Function '...'.
A function with the id 'X' name already exists.

This happens when the worker process starts and initializes twice on the first cold POST. Two reliable workarounds:

Warm up with a GET first before issuing any write/POST request.
Use dotnet run instead of func start. Core Tools itself warns:

Running 'func start' directly against a .NET Isolated project may not correctly load function extensions. Use 'dotnet run' instead.

Recommended local workflow

Prefer dotnet run for .NET-isolated projects so extensions load correctly.
If you must use func start, hit a health/GET endpoint once to warm the host before triggering functions that write.
Observed with Core Tools 4.12.1 / .NET 10 isolated.

Related tooling gotcha

On the .NET 10 SDK, dotnet new sln creates an XML .slnx file (not .sln). Build and reference it by its .slnx name, e.g. dotnet build App.slnx. Using the .sln name fails with MSB1009.