Add setup-local-sdk skill for global.json paths feature

.github/CODEOWNERS

@@ -21,6 +21,9 @@
 /plugins/dotnet/skills/nuget-trusted-publishing/ @lewing @kartheekp-ms @dotnet/skills-csharp-language-reviewers
 /tests/dotnet/nuget-trusted-publishing/ @lewing @kartheekp-ms @dotnet/skills-csharp-language-reviewers
 
+/plugins/dotnet/skills/setup-local-sdk/ @jfversluis @Redth @dotnet/skills-csharp-language-reviewers
+/tests/dotnet/setup-local-sdk/ @jfversluis @Redth @dotnet/skills-csharp-language-reviewers
+
 /plugins/dotnet/agents/optimizing-dotnet-performance.agent.md @dotnet/appmodel
 
 /plugins/dotnet-ai/skills/mcp-csharp-create/ @leslierichardson95 @cathysull

plugins/dotnet/skills/setup-local-sdk/SKILL.md

@@ -0,0 +1,373 @@
+---
+name: setup-local-sdk
+license: MIT
+description: >
+  Install a .NET SDK locally for safe preview testing, specific-version pinning, or
+  reproducible team setups — without modifying the system-wide installation.
+  USE FOR: trying .NET previews safely, testing specific SDK versions, installing MAUI
+  or other workloads on a preview, updating or replacing an existing local SDK,
+  creating reproducible team/CI install scripts, configuring global.json paths.
+  DO NOT USE FOR: system-wide SDK installs, .NET hosts older than 10, runtime-only
+  installs, or projects not using SDK-style commands.
+---
+
+# setup-local-sdk
+
+## Purpose
+
+Guide the user through installing a .NET SDK into a project-local `.dotnet/`
+directory and wiring it up via the `global.json` `paths` feature (.NET 10+).
+The examples use .NET 11, but this works with any version — prerelease or stable.
+
+The result is a fully isolated SDK that:
+- Does **not** modify the system-wide .NET installation.
+- Is picked up automatically by `dotnet` commands from the project root.
+- Can be deleted to revert (`rm -rf .dotnet/` or `Remove-Item -Recurse -Force .\.dotnet`).
+
+## When NOT to use
+
+- User wants a **system-wide** install — direct to the official installer.
+- Host `dotnet` is **older than v10** — `paths` doesn't exist; explain and stop.
+- User needs a **runtime-only** install — `paths` applies to SDK resolution only.
+
+## Inputs / Prerequisites
+
+| Input | Required | Default | Notes |
+|---|---|---|---|
+| Channel or version | No | `11.0` | e.g. `11.0`, `STS`, `LTS`, or an exact version like `11.0.100-preview.2.26159.112` |
+| Quality | No | `preview` | One of: `daily`, `preview`, `GA` |
+| jq | No | — | Optional for bash team scripts when patching an existing `global.json`; without it, do not overwrite the file |
+
+### Prerequisites
+
+1. **A .NET 10+ SDK is installed globally** — run `dotnet --version`; major ≥ 10.
+2. **curl** (macOS/Linux) or **PowerShell** (Windows) is available.
+
+## Workflow
+
+### Step 1 — Clarify what to install
+
+If the user didn't specify, ask what .NET SDK version they want (e.g., "latest
+.NET 11 preview" or an exact version like `11.0.100-preview.2.26159.112`).
+Map the answer to `--channel`/`--quality` or `--version` flags.
+
+### Step 2 — Verify .NET 10+ host
+
+If the user already provided `dotnet --version` output, treat that as the
+authoritative version for their machine. Do not override it with the agent
+workspace's version; if the two differ, explain that the workspace differs and
+continue advising for the user's machine.
+
+```bash
+dotnet --version
+```
+
+If major version < 10, stop before downloading anything: the `paths` feature
+requires a .NET 10+ host SDK. Tell the user to install .NET 10 or later
+system-wide first, then return to the local SDK setup.
+
+### Step 3 — Detect operating system
+
+Run `uname -s 2>/dev/null`. If it succeeds (including `MINGW*`, `MSYS*`, `CYGWIN*` —
+these are bash-capable environments like Git Bash) → use bash/`dotnet-install.sh`.
+If it fails (native Windows without Git Bash) → use PowerShell/`dotnet-install.ps1`.
+
+### Step 4 — Check for existing local SDK
+
+```bash
+test -d .dotnet && echo "exists" || echo "not found"
+```
+
+If `.dotnet/` exists, ask: update with the new version, or skip and keep it?
+
+### Step 5 — Download and run the install script
+
+**macOS / Linux:**
+
+```bash
+INSTALL_SCRIPT="$(mktemp)"
+trap 'rm -f "$INSTALL_SCRIPT"' EXIT
+curl -fsSL https://dot.net/v1/dotnet-install.sh -o "$INSTALL_SCRIPT"
+bash "$INSTALL_SCRIPT" --channel <CHANNEL> --quality <QUALITY> --install-dir .dotnet
+```
+
+**Windows (PowerShell):**
+
+```powershell
+$installScript = Join-Path $env:TEMP "dotnet-install-$([guid]::NewGuid()).ps1"
+try {
+    Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1' -OutFile $installScript
+    & $installScript -Channel <CHANNEL> -Quality <QUALITY> -InstallDir .dotnet
+}
+finally {
+    if (Test-Path -LiteralPath $installScript) {
+        Remove-Item -LiteralPath $installScript -Force
+    }
+}
+```
+
+For exact versions: use `--version <VERSION>` (bash) or `-Version <VERSION>` (PowerShell)
+instead of channel/quality flags. The install script is from Microsoft's official
+URL (https://dot.net/v1/dotnet-install.sh).
+
+### Step 6 — Identify the installed version
+
+```bash
+./.dotnet/dotnet --version          # macOS/Linux
+.\.dotnet\dotnet.exe --version      # Windows
+```
+
+Record the exact version string (e.g., `11.0.100-preview.2.26159.112`) for `global.json`.
+
+### Step 7 — Create or update global.json
+
+```json
+{
+  "sdk": {
+    "version": "<INSTALLED_VERSION>",
+    "allowPrerelease": true,
+    "rollForward": "latestFeature",
+    "paths": [".dotnet", "$host$"],
+    "errorMessage": "Required .NET SDK not found. Run ./install-dotnet.sh (or .ps1) to install it locally."
+  }
+}
+```
+
+- `paths`: `.dotnet` first (local priority), `$host$` = system-wide fallback.
+- `rollForward: "latestFeature"`: use for latest-preview or floating feature-band installs.
+- Exact version requests: use `rollForward: "disable"` so SDK resolution doesn't move to a different feature band.
+- `allowPrerelease`: set to `true` only when installing a prerelease SDK. Omit for stable versions.
+- `errorMessage`: include only when team install scripts are created (Step 10). Otherwise omit.
+
+If `global.json` already exists, **merge** carefully: preserve existing properties (`msbuild-sdks`,
+`tools`, etc.) and only add/update the `sdk` section. Read the existing file first, update/add
+the `sdk` object, then write it back. This ensures cross-project config (e.g., MSBuild settings)
+isn't lost. Always back up the original file (e.g., `global.json.bak`) before modifying.
+
+**Minimal config** (when version pinning isn't needed):
+`{"sdk":{"paths":[".dotnet","$host$"]}}`
+
+### Step 8 — Update .gitignore
+
+**macOS / Linux (or Git Bash):**
+
+```bash
+grep -qxF '.dotnet/' .gitignore 2>/dev/null || printf '\n.dotnet/\n' >> .gitignore
+```
+
+**Windows (PowerShell):**
+
+```powershell
+if (-not (Test-Path .gitignore) -or -not (Select-String -Path .gitignore -Pattern '^\.dotnet/$' -Quiet)) {
+    Add-Content -Path .gitignore -Value '.dotnet/'
+}
+```
+
+### Step 9 — Install workloads (if requested)
+
+Only do this after `global.json` and `.gitignore` are complete, so a slow or
+platform-limited workload install does not prevent the base local SDK setup from
+being usable.
+
+If the user mentioned MAUI, mobile, workload, Blazor WASM, or cross-platform,
+install using the **local** binary (no sudo needed):
+
+```bash
+./.dotnet/dotnet workload install <workload>       # macOS/Linux
+.\.dotnet\dotnet.exe workload install <workload>   # Windows
+```
+
+Verify: `./.dotnet/dotnet workload list` (or `.\.dotnet\dotnet.exe workload list`).
+
+For MAUI, pick a workload supported by the current OS and target platform. On
+Linux, the full `maui` meta-workload is not available; use a supported workload
+such as `maui-android` when Android is the target, or explain the platform
+limitation and ask which target to configure.
+
+> **Always use the local dotnet binary for workload commands.** Workload metadata
+> is stored relative to the host process's dotnet root. The system `dotnet` puts
+> metadata in the wrong location. (See [dotnet/sdk#49825](https://github.com/dotnet/sdk/issues/49825).)
+
+### Step 10 — Create team install scripts
+
+Create if user mentioned "team", "share", "CI", "scripts", etc. Otherwise offer.
+These examples back up `global.json` and preserve existing settings. The bash script
+uses `jq` when an existing `global.json` must be patched; if `jq` is unavailable,
+it refuses to overwrite the file and prints the settings to merge manually.
+Adapt script variables to the install choice from Step 1: exact versions should
+use `--version` / `-Version` and `rollForward: "disable"`; channel installs should
+use channel/quality and only set `allowPrerelease: true` for prerelease SDKs.
+
+**install-dotnet.sh:**
+
+```bash
+#!/usr/bin/env bash
+set -euo pipefail
+INSTALL_DIR=".dotnet"
+CHANNEL="11.0"
+QUALITY="preview"
+VERSION=""
+ROLL_FORWARD="latestFeature"
+ALLOW_PRERELEASE="true"
+WORKLOADS=("${@}")
+ERROR_MESSAGE="Required .NET SDK not found. Run ./install-dotnet.sh (or .ps1) to install it locally."
+INSTALL_SCRIPT="$(mktemp)"
+GLOBAL_JSON_TMP=""
+cleanup() {
+    rm -f "$INSTALL_SCRIPT"
+    [ -n "$GLOBAL_JSON_TMP" ] && rm -f "$GLOBAL_JSON_TMP"
+}
+trap cleanup EXIT
+curl -fsSL https://dot.net/v1/dotnet-install.sh -o "$INSTALL_SCRIPT"
+INSTALL_ARGS=(--install-dir "$INSTALL_DIR")
+if [ -n "$VERSION" ]; then
+    INSTALL_ARGS+=(--version "$VERSION")
+    ROLL_FORWARD="disable"
+else
+    INSTALL_ARGS+=(--channel "$CHANNEL" --quality "$QUALITY")
+fi
+bash "$INSTALL_SCRIPT" "${INSTALL_ARGS[@]}"
+SDK_VERSION=$("$INSTALL_DIR/dotnet" --version)
+write_global_json() {
+    if [ -f global.json ]; then
+        cp global.json global.json.bak
+        if ! command -v jq >/dev/null 2>&1; then
+            echo "global.json exists; install succeeded, but this script will not overwrite it without jq." >&2
+            echo "Merge these sdk settings manually so existing global.json properties are preserved:" >&2
+            cat >&2 <<EOF
+{
+  "sdk": {
+    "version": "$SDK_VERSION",
+    "allowPrerelease": $ALLOW_PRERELEASE,
+    "rollForward": "$ROLL_FORWARD",
+    "paths": [".dotnet", "\$host\$"],
+    "errorMessage": "$ERROR_MESSAGE"
+  }
+}
+EOF
+            exit 1
+        fi
+        GLOBAL_JSON_TMP="$(mktemp)"
+        jq --arg version "$SDK_VERSION" --arg rollForward "$ROLL_FORWARD" --argjson allowPrerelease "$ALLOW_PRERELEASE" --arg errorMessage "$ERROR_MESSAGE" '
+          .sdk = ((.sdk // {}) + {
+            version: $version,
+            allowPrerelease: $allowPrerelease,
+            rollForward: $rollForward,
+            paths: [".dotnet", "$host$"],
+            errorMessage: $errorMessage
+          })
+        ' global.json > "$GLOBAL_JSON_TMP"
+        mv "$GLOBAL_JSON_TMP" global.json
+        GLOBAL_JSON_TMP=""
+    else
+        cat > global.json <<EOF
+{
+  "sdk": {
+    "version": "$SDK_VERSION",
+    "allowPrerelease": $ALLOW_PRERELEASE,
+    "rollForward": "$ROLL_FORWARD",
+    "paths": [".dotnet", "\$host\$"],
+    "errorMessage": "$ERROR_MESSAGE"
+  }
+}
+EOF
+    fi
+}
+write_global_json
+grep -qxF '.dotnet/' .gitignore 2>/dev/null || printf '\n.dotnet/\n' >> .gitignore
+[ ${#WORKLOADS[@]} -gt 0 ] && "$INSTALL_DIR/dotnet" workload install "${WORKLOADS[@]}"
+echo "Done. SDK: $SDK_VERSION"
+```
+
+```bash
+chmod +x install-dotnet.sh
+```
+
+**install-dotnet.ps1:**
+
+```powershell
+param([string[]]$Workloads = @())
+$ErrorActionPreference = 'Stop'
+$installDir = '.dotnet'; $channel = '11.0'; $quality = 'preview'
+$version = ''; $rollForward = 'latestFeature'; $allowPrerelease = $true
+$errorMessage = 'Required .NET SDK not found. Run ./install-dotnet.sh (or .ps1) to install it locally.'
+$installScript = Join-Path $env:TEMP "dotnet-install-$([guid]::NewGuid()).ps1"
+try {
+    Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1' -OutFile $installScript
+    $installArgs = @('-InstallDir', $installDir)
+    if ($version) {
+        $installArgs += @('-Version', $version)
+        $rollForward = 'disable'
+    } else {
+        $installArgs += @('-Channel', $channel, '-Quality', $quality)
+    }
+    & $installScript @installArgs
+}
+finally {
+    if (Test-Path -LiteralPath $installScript) {
+        Remove-Item -LiteralPath $installScript -Force
+    }
+}
+$sdkVersion = & "$installDir\dotnet.exe" --version
+$globalJson = if (Test-Path 'global.json') {
+    Copy-Item 'global.json' 'global.json.bak'
+    Get-Content -Path 'global.json' -Raw | ConvertFrom-Json
+} else {
+    [pscustomobject]@{}
+}
+if (-not $globalJson.PSObject.Properties['sdk']) {
+    $globalJson | Add-Member -MemberType NoteProperty -Name 'sdk' -Value ([pscustomobject]@{})
+}
+$updates = [ordered]@{
+    version = $sdkVersion
+    allowPrerelease = $allowPrerelease
+    rollForward = $rollForward
+    paths = @('.dotnet', '$host$')
+    errorMessage = $errorMessage
+}
+foreach ($entry in $updates.GetEnumerator()) {
+    $property = $globalJson.sdk.PSObject.Properties[$entry.Key]
+    if ($property) {
+        $property.Value = $entry.Value
+    } else {
+        $globalJson.sdk | Add-Member -MemberType NoteProperty -Name $entry.Key -Value $entry.Value
+    }
+}
+$globalJson | ConvertTo-Json -Depth 10 | Set-Content -Path 'global.json' -Encoding UTF8
+if (-not (Test-Path .gitignore) -or -not (Select-String -Path .gitignore -Pattern '^\.dotnet/$' -Quiet)) {
+    Add-Content -Path .gitignore -Value '.dotnet/'
+}
+if ($Workloads.Count -gt 0) { & "$installDir\dotnet.exe" workload install @Workloads }
+Write-Host "Done. SDK: $sdkVersion"
+```
+
+Commit these scripts to the repo so teammates can run them.
+
+### Step 11 — Verify SDK resolution
+
+```bash
+dotnet --version
+```
+
+Output should match the locally installed version. If not, check: global.json
+location, `paths` array contents, host dotnet version ≥ 10.
+
+### Step 12 — Summarize and explain cleanup
+
+Tell the user: SDK installed, global.json configured, .dotnet/ gitignored, system
+install untouched. Cleanup: delete `.dotnet/`, remove `paths`/`errorMessage` from
+global.json, optionally delete install scripts. Include the final `global.json`
+`sdk` values (or a short snippet) so the user can see the configured version,
+`paths`, and any `errorMessage`. If workloads were requested, include the local
+`dotnet workload install ...` command used and the workload verification result
+or the exact blocker if the workload could not be installed.
+
+## Common pitfalls
+
+| Pitfall | Cause | Fix |
+|---|---|---|
+| `paths` ignored | Host `dotnet` < v10 | Install .NET 10+ system-wide |
+| Wrong SDK resolves | `global.json` in parent directory | Check for global.json up the tree |
+| Teammates get "SDK not found" | `.dotnet/` gitignored, no install script run | Use `errorMessage` in global.json |
+| Workloads missing | Used system `dotnet` instead of local | Use `./.dotnet/dotnet workload install` |
+| `dotnet app.dll` wrong runtime | `paths` is SDK-only, not apphost | Use `dotnet run` or set `DOTNET_ROOT` |