Unity Package Management (headless, via the C# Client API)

Add, remove, upgrade, and discover UPM (Unity Package Manager) packages programmatically with UnityEditor.PackageManager.Client, driven headless from the terminal or CI. Do not hand-edit Packages/manifest.json — the Client API resolves dependencies and compatible versions correctly, whereas manual edits routinely break resolution.

This complements the unity-cli skill (editor install, project creation, build/test): the CLI has no package-management command, so all package work goes through the Editor's C# API.

When to use

• Add / remove / upgrade one or more packages in an existing or freshly-created project.
• Set up a project's packages non-interactively in CI.
• Verify a package id exists, or find its available versions, before depending on it.
• Decide which packages a game actually needs — see references/select-packages.md.

Choosing what to install

Install what the project actually needs, not everything; prefer packages the chosen template already provides (URP templates already include the render pipeline, Input System, etc.). The genre / look / platform / monetization → package mapping, plus how to search the registry, is in references/select-packages.md. Produce a deduplicated list of package ids and read it back to the user before installing.

The -quit problem — why NOT unity run for installs

Client.Add / Client.AddAndRemove are asynchronous: they return a Request that only completes on later EditorApplication.update ticks (the UPM child process marshals its result back on the Editor's main-loop pump, so a blocking while (!req.IsCompleted) busy-wait deadlocks it). The Editor must stay alive after -executeMethod returns, until the request finishes.

unity run cannot be used for the installer: its default path injects -quit (see the reserved flags in the unity-cli skill). With -quit, the Editor quits the instant the method returns — before UPM resolves — so packages never install and the callback never runs.

Solution: launch the Editor binary directly in -batchmode without -quit. The Editor stays alive, EditorApplication.update keeps ticking, the poll callback runs, and it calls EditorApplication.Exit(code) itself when done — which both quits and sets the process exit code.

The installer script

Write this to Assets/Editor/ProjectBootstrap/PackageInstaller.cs. It must live under an Editor/ folder (or an Editor-only assembly) because it uses UnityEditor.

using System.Linq;
using UnityEditor;
using UnityEditor.PackageManager;
using UnityEditor.PackageManager.Requests;
using UnityEngine;

namespace ProjectBootstrap
{
    // Installs (and optionally removes) a fixed set of packages via the PackageManager
    // Client API, headless-safe.
    public static class PackageInstaller
    {
        // EDIT this list to match the package selection (see references/select-packages.md).
        static readonly string[] PackagesToAdd =
        {
            "com.unity.inputsystem",
            "com.unity.cinemachine",
            "com.unity.render-pipelines.universal",
            // "com.unity.package@1.2.3"  // pin a version with @ when a minimum is required
        };

        // Optionally drop packages in the same resolution pass (e.g. a template default you don't want).
        static readonly string[] PackagesToRemove = { };

        const double TimeoutSeconds = 600; // UPM resolution + downloads can be slow

        static AddAndRemoveRequest _request;
        static double _deadline;

        // Invoke with: -executeMethod ProjectBootstrap.PackageInstaller.Install  (NO -quit)
        public static void Install()
        {
            if (PackagesToAdd.Length == 0 && PackagesToRemove.Length == 0)
            {
                Debug.Log("[PackageInstaller] Nothing to do.");
                EditorApplication.Exit(0);
                return;
            }

            Debug.Log($"[PackageInstaller] Adding: {string.Join(", ", PackagesToAdd)}");
            _request = Client.AddAndRemove(packagesToAdd: PackagesToAdd, packagesToRemove: PackagesToRemove);
            _deadline = EditorApplication.timeSinceStartup + TimeoutSeconds;
            EditorApplication.update += Poll;
        }

        static void Poll()
        {
            if (_request == null) return;

            if (!_request.IsCompleted)
            {
                if (EditorApplication.timeSinceStartup > _deadline)
                {
                    EditorApplication.update -= Poll;
                    Debug.LogError("[PackageInstaller] Timed out waiting for UPM.");
                    EditorApplication.Exit(2);
                }
                return;
            }

            EditorApplication.update -= Poll;

            if (_request.Status == StatusCode.Success)
            {
                var names = _request.Result.Select(p => $"{p.name}@{p.version}");
                Debug.Log($"[PackageInstaller] Resolved: {string.Join(", ", names)}");
                EditorApplication.Exit(0);
            }
            else
            {
                Debug.LogError($"[PackageInstaller] Failed: {_request.Error?.message}");
                EditorApplication.Exit(1);
            }
        }
    }
}

AddAndRemove installs the whole set in a single UPM resolution pass — faster and less error-prone than one Client.Add per package.

Add / remove / upgrade with one script:

• Add: list the id in PackagesToAdd.
• Remove: list the id in PackagesToRemove.
• Upgrade / pin: add the id with @<version> (e.g. com.unity.cinemachine@2.9.7). Without a version, resolution picks the latest compatible release.

Discovering / verifying packages

To confirm an id exists or list its versions before adding it, search the registry. The in-Editor Client.SearchAll() / Client.Search("<id>") calls are also async, so they use the same poll-and-Exit pattern and the same headless run as the installer. Write Assets/Editor/ProjectBootstrap/PackageSearch.cs:

using System.Linq;
using UnityEditor;
using UnityEditor.PackageManager;
using UnityEditor.PackageManager.Requests;
using UnityEngine;

namespace ProjectBootstrap
{
    public static class PackageSearch
    {
        const double TimeoutSeconds = 120;
        static SearchRequest _request;
        static double _deadline;

        // Invoke with: -executeMethod ProjectBootstrap.PackageSearch.SearchAll  (NO -quit)
        public static void SearchAll()
        {
            _request = Client.SearchAll();                 // or Client.Search("com.unity.cinemachine")
            _deadline = EditorApplication.timeSinceStartup + TimeoutSeconds;
            EditorApplication.update += Poll;
        }

        static void Poll()
        {
            if (_request == null) return;
            if (!_request.IsCompleted)
            {
                if (EditorApplication.timeSinceStartup > _deadline)
                {
                    EditorApplication.update -= Poll;
                    Debug.LogError("[PackageSearch] Timed out.");
                    EditorApplication.Exit(2);
                }
                return;
            }
            EditorApplication.update -= Poll;

            if (_request.Status == StatusCode.Success)
            {
                foreach (var p in _request.Result.OrderBy(p => p.name))
                    Debug.Log($"[PackageSearch] {p.name}@{p.versions.latestCompatible}  {p.displayName}");
                Debug.Log($"[PackageSearch] {_request.Result.Length} packages found.");
                EditorApplication.Exit(0);
            }
            else
            {
                Debug.LogError($"[PackageSearch] Failed: {_request.Error?.message}");
                EditorApplication.Exit(1);
            }
        }
    }
}

_request.Result is a PackageInfo[]; each entry exposes name, displayName, description, and versions (.latest, .latestCompatible, .all). For a terminal-only check without the Editor (a known id, not free-text search), query the registry directly — see references/select-packages.md.

Run it headless (direct Editor invocation, no -quit)

Resolve the Editor binary from the version, then run it in batch mode. The script owns quitting via EditorApplication.Exit, so do not pass -quit:

VERSION="<version>"          # e.g. 6000.0.47f1 (or an installed version)
PROJECT="<project-path>"
METHOD="ProjectBootstrap.PackageInstaller.Install"   # or ...PackageSearch.SearchAll

# Install directory of that editor (Hub layout), via the unity CLI
ED=$(unity editors path "$VERSION" --format json | python3 -c "import sys,json;print(json.load(sys.stdin)['data']['path'])")

# Resolve the executable per-OS (handles both "dir containing Unity.app" and the ".app" itself)
case "$(uname)" in
  Darwin) if [ -d "$ED/Unity.app" ]; then UNITY_BIN="$ED/Unity.app/Contents/MacOS/Unity";
          elif [[ "$ED" == *.app ]]; then UNITY_BIN="$ED/Contents/MacOS/Unity";
          else UNITY_BIN="$ED/Unity"; fi ;;
  Linux)  UNITY_BIN="$ED/Editor/Unity" ;;
  *)      UNITY_BIN="$ED/Editor/Unity.exe" ;;   # Windows (Git Bash / MSYS); use Editor\Unity.exe in PowerShell
esac

"$UNITY_BIN" -batchmode -projectPath "$PROJECT" -executeMethod "$METHOD" -logFile -
echo "Exit code: $?"   # 0 = success, 1 = UPM error, 2 = timeout

-logFile - streams the Editor log (including the [PackageInstaller] / [PackageSearch] lines) to stdout so you can watch resolution progress and read any UPM error. If unity editors path output shape differs on your build, get the directory from unity editors --installed --format json instead.

Verify

# Every requested id should appear as a dependency
cat "<project-path>/Packages/manifest.json"

Confirm the run exited 0 and each package from the list is present in manifest.json. If a package fails to resolve, _request.Error.message is logged; read it and check the id/version against the registry. The Editor's own log (including the [PackageInstaller] lines) is the stdout you streamed with -logFile - above — read it there, not via unity logs (which shows the CLI's own log, not the Editor's).

Import & save headlessly (generate .meta files)

After a script or tool writes new .cs/asset files, Unity must import them so it generates the .meta file each asset needs — and every .cs/asset MUST be committed together with its .meta. Merely opening the project once (unity open "<project-path>") imports and generates them; use this method when you need it headless (in a script or CI).

Unlike the package installer, this is synchronous — it finishes before returning — so it's safe to run via unity run (its injected -quit is harmless; the method also calls EditorApplication.Exit for a clean exit code). Write Assets/Editor/ProjectBootstrap/ProjectSaver.cs:

using UnityEditor;
using UnityEngine;

namespace ProjectBootstrap
{
    public static class ProjectSaver
    {
        // Invoke with: -executeMethod ProjectBootstrap.ProjectSaver.SaveAll
        public static void SaveAll()
        {
            AssetDatabase.Refresh(ImportAssetOptions.ForceUpdate);
            AssetDatabase.SaveAssets();
            Debug.Log("[ProjectSaver] Assets imported and saved.");
            EditorApplication.Exit(0);
        }
    }
}

unity run "<project-path>" --editor-version <version> \
  -- -executeMethod ProjectBootstrap.ProjectSaver.SaveAll

Notes

• These editor scripts are a bootstrap convenience. Leave them in Assets/Editor/ProjectBootstrap/ (they do nothing unless invoked) or delete them after setup — your call; mention it to the user.
• All scripts live under Editor/ because they use UnityEditor; they never ship in a build.
• Monetization / backend packages (com.unity.purchasing, com.unity.services.levelplay, the UGS packages) install through this same mechanism, but do the actual integration via the dedicated skills: implement-in-app-purchases, levelplay-unity-integration, build-live-game.