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+status: proposed
+date: 2026-03-23
+contact: sergeymenshykh
+deciders: rbarreto, westey-m, eavanvalkenburg
+---
+
+# Agent Skills: Multi-Source Architecture
+
+## Context and Problem Statement
+
+The Agent Framework needs a skills system that lets agents discover and use domain-specific knowledge, reference documents, and executable scripts. Skills can originate from different sources — filesystem directories (SKILL.md files), inline C# code, or reusable class libraries — and the framework must support all three uniformly while allowing extensibility, composition, and filtering.
+
+## Decision Drivers
+
+- Skills must be definable from multiple sources: filesystem, inline code, reusable classes, etc
+- Common abstractions are needed so the provider and builder work uniformly regardless of skill origin
+- File-based scripts must support user-defined executors, enabling custom runtimes and languages; code/class-based scripts execute in-process as C# delegates
+- Skills must be filterable so consumers can include or exclude specific skills based on defined criteria
+- Multiple skill sources must be composable into a single provider
+- It must be possible to add custom skill sources (e.g., databases, REST APIs, package registries) by implementing a common abstraction
+
+## Architecture
+
+### Model-Facing Tools
+
+Skills are presented to the model as up to three tools that progressively disclose skill content. The system prompt lists available skill names and descriptions; the model then calls these tools on demand:
+
+- **`load_skill(skillName)`** — returns the full skill body (instructions, listed resources, listed scripts)
+- **`read_skill_resource(skillName, resourceName)`** — reads a supplementary resource (file-based or code-defined) associated with a skill
+- **`run_skill_script(skillName, scriptName, arguments?)`** — executes a script associated with a skill; only registered when at least one skill contains scripts
+
+Each tool delegates to the corresponding method on the resolved `AgentSkill` — calling `Resource.ReadAsync()` or `Script.RunAsync()` respectively.
+
+If skills have no scripts defined, the `run_skill_script` tool is **not advertised** to the model and instructions related to script execution are **not included** in the default skills instructions.
+
+### Abstract Base Types
+
+The architecture defines four abstract base types that all skill variants implement:
+
+```csharp
+public abstract class AgentSkill
+{
+    public abstract AgentSkillFrontmatter Frontmatter { get; }
+    public abstract string Content { get; }
+    public abstract IReadOnlyList<AgentSkillResource>? Resources { get; }
+    public abstract IReadOnlyList<AgentSkillScript>? Scripts { get; }
+}
+
+public abstract class AgentSkillResource
+{
+    public string Name { get; }
+    public string? Description { get; }
+    public abstract Task<object?> ReadAsync(IServiceProvider? serviceProvider = null, CancellationToken cancellationToken = default);
+}
+
+public abstract class AgentSkillScript
+{
+    public string Name { get; }
+    public string? Description { get; }
+    public abstract Task<object?> RunAsync(AgentSkill skill, AIFunctionArguments arguments, CancellationToken cancellationToken = default);
+}
+
+public abstract class AgentSkillsSource
+{
+    public abstract Task<IList<AgentSkill>> GetSkillsAsync(CancellationToken cancellationToken = default);
+}
+```
+
+Skill metadata is captured via `AgentSkillFrontmatter`:
+
+```csharp
+public sealed class AgentSkillFrontmatter
+{
+    public AgentSkillFrontmatter(string name, string description) { ... }
+
+    public string Name { get; }
+    public string Description { get; }
+    public string? License { get; set; }
+    public string? Compatibility { get; set; }
+    public string? AllowedTools { get; set; }
+    public AdditionalPropertiesDictionary? Metadata { get; set; }
+}
+```
+
+The type hierarchy at a glance:
+
+```
+AgentSkill (abstract)                   AgentSkillsSource (abstract)
+├── AgentFileSkill                      ├── AgentFileSkillsSource          (public)
+└── [Programmatic]                      ├── AgentInMemorySkillsSource      (public)
+    ├── AgentInlineSkill                ├── AggregatingAgentSkillsSource     (public)
+    └── AgentClassSkill (abstract)      └── DelegatingAgentSkillsSource    (abstract, public)
+                                            ├── FilteringAgentSkillsSource  (public)
+AgentSkillResource (abstract)               ├── CachingAgentSkillsSource    (public)
+├── AgentFileSkillResource                  └── DeduplicatingAgentSkillsSource (public)
+└── AgentInlineSkillResource
+                                        AgentSkillScript (abstract)
+                                        ├── AgentFileSkillScript
+                                        └── AgentInlineSkillScript
+```
+
+There are two top-level categories of skills:
+
+1. **File-Based Skills** — discovered from `SKILL.md` files on the filesystem. Resources and scripts are files in subdirectories.
+2. **Programmatic Skills** — defined in C# code. These are further divided into:
+   - **Inline Skills** — built at runtime via the `AgentInlineSkill` class and its fluent API. Ideal for quick, agent-specific skill definitions.
+   - **Class-Based Skills** — defined as reusable C# classes that subclass `AgentClassSkill`. Ideal for packaging skills as shared libraries or NuGet packages.
+
+Both programmatic skill types use `AgentInlineSkillResource` and `AgentInlineSkillScript` for their resources and scripts. They are typically served by `AgentInMemorySkillsSource`, which accepts any `AgentSkill` and is not limited to programmatic skills.
+
+### File-Based Skills
+
+File-based skills are authored as `SKILL.md` files on disk. Resources and scripts are discovered from corresponding subfolders within the skill directory.
+
+**`AgentFileSkill`** — A filesystem-based skill discovered from a directory containing a `SKILL.md` file. Parsed from YAML frontmatter; content is the raw markdown body. Resources and scripts are discovered from files in corresponding subfolders:
+
+```csharp
+public sealed class AgentFileSkill : AgentSkill
+{
+    internal AgentFileSkill(
+        AgentSkillFrontmatter frontmatter, string content, string path,
+        IReadOnlyList<AgentSkillResource>? resources = null,
+        IReadOnlyList<AgentSkillScript>? scripts = null) { ... }
+}
+```
+
+**`AgentFileSkillResource`** — A file-based skill resource. Reads content from a file on disk relative to the skill directory:
+
+```csharp
+internal sealed class AgentFileSkillResource : AgentSkillResource
+{
+    public AgentFileSkillResource(string name, string fullPath) { ... }
+
+    public string FullPath { get; }
+
+    public override Task<object?> ReadAsync(IServiceProvider? serviceProvider = null, CancellationToken cancellationToken = default)
+    {
+        return File.ReadAllTextAsync(FullPath, Encoding.UTF8, cancellationToken);
+    }
+}
+```
+
+**`AgentFileSkillScript`** — A file-based skill script that represents a script file on disk. Delegates execution to an external `AgentFileSkillScriptRunner` callback (e.g., runs Python/shell via `Process.Start`). Throws `NotSupportedException` if no executor is configured:
+
+```csharp
+public delegate Task<object?> AgentFileSkillScriptRunner(
+    AgentFileSkill skill, AgentFileSkillScript script,
+    AIFunctionArguments arguments, CancellationToken cancellationToken);
+
+public sealed class AgentFileSkillScript : AgentSkillScript
+{
+    private readonly AgentFileSkillScriptRunner _executor;
+
+    internal AgentFileSkillScript(string name, string fullPath, AgentFileSkillScriptRunner executor)
+        : base(name) { ... }
+
+    public override async Task<object?> RunAsync(AgentSkill skill, AIFunctionArguments arguments, ...)
+    {
+
+        return await _executor(fileSkill, this, arguments, cancellationToken);
+    }
+}
+```
+
+The executor can be provided at the **provider level** via `AgentSkillsProviderBuilder.UseFileScriptRunner(executor)` and optionally overridden for a **particular file skill** or for a **set of skills** at the file skill source level, giving fine-grained control over how different scripts are executed.
+
+**`AgentFileSkillsSource`** — A skill source that discovers skills from filesystem directories containing `SKILL.md` files. Recursively scans directories (max 2 levels), validates frontmatter, and enforces path traversal and symlink security checks:
+
+```csharp
+public sealed partial class AgentFileSkillsSource : AgentSkillsSource
+{
+    public AgentFileSkillsSource(
+        IEnumerable<string> skillPaths,
+        AgentFileSkillScriptRunner scriptRunner,
+        AgentFileSkillsSourceOptions? options = null,
+        ILoggerFactory? loggerFactory = null) { ... }
+}
+```
+
+**`AgentFileSkillsSourceOptions`** — Configuration options for `AgentFileSkillsSource`. Allows customizing the allowed file extensions for resources and scripts without adding constructor parameters:
+
+```csharp
+public sealed class AgentFileSkillsSourceOptions
+{
+    public IEnumerable<string>? AllowedResourceExtensions { get; set; }
+    public IEnumerable<string>? AllowedScriptExtensions { get; set; }
+}
+```
+
+**Example** — A file-based skill on disk and how it is added to a source:
+
+```
+skills/
+└── unit-converter/
+    ├── SKILL.md               # frontmatter + instructions
+    ├── resources/
+    │   └── conversion-table.csv   # discovered as a resource
+    └── scripts/
+        └── convert.py             # discovered as a script
+```
+
+```csharp
+var source = new AgentFileSkillsSource(skillPaths: ["./skills"], scriptRunner: SubprocessScriptRunner.RunAsync);
+
+var provider = new AgentSkillsProvider(source);
+
+AIAgent agent = chatClient.AsAIAgent(new ChatClientAgentOptions
+{
+    AIContextProviders = [provider],
+});
+```
+
+### Programmatic Skills
+
+Programmatic skills are defined in C# code rather than discovered from the filesystem. There are two kinds: **inline** and **class-based**. Both use `AgentInlineSkillResource` and `AgentInlineSkillScript` for resources and scripts, and are held by a single `AgentInMemorySkillsSource`.
+
+**`AgentInMemorySkillsSource`** — A general-purpose skill source that holds any `AgentSkill` instances in memory. Although commonly used for programmatic skills (`AgentInlineSkill` and `AgentClassSkill`), it accepts any `AgentSkill` subclass and is not restricted to code-defined skills:
+
+```csharp
+public sealed class AgentInMemorySkillsSource : AgentSkillsSource
+{
+    public AgentInMemorySkillsSource(
+        IEnumerable<AgentSkill> skills,
+        ILoggerFactory? loggerFactory = null) { ... }
+}
+```
+
+#### Inline Skills
+
+Inline skills are built at runtime via the `AgentInlineSkill` class and its fluent API. They are ideal for quick, agent-specific skill definitions where a full class hierarchy would be overkill.
+
+**`AgentInlineSkill`** — A skill defined entirely in code. Resources can be static values or functions; scripts are always functions. Constructed with name, description, and instructions, then extended with resources and scripts:
+
+```csharp
+public sealed class AgentInlineSkill : AgentSkill
+{
+    public AgentInlineSkill(string name, string description, string instructions, string? license = null, string? compatibility = null, ...) { ... }
+    public AgentInlineSkill(AgentSkillFrontmatter frontmatter, string instructions) { ... }
+
+    public AgentInlineSkill AddResource(object value, string name, string? description = null);
+    public AgentInlineSkill AddResource(Delegate handler, string name, string? description = null);
+    public AgentInlineSkill AddScript(Delegate handler, string name, string? description = null);
+}
+```
+
+**`AgentInlineSkillResource`** — A skill resource that wraps a static value:
+
+```csharp
+public sealed class AgentInlineSkillResource : AgentSkillResource
+{
+    public AgentInlineSkillResource(object value, string name, string? description = null)
+        : base(name, description)
+    {
+        _value = value;
+    }
+
+    public override Task<object?> ReadAsync(IServiceProvider? serviceProvider = null, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult<object?>(_value);
+    }
+}
+```
+
+**`AgentInlineSkillResource`** — A skill resource backed by a delegate. The delegate is invoked via an `AIFunction` each time `ReadAsync` is called, producing a dynamic (computed) value:
+
+```csharp
+public sealed class AgentInlineSkillResource : AgentSkillResource
+{
+    public AgentInlineSkillResource(Delegate handler, string name, string? description = null)
+        : base(name, description)
+    {
+        _function = AIFunctionFactory.Create(handler, name: name);
+    }
+
+    public override async Task<object?> ReadAsync(IServiceProvider? serviceProvider = null, CancellationToken cancellationToken = default)
+    {
+        return await _function.InvokeAsync(new AIFunctionArguments() { Services = serviceProvider }, cancellationToken);
+    }
+}
+```
+
+**`AgentInlineSkillScript`** — A skill script backed by a delegate via an `AIFunction`:
+
+```csharp
+public sealed class AgentInlineSkillScript : AgentSkillScript
+{
+    private readonly AIFunction _function;
+
+    public AgentInlineSkillScript(Delegate handler, string name, string? description = null)
+        : base(name, description)
+    {
+        _function = AIFunctionFactory.Create(handler, name: name);
+    }
+
+    public JsonElement? ParametersSchema => _function.JsonSchema;
+
+    public override async Task<object?> RunAsync(AgentSkill skill, AIFunctionArguments arguments, ...)
+    {
+        return await _function.InvokeAsync(arguments, cancellationToken);
+    }
+}
+```
+
+**Example** — Creating an inline skill with a resource and script, then adding it to a source:
+
+```csharp
+var skill = new AgentInlineSkill(
+        name: "unit-converter",
+        description: "Converts between measurement units.",
+        instructions: """
+            Use this skill to convert values between metric and imperial units.
+            Refer to the conversion-table resource for supported unit pairs.
+            Run the convert script to perform conversions.
+            """
+    )
+    .AddResource("kg=2.205lb, m=3.281ft, L=0.264gal", "conversion-table", "Supported unit pairs")
+    .AddScript(Convert, "convert", "Converts a value between units");
+
+var source = new AgentInMemorySkillsSource([skill]);
+
+var provider = new AgentSkillsProvider(source);
+
+AIAgent agent = chatClient.AsAIAgent(new ChatClientAgentOptions
+{
+    AIContextProviders = [provider],
+});
+
+static string Convert(double value, double factor)
+    => JsonSerializer.Serialize(new { result = Math.Round(value * factor, 4) });
+```
+
+#### Class-Based Skills
+
+Class-based skills are designed for packaging skills as reusable libraries. Users subclass `AgentClassSkill` and override properties. Unlike inline skills, class-based skills are self-contained, can live in shared libraries or NuGet packages, and are well-suited for dependency injection.
+
+**`AgentClassSkill`** — An abstract base class for defining skills as reusable C# classes that bundle all skill components (frontmatter, instructions, resources, scripts) together. Designed for packaging skills as distributable libraries:
+
+```csharp
+public abstract class AgentClassSkill : AgentSkill
+{
+    public abstract string Instructions { get; }
+
+    // Content is auto-synthesized from Frontmatter + Instructions + Resources + Scripts
+    public override string Content =>
+        SkillContentBuilder.BuildContent(Frontmatter.Name, Frontmatter.Description,
+            SkillContentBuilder.BuildBody(Instructions, Resources, Scripts));
+}
+```
+
+**Example** — Defining a class-based skill and adding it to a source:
+
+```csharp
+public class UnitConverterSkill : AgentClassSkill
+{
+    public override AgentSkillFrontmatter Frontmatter { get; } =
+        new("unit-converter", "Converts between measurement units.");
+
+    public override string Instructions => """
+        Use this skill to convert values between metric and imperial units.
+        Refer to the conversion-table resource for supported unit pairs.
+        Run the convert script to perform conversions.
+        """;
+
+    public override IReadOnlyList<AgentSkillResource>? Resources { get; } =
+    [
+        new AgentInlineSkillResource("kg=2.205lb, m=3.281ft", "conversion-table"),
+    ];
+
+    public override IReadOnlyList<AgentSkillScript>? Scripts { get; } =
+    [
+        new AgentInlineSkillScript(Convert, "convert"),
+    ];
+
+    private static string Convert(double value, double factor)
+        => JsonSerializer.Serialize(new { result = Math.Round(value * factor, 4) });
+}
+
+var source = new AgentInMemorySkillsSource([new UnitConverterSkill()]);
+
+var provider = new AgentSkillsProvider(source);
+
+AIAgent agent = chatClient.AsAIAgent(new ChatClientAgentOptions
+{
+    AIContextProviders = [provider],
+});
+```
+
+## Filtering, Caching, and Deduplication
+
+The following subsections present alternative approaches for handling filtering, caching, and deduplication of skills across multiple sources.
+
+### Via Composition
+
+In this approach, the `AgentSkillsProvider` accepts a **single** `AgentSkillsSource`. Multiple sources are composed externally via an aggregate source, and cross-cutting concerns like filtering, caching, and deduplication are implemented as **source decorators** — subclasses of `DelegatingAgentSkillsSource` that intercept `GetSkillsAsync()`.
+
+**`FilteringAgentSkillsSource`** — A decorator that applies filter logic before returning results. The decorator pattern keeps filtering orthogonal to source implementations and allows composing multiple filters:
+
+```csharp
+public sealed class FilteringAgentSkillsSource : DelegatingAgentSkillsSource
+{
+    private readonly Func<AgentSkill, bool> _predicate;
+
+    public FilteringAgentSkillsSource(AgentSkillsSource innerSource, Func<AgentSkill, bool> predicate)
+        : base(innerSource)
+    {
+        _predicate = predicate;
+    }
+
+    public override async Task<IList<AgentSkill>> GetSkillsAsync(CancellationToken cancellationToken = default)
+    {
+        var skills = await this.InnerSource.GetSkillsAsync(cancellationToken);
+        return skills.Where(_predicate).ToList();
+    }
+}
+```
+
+**`CachingAgentSkillsSource`** — A decorator that caches skills after the first load, keeping the provider stateless and giving consumers control over caching granularity per source. For example, file-based skills (expensive to discover) can be cached while code-defined skills remain uncached:
+
+```csharp
+public sealed class CachingAgentSkillsSource : DelegatingAgentSkillsSource
+{
+    private IList<AgentSkill>? _cached;
+
+    public CachingAgentSkillsSource(AgentSkillsSource innerSource)
+        : base(innerSource)
+    {
+    }
+
+    public override async Task<IList<AgentSkill>> GetSkillsAsync(CancellationToken cancellationToken = default)
+    {
+        return _cached ??= await this.InnerSource.GetSkillsAsync(cancellationToken);
+    }
+}
+```
+
+**Deduplication** is similarly implemented as a decorator (`DeduplicatingAgentSkillsSource`) that deduplicates by name (case-insensitive, first-one-wins) and logs a warning for skipped duplicates.
+
+**Example** — Combining file-based and code-defined sources with filtering and caching:
+
+```csharp
+var fileSource = new CachingAgentSkillsSource(new AgentFileSkillsSource(["./skills"]));
+var codeSource = new AgentInMemorySkillsSource([myCodeSkill]);
+
+var compositeSource = new FilteringAgentSkillsSource(
+    new AggregatingAgentSkillsSource([fileSource, codeSource]),
+    filter: s => s.Frontmatter.Name != "internal");
+
+var provider = new AgentSkillsProvider(compositeSource);
+
+AIAgent agent = chatClient.AsAIAgent(new ChatClientAgentOptions
+{
+    AIContextProviders = [provider],
+});
+```
+
+**Pros:**
+- Clean single-responsibility: the provider serves skills, sources provide them.
+- Caching, filtering, and deduplication are composable as source decorators — each concern is a separate, testable wrapper.
+
+**Cons:**
+- DI is less flexible: multiple `AgentSkillsSource` implementations registered in the container cannot be auto-injected into the provider. The consumer must manually compose them via an aggregate source.
+- Increased public API surface: requires additional public classes (aggregate source, caching decorators, filtering decorators) that consumers need to learn and use.
+
+### Via AgentSkillsProvider
+
+In this approach, the `AgentSkillsProvider` accepts **`IEnumerable<AgentSkillsSource>`** and handles aggregation, filtering, caching, and deduplication internally.
+
+The provider aggregates skills from all registered sources, deduplicates by name (case-insensitive, first-one-wins), caches the result after the first load, and optionally applies filtering via a predicate on `AgentSkillsProviderOptions`. Duplicate skill names are logged as warnings.
+
+**Example** — Registering multiple sources directly with the provider:
+
+```csharp
+// Conceptual example — in practice, use AgentSkillsProviderBuilder
+var fileSource = new AgentFileSkillsSource(["./skills"]);
+var codeSource = new AgentInMemorySkillsSource([myCodeSkill]);
+
+var provider = new AgentSkillsProvider(
+    sources: [fileSource, codeSource],
+    options: new AgentSkillsProviderOptions
+    {
+        Filter = s => s.Frontmatter.Name != "internal",
+    });
+
+AIAgent agent = chatClient.AsAIAgent(new ChatClientAgentOptions
+{
+    AIContextProviders = [provider],
+});
+```
+
+**Pros:**
+- DI-friendly: register multiple `AgentSkillsSource` implementations in the container, and they are all auto-injected into `AgentSkillsProvider` via `IEnumerable<AgentSkillsSource>`.
+- Smaller public API surface: no need for aggregate source, caching decorators, or filtering decorator classes — these concerns are handled internally by the provider.
+
+**Cons:**
+- The provider takes on multiple responsibilities — aggregation, caching, deduplication, and filtering.
+- Less granular caching control: caching is all-or-nothing across sources rather than per-source as with decorators.
+- Less extensible: new behaviors (e.g., ordering, TTL expiration) require modifying the provider rather than adding a decorator.
+
+### Builder Pattern
+
+**`AgentSkillsProviderBuilder`** provides a fluent API for composing skills from multiple sources. The builder centralizes configuration — script executors, approval callbacks, prompt templates, and filtering — so consumers don't need to know the underlying source types.
+
+The builder internally decides how to wire up the object graph: it creates the appropriate source instances, applies caching and filtering, and returns a fully configured `AgentSkillsProvider`. This keeps the setup code concise while still allowing fine-grained control when needed.
+
+**Example** — Using the builder to combine multiple source types with configuration:
+
+```csharp
+var provider = new AgentSkillsProviderBuilder()
+    .UseFileSkill("./skills")                           // file-based source
+    .UseInlineSkills(codeSkill)                            // code-defined source
+    .UseClassSkills(new ClassSkill())                    // class-based source
+    .UseFileScriptRunner(SubprocessScriptRunner.RunAsync) // script runner
+    .UseScriptApproval()                                // optional human-in-the-loop
+    .UsePromptTemplate(customTemplate)                  // optional prompt customization
+    .UseFilter(s => s.Frontmatter.Name != "internal")   // optional skill filtering
+    .Build();
+
+AIAgent agent = chatClient.AsAIAgent(new ChatClientAgentOptions
+{
+    AIContextProviders = [provider],
+});
+```
+
+## Adding a Custom Skill Type
+
+The skills framework is designed for extensibility. While file-based and inline skills cover common
+scenarios, you can introduce entirely new skill types by subclassing the four base classes:
+
+| Base class            | Purpose                                             |
+|-----------------------|-----------------------------------------------------|
+| `AgentSkillsSource`   | Discovers and loads skills from a particular origin  |
+| `AgentSkill`          | Holds metadata, content, resources, and scripts      |
+| `AgentSkillResource`  | Provides supplementary content to a skill            |
+| `AgentSkillScript`    | Represents an executable action within a skill       |
+
+The example below implements a **cloud-based skill type** where skills, resources, and scripts are
+all stored in and executed through a remote cloud service (e.g., Azure Blob Storage + Azure Functions).
+
+### Step 1 — Define a custom resource
+
+A `CloudSkillResource` reads resource content from a cloud storage endpoint instead of the local
+filesystem:
+
+```csharp
+/// <summary>
+/// A skill resource backed by a cloud storage endpoint.
+/// </summary>
+public sealed class CloudSkillResource : AgentSkillResource
+{
+    private readonly HttpClient _httpClient;
+
+    public CloudSkillResource(string name, Uri blobUri, HttpClient httpClient, string? description = null)
+        : base(name, description)
+    {
+        BlobUri = blobUri ?? throw new ArgumentNullException(nameof(blobUri));
+        _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+    }
+
+    /// <summary>
+    /// Gets the URI of the cloud blob that holds this resource's content.
+    /// </summary>
+    public Uri BlobUri { get; }
+
+    /// <inheritdoc/>
+    public override async Task<object?> ReadAsync(
+        IServiceProvider? serviceProvider = null,
+        CancellationToken cancellationToken = default)
+    {
+        return await _httpClient.GetStringAsync(BlobUri, cancellationToken).ConfigureAwait(false);
+    }
+}
+```
+
+### Step 2 — Define a custom script
+
+A `CloudSkillScript` executes a script by calling a cloud function endpoint, passing arguments as
+the request body:
+
+```csharp
+/// <summary>
+/// A skill script executed via a cloud function endpoint.
+/// </summary>
+public sealed class CloudSkillScript : AgentSkillScript
+{
+    private readonly HttpClient _httpClient;
+
+    public CloudSkillScript(string name, Uri functionUri, HttpClient httpClient, string? description = null)
+        : base(name, description)
+    {
+        FunctionUri = functionUri ?? throw new ArgumentNullException(nameof(functionUri));
+        _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+    }
+
+    /// <summary>
+    /// Gets the URI of the cloud function that runs this script.
+    /// </summary>
+    public Uri FunctionUri { get; }
+
+    /// <inheritdoc/>
+    public override async Task<object?> RunAsync(
+        AgentSkill skill,
+        AIFunctionArguments arguments,
+        CancellationToken cancellationToken = default)
+    {
+        var json = JsonSerializer.Serialize(arguments);
+        using var content = new StringContent(json, Encoding.UTF8, "application/json");
+        var response = await _httpClient.PostAsync(FunctionUri, content, cancellationToken)
+            .ConfigureAwait(false);
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
+    }
+}
+```
+
+### Step 3 — Define a custom skill
+
+A `CloudSkill` bundles cloud-specific metadata (e.g., the base endpoint) with the standard skill
+shape:
+
+```csharp
+/// <summary>
+/// An <see cref="AgentSkill"/> whose content, resources, and scripts are stored in a cloud service.
+/// </summary>
+public sealed class CloudSkill : AgentSkill
+{
+    public CloudSkill(
+        AgentSkillFrontmatter frontmatter,
+        string content,
+        Uri endpoint,
+        IReadOnlyList<AgentSkillResource>? resources = null,
+        IReadOnlyList<AgentSkillScript>? scripts = null)
+    {
+        Frontmatter = frontmatter ?? throw new ArgumentNullException(nameof(frontmatter));
+        Content = content ?? throw new ArgumentNullException(nameof(content));
+        Endpoint = endpoint ?? throw new ArgumentNullException(nameof(endpoint));
+        Resources = resources;
+        Scripts = scripts;
+    }
+
+    /// <inheritdoc/>
+    public override AgentSkillFrontmatter Frontmatter { get; }
+
+    /// <inheritdoc/>
+    public override string Content { get; }
+
+    /// <summary>
+    /// Gets the base cloud endpoint for this skill.
+    /// </summary>
+    public Uri Endpoint { get; }
+
+    /// <inheritdoc/>
+    public override IReadOnlyList<AgentSkillResource>? Resources { get; }
+
+    /// <inheritdoc/>
+    public override IReadOnlyList<AgentSkillScript>? Scripts { get; }
+}
+```
+
+### Step 4 — Define a custom source
+
+A `CloudSkillsSource` discovers skills from a cloud catalog API and constructs `CloudSkill`
+instances with their associated resources and scripts:
+
+```csharp
+/// <summary>
+/// A skill source that discovers and loads skills from a cloud catalog API.
+/// </summary>
+public sealed class CloudSkillsSource : AgentSkillsSource
+{
+    private readonly Uri _catalogUri;
+    private readonly HttpClient _httpClient;
+
+    public CloudSkillsSource(Uri catalogUri, HttpClient httpClient)
+    {
+        _catalogUri = catalogUri ?? throw new ArgumentNullException(nameof(catalogUri));
+        _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+    }
+
+    /// <inheritdoc/>
+    public override async Task<IList<AgentSkill>> GetSkillsAsync(
+        CancellationToken cancellationToken = default)
+    {
+        // Fetch the skill catalog from the cloud service.
+        var json = await _httpClient.GetStringAsync(_catalogUri, cancellationToken)
+            .ConfigureAwait(false);
+        var catalog = JsonSerializer.Deserialize<CloudSkillCatalog>(json)!;
+
+        var skills = new List<AgentSkill>();
+
+        foreach (var entry in catalog.Skills)
+        {
+            var frontmatter = new AgentSkillFrontmatter(entry.Name, entry.Description);
+
+            // Build cloud-backed resources.
+            var resources = entry.Resources
+                .Select(r => new CloudSkillResource(r.Name, r.BlobUri, _httpClient, r.Description))
+                .ToList<AgentSkillResource>();
+
+            // Build cloud-backed scripts.
+            var scripts = entry.Scripts
+                .Select(s => new CloudSkillScript(s.Name, s.FunctionUri, _httpClient, s.Description))
+                .ToList<AgentSkillScript>();
+
+            skills.Add(new CloudSkill(frontmatter, entry.Content, entry.Endpoint, resources, scripts));
+        }
+
+        return skills;
+    }
+}
+```
+
+### Step 5 — Register with the builder
+
+Use `UseSource` to wire the custom source into the provider:
+
+```csharp
+var httpClient = new HttpClient();
+
+var provider = new AgentSkillsProviderBuilder()
+    .UseSource(new CloudSkillsSource(
+        new Uri("https://my-service.example.com/skills/catalog"),
+        httpClient))
+    // Mix with other source types if needed:
+    .UseFileSkill("/local/skills", scriptRunner)
+    .UseInlineSkills(someInlineSkill)
+    .Build();
+```
+
+The `AgentSkillsProvider` handles all skill types uniformly — any combination of file-based, inline,
+class-based, and custom skills can coexist in the same provider. Custom skills automatically
+participate in the model-facing tools (`load_skill`, `read_skill_resource`, `run_skill_script`),
+filtering, deduplication, and caching — no additional integration work is required.
+
+## Script Representation: `AgentSkillScript` vs `AIFunction`
+
+Two approaches were considered for representing executable scripts within skills:
+
+### Option A — Custom `AgentSkillScript` abstract base class (original design)
+
+Scripts are modeled as a custom `AgentSkillScript` abstract class with `Name`, `Description`, and
+`RunAsync(AgentSkill, AIFunctionArguments, CancellationToken)`. Concrete implementations:
+`AgentInlineSkillScript` (wraps a delegate/`AIFunction`) and `AgentFileSkillScript` (wraps a file path + executor delegate).
+
+```csharp
+// Base type
+public abstract class AgentSkillScript
+{
+    public string Name { get; }
+    public string? Description { get; }
+    public abstract Task<object?> RunAsync(AgentSkill skill, AIFunctionArguments arguments, CancellationToken cancellationToken = default);
+}
+
+// AgentSkill exposes scripts as:
+public abstract IReadOnlyList<AgentSkillScript>? Scripts { get; }
+
+// Inline script wraps an AIFunction internally
+var script = new AgentInlineSkillScript(ConvertUnits, "convert");
+
+// Pre-built AIFunction must be wrapped
+var script = new AgentInlineSkillScript(myAIFunction);
+
+// Class-based skill declares scripts as:
+public override IReadOnlyList<AgentSkillScript>? Scripts { get; } =
+[
+    new AgentInlineSkillScript(ConvertUnits, "convert"),
+];
+
+// Provider executes scripts by passing the owning skill:
+await script.RunAsync(skill, arguments, cancellationToken);
+```
+
+**Pros:**
+
+- **Explicit skill context at execution time.** `RunAsync` receives the owning `AgentSkill`, so any script can access skill metadata or resources during execution without requiring construction-time wiring.
+- **Self-contained abstraction.** A dedicated type communicates clearly that scripts are a skills-framework concept, separate from general-purpose AI functions.
+- **Easier extensibility for custom script types.** Third-party implementations can subclass `AgentSkillScript` and access the owning skill in `RunAsync` without special setup.
+
+**Cons:**
+
+- **Wrapper overhead.** `AgentInlineSkillScript` is a thin pass-through around `AIFunction` — it adds a class, a constructor, and an indirection layer for no behavioral difference.
+- **Parallel abstraction.** `AgentSkillScript` and `AIFunction` serve overlapping purposes (named callable with arguments), creating two parallel hierarchies for the same concept.
+- **Friction for consumers.** Users who already have `AIFunction` instances must wrap them in `AgentInlineSkillScript` to use them as scripts, adding ceremony.
+
+### Option B — Reuse `AIFunction` directly
+
+Scripts are represented as `AIFunction` (from `Microsoft.Extensions.AI`). `AgentSkill.Scripts` returns
+`IReadOnlyList<AIFunction>?`. `AgentInlineSkillScript` is eliminated entirely — callers use
+`AIFunctionFactory.Create(delegate, name: ...)` or pass `AIFunction` instances directly.
+`AgentFileSkillScript` becomes an `AIFunction` subclass that captures its owning `AgentFileSkill` via
+an internal back-reference set during construction.
+
+```csharp
+// AgentSkill exposes scripts as AIFunction directly:
+public abstract IReadOnlyList<AIFunction>? Scripts { get; }
+
+// Inline scripts use AIFunctionFactory — no wrapper class needed
+var skill = new AgentInlineSkill("my-skill", "desc", "instructions");
+skill.AddScript(ConvertUnits, "convert");           // delegate
+skill.AddScript(myAIFunction);                       // pre-built AIFunction — no wrapping
+
+// Class-based skill declares scripts as:
+public override IReadOnlyList<AIFunction>? Scripts { get; } =
+[
+    AIFunctionFactory.Create(ConvertUnits, name: "convert"),
+];
+
+// Provider executes scripts via standard AIFunction invocation:
+await script.InvokeAsync(arguments, cancellationToken);
+
+// File-based scripts extend AIFunction and capture the owning skill internally:
+public sealed class AgentFileSkillScript : AIFunction
+{
+    internal AgentFileSkill? Skill { get; set; }   // set by AgentFileSkill constructor
+
+    protected override async ValueTask<object?> InvokeCoreAsync(
+        AIFunctionArguments arguments, CancellationToken cancellationToken)
+    {
+        return await _executor(Skill!, this, arguments, cancellationToken);
+    }
+}
+```
+
+**Pros:**
+
+- **Fewer types.** Eliminates `AgentSkillScript` and `AgentInlineSkillScript`, reducing the public API surface by two classes.
+- **Seamless interop.** Any `AIFunction` — whether from `AIFunctionFactory`, a custom subclass, or an external library — can be used as a skill script with zero wrapping.
+- **Consistent with `Microsoft.Extensions.AI` ecosystem.** Scripts share the same type as tool functions used by `IChatClient` and `FunctionInvokingChatClient`, reducing conceptual overhead for developers already familiar with the ecosystem.
+
+**Cons:**
+
+- **No owning-skill context in invocation signature.** `AIFunction.InvokeAsync` does not accept an `AgentSkill` parameter, so `AgentFileSkillScript` must capture its owning skill via an internal setter during construction. This adds a construction-order dependency: the skill must set the back-reference on its scripts.
+- **Custom script types lose automatic skill access.** Third-party `AIFunction` subclasses that need the owning skill must implement their own mechanism (e.g., constructor injection, closure capture) instead of receiving it as a method parameter.
+- **Semantic overloading.** `AIFunction` now means both "a tool the model can call" and "a script within a skill", which could blur the distinction for framework users.
+
+## Resource Representation: `AgentSkillResource` vs `AIFunction`
+
+Two approaches were considered for representing skill resources (supplementary content such as references, assets, or dynamic data):
+
+### Option A — Custom `AgentSkillResource` abstract base class (original design)
+
+Resources are modeled as a custom `AgentSkillResource` abstract class with `Name`, `Description`, and
+`ReadAsync(IServiceProvider?, CancellationToken)`. Concrete implementations:
+`AgentInlineSkillResource` (static value, delegate, or `AIFunction` wrapper) and `AgentFileSkillResource` (reads file content from disk).
+
+```csharp
+// Base type
+public abstract class AgentSkillResource
+{
+    public string Name { get; }
+    public string? Description { get; }
+    public abstract Task<object?> ReadAsync(IServiceProvider? serviceProvider = null, CancellationToken cancellationToken = default);
+}
+
+// AgentSkill exposes resources as:
+public abstract IReadOnlyList<AgentSkillResource>? Resources { get; }
+
+// Static resource
+var resource = new AgentInlineSkillResource("static content", "my-resource");
+
+// Dynamic resource (delegate)
+var resource = new AgentInlineSkillResource((IServiceProvider sp) => GetData(sp), "my-resource");
+
+// Pre-built AIFunction must be wrapped
+var resource = new AgentInlineSkillResource(myAIFunction);
+
+// Class-based skill declares resources as:
+public override IReadOnlyList<AgentSkillResource>? Resources { get; } =
+[
+    new AgentInlineSkillResource("# Conversion Tables\n...", "conversion-table"),
+];
+
+// Provider reads resources via:
+await resource.ReadAsync(serviceProvider, cancellationToken);
+```
+
+**Pros:**
+
+- **Clear semantic distinction.** A dedicated `AgentSkillResource` type distinguishes resources (data providers) from scripts (executable actions), making the API self-documenting.
+- **Purpose-built API.** `ReadAsync` communicates intent better than `InvokeAsync` for a data-access operation.
+
+**Cons:**
+
+- **Wrapper overhead.** `AgentInlineSkillResource` wraps `AIFunction` internally for delegate/function cases — adding a class and indirection for no behavioral difference.
+- **Parallel abstraction.** `AgentSkillResource` and `AIFunction` serve overlapping purposes (named callable that returns data), creating two parallel hierarchies.
+- **Friction for consumers.** Users who already have `AIFunction` instances must wrap them in `AgentInlineSkillResource`, adding ceremony.
+
+### Option B — Reuse `AIFunction` directly
+
+Resources are represented as `AIFunction`. `AgentSkill.Resources` returns `IReadOnlyList<AIFunction>?`.
+`AgentInlineSkillResource` becomes an `AIFunction` subclass (retained as a convenience for the static-value
+pattern: `new AgentInlineSkillResource("data", "name")`). `AgentFileSkillResource` becomes an `AIFunction`
+subclass that reads file content.
+
+```csharp
+// AgentSkill exposes resources as AIFunction directly:
+public abstract IReadOnlyList<AIFunction>? Resources { get; }
+
+// Static resource — AgentInlineSkillResource is retained as a convenience AIFunction subclass
+var resource = new AgentInlineSkillResource("static content", "my-resource");
+
+// Dynamic resource — AgentInlineSkillResource wraps delegate as AIFunction
+var resource = new AgentInlineSkillResource((IServiceProvider sp) => GetData(sp), "my-resource");
+
+// Pre-built AIFunction can be used directly — no wrapping needed
+skill.AddResource(myAIFunction);
+
+// Class-based skill declares resources as:
+public override IReadOnlyList<AIFunction>? Resources { get; } =
+[
+    new AgentInlineSkillResource("# Conversion Tables\n...", "conversion-table"),
+];
+
+// Provider reads resources via standard AIFunction invocation:
+await resource.InvokeAsync(arguments, cancellationToken);
+
+// File-based resources extend AIFunction directly:
+internal sealed class AgentFileSkillResource : AIFunction
+{
+    public string FullPath { get; }
+
+    protected override async ValueTask<object?> InvokeCoreAsync(
+        AIFunctionArguments arguments, CancellationToken cancellationToken)
+    {
+        return await File.ReadAllTextAsync(FullPath, Encoding.UTF8, cancellationToken);
+    }
+}
+```
+
+**Pros:**
+
+- **Fewer base types.** Eliminates the `AgentSkillResource` abstract class, reducing the public API surface.
+- **Seamless interop.** Any `AIFunction` can be used as a skill resource with zero wrapping.
+
+**Cons:**
+
+- **Loss of semantic distinction.** Resources and scripts are now both `AIFunction`, which could make it less obvious which list a function belongs to when reading code.
+- **Static values require a wrapper.** Unlike the original `ReadAsync` which could return a stored value directly, `AIFunction.InvokeAsync` implies invocation. `AgentInlineSkillResource` is retained as a convenience subclass to handle the static-value case, so this is not eliminated — just moved to a different class.
+
+## Decision Outcome
+
+### 1. Keep `AgentSkillResource` and `AgentSkillScript` (Option A for both sections)
+
+We are staying with the custom `AgentSkillResource` and `AgentSkillScript` model classes instead of reusing `AIFunction`:
+
+- **Resources have no parameters.** If a consumer provides an `AIFunction` with parameters, those parameters will never be advertised to the LLM, and the resulting call will fail.
+- **Approval breaks for `AIFunction`-based representations.** When a resource or script represented by an `AIFunction` is configured with approval, the second approval invocation will not work correctly.
+- **Injecting the owning skill into an `AIFunction`-based script is problematic.** Constructor injection would introduce a circular reference between the skill and the script. An internal property setter is possible but adds coupling.
+
+### 2. Make all agent skill classes internal
+
+All agent-skill-related classes are made `internal` to minimize the public API surface while the feature matures. We can reconsider and promote types to `public` later based on community signal.
+
+This leaves two public entry points:
+
+- **`AgentSkillsProvider`** — use directly when all skills come from a single source and filtering is not needed.
+- **`AgentSkillsProviderBuilder`** — use when mixing skill types or when filtering support is required.
+
+### 3. Caching at provider level
+
+Caching of tools and instructions is implemented inside `AgentSkillsProvider` rather than as an external decorator. Recreating tools and instructions on every provider call is wasteful, and a caching decorator sitting outside the provider would not have the information needed to cache them effectively.