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diff --git a/‎dotnet/src/Canvas.cs‎
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+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------------------------------------------*/
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Threading;
+using System.Threading.Tasks;
+using GitHub.Copilot.Rpc;
+
+namespace GitHub.Copilot;
+
+/// <summary>
+/// Declarative metadata for a single canvas, sent over the wire on
+/// <c>session.create</c> / <c>session.resume</c>.
+/// </summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasDeclaration
+{
+    /// <summary>Canvas identifier, unique within the declaring connection.</summary>
+    [JsonPropertyName("id")]
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>Human-readable name shown in host UI and canvas pickers.</summary>
+    [JsonPropertyName("displayName")]
+    public string DisplayName { get; set; } = string.Empty;
+
+    /// <summary>Short, single-sentence description shown to the agent in canvas catalogs.</summary>
+    [JsonPropertyName("description")]
+    public string Description { get; set; } = string.Empty;
+
+    /// <summary>JSON Schema for the <c>input</c> payload accepted by <c>canvas.open</c>.</summary>
+    [JsonPropertyName("inputSchema")]
+    public JsonElement? InputSchema { get; set; }
+
+    /// <summary>Agent-callable actions this canvas exposes.</summary>
+    [JsonPropertyName("actions")]
+    public IList<CanvasAction>? Actions { get; set; }
+}
+
+/// <summary>
+/// Stable extension identity for session participants that provide canvases.
+/// </summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class ExtensionInfo
+{
+    /// <summary>Extension namespace/source, e.g. <c>"github-app"</c>.</summary>
+    [JsonPropertyName("source")]
+    public string Source { get; set; } = string.Empty;
+
+    /// <summary>Stable provider name within the source namespace.</summary>
+    [JsonPropertyName("name")]
+    public string Name { get; set; } = string.Empty;
+}
+
+/// <summary>Response returned from <see cref="ICanvasHandler.OnOpenAsync"/>.</summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasOpenResponse
+{
+    /// <summary>URL the host should render. Optional for canvases with no visual surface.</summary>
+    [JsonPropertyName("url")]
+    public string? Url { get; set; }
+
+    /// <summary>Provider-supplied title shown in host chrome.</summary>
+    [JsonPropertyName("title")]
+    public string? Title { get; set; }
+
+    /// <summary>Provider-supplied status text shown in host chrome.</summary>
+    [JsonPropertyName("status")]
+    public string? Status { get; set; }
+}
+
+/// <summary>Host capabilities passed to canvas provider callbacks.</summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasHostContext
+{
+    /// <summary>Host capability details.</summary>
+    [JsonPropertyName("capabilities")]
+    public CanvasHostCapabilities Capabilities { get; set; } = new();
+}
+
+/// <summary>Host capability details passed to canvas provider callbacks.</summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasHostCapabilities
+{
+    /// <summary>Whether the host supports canvas rendering.</summary>
+    [JsonPropertyName("canvases")]
+    public bool Canvases { get; set; }
+}
+
+/// <summary>Context handed to <see cref="ICanvasHandler.OnOpenAsync"/>.</summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasOpenContext
+{
+    /// <summary>Session that requested the canvas.</summary>
+    public string SessionId { get; init; } = string.Empty;
+
+    /// <summary>Owning provider identifier.</summary>
+    public string ExtensionId { get; init; } = string.Empty;
+
+    /// <summary>Canvas id from the declaring <see cref="CanvasDeclaration"/>.</summary>
+    public string CanvasId { get; init; } = string.Empty;
+
+    /// <summary>Stable instance id supplied by the runtime.</summary>
+    public string InstanceId { get; init; } = string.Empty;
+
+    /// <summary>Validated input payload.</summary>
+    public JsonElement Input { get; init; }
+
+    /// <summary>Host capabilities supplied by the runtime.</summary>
+    public CanvasHostContext? Host { get; init; }
+}
+
+/// <summary>Context handed to <see cref="ICanvasHandler.OnActionAsync"/>.</summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasActionContext
+{
+    /// <summary>Session that invoked the action.</summary>
+    public string SessionId { get; init; } = string.Empty;
+
+    /// <summary>Owning provider identifier.</summary>
+    public string ExtensionId { get; init; } = string.Empty;
+
+    /// <summary>Canvas id targeted by the action.</summary>
+    public string CanvasId { get; init; } = string.Empty;
+
+    /// <summary>Instance id targeted by the action.</summary>
+    public string InstanceId { get; init; } = string.Empty;
+
+    /// <summary>Action name from <see cref="CanvasAction.Name"/>.</summary>
+    public string ActionName { get; init; } = string.Empty;
+
+    /// <summary>Validated input payload.</summary>
+    public JsonElement Input { get; init; }
+
+    /// <summary>Host capabilities supplied by the runtime.</summary>
+    public CanvasHostContext? Host { get; init; }
+}
+
+/// <summary>Context handed to a canvas's close lifecycle hook.</summary>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasLifecycleContext
+{
+    /// <summary>Session owning the canvas instance.</summary>
+    public string SessionId { get; init; } = string.Empty;
+
+    /// <summary>Owning provider identifier.</summary>
+    public string ExtensionId { get; init; } = string.Empty;
+
+    /// <summary>Canvas id from the declaring <see cref="CanvasDeclaration"/>.</summary>
+    public string CanvasId { get; init; } = string.Empty;
+
+    /// <summary>Instance id this lifecycle event applies to.</summary>
+    public string InstanceId { get; init; } = string.Empty;
+
+    /// <summary>Host capabilities supplied by the runtime.</summary>
+    public CanvasHostContext? Host { get; init; }
+}
+
+/// <summary>Structured error returned from canvas handlers.</summary>
+/// <remarks>
+/// Throw this from <see cref="ICanvasHandler"/> implementations to surface a
+/// machine-readable error code to the runtime. Any other exception is wrapped
+/// in a generic <c>canvas_handler_error</c> envelope.
+/// </remarks>
+[Experimental(Diagnostics.Experimental)]
+public sealed class CanvasError : Exception
+{
+    /// <summary>Initializes a new <see cref="CanvasError"/>.</summary>
+    /// <param name="code">Machine-readable error code.</param>
+    /// <param name="message">Human-readable message.</param>
+    public CanvasError(string code, string message) : base(message)
+    {
+        Code = code;
+    }
+
+    /// <summary>Machine-readable error code.</summary>
+    public string Code { get; }
+
+    /// <summary>
+    /// Default error returned when a custom action has no handler.
+    /// </summary>
+    public static CanvasError NoHandler() => new(
+        "canvas_action_no_handler",
+        "No handler implemented for this canvas action");
+}
+
+/// <summary>
+/// Internal helpers used by the session runtime to translate <see cref="CanvasError"/>
+/// (and other handler-thrown exceptions) into structured JSON-RPC error responses.
+/// </summary>
+internal static class CanvasErrorHelpers
+{
+    private const int InternalError = -32603;
+
+    public static LocalRpcInvocationException HandlerUnset() => Build(
+        "canvas_handler_unset",
+        "No canvas handler is registered on this session");
+
+    public static LocalRpcInvocationException HandlerError(string message) => Build(
+        "canvas_handler_error",
+        message);
+
+    public static LocalRpcInvocationException ToRpcException(CanvasError error) => Build(error.Code, error.Message);
+
+    private static LocalRpcInvocationException Build(string code, string message)
+    {
+        var json = JsonSerializer.Serialize(
+            new CanvasErrorPayload { Code = code, Message = message },
+            CanvasJsonContext.Default.CanvasErrorPayload);
+        using var doc = JsonDocument.Parse(json);
+        return new LocalRpcInvocationException(InternalError, message, doc.RootElement.Clone());
+    }
+
+    internal sealed class CanvasErrorPayload
+    {
+        [JsonPropertyName("code")]
+        public string Code { get; set; } = string.Empty;
+
+        [JsonPropertyName("message")]
+        public string Message { get; set; } = string.Empty;
+    }
+}
+
+[JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
+[JsonSerializable(typeof(CanvasErrorHelpers.CanvasErrorPayload))]
+internal partial class CanvasJsonContext : JsonSerializerContext;
+
+/// <summary>
+/// Provider-side canvas lifecycle handler.
+/// </summary>
+/// <remarks>
+/// A session installs a single <see cref="ICanvasHandler"/> via
+/// <c>SessionConfigBase.CanvasHandler</c>. The handler receives every
+/// inbound <c>canvas.open</c> / <c>canvas.close</c> / <c>canvas.action.invoke</c>
+/// JSON-RPC request the runtime issues for this session and decides — typically
+/// by inspecting <see cref="CanvasOpenContext.CanvasId"/> — which
+/// application-side canvas should handle the call.
+/// <para>
+/// The SDK does not maintain a per-canvas registry; multiplexing across
+/// declared canvases is the implementor's responsibility.
+/// </para>
+/// <para>
+/// Implementations targeting <c>netstandard2.0</c> cannot rely on default
+/// interface methods; derive from <see cref="CanvasHandlerBase"/> to inherit
+/// sensible defaults for <see cref="OnCloseAsync"/> and <see cref="OnActionAsync"/>.
+/// </para>
+/// </remarks>
+[Experimental(Diagnostics.Experimental)]
+public interface ICanvasHandler
+{
+    /// <summary>Open a new canvas instance.</summary>
+    Task<CanvasOpenResponse> OnOpenAsync(CanvasOpenContext context, CancellationToken cancellationToken);
+
+    /// <summary>Canvas was closed by the user or agent. Default: no-op.</summary>
+    Task OnCloseAsync(CanvasLifecycleContext context, CancellationToken cancellationToken);
+
+    /// <summary>
+    /// Handle a non-lifecycle action declared by the canvas.
+    /// Default: throws <see cref="CanvasError.NoHandler"/>.
+    /// </summary>
+    Task<object?> OnActionAsync(CanvasActionContext context, CancellationToken cancellationToken);
+}
+
+/// <summary>
+/// Convenience base class for <see cref="ICanvasHandler"/> that supplies
+/// default no-op / no-handler implementations of the optional callbacks.
+/// </summary>
+[Experimental(Diagnostics.Experimental)]
+public abstract class CanvasHandlerBase : ICanvasHandler
+{
+    /// <inheritdoc />
+    public abstract Task<CanvasOpenResponse> OnOpenAsync(CanvasOpenContext context, CancellationToken cancellationToken);
+
+    /// <inheritdoc />
+    public virtual Task OnCloseAsync(CanvasLifecycleContext context, CancellationToken cancellationToken)
+#if NET8_0_OR_GREATER
+        => Task.CompletedTask;
+#else
+        => Task.FromResult<object?>(null);
+#endif
+
+    /// <inheritdoc />
+    public virtual Task<object?> OnActionAsync(CanvasActionContext context, CancellationToken cancellationToken)
+        => Task.FromException<object?>(CanvasError.NoHandler());
+}