|
| 1 | +--- |
| 2 | +title: Identity and Role Propagation |
| 3 | +author: halter73 |
| 4 | +description: How to access caller identity and roles in MCP tool, prompt, and resource handlers. |
| 5 | +uid: identity |
| 6 | +--- |
| 7 | + |
| 8 | +# Identity and Role Propagation |
| 9 | + |
| 10 | +When building production MCP servers, you often need to know _who_ is calling a tool so you can enforce permissions, filter data, or audit access. The MCP C# SDK provides built-in support for propagating the caller's identity from the transport layer into your tool, prompt, and resource handlers — no custom headers or workarounds required. |
| 11 | + |
| 12 | +## How Identity Flows Through the SDK |
| 13 | + |
| 14 | +When a client sends a request over an authenticated HTTP transport (Streamable HTTP or SSE), the ASP.NET Core authentication middleware populates `HttpContext.User` with a `ClaimsPrincipal`. The SDK's transport layer automatically copies this `ClaimsPrincipal` into `JsonRpcMessage.Context.User`, which then flows through message filters, request filters, and finally into the handler or tool method. |
| 15 | + |
| 16 | +``` |
| 17 | +HTTP Request (with auth token) |
| 18 | + → ASP.NET Core Authentication Middleware (populates HttpContext.User) |
| 19 | + → MCP Transport (copies User into JsonRpcMessage.Context.User) |
| 20 | + → Message Filters (context.User available) |
| 21 | + → Request Filters (context.User available) |
| 22 | + → Tool / Prompt / Resource Handler (ClaimsPrincipal injected as parameter) |
| 23 | +``` |
| 24 | + |
| 25 | +This means you can access the authenticated user's identity at every stage of request processing. |
| 26 | + |
| 27 | +## Direct `ClaimsPrincipal` Parameter Injection (Recommended) |
| 28 | + |
| 29 | +The simplest and recommended approach is to declare a `ClaimsPrincipal` parameter on your tool method. The SDK automatically injects the authenticated user without including it in the tool's input schema: |
| 30 | + |
| 31 | +```csharp |
| 32 | +[McpServerToolType] |
| 33 | +public class UserAwareTools |
| 34 | +{ |
| 35 | + [McpServerTool, Description("Returns a personalized greeting.")] |
| 36 | + public string Greet(ClaimsPrincipal? user, string message) |
| 37 | + { |
| 38 | + var userName = user?.Identity?.Name ?? "anonymous"; |
| 39 | + return $"{userName}: {message}"; |
| 40 | + } |
| 41 | +} |
| 42 | +``` |
| 43 | + |
| 44 | +This pattern works the same way for prompts and resources: |
| 45 | + |
| 46 | +```csharp |
| 47 | +[McpServerPromptType] |
| 48 | +public class UserAwarePrompts |
| 49 | +{ |
| 50 | + [McpServerPrompt, Description("Creates a user-specific prompt.")] |
| 51 | + public ChatMessage PersonalizedPrompt(ClaimsPrincipal? user, string topic) |
| 52 | + { |
| 53 | + var userName = user?.Identity?.Name ?? "user"; |
| 54 | + return new(ChatRole.User, $"As {userName}, explain {topic}."); |
| 55 | + } |
| 56 | +} |
| 57 | +``` |
| 58 | + |
| 59 | +### Why This Works |
| 60 | + |
| 61 | +The SDK registers `ClaimsPrincipal` as one of the built-in services available during request processing. When a tool, prompt, or resource method declares a `ClaimsPrincipal` parameter, the SDK: |
| 62 | + |
| 63 | +1. Excludes it from the generated JSON schema (clients never see it). |
| 64 | +2. Automatically resolves it from the current request's `User` property at invocation time. |
| 65 | +3. Passes `null` if no authenticated user is present (when the parameter is nullable). |
| 66 | + |
| 67 | +This behavior is transport-agnostic. For HTTP transports, the `ClaimsPrincipal` comes from ASP.NET Core authentication. For other transports (like stdio), it will be `null` unless you set it explicitly via a message filter. |
| 68 | + |
| 69 | +## Accessing Identity in Filters |
| 70 | + |
| 71 | +Both message filters and request-specific filters expose the user via `context.User`: |
| 72 | + |
| 73 | +```csharp |
| 74 | +services.AddMcpServer() |
| 75 | + .WithRequestFilters(requestFilters => |
| 76 | + { |
| 77 | + requestFilters.AddCallToolFilter(next => async (context, cancellationToken) => |
| 78 | + { |
| 79 | + // Access user identity in a filter |
| 80 | + var userName = context.User?.Identity?.Name; |
| 81 | + var logger = context.Services?.GetService<ILogger<Program>>(); |
| 82 | + logger?.LogInformation("Tool called by: {User}", userName ?? "anonymous"); |
| 83 | + |
| 84 | + return await next(context, cancellationToken); |
| 85 | + }); |
| 86 | + }) |
| 87 | + .WithTools<UserAwareTools>(); |
| 88 | +``` |
| 89 | + |
| 90 | +## Role-Based Access with `[Authorize]` Attributes |
| 91 | + |
| 92 | +For declarative authorization, you can use standard ASP.NET Core `[Authorize]` attributes on your tools, prompts, and resources. This requires calling `AddAuthorizationFilters()` during server configuration: |
| 93 | + |
| 94 | +```csharp |
| 95 | +services.AddMcpServer() |
| 96 | + .WithHttpTransport() |
| 97 | + .AddAuthorizationFilters() |
| 98 | + .WithTools<RoleProtectedTools>(); |
| 99 | +``` |
| 100 | + |
| 101 | +Then decorate your tools with role requirements: |
| 102 | + |
| 103 | +```csharp |
| 104 | +[McpServerToolType] |
| 105 | +public class RoleProtectedTools |
| 106 | +{ |
| 107 | + [McpServerTool, Description("Available to all authenticated users.")] |
| 108 | + [Authorize] |
| 109 | + public string GetData(string query) |
| 110 | + { |
| 111 | + return $"Data for: {query}"; |
| 112 | + } |
| 113 | + |
| 114 | + [McpServerTool, Description("Admin-only operation.")] |
| 115 | + [Authorize(Roles = "Admin")] |
| 116 | + public string AdminOperation(string action) |
| 117 | + { |
| 118 | + return $"Admin action: {action}"; |
| 119 | + } |
| 120 | + |
| 121 | + [McpServerTool, Description("Public tool accessible without authentication.")] |
| 122 | + [AllowAnonymous] |
| 123 | + public string PublicInfo() |
| 124 | + { |
| 125 | + return "This is public information."; |
| 126 | + } |
| 127 | +} |
| 128 | +``` |
| 129 | + |
| 130 | +When authorization fails, the SDK automatically: |
| 131 | + |
| 132 | +- **For list operations**: Removes unauthorized items from the results so users only see what they can access. |
| 133 | +- **For individual operations**: Returns a JSON-RPC error indicating access is forbidden. |
| 134 | + |
| 135 | +See [Filters](xref:filters) for more details on authorization filters and their execution order. |
| 136 | + |
| 137 | +## Using `IHttpContextAccessor` (HTTP-Only Alternative) |
| 138 | + |
| 139 | +If you need access to the full `HttpContext` (not just the user), you can inject `IHttpContextAccessor` into your tool class. This gives you access to HTTP headers, query strings, and other request metadata: |
| 140 | + |
| 141 | +```csharp |
| 142 | +[McpServerToolType] |
| 143 | +public class HttpContextTools(IHttpContextAccessor contextAccessor) |
| 144 | +{ |
| 145 | + [McpServerTool, Description("Returns data filtered by caller identity.")] |
| 146 | + public string GetFilteredData(string query) |
| 147 | + { |
| 148 | + var httpContext = contextAccessor.HttpContext |
| 149 | + ?? throw new InvalidOperationException("No HTTP context available."); |
| 150 | + var userName = httpContext.User.Identity?.Name ?? "anonymous"; |
| 151 | + return $"{userName}: results for '{query}'"; |
| 152 | + } |
| 153 | +} |
| 154 | +``` |
| 155 | + |
| 156 | +> [!IMPORTANT] |
| 157 | +> `IHttpContextAccessor` only works with HTTP transports. For transport-agnostic identity access, use `ClaimsPrincipal` parameter injection instead. |
| 158 | +
|
| 159 | +See [HTTP Context](xref:httpcontext) for more details, including important caveats about stale `HttpContext` with the legacy SSE transport. |
| 160 | + |
| 161 | +## Transport Considerations |
| 162 | + |
| 163 | +| Transport | Identity Source | Notes | |
| 164 | +| --- | --- | --- | |
| 165 | +| Streamable HTTP | ASP.NET Core authentication middleware populates `HttpContext.User`, which the transport copies to each request. | Recommended for production. Each request carries fresh authentication context. | |
| 166 | +| SSE | Same as Streamable HTTP, but the `HttpContext` is tied to the long-lived SSE connection. | The `ClaimsPrincipal` parameter injection still works correctly, but `IHttpContextAccessor` may return stale claims if the client's token was refreshed after the SSE connection was established. | |
| 167 | +| Stdio | No built-in authentication. `ClaimsPrincipal` is `null` unless set via a message filter. | For process-level identity, you can set the user in a message filter based on environment variables or other process-level context. | |
| 168 | + |
| 169 | +### Setting Identity for Stdio Transport |
| 170 | + |
| 171 | +For stdio-based servers where the caller's identity comes from the process environment rather than HTTP authentication, you can set the user in a message filter: |
| 172 | + |
| 173 | +```csharp |
| 174 | +services.AddMcpServer() |
| 175 | + .WithMessageFilters(messageFilters => |
| 176 | + { |
| 177 | + messageFilters.AddIncomingFilter(next => async (context, cancellationToken) => |
| 178 | + { |
| 179 | + // Set user based on process-level context |
| 180 | + var role = Environment.GetEnvironmentVariable("MCP_USER_ROLE") ?? "default"; |
| 181 | + context.User = new ClaimsPrincipal(new ClaimsIdentity( |
| 182 | + [new Claim(ClaimTypes.Name, "stdio-user"), new Claim(ClaimTypes.Role, role)], |
| 183 | + "StdioAuth", ClaimTypes.Name, ClaimTypes.Role)); |
| 184 | + |
| 185 | + await next(context, cancellationToken); |
| 186 | + }); |
| 187 | + }) |
| 188 | + .WithTools<UserAwareTools>(); |
| 189 | +``` |
| 190 | + |
| 191 | +## Full Example: Protected HTTP Server |
| 192 | + |
| 193 | +For a complete example of an MCP server with JWT authentication, OAuth resource metadata, and protected tools, see the [ProtectedMcpServer sample](https://github.com/modelcontextprotocol/csharp-sdk/tree/main/samples/ProtectedMcpServer). |
| 194 | + |
| 195 | +The sample demonstrates: |
| 196 | + |
| 197 | +- Configuring JWT Bearer authentication |
| 198 | +- Setting up MCP authentication with resource metadata |
| 199 | +- Using `RequireAuthorization()` to protect the MCP endpoint |
| 200 | +- Implementing weather tools that require authentication |
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