Add ResourceNotFound error code (-32002) (#1062)

Author: Copilot

src/ModelContextProtocol.Core/McpErrorCode.cs

@@ -6,12 +6,29 @@ namespace ModelContextProtocol;
 public enum McpErrorCode
 {
     /// <summary>
-    /// Indicates that the JSON received could not be parsed.
+    /// Indicates that the requested resource could not be found.
     /// </summary>
     /// <remarks>
-    /// This error occurs when the input contains malformed JSON or incorrect syntax.
+    /// This error should be used when a resource URI does not match any available resource on the server.
+    /// It allows clients to distinguish between missing resources and other types of errors.
     /// </remarks>
-    ParseError = -32700,
+    ResourceNotFound = -32002,
+
+    /// <summary>
+    /// Indicates that URL-mode elicitation is required to complete the requested operation.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This error is returned when a server operation requires additional user input through URL-mode elicitation
+    /// before it can proceed. The error data must include the `data.elicitations` payload describing the pending
+    /// elicitation(s) for the client to present to the user.
+    /// </para>
+    /// <para>
+    /// Common scenarios include OAuth authorization and other out-of-band flows that cannot be completed inside
+    /// the MCP client.
+    /// </para>
+    /// </remarks>
+    UrlElicitationRequired = -32042,
 
     /// <summary>
     /// Indicates that the JSON payload does not conform to the expected Request object structure.
@@ -58,18 +75,10 @@ public enum McpErrorCode
     InternalError = -32603,
 
     /// <summary>
-    /// Indicates that URL-mode elicitation is required to complete the requested operation.
+    /// Indicates that the JSON received could not be parsed.
     /// </summary>
     /// <remarks>
-    /// <para>
-    /// This error is returned when a server operation requires additional user input through URL-mode elicitation
-    /// before it can proceed. The error data must include the `data.elicitations` payload describing the pending
-    /// elicitation(s) for the client to present to the user.
-    /// </para>
-    /// <para>
-    /// Common scenarios include OAuth authorization and other out-of-band flows that cannot be completed inside
-    /// the MCP client.
-    /// </para>
+    /// This error occurs when the input contains malformed JSON or incorrect syntax.
     /// </remarks>
-    UrlElicitationRequired = -32042,
+    ParseError = -32700,
 }

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -273,7 +273,7 @@ subscribeHandler is null && unsubscribeHandler is null && resources is null &&
 
         listResourcesHandler ??= (static async (_, __) => new ListResourcesResult());
         listResourceTemplatesHandler ??= (static async (_, __) => new ListResourceTemplatesResult());
-        readResourceHandler ??= (static async (request, _) => throw new McpProtocolException($"Unknown resource URI: '{request.Params?.Uri}'", McpErrorCode.InvalidParams));
+        readResourceHandler ??= (static async (request, _) => throw new McpProtocolException($"Unknown resource URI: '{request.Params?.Uri}'", McpErrorCode.ResourceNotFound));
         subscribeHandler ??= (static async (_, __) => new EmptyResult());
         unsubscribeHandler ??= (static async (_, __) => new EmptyResult());
         var listChanged = resourcesCapability?.ListChanged;

tests/ModelContextProtocol.TestServer/Program.cs

@@ -434,7 +434,7 @@ private static void ConfigureResources(McpServerOptions options)
             }
 
             ResourceContents contents = resourceContents.FirstOrDefault(r => r.Uri == request.Params.Uri)
-                ?? throw new McpProtocolException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                ?? throw new McpProtocolException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.ResourceNotFound);
 
             return new ReadResourceResult
             {

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -273,7 +273,7 @@ static CreateMessageRequestParams CreateRequestSamplingParams(string context, st
                 }
 
                 ResourceContents? contents = resourceContents.FirstOrDefault(r => r.Uri == request.Params.Uri) ??
-                    throw new McpProtocolException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.ResourceNotFound);
 
                 return new ReadResourceResult
                 {

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsResourcesTests.cs

@@ -108,7 +108,7 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
                     };
             }
 
-            throw new McpProtocolException($"Resource not found: {request.Params?.Uri}");
+            throw new McpProtocolException($"Resource not found: {request.Params?.Uri}", McpErrorCode.ResourceNotFound);
         })
         .WithResources<SimpleResources>();
     }
@@ -249,6 +249,7 @@ public async Task Throws_Exception_On_Unknown_Resource()
             cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Contains("Resource not found", e.Message);
+        Assert.Equal(McpErrorCode.ResourceNotFound, e.ErrorCode);
     }
 
     [Fact]

tests/ModelContextProtocol.Tests/Configuration/McpServerResourceRoutingTests.cs

@@ -36,7 +36,7 @@ public async Task MultipleTemplatedResources_MatchesCorrectResource()
         Assert.Equal("params: a, b, c", ((TextResourceContents)paramsResult.Contents[0]).Text);
 
         var mcpEx = await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ReadResourceAsync("test://params{?a1,a2,a3}", null, TestContext.Current.CancellationToken));
-        Assert.Equal(McpErrorCode.InvalidParams, mcpEx.ErrorCode);
+        Assert.Equal(McpErrorCode.ResourceNotFound, mcpEx.ErrorCode);
         Assert.Equal("Request failed (remote): Unknown resource URI: 'test://params{?a1,a2,a3}'", mcpEx.Message);
     }
 }

tests/ModelContextProtocol.Tests/McpProtocolExceptionDataTests.cs

@@ -33,7 +33,7 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
             switch (toolName)
             {
                 case "throw_with_serializable_data":
-                    throw new McpProtocolException("Resource not found", (McpErrorCode)(-32002))
+                    throw new McpProtocolException("Resource not found", McpErrorCode.ResourceNotFound)
                     {
                         Data =
                         {
@@ -43,7 +43,7 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
                     };
 
                 case "throw_with_nonserializable_data":
-                    throw new McpProtocolException("Resource not found", (McpErrorCode)(-32002))
+                    throw new McpProtocolException("Resource not found", McpErrorCode.ResourceNotFound)
                     {
                         Data =
                         {
@@ -55,7 +55,7 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
                     };
 
                 case "throw_with_only_nonserializable_data":
-                    throw new McpProtocolException("Resource not found", (McpErrorCode)(-32002))
+                    throw new McpProtocolException("Resource not found", McpErrorCode.ResourceNotFound)
                     {
                         Data =
                         {
@@ -79,7 +79,7 @@ public async Task Exception_With_Serializable_Data_Propagates_To_Client()
             await client.CallToolAsync("throw_with_serializable_data", cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): Resource not found", exception.Message);
-        Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
+        Assert.Equal(McpErrorCode.ResourceNotFound, exception.ErrorCode);
 
         // Verify the data was propagated to the exception
         // The Data collection should contain the expected keys
@@ -113,7 +113,7 @@ public async Task Exception_With_NonSerializable_Data_Still_Propagates_Error_To_
             await client.CallToolAsync("throw_with_nonserializable_data", cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): Resource not found", exception.Message);
-        Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
+        Assert.Equal(McpErrorCode.ResourceNotFound, exception.ErrorCode);
 
         // Verify that only the serializable data was propagated (non-serializable was filtered out)
         var hasUri = false;
@@ -142,7 +142,7 @@ public async Task Exception_With_Only_NonSerializable_Data_Still_Propagates_Erro
             await client.CallToolAsync("throw_with_only_nonserializable_data", cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): Resource not found", exception.Message);
-        Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
+        Assert.Equal(McpErrorCode.ResourceNotFound, exception.ErrorCode);
 
         // When all data is non-serializable, the Data collection should be empty
         // (the server's ConvertExceptionData returns null when no serializable data exists)

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -681,7 +681,7 @@ await transport.SendMessageAsync(
     public async Task Can_Handle_Call_Tool_Requests_With_McpProtocolException_And_Data()
     {
         const string ErrorMessage = "Resource not found";
-        const McpErrorCode ErrorCode = (McpErrorCode)(-32002);
+        const McpErrorCode ErrorCode = McpErrorCode.ResourceNotFound;
         const string ResourceUri = "file:///path/to/resource";
 
         await using var transport = new TestServerTransport();