fix: report refused calls as error results instead of exceptions

Tools/SqlTools.cs

@@ -1,4 +1,4 @@
-using ModelContextProtocol;
+using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 using SqlMcp.Models;
 using SqlMcp.Services;
@@ -23,15 +23,15 @@ public sealed class SqlTools(QueryService svc)
 
     [McpServerTool(Name = "list_tables", ReadOnly = true, Idempotent = true)]
     [Description("List the tables and views the database login can read, as JSON. System schemas are left out unless named.")]
-    public async Task<string> ListTables(
+    public async Task<CallToolResult> ListTables(
         [Description("Database name from list_databases.")] string dbName,
         [Description("Only list this schema (on MySQL, the database). Optional.")] string? schema = null,
         CancellationToken cancellationToken = default) =>
         Serialize(await svc.ListTablesAsync(dbName, schema, cancellationToken));
 
     [McpServerTool(Name = "describe_table", ReadOnly = true, Idempotent = true)]
     [Description("Describe a table or view's columns: name, data type, nullability and primary key, as JSON.")]
-    public async Task<string> DescribeTable(
+    public async Task<CallToolResult> DescribeTable(
         [Description("Database name from list_databases.")] string dbName,
         [Description("Table or view name, optionally schema-qualified (\"sales.orders\"). Case-insensitive.")] string table,
         [Description("Schema containing the table. Optional; needed only when the name exists in more than one schema.")] string? schema = null,
@@ -42,20 +42,19 @@ public async Task<string> DescribeTable(
     [Description("Execute a read-only SELECT query on the specified database and return the columns and rows as JSON. " +
                  "Pass values through parameters rather than writing them into the SQL: they are bound by the database, " +
                  "so quotes in values need no escaping and can't change the query.")]
-    public async Task<string> QueryDatabase(
+    public async Task<CallToolResult> QueryDatabase(
         [Description("Database name from list_databases.")] string dbName,
         [Description("SELECT query to execute. Refer to parameters as @name on SQL Server, Postgres and MySQL, and as :name on Oracle.")] string sql,
         [Description("Values for the query's placeholders, by name without the @ or :, e.g. {\"customer\": \"O'Brien\", \"minTotal\": 100}. " +
                      "Values may be strings, numbers, booleans or null. Optional.")] Dictionary<string, JsonElement>? parameters = null,
         CancellationToken cancellationToken = default) =>
         Serialize(await svc.ExecuteAsync(new QueryRequest(dbName, sql, parameters), cancellationToken));
 
-    private static string Serialize<T>(QueryService.Result<T, string> result)
+    // An expected failure (unknown database, rejected statement, refused login) is an isError result, not an
+    // exception: the SDK would log a thrown one as unhandled, with a stack trace, and prefix its message.
+    private static CallToolResult Serialize<T>(QueryService.Result<T, string> result) => new()
     {
-        // McpException messages are returned to the client as an isError tool result.
-        if (result.IsError)
-            throw new McpException(result.Error!);
-
-        return JsonSerializer.Serialize(result.Value, JsonOptions);
-    }
+        Content = [new TextContentBlock { Text = result.IsError ? result.Error! : JsonSerializer.Serialize(result.Value, JsonOptions) }],
+        IsError = result.IsError,
+    };
 }

tests/SqlMcp.Tests/McpToolsTests.cs

@@ -6,23 +6,26 @@ namespace SqlMcp.Tests;
 
 public class McpToolsTests
 {
-    private static async Task<JsonElement> ListToolsAsync()
+    private static async Task<JsonElement> ListToolsAsync() =>
+        (await PostAsync("""{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}""")).GetProperty("tools");
+
+    private static async Task<JsonElement> PostAsync(string body)
     {
         using var client = new WebApplicationFactory<Program>().CreateClient();
         var request = new HttpRequestMessage(HttpMethod.Post, "/mcp")
         {
-            Content = new StringContent("""{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}""", Encoding.UTF8, "application/json")
+            Content = new StringContent(body, Encoding.UTF8, "application/json")
         };
         request.Headers.Accept.ParseAdd("application/json");
         request.Headers.Accept.ParseAdd("text/event-stream");
 
         var response = await client.SendAsync(request);
         response.EnsureSuccessStatusCode();
-        var body = await response.Content.ReadAsStringAsync();
+        var text = await response.Content.ReadAsStringAsync();
 
         // The response is either plain JSON or a single server-sent event.
-        var json = body.Split('\n').FirstOrDefault(l => l.StartsWith("data: "))?["data: ".Length..] ?? body;
-        return JsonDocument.Parse(json).RootElement.GetProperty("result").GetProperty("tools");
+        var json = text.Split('\n').FirstOrDefault(l => l.StartsWith("data: "))?["data: ".Length..] ?? text;
+        return JsonDocument.Parse(json).RootElement.GetProperty("result");
     }
 
     [Theory]
@@ -40,4 +43,14 @@ public async Task Advertises_read_only_tools_with_their_required_arguments(strin
         var actual = schema.TryGetProperty("required", out var r) ? r.EnumerateArray().Select(e => e.GetString()!).ToArray() : [];
         Assert.Equal(required.Order(), actual.Order());
     }
+
+    [Fact]
+    public async Task A_refused_call_is_an_error_result_with_just_the_reason()
+    {
+        var result = await PostAsync("""{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"query_database","arguments":{"dbName":"nope","sql":"SELECT 1"}}}""");
+
+        Assert.True(result.GetProperty("isError").GetBoolean());
+        var text = result.GetProperty("content")[0].GetProperty("text").GetString()!;
+        Assert.StartsWith("Unknown database 'nope'", text);
+    }
 }