Diagnostics

Generator diagnostics for MCP server validation.

The generator reports diagnostics at build time for invalid MCP server declarations.

Rules

Id	Severity	Description
QA0001	Error	[McpServer] class must be partial
QA0002	Error	[McpServer] class must not be static
QA0003	Error	[McpServer] class must not be generic
QA0004	Error	[Tool] method must be inside [McpServer] class
QA0005	Error	[Tool] method must not be static
QA0006	Error	[Tool] method must not be generic
QA0007	Error	[Tool] method has unsupported return type
QA0008	Error	[Tool] parameter has unsupported type
QA0009	Warning	[Tool] parameter missing [Description]
QA0010	Warning	[McpServer] class has no [Tool] methods
QA0011	Error	Duplicate tool name in same server
QA0012	Warning	All tool safety hints are Unset
QA0013	Error	[Resource] method has invalid return type
QA0014	Error	Duplicate resource URI in same server
QA0015	Error	[Prompt] method has invalid return type
QA0016	Error	Duplicate prompt name in same server

Examples

QA0001 — Class must be partial

// Bad: not partial
[McpServer]
public class MyServer { }

// Good: partial
[McpServer]
public partial class MyServer { }

QA0004 — Orphaned tool

// Bad: no [McpServer] on containing class
public partial class NotAServer
{
    [Tool]
    public string Echo(string input) => input;
}

QA0009 — Missing description

[McpServer]
public partial class MyServer
{
    [Tool]
    public string Echo(string input) => input;
    //                  ^ QA0009: parameter 'input' has no [Description]
}

Add [Description] to suppress:

[Tool]
public string Echo([Description("Text to echo")] string input) => input;

QA0012 — All safety hints Unset

Only fires when all four hints are explicitly set to ToolHint.Unset:

// Triggers QA0012
[Tool(ReadOnly = ToolHint.Unset, Idempotent = ToolHint.Unset,
      Destructive = ToolHint.Unset, OpenWorld = ToolHint.Unset)]
public string Echo(string input) => input;

// No diagnostic — hints left at defaults
[Tool]
public string Echo(string input) => input;

QA0013 — Resource invalid return type

[Resource] methods must return string, Task<string>, or ValueTask<string>.

QA0014 — Duplicate resource URI

Two methods in the same server cannot share a [Resource] URI.

QA0015 — Prompt invalid return type

[Prompt] methods must return string, Task<string>, PromptResult, or Task<PromptResult>.

QA0016 — Duplicate prompt name

Two methods in the same server cannot share a [Prompt] name.

Configuration

[*.cs]
dotnet_diagnostic.QA0009.severity = none