Tool calls fail with JSON deserialization error when _meta.progressToken is sent as a number

[BriCass]

When an MCP client sends a tools/call request where _meta.progressToken is a numeric value (e.g., 1 or 42), the server throws: MCP error -32000: The JSON value could not be converted to System.String. Path: $._meta.progressToken | LineNumber: 0 | BytePositionInLine: 163.

The MetaData.ProgressToken property appears to be typed as string, but the MCP specification defines progressToken as ProgressToken which is string | number
(https://spec.modelcontextprotocol.io/specification/2025-03-26/basic/utilities/progress/). Claude Code (Anthropic's CLI) sends progressToken as a numeric value, which causes
System.Text.Json to reject the deserialization.

Steps to Reproduce:

1. Create an MCP server using MCPSharp v1.0.11
2. Connect via a client that sends numeric progressToken values (e.g., Claude Code)
3. Call any tool

Expected: Tool executes successfully, progressToken accepts both string and number values per the MCP spec.

Actual: All tool calls fail with the deserialization error above.

Suggested Fix:

Change ProgressToken in the MetaData class from string to a type that accepts both, e.g.:

// Option A: Use object
[JsonPropertyName("progressToken")]
public object? ProgressToken { get; set; }

// Option B: Use JsonElement for deferred parsing
[JsonPropertyName("progressToken")]
public JsonElement? ProgressToken { get; set; }

// Option C: Custom JsonConverter that coerces number to string
[JsonConverter(typeof(StringOrNumberConverter))]
[JsonPropertyName("progressToken")]
public string? ProgressToken { get; set; }

Environment:

• MCPSharp v1.0.11
• .NET 9.0
• Client: Claude Code (Anthropic CLI)