MEAI adapter: HostedMcpServerTool requests are always rejected

[johanjeppson]

Environment

• Anthropic 12.44.0
• Microsoft.Extensions.AI.Abstractions 10.9.0
• .NET 10

Repro

Any HostedMcpServerTool passed through the beta AsIChatClient:

var client = new AnthropicClient(new ClientOptions { ApiKey = apiKey }).Beta.AsIChatClient();

var options = new ChatOptions
{
    ModelId = "claude-haiku-4-5",
    MaxOutputTokens = 1024,
    Tools = [new HostedMcpServerTool("microsoft_learn", "https://learn.microsoft.com/api/mcp")]
};

var response = await client.GetResponseAsync(
    [new ChatMessage(ChatRole.User, "What tools are available?")], options);

Actual

Anthropic.Exceptions.AnthropicBadRequestException: Status Code: BadRequest
{"type":"error","error":{"type":"invalid_request_error","message":"MCP server 'mcp' is defined but not referenced by any `mcp_toolset` in `tools`."}}

Note the server is reported as 'mcp' even though it was configured as microsoft_learn.

Cause

Three issues in the HostedMcpServerTool branch of the beta tool mapping
(AnthropicBetaClientExtensions):

1. Wrong name property. The server definition is built with Name = mcp.Name — but
   HostedMcpServerTool.Name (the AITool.Name override) is hardcoded to the literal "mcp"
   in Microsoft.Extensions.AI 10.9; the configured server name is ServerName, which the
   mapping never reads. Every server serializes as "name": "mcp", so multiple servers also
   collide.
2. No mcp_toolset is emitted. The mapping adds the server to mcp_servers and opts into
   the mcp-client-2025-11-20 beta, but never adds a BetaMcpToolset to tools. That API
   shape requires every mcp_servers entry to be referenced by exactly one MCPToolset
   (docs — validation rules),
   so the request is rejected unconditionally.
3. Deprecated allowlist shape. AllowedTools is mapped to tool_configuration on the
   server definition, which is deprecated under mcp-client-2025-11-20 — the allowlist now
   belongs in the toolset's default_config/configs
   (migration guide).

The unit tests pin the broken output rather than catching it: GetResponseAsync_WithHostedMcpServerTool
asserts (against a verbatim mock) that the request contains "name": "mcp" with no toolset, and
GetResponseAsync_WithMultipleHostedMcpServerTools asserts both servers serialize as "name": "mcp" —
so CI passes while every live call fails.

Expected

case HostedMcpServerTool mcp:
    (betaHeaders ??= []).Add("mcp-client-2025-11-20");

    // The wire-level server name must be the configured ServerName — AITool.Name is the
    // fixed tool-type identifier "mcp" and would collide across multiple servers.
    (mcpServers ??= []).Add(new() { Name = mcp.ServerName, Url = mcp.ServerAddress });

    // mcp-client-2025-11-20 requires every mcp_servers entry to be referenced by exactly
    // one mcp_toolset in `tools`; tool_configuration on the server definition is deprecated.
    // AllowedTools maps to the documented allowlist pattern: disabled by default, listed
    // tools explicitly enabled.
    (createdTools ??= []).Add(
        mcp.AllowedTools is { Count: > 0 } allowedTools
            ? new BetaMcpToolset(mcp.ServerName)
            {
                DefaultConfig = new() { Enabled = false },
                Configs = allowedTools.ToDictionary(
                    static tool => tool,
                    static _ => new BetaMcpToolConfig { Enabled = true }),
            }
            : new BetaMcpToolset(mcp.ServerName)
    );
    break;

This makes HostedMcpServerTool usable end-to-end through AsIChatClient, matching the
AsAITool docs' guidance that the portable MEAI tool types are preferred over raw tool unions.

One migration note: callers currently working around this by passing HostedMcpServerTool
plus a hand-wrapped BetaMcpToolset (via AsAITool) for the same server would then send two
toolsets referencing one server, which the API also rejects — the fix may want to skip emitting
a toolset when one already exists for that ServerName, or call this out as a breaking note.

The test expectations in AnthropicClientBetaExtensionsTests need updating alongside, e.g.:

     "mcp_servers": [{
-        "name": "mcp",
+        "name": "my-mcp-server",
         "type": "url",
         "url": "https://mcp.example.com/server"
-    }]
+    }],
+    "tools": [{
+        "type": "mcp_toolset",
+        "mcp_server_name": "my-mcp-server"
+    }]

[z0rimo]

I'd like to take a look at this. I've confirmed the issue is still present on the current next branch, and I'm planning to work on a focused fix for the HostedMcpServerTool mapping along with the existing adapter tests.

I'll open a PR referencing this issue once it's ready.