把每個 MCP 伺服器都當作陌生方：.NET 10 聊天主機的 7 項客戶端規則

Connecting a .NET chat app to an MCP server is now a short job. McpClient.CreateAsync, ListToolsAsync(), put the tools in ChatOptions.Tools, add UseFunctionInvocation(), and the model starts calling tools. It's a great demo.

What the demo skips is that every one of those tools belongs to someone else. A server can offer tools you never meant to expose, reuse a name another server already uses, take 30 seconds to answer, return 14 KB of text, or return a page that tells your model to ignore its instructions. And some days the server just isn't there.

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These are the seven rules I now follow on the client side, the chat host that consumes MCP servers. They come from a host I built on the official MCP C# SDK (ModelContextProtocol 2.2.0) and Microsoft.Extensions.AI 10.11 in .NET 10, with 70 offline tests (43 unit, 27 integration). The code below is the shape of each rule, not the full implementation; the omitted parts are marked.

Rule 1: Don't hand your environment to a child process

A stdio server is a process your app launches. By default, StdioClientTransport passes it your host's entire environment, including whatever API keys live there. The server you installed from a package registry five minutes ago doesn't need your model key.

new StdioClientTransport(new StdioClientTransportOptions
{
    Name = server.Name,
    Command = server.Command!,
    InheritEnvironmentVariables = false,
    EnvironmentVariables = StdioClientTransportOptions.GetDefaultEnvironmentVariables(),
    // Arguments and logger omitted here
}, loggerFactory);

GetDefaultEnvironmentVariables() is the SDK's small curated set (things like PATH and HOME). Start there and add only what a given server really needs. For remote servers I also pin the HTTP transport to Streamable HTTP instead of letting it auto-detect and possibly fall back to legacy SSE.

Rule 2: Nothing is exposed unless you named it

Never pass a server's tool list straight to the model. Each server in config gets an exact, case-sensitive allowlist, and an empty list exposes nothing. Fail closed.

Don't build this on tool annotations either. The MCP spec says clients must treat annotations as untrusted unless they come from trusted servers, so readOnlyHint: true is a claim, not a guarantee.

var allowed = new HashSet<string>(server.AllowedTools, StringComparer.Ordinal);
// for each discovered tool: Exposed = allowed.Contains(name),
// otherwise hidden with reason "not in allowlist" (full policy omitted here)

Hidden tools still show up in a status endpoint, so you can see what a server offered and you said no to.

Rule 3: Rename tools for the model, not for the server

Two servers can both have a tool called search, and MCP names can be up to 128 characters with dots, while common model providers want function names of letters, digits, _ and -, at most 64 characters.

So the model sees {prefix}_{tool}, sanitized and cut to 64: weather_search and docs_search instead of two search tools. Prefixes are validated as unique at startup, and a final pass hides any collision that sanitizing still produces.

The renaming lives in a wrapper built on DelegatingAIFunction, which is also where rules 4 to 6 happen:

public sealed class GuardedMcpTool : DelegatingAIFunction
{
    public override string Name => _exposedName;   // e.g. "docs_search"
    public override string Description => $"[{_server} server] {_tool.Description}";

    protected override async ValueTask<object?> InvokeCoreAsync(
        AIFunctionArguments args, CancellationToken ct)
    {
        // timeout, raw CallAsync, truncate, fence, audit
        // (full implementation in the complete project)
    }
}

Rule 4: Put a clock on every call, and don't trust it to stop the server

Every tool call gets its own timeout (10 seconds by default in my host), linked to the request's token. Two details matter:

• Race the call against the session. If the server process dies mid-call, the pending request can just hang until the timeout. Racing the call against McpClient.Completion turns that into an immediate "server unavailable".
• Your timeout frees the chat turn, not the server. In my tests, cancelling the client call did not stop the server-side work: the cancellation notification never left the client. When I built this it was a known SDK issue (csharp-sdk #1365). Servers need their own timeouts.

A timeout comes back to the model as a fenced error ("timed out, don't retry in this turn"), not as an exception that kills the request. A cancellation from the caller is different: that one is rethrown.

Rule 5: Cap what comes back, and say so

Tool output goes into your prompt, so it's a cost and a risk. The formatter pulls text out of the content blocks, swaps images and audio for a placeholder, falls back to structured JSON when there's no text, and cuts the result to a character budget (4,000 by default). The cut is announced in-band, like ...[truncated: showing 600 of 14377 chars], so the model knows it's looking at partial output.

Characters aren't tokens. It's a cheap, predictable ceiling, which is what you want here.

Rule 6: Fence results as data, and rely on the allowlist

Every result goes back wrapped in a <tool_result ... trust="untrusted"> fence, and any <tool_result or </tool_result inside the payload is neutralized so a server can't close the fence early. The system prompt says what's inside the fence is data, never instructions. A small regex tripwire flags phrases like "ignore previous instructions" or "call the X tool" and logs a warning.

Be honest about what that buys you. Fencing and a regex don't stop prompt injection; they make it less likely and much more visible. The real protection is rule 2: in my sample, the poisoned result asks the model to call a delete tool that was never exposed, so there's nothing to trigger. Destructive tools you really need belong behind human approval.

While you're there, audit every call: server, real and exposed name, outcome, duration, result size, truncation and injection flags, and the argument names. Not the values; they carry customer data.

Rule 7: A server that's down is degraded service, not an outage

All enabled servers connect in parallel, each with its own connect timeout (5 seconds by default). A failure doesn't throw. The server is marked unavailable with a short reason and isn't retried until a cooldown passes (30 seconds by default), so one dead server doesn't add a connect timeout to every message.

The chat continues with whatever tools are left, and the system prompt names the missing servers so the model says so instead of inventing an answer. The chat endpoint still returns 200, with the missing servers listed in unavailableServers.

How to test all of this without a network

You don't need API keys to test a guarded MCP host, and you can still put real MCP on the wire:

1. In-memory servers over System.IO.Pipelines, with StreamServerTransport on one end and StreamClientTransport on the other.
2. A real child process: a tiny stdio server project the tests launch through StdioClientTransport.
3. Real Streamable HTTP against an ASP.NET Core MCP server running on TestServer.
4. A fake model that picks a tool by keyword and summarizes the fenced result without ever acting on what's inside it.

Make the sample servers awkward on purpose: duplicate tool names, a tool that must never be exposed, a slow tool, a huge result and a poisoned one. Then test three kinds of "server down" (unknown, missing command, refused port) and a crash in the middle of a session.

The checklist

• [ ] Do stdio servers get a curated environment instead of yours?
• [ ] Does every server have an exact allowlist, with empty meaning nothing?
• [ ] Are annotations treated as claims, not permissions?
• [ ] Are exposed names prefixed, sanitized, 64 characters max and collision-free?
• [ ] Does every call have a timeout, and do the servers have their own?
• [ ] Is output capped, with the truncation visible to the model?
• [ ] Are results fenced as untrusted data?
• [ ] Do you audit argument names but not values?
• [ ] Does the chat keep answering when a server is down?

What I didn't verify

All 70 tests run offline against a fake model, so I haven't measured how a real model behaves with the fenced results. Plugging one in means registering its IChatClient before the MCP services, without its own UseFunctionInvocation().

Keep building with Tech Skill Builder

The complete, working .NET 10 solution behind this post is in Tech Skill Builder: the guarded MCP host (Core, minimal API and CLI), the sample servers and the stdio companion, all 70 offline tests, and a step-by-step PDF guide. No API key needed to run it.

Tech Skill Builder is $5/month for a very limited time (normally $10) — grab it now before it goes back to $10. You get 20+ complete, tested .NET + AI projects with full source code and step-by-step PDF guides, and a new one is added every day. 👉 Join Tech Skill Builder

Which MCP server would you be most nervous to give your chat app, and what would you lock down first?