Keep a response from flooding the agent

A list endpoint with no pagination is harmless to your backend and fatal to an agent. One GET /orders that returns 40 000 rows fills the agent's context, and the agent cannot undo that. sezzlee puts a budget and a deadline on every invocation, with defaults that work.

When the response is too large

The response is refused, not truncated. No prefix, no preview:

{
  "error": "response_too_large",
  "message": "The response is 1843200 bytes; the limit is 262144 bytes. It is refused, not truncated: no part of the body was returned. The body is an array of 8412 items. Narrow it and call again: limit, status.",
  "retryable": false,
  "payload": {
    "bytes": 1843200,
    "limit": 262144,
    "shape": { "kind": "array", "count": 8412 }
  },
  "fields": [
    { "name": "limit", "message": "Maximum number of rows to return." },
    { "name": "status", "message": "Filters by order status." }
  ]
}

shape tells the agent how big the answer was, often enough to answer "how many refunds are there?" without calling again. fields names your endpoint's narrowing arguments, read from its published input schema, so the retry is a specific one.

When the backend is slow

{
  "error": "invoke_timeout",
  "message": "The backend did not answer within 30000 ms and the call was abandoned. The operation may already have been applied; re-read before retrying.",
  "retryable": true
}

A timeout frees the agent. It does not cancel your handler. sezzlee delivers a cancellation signal, but neither runtime preempts running code. A handler that ignores its signal runs to completion and its result is discarded, and a write it already committed stays committed. To make a timeout stop work, the handler has to cooperate:

[HttpGet("/reports/heavy")]
public async Task<IActionResult> Heavy(CancellationToken cancellationToken) =>
    Ok(await _db.Reports.ToListAsync(cancellationToken));

On ASP.NET this is usually free, because CancellationToken is already threaded through EF Core and HttpClient. On Node it usually is not, because AbortSignal is not conventional in Nest handler signatures.

Change the numbers

builder.Services.AddSezzlee(options =>
{
    options.Invoke.MaxResponseBytes = 512 * 1024;
    options.Invoke.Timeout = TimeSpan.FromSeconds(10);
});

The defaults are 262 144 bytes and 30 000 ms in both SDKs. Do not raise the timeout above one minute. A stock MCP client cancels at 60 seconds first, so a larger number is unreachable.

A report endpoint that legitimately returns megabytes should not raise the budget for everything else. Override it per endpoint with a delegate. Returning null or undefined falls back to the global value:

options.Invoke.MaxResponseBytesFor = target =>
    target.Route.StartsWith("/reports/", StringComparison.Ordinal) ? 4 * 1024 * 1024 : null;

Every meta-tool answer passes the budget, not just invoke_tool. A DTO deep enough has an input schema large enough to flood the agent on its own, and load_tool is guarded too. If you hit that, the fix is the schema depth budget, not a larger response budget. The normative rules are in invoke-semantics.md.