RocketChat/Rocket.Chat · error · Error

MCP tool response exceeds the 5 MiB limit

Error message

MCP tool response exceeds the 5 MiB limit

What it means

Fast-fail guard in readResponseText: before reading the body of the loopback REST response for an MCP tool call, it parses the content-length header and throws when the declared size already exceeds MAX_TOOL_RESPONSE_BYTES (5 MiB). The oversized single-tool response is rejected without transferring its body.

Source

Thrown at apps/meteor/ee/server/api/mcp/dispatch.ts:39

export const createMcpResponseBudget = (maxBytes = MAX_TOOL_RESPONSE_BYTES): McpResponseBudget => {
	let remainingBytes = maxBytes;
	const formattedLimit = formatByteLimit(maxBytes);

	return {
		consume(bytes) {
			if (bytes > remainingBytes) {
				throw new Error(`MCP batch response exceeds the ${formattedLimit} limit`);
			}

			remainingBytes -= bytes;
		},
	};
};

const readResponseText = async (response: Response, responseBudget?: McpResponseBudget): Promise<string> => {
	const contentLength = Number(response.headers.get('content-length'));
	if (Number.isFinite(contentLength) && contentLength > MAX_TOOL_RESPONSE_BYTES) {
		throw new Error('MCP tool response exceeds the 5 MiB limit');
	}

	if (!response.body) {
		return '';
	}

	const reader = response.body.getReader();
	const decoder = new TextDecoder();
	const chunks: string[] = [];
	let receivedBytes = 0;

	while (true) {
		const { done, value } = await reader.read();
		if (done) {
			break;
		}

		receivedBytes += value.byteLength;

View on GitHub (pinned to b2c16d5842)

Solutions

1. Call the tool with stricter pagination args (count/offset) so the response stays under 5 MiB
2. Use a narrower filter or a tool that returns summaries instead of full objects
3. Return a reference (file URL, export job id) instead of inline bulk data

Example fix

// before
const result = await callTool('channels_list', {}); // full dataset, content-length > 5 MiB

// after
const result = await callTool('channels_list', { count: 100, offset: 0 });

const response = await fetch(url);
const contentLength = Number(response.headers.get('content-length'));
if (Number.isFinite(contentLength) && contentLength > 5 * 1024 * 1024) {
  throw new Error('tool response would exceed 5 MiB - narrow the query (count/offset)');
}
const text = await response.text();

try {
  return await callTool(name, args);
} catch (error) {
  if (error instanceof Error && error.message.includes('MCP tool response exceeds the 5 MiB limit')) {
    return callTool(name, { ...args, count: Math.min(args.count ?? 100, 100) });
  }
  throw error;
}

When it happens

Trigger: dispatchTool fetches the REST endpoint behind an MCP tool and that endpoint declares content-length greater than 5,242,880 bytes (e.g. a list endpoint returning thousands of records because no count was passed).

Common situations: Tools mapped to export/list endpoints that ignore pagination params; test workspaces seeded with large datasets queried without a count; tool args not forwarded to the REST query.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

• MCP batch response exceeds the ${formattedLimit} limit
• FileUpload_Error
• Integration payload must be a JSON object, not an array or p
• could-not-access-webdav
• Failed to complete OAuth handshake with ${this.name} at ${th

AI-assisted analysis of RocketChat/Rocket.Chat@b2c16d5842 (2026-08-18). Data as JSON: /api/errors/448ced457321469c. Report an issue: GitHub.