different-ai/openwork · error · RemoteMcpAppError

app_too_large

app_too_large

Error message

Remote MCP Apps must be ${REMOTE_MCP_APP_MAX_BYTES / 1024} KiB or smaller.

What it means

This error is thrown by boundedResponseText in ee/apps/den-api/src/remote-mcp-apps.ts when downloading a Remote MCP App whose HTML exceeds REMOTE_MCP_APP_MAX_BYTES. The library checks the declared content-length header before streaming; if it is present, finite, and larger than the cap, it cancels the body and throws a 413 app_too_large error to avoid buffering oversized payloads.

Source

Thrown at ee/apps/den-api/src/remote-mcp-apps.ts:245

  const mimeType = rawMimeType?.trim().toLowerCase()
  if (mimeType !== "text/html" && mimeType !== "application/xhtml+xml") {
    throw new RemoteMcpAppError(422, "invalid_content_type", "Remote MCP Apps must be served as text/html or application/xhtml+xml.")
  }
  for (const parameter of parameters) {
    const match = parameter.trim().match(/^charset\s*=\s*(?:"([^"]+)"|'([^']+)'|([^\s]+))$/i)
    const charset = (match?.[1] ?? match?.[2] ?? match?.[3])?.toLowerCase()
    if (charset && charset !== "utf-8" && charset !== "utf8") {
      throw new RemoteMcpAppError(422, "invalid_encoding", "Remote MCP Apps must be UTF-8 HTML.")
    }
  }
  return mimeType
}

async function boundedResponseText(response: Response) {
  const declaredLength = Number(response.headers.get("content-length"))
  if (Number.isFinite(declaredLength) && declaredLength > REMOTE_MCP_APP_MAX_BYTES) {
    await response.body?.cancel()
    throw new RemoteMcpAppError(413, "app_too_large", `Remote MCP Apps must be ${REMOTE_MCP_APP_MAX_BYTES / 1024} KiB or smaller.`)
  }
  if (!response.body) return ""
  const reader = response.body.getReader()
  const chunks: Uint8Array[] = []
  let total = 0
  try {
    while (true) {
      const { done, value } = await reader.read()
      if (done) break
      total += value.byteLength
      if (total > REMOTE_MCP_APP_MAX_BYTES) {
        await reader.cancel()
        throw new RemoteMcpAppError(413, "app_too_large", `Remote MCP Apps must be ${REMOTE_MCP_APP_MAX_BYTES / 1024} KiB or smaller.`)
      }
      chunks.push(value)
    }
  } finally {
    reader.releaseLock()

View on GitHub (pinned to 2b7df46e8a)

Solutions

Reduce the app HTML size below REMOTE_MCP_APP_MAX_BYTES (minify, remove inlined assets, split resources).
Host large assets externally and reference them by URL instead of inlining.
Verify the content-length header is accurate; fix misconfigured proxies/CDNs that inflate it.
If the limit must change, adjust REMOTE_MCP_APP_MAX_BYTES and redeploy the den-api service.

Defensive patterns

Strategy: validation

Validate before calling

const head = await fetch(sourceUrl, { method: "HEAD" });
const len = Number(head.headers.get("content-length"));
const MAX = REMOTE_MCP_APP_MAX_BYTES;
if (Number.isFinite(len) && len > MAX) throw new Error(`App HTML is ${len} bytes; limit is ${MAX}.`);

Try / catch

try {
  await importRemoteMcpApp({ sourceUrl, ... });
} catch (e) {
  if (e instanceof RemoteMcpAppError && e.code === "app_too_large") {
    // shrink the HTML bundle or externalize assets, then retry
  } else throw e;
}

Prevention

Keep single-file HTML apps well under the size limit; minify JS/CSS.
Externalize large assets (images, fonts, libraries) to URLs.
Set correct content-length headers on the host so failures surface early.
CI-check the built app HTML size before registering it as a Remote MCP App.

When it happens

Trigger: Calling fetchRemoteMcpApp (via the html caller) with a sourceUrl whose response declares a content-length header greater than REMOTE_MCP_APP_MAX_BYTES. Any download of a remote MCP app HTML document larger than the configured KiB limit.

Common situations: Publishing a very large single-file HTML app (inlined JS/CSS/assets) to a static host and registering it as a Remote MCP App source URL; a host that inflates content-length; limit lowered in config while existing apps still exceed it.

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

AI-assisted analysis of different-ai/openwork@2b7df46e8a (2026-09-01). Data as JSON: /api/errors/bcfb06c6a11cb980. Report an issue: GitHub.