An MCP server can reject a request before the SDK sees it. In the Express setup described by Imran Siddique for SDK 1.30.1, the JSON parser may refuse an oversized body before the transport applies its own 4 MiB request-body cap. Separately, the SDK limits a JSON-RPC batch to 100 messages. These controls address different risks; changing the SDK body-size setting will not override an upstream parser rejection.
What the two limits control
The SDK’s request-body limit controls bytes when the SDK itself reads the incoming request stream. The batch limit controls how many JSON-RPC messages are in a batch. A small body can contain many short messages, and a large body can contain fewer messages, so neither setting substitutes for the other.
| Control | What it limits | When it applies |
|---|---|---|
| Request-body size | Bytes in the HTTP request body; the SDK changelog gives a 4 MiB default for SDK-owned reads. | When the SDK reads the request stream. A caller-provided parsed body skips this SDK read limit, according to the changelog. |
| JSON-RPC batch size | Number of messages in a batch; the changelog gives a limit of 100. | During batch validation, including when the caller supplies an already parsed body. |
The 1.30.1 defaults and reported behavior are described in Siddique’s article. The official SDK changelog confirms the distinction between the bounded SDK read and batch validation. Because that changelog is on the current main branch and includes later changes, it is not, by itself, proof of every detail in the published 1.30.1 package.
Why Express can return 413 before the SDK
Express applications commonly parse JSON in middleware before routing the parsed body to a transport. If that parser rejects the request, the SDK never gets the chance to read the stream or apply its own body-size setting. Raising the SDK limit alone therefore cannot fix a request rejected upstream.
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Siddique reports this interaction for the documented 1.x Express path and describes different response handling and observability depending on which layer rejects the request. Those response details reflect the author’s stated setup, not an independently reproduced test here. Treat them as version- and configuration-specific, not as a rule for every MCP server.
The current official Express adapter source exposes a jsonLimit option passed to express.json({ limit }) and documents Express’s built-in default as 100kb. See the current adapter source. Adapter options have evolved, so check the source or documentation for the exact version you have installed rather than assuming current main-branch behavior applies to an older package.
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Find the control that is rejecting your request
- Identify the installed SDK generation and version. Determine whether your application uses the 1.x monolithic SDK, a v2 split package, or a custom Express integration. The relevant adapter options can differ by version.
- Trace the request path. Check whether
express.json()or another middleware parses the body before the MCP transport receives it. If the transport receives a parsed object, the SDK’s bounded stream read is not the control limiting those bytes. - Inspect the first rejection point. Note the HTTP status, response content type and body, and which middleware or transport logs the failure. A parser-level rejection and an SDK-level rejection may have different handlers and monitoring hooks.
- Configure both relevant limits. Set the Express parser limit using the option supported by your installed adapter or middleware. Set the SDK body limit separately for requests whose stream the SDK reads. Choose compatible byte limits based on the requests your service needs to accept.
- Test the controls independently. Send one body that exceeds the intended byte limit and one batch that exceeds the message-count limit. Check which component rejects each request and confirm the response and logs are visible at the layer you expect.
How to interpret an unexpected 413
If a request below the value you configured for the SDK still receives HTTP 413, that does not establish that the SDK ignored its setting. An upstream parser may have a lower limit and may reject the body before the transport handles it. Check the configured parser limit, the adapter version, middleware order, and the handler that produced the response.
If the body is accepted but a batch is rejected, check the number of JSON-RPC messages rather than increasing the byte limit. The SDK changelog describes batch validation as a separate control that remains applicable to caller-provided parsed bodies.
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