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A request that looks short by character count can still exceed a size limit because HTTP bodies are measured in bytes, not abstract characters. But the title alone does not identify the API or explain its particular 21 KB and 32 KB figures. The useful first step is to measure the exact serialized bytes being sent, then find which component rejected them and check that component’s limit.
Why can a short-looking request exceed a size limit?
Character count and transmitted body size are different measurements. A programming language’s string-length function may count code units, Unicode code points, or another representation-specific unit. HTTP framing, by contrast, deals in octets—8-bit bytes. RFC 9112, the IETF HTTP/1.1 specification, says a Content-Length header can provide the anticipated size of octets for the content: RFC 9112, Section 6.
With UTF-8, characters can require different numbers of bytes. Serialization can add more: JSON includes quotes, braces, commas, and sometimes escape sequences; form URL encoding may replace characters with percent-encoded sequences. Consequently, the string length you see before serialization may not match the body sent over the network.
Measure the representation that is actually sent
Do not estimate from the text displayed in an editor or log. Measure the exact outgoing body after serialization and encoding. The request’s content type, charset, escaping rules, and any compression can affect what is transmitted or what a receiving component measures. OpenAPI documents how form encoding can represent characters as percent-encoded sequences: OpenAPI Specification. Salesforce describes UTF-8 as the default request-body encoding in its External Services schema documentation: Salesforce External Services schema. The e-Gov API documentation also specifies UTF-8 for its messages: e-Gov API. These are examples, not a guarantee that every service uses the same encoding or representation.
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Do 21 KB and 32 KB identify the API’s limit?
No. Without the API or service name, operation, request format, and error response, those figures cannot be attributed to a particular product. Limits differ by service and by the layer enforcing them. A gateway, firewall, application, or messaging service may each measure a different object and apply a separate threshold.
Examples of limits that are not interchangeable
| Service and measured object | Documented size | What the figure means |
|---|---|---|
| Amazon API Gateway WebSocket messages and frames | 128 KB maximum message payload; 32 KB maximum frame size | A message larger than 32 KB must be split into frames no larger than 32 KB. Otherwise, the connection closes with code 1009. This is a WebSocket message/frame rule, not evidence about the unnamed API in the title. Amazon API Gateway quotas |
| Twilio Programmable Chat message body | 32 KiB | Twilio error 50504 describes an over-limit body. Its guidance is to reduce the serialized body or validate message length before creating or updating it. Twilio error 50504 |
| AWS WAF request-body inspection | 16 KB (16,384 bytes) default for specified protected resource types; 8 KB (8,192 bytes) fixed for Application Load Balancer and AWS AppSync | These are WAF inspection-size limits, not universal HTTP request-body limits. A WAF inspection limit does not by itself establish the maximum body accepted by the application behind it. AWS WAF AssociationConfig API reference |
These examples show why the number alone is insufficient: one limit concerns WebSocket frames, another a chat message body, and another the portion of a request body inspected by a firewall. The 21 KB figure in the title is not established by these examples.
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How to diagnose the failure
- Capture the outgoing request. Inspect the body after the client has serialized it. Record the content type and charset, and determine whether the body is compressed or encoded before transmission.
- Measure bytes, not just characters. Count the bytes of the exact body representation sent. If you can capture the request at the network or client-library boundary, measure that representation rather than the source string.
- Identify the rejecting component. Use the error response, status code, logs, and request path to determine whether the client, gateway, WAF, application, or messaging service rejected the request. A connection close, for example, is different from an application-level validation error.
- Check the current limit for that component and operation. Confirm whether its threshold applies to an HTTP request body, a serialized message, a WebSocket message or frame, or an inspected portion of the body. Do not assume a limit shown for one product or layer applies to another.
- Reduce or split the payload only under the service’s rules. If the service supports splitting, follow its required frame or chunk sizes. Otherwise, reduce the serialized payload or remove unnecessary fields. Changing a character-count check alone will not fix a byte-limit rejection.
What information would establish the specific cause?
To connect the reported 21 KB limit and 32 KB failure to a real service, you need the API or service name, the operation, the request content type and charset, the measured outgoing body size, and the full error response or code. Without those details, confusing bytes with characters is a plausible general cause of a size mismatch—not a verified explanation for this particular incident.
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