Teams error 413 MessageSizeTooBig: MessageSizeTooBig - payload over the size cap
Last verified against Microsoft Learn - Status codes from bot conversational APIs
Message size too large.What error 413 MessageSizeTooBig means
Your activity exceeded the platform's payload cap. The status table documents 413 as code MessageSizeTooBig, "The size of the incoming request was too large," retry No, action "Reduce the payload size." The formatting guide pins the numbers: "The agent message size limit is 100 KB," measured approximately - "it includes the message itself (text, image links, etc.), @-mentions, and reactions encoded as UTF-16" but "doesn't include base64 encoded image" - and "it's recommended to ensure that the size of the message itself is within 80 KB to guarantee successful message delivery." On overflow "the agent receives a 413 status code (RequestEntityTooLarge), which contains the error code MessageSizeTooBig."
The usual offenders are machine-generated Adaptive Cards: a table rendered as hundreds of TextBlocks, a choice set with thousands of options, or raw data pasted into card JSON. Note the UTF-16 accounting - non-ASCII text costs more than its byte length in UTF-8 suggests - and remember the limit applies to the serialized activity as a whole, not just the visible text.
What it looks like
HTTP/1.1 413 Request Entity Too Large
{
"error": {
"code": "MessageSizeTooBig",
"message": "Message size too large."
}
}Why it happens
- An Adaptive Card generated from unbounded data (long tables, huge choice sets, embedded raw JSON).
- Message text assembled from logs or documents without truncation.
- Large entity arrays (mentions of many users) inflating the serialized activity.
- Payload sized against UTF-8 byte length while the platform counts UTF-16.
How to fix Teams error 413 MessageSizeTooBig
- 1Measure the serialized activity size before sending; enforce the documented 80 KB working ceiling in code.
- 2Paginate: split long content into multiple messages or a card with a 'show more' flow.
- 3Move bulk content out of the message - host it and link it, or render a summary card.
- 4For dynamic cards, cap list lengths and truncate text fields at generation time with an explicit ellipsis.
How to stop it recurring
Put a size guard in the one code path every outbound message flows through, and log near-misses (say, over 60 KB) so growth is visible before it becomes failure. Design cards for summaries with links to detail rather than full data dumps. All Teams size ceilings, including this one and the incoming-webhook 28 KB cap, are tabulated in message size limits.
Official reference: Microsoft Learn - Status codes from bot conversational APIs. See all Teams error codes or the Teams limits and quotas.
Related codes
Error 413 MessageSizeTooBig - quick answers
What does Teams error 413 MessageSizeTooBig mean?
Your activity exceeded the platform's payload cap. The status table documents 413 as code MessageSizeTooBig , "The size of the incoming request was too large," retry No , action "Reduce the payload size." The formatting guide pins the numbers: "The agent message size limit is 100 KB," measured approximately - "it includes the message itself (text, image links, etc.), @-mentions, and reactions encoded as UTF-16" but "
How do I fix Teams error 413 MessageSizeTooBig?
1. Measure the serialized activity size before sending; enforce the documented 80 KB working ceiling in code. 2. Paginate: split long content into multiple messages or a card with a 'show more' flow. 3. Move bulk content out of the message - host it and link it, or render a summary card. 4. For dynamic cards, cap list lengths and truncate text fields at generation time with an explicit ellipsis.
Should I retry after error 413 MessageSizeTooBig?
No. Retrying the same request produces the same error; the condition has to be fixed first. Treat it as a permanent failure for that message and surface it, rather than looping.
Stop debugging Teams by hand
Connect the channel through Conferbot: tokens, webhooks and retries are handled, failures show as readable status.