Create a Message changedapi/beta/messages/create
Nearest release: v2.1.322, published 2 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+27added
Lines−3removed
From line
131
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First seen
14 Aug 2026
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Recorded edits34to this page, all time
The whole hunk
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from line 131
131131
132132 - `"webhooks-2026-10-07"`
133133
134 - `"admin-user-sign-out-2026-10-08"`
135
134136 - `"message-threads-2026-08-12"`
135137
136138 - `"analytics-usage-cost-2026-08-24"`
from line 3128
31263128
31273129 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.
31283130
3131 - `"claude-fable-5-2"`
3132
3133 Built for the most ambitious work in coding, enterprise workflows, and the sciences
3134
3135 - `"claude-mythos-5-2"`
3136
3137 Built for the most demanding cybersecurity and life sciences research, available through verification programs
3138
31293139 - `"claude-haiku-5-5"`
31303140
31313141 Fastest model for high-volume, real-time tasks
from line 3486
34763486 Accepted in `messages[].content` and not rendered into the prompt; not
34773487 validated against the request's `fallbacks` chain or top-level `model`.
34783488
3479 Echo the assistant turn back verbatim, including this block in its
3489 When you send the assistant turn back, keep this block in its
34803490 original position. The block marks the boundary between content produced
34813491 before and after a fallback hop, and the server relies on that boundary
34823492 to validate the turn: when thinking runs flank the boundary, omitting
from line 3493
34833493 the block merges them into one span the server cannot validate (the
34843494 request is rejected), and moving it into the middle of a single run is
34853495 likewise rejected; between non-thinking blocks the block's placement has
3486 no validation effect.
3496 no effect on thinking validation.
34873497
3498 Some blocks from before the turn's final `fallback` block must be left
3499 out when you send the turn back, and no `tool_result` is sent for a
3500 `tool_use` block you left out.
3501
3502 See the [refusals and fallback guide](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback?sdk_languages=http#continuing-the-conversation) to learn which blocks to keep.
3503
34883504 - `type: "fallback"`
34893505
34903506 - `from: BetaFallbackInfoParam`
from line 7016
70007016
70017017 Citations configuration for fetched documents. Citations are disabled by default.
70027018
7003 - `defer_loading: optional boolean`
70047019
7005 If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.
7006
7007 - `max_content_tokens: optional number or null`
7008
7009 Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.
7010
7011 minimum: 1
7012
7013 - `max_uses: optional number or null`
7014
7015 Maximum number of times the tool can be used in the API request.
7016
7017 minimum: 1
7018
7019 - `response_inclusion: optional "full" or "excluded"`
7020
7021 How this tool's result blocks appear in the API respo
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