mid-conversation-system-messages changedbuild-with-claude/mid-conversation-system-messages
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7474 ```
7575
7676 ```bash CLI
77 ant beta:messages create --beta inline-tools-2026-09-15 \
78 --transform 'content.#(type=="text").text' --raw-output <<'YAML'
77 ant beta:messages create --beta inline-tools-2026-09-15 <<'YAML'
7978 model: claude-opus-5-5
8079 max_tokens: 1024
8180 tools:
from line 454
455454
456455### Define tools in a message (beta)
457456
458With the `inline-tools-2026-09-15` beta header, a `tool_addition` block can define a tool by value, carrying its full definition, instead of naming it by reference. This lets you introduce a tool that is unknown at the start of the conversation, or whose schema changes later, by appending a `role: "system"` message. The `tools` array and every earlier message stay exactly as sent, so the prompt cache still hits and only the appended message is processed as new input. The one exception, a `tools` array with no non-deferred tool, is covered in the rules below. The header also covers adding and removing tools by reference, so you don't need to send `mid-conversation-tool-changes-2026-07-01` as well.
457With the `inline-tools-2026-09-15` beta header, a `tool_addition` block can define a tool by value, carrying its full definition, instead of naming it by reference. This lets you introduce a tool that is unknown at the start of the conversation, or whose schema changes later, by appending a `role: "system"` message. The `tools` array and every earlier message stay exactly as sent, so the prompt cache still hits and only the appended message is processed as new input. The one exception, a `tools` array with no non-deferred tool, is covered in the rules that follow. The header also covers adding and removing tools by reference, so you don't need to send `mid-conversation-tool-changes-2026-07-01` as well.
459458
460459Wrap the definition in a `tool` object of type `tool_definition`. The `definition` is a `tools` entry, such as a custom tool or an Anthropic-defined client or server tool, with its usual configuration, including `cache_control` and `defer_loading`. During the beta, some tool types (the computer use tool among them) can't be defined in a message yet and return a 400 error that says so; declare those in `tools` and add them by reference. For example, to define a custom tool mid-conversation:
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16771676 ```
16781677
16791678 ```bash CLI
1680 ant messages create --transform 'content.#(type=="text").text' --raw-output <<'YAML'
1679 ant messages create <<'YAML'
16811680 model: claude-opus-5-5
16821681 max_tokens: 1024
16831682 cache_control:
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21392138 ```
21402139
21412140 ```bash CLI
2142 ant beta:messages create --beta mid-conversation-system-clear-at-2026-08-21 \
2143 --transform 'content.#(type=="text").text' --raw-output <<'YAML'
2141 ant beta:messages create --beta mid-conversation-system-clear-at-2026-08-21 <<'YAML'
21442142 model: claude-fable-5-1
21452143 max_tokens: 4096
21462144 messages:
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