mid-conversation-system-messages changedbuild-with-claude/mid-conversation-system-messages
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476476With 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.
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478Wrap 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:
478Wrap 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, the computer use and browser use tools can’t be defined in a message 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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480480```json
481481{
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