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mid-conversation-system-messages changedbuild-with-claude/mid-conversation-system-messages

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2121 supportedPlatforms:
2222 Claude API: stable
2323 Claude Platform on AWS: stable
24 Claude Models on Google Cloud (Anthropic managed): stable
2425 Amazon Bedrock: stable
2526 Claude Models on Google Cloud (Google managed): stable
2627---
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3536Mid-conversation system messages close that gap. You append a `{"role": "system"}` message at the point in the conversation where the new instruction becomes relevant, instead of editing the top-level `system` field. The cached prefix stays the same, so the next request still reads it from cache, and the new instruction is still applied as a system instruction rather than as ordinary user text.
3637 
3738<Note>
38 Mid-conversation system messages are available on the Claude API, [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), and [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed).
39 Mid-conversation system messages are available on Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, Claude Opus 4.8, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. No beta header is required for mid-conversation system messages. This feature is not available on Claude Sonnet 5. Use the top-level `system` field there instead.
3940 
40 This feature is available on Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, Claude Opus 4.8, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. No beta header is required for mid-conversation system messages. This feature is not available on Claude Sonnet 5. Use the top-level `system` field there instead.
41 [Mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) are in beta on the same models. On the Claude API, Claude Platform on AWS, and Claude Models on Google Cloud (Anthropic managed), send the `inline-tools-2026-09-15` beta header, which also covers [defining a tool inside a `tool_addition` block](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#define-tools-in-a-message-beta). [Adding an MCP server that way](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#add-an-mcp-server-mid-conversation-beta) needs a second beta header, `mcp-client-2026-09-15`, which is available on the Claude API. The `mid-conversation-tool-changes-2026-07-01` header works for changes that name a tool by reference, on the Claude API, Amazon Bedrock, Claude Platform on AWS, Claude Models on Google Cloud (Anthropic managed), and Claude Models on Google Cloud (Google managed).
4142 
42 [Mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) are in beta on the same models. On the Claude API, send the `inline-tools-2026-09-15` beta header, which also covers [defining a tool inside a `tool_addition` block](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#define-tools-in-a-message-beta). [Adding an MCP server that way](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#add-an-mcp-server-mid-conversation-beta) needs a second beta header, `mcp-client-2026-09-15`, which is available on the Claude API. The `mid-conversation-tool-changes-2026-07-01` header works for changes that name a tool by reference, on the Claude API, Amazon Bedrock, and Claude Models on Google Cloud (Google managed).
43 
4443 [Turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) (`clear_at`) are in beta and require the `mid-conversation-system-clear-at-2026-08-21` beta header, on the same models and platforms as mid-conversation system messages.
4544</Note>
4645 
4746## Mid-conversation tool changes
4847 
49The `tools` array sits even earlier in the hashed request prefix than the top-level `system` field, so editing it invalidates the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) for the entire conversation. Mid-conversation tool changes are the tools counterpart to mid-conversation system messages. Instead of fixing the tool list for the lifetime of the conversation, you change which tools are offered to the model between turns: declare the full tool set in `tools` up front, then use `tool_addition` and `tool_removal` blocks to offer a tool to the model, or withdraw it, from a specific point in the conversation onward. The `tools` array itself never changes, so the cached prefix stays intact. Mid-conversation tool changes are in beta and use the `inline-tools-2026-09-15` beta header on the Claude API.
48The `tools` array sits even earlier in the hashed request prefix than the top-level `system` field, so editing it invalidates the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) for the entire conversation. Mid-conversation tool changes are the tools counterpart to mid-conversation system messages. Instead of fixing the tool list for the lifetime of the conversation, you change which tools are offered to the model between turns: declare the full tool set in `tools` up front, then use `tool_addition` and `tool_removal` blocks to offer a tool to the model, or withdraw it, from a specific point in the conversation onward. The `tools` array itself never changes, so the cached prefix stays intact. Mid-conversation tool changes are in beta and use the `inline-tools-2026-09-15` beta header on the Claude API, Claude Platform on AWS, and Claude Models on Google Cloud (Anthropic managed).
5049 
5150`tool_addition` and `tool_removal` are content blocks in the `content` array of a `role: "system"` message, and they can be mixed with `text` blocks in the same message. The message follows the placement rules for any mid-conversation system message, with one extra restriction after a paused turn (see [Limitations](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations)), and the change applies from that point in the conversation onward. Each block’s `tool` field references a tool rather than defining one: `{"type": "tool_reference", "name": "..."}` names a tool declared in the request’s `tools` array, and [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector) tools can be referenced individually with `mcp_tool_reference` (`server_name` and `name`) or as a whole toolset with `mcp_toolset_reference` (`server_name`). Referencing a name that is not declared in `tools` returns a 400 error (on the Claude API, with `error.details.error_code` set to `tool_reference_unresolved`). A `tool_addition` block can instead [carry the tool’s full definition](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#define-tools-in-a-message-beta), which the `mid-conversation-tool-changes-2026-07-01` header doesn’t support.
5251 
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21162115messages.3.content.0: cache_control is not permitted on a turn-scoped system message (clear_at: 'next_user_message')
21172116```
21182117 
2119The first is the error returned without the beta header. On Claude Models on Google Cloud (Google managed) and [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), send the beta value in the `anthropic-beta` header, as on the Claude API (see [Beta headers](https://platform.claude.com/docs/en/api/beta-headers)). With the Amazon Bedrock [InvokeModel API](https://platform.claude.com/docs/en/build-with-claude/claude-on-amazon-bedrock-legacy), put it in the `anthropic_beta` array of the request body instead.
2118The first is the error returned without the beta header. On [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), Claude Platform on AWS, Claude Models on Google Cloud (Anthropic managed), and Claude Models on Google Cloud (Google managed), send the beta value in the `anthropic-beta` header, as on the Claude API (see [Beta headers](https://platform.claude.com/docs/en/api/beta-headers)). With the Amazon Bedrock [InvokeModel API](https://platform.claude.com/docs/en/build-with-claude/claude-on-amazon-bedrock-legacy), put it in the `anthropic_beta` array of the request body instead.
21202119 
21212120Through the SDK, set `clear_at` (csharp, go: `ClearAt`; java: `.clearAt()`; php: `clearAt`) on the `role: "system"` entry in `messages` and send the beta header. The following example appends a turn-scoped reminder after the user turn; on the next request, once a later user message exists, the reminder stays in the array but no longer renders:
21222121 
from line 2396
23972396* Availability: Available
23982397* Usage: Append a message with `role` `"system"` to `messages` at the point where the instruction starts to apply.
23992398 
2400| Status | Cloud platforms | Models |
2401| --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
2402| Available | Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed) | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
2399| Status | Cloud platforms | Models |
2400| --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
2401| Available | Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed) | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
24032402 
24042403#### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
24052404 
from line 2405
24062405* Availability: Available
24072406* Usage: Send a `system.message` event. It applies to that turn and all later turns; the agent’s own `system` field cannot change during a session.
24082407 
2409| Status | Cloud platforms | Models |
2410| --------- | ------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
2411| Available | Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws) | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
2408| Status | Cloud platforms | Models |
2409| --------- | ----------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
2410| Available | All supported platforms | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
24122411 
24132412### Data handling
24142413 
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