mid-conversation-system-messages changedbuild-with-claude/mid-conversation-system-messages
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88 eligibility: eligible
99 note: Excludes [Covered Models](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements). Request-shape capability of the Messages API; mid-conversation system messages flow through the standard inference path and nothing is stored server-side after the response.
1010 supportedModels:
11 - claude-fable-5-2
12 - claude-mythos-5-2
1113 - claude-fable-5-1
1214 - claude-mythos-5-1
1315 - claude-fable-5
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3638Mid-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.
3739
3840<Note>
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.
41 Mid-conversation system messages are available on Claude Fable 5.2, [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), 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.
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4143 [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).
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20192021}
20202022```
20212023
2022The main use is a per-turn reminder in a tool loop. Each time you want the model to see the reminder, append it after the `tool_result` message. Leave every earlier copy where it is. The model sees only the copies that come after the last user message, so the reminder never piles up. Nothing earlier in `messages` changes, so the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) keeps matching. On Claude 5.1 and later models, this also keeps later [thinking blocks valid](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Deleting an earlier reminder would change the conversation before those blocks, so they would fail the prefix check. A cleared message stays in the array and leaves the conversation before those blocks unchanged.
2024The main use is a per-turn reminder in a tool loop. Each time you want the model to see the reminder, append it after the `tool_result` message. Leave every earlier copy where it is. The model sees only the copies that come after the last user message, so the reminder never piles up. Nothing earlier in `messages` changes, so the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) keeps matching. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, this also keeps later [thinking blocks valid](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Deleting an earlier reminder would change the conversation before those blocks, so they would fail the prefix check. A cleared message stays in the array and leaves the conversation before those blocks unchanged.
20232025
20242026The following request is a later step of an agent loop. `messages[3]` rendered on the earlier request, when it was the last message in the array. Once `messages[5]` (a later user message) exists, `messages[3]` is cleared: the cleared message stays in the array, so the conversation before the thinking block in `messages[4]` is unchanged, but the model no longer receives its text. `messages[6]` and `messages[7]` both render, in order.
20252027
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20962098
20972099Rules for turn-scoped messages:
20982100
2099* **Resend cleared messages verbatim.** A cleared message is still part of the conversation history. Rebuilding it from current state (a fresh token count, a timestamp), dropping it as redundant, or changing its `clear_at` value is an edit to an earlier message. On Claude 5.1 and later models, every thinking block produced after it fails the [prefix check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation).
2101* **Resend cleared messages verbatim.** A cleared message is still part of the conversation history. Rebuilding it from current state (a fresh token count, a timestamp), dropping it as redundant, or changing its `clear_at` value is an edit to an earlier message. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, every thinking block produced after it fails the [prefix check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation).
21002102* **Text only.** `content` is one or more `text` blocks (or a string). `tool_addition` and `tool_removal` blocks return a 400 error on a turn-scoped message, and so does `output_config`. Use a separate `role: "system"` message without `clear_at` for those.
21012103* **No `cache_control` on its blocks.** A cleared message is never part of a cache key, so a breakpoint on it could never match. Put the breakpoint on the last block of the preceding user turn instead, as the example does. The top-level [automatic caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#automatic-caching) field skips turn-scoped messages when it picks a breakpoint. On the request that clears a message, the reusable cached prefix ends at the user turn before it, so only the one assistant turn between that message and the new user message is reprocessed.
21022104* **Placement rules still apply**, cleared or not. A turn-scoped message must follow a `user` turn (or an `assistant` turn ending in a server tool result) and precede an `assistant` turn or end the array, like any mid-conversation system message. One that ends the array always renders. One followed directly by another `user` message is a 400 error, not a cleared message: put all of a tool round’s results in one user message and the reminders after it.
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23522354* **Append the system message after the breakpoint.** Because it comes after the cached prefix, it does not change the prefix hash and the cache still hits.
23532355* **A mid-conversation system message is itself cacheable.** Once it is in the conversation, it becomes part of the stable history. On the next turn you can move your cache breakpoint past it (or rely on [automatic caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#automatic-caching) to do so) and the system message is read from cache like any other turn.
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2355Avoid editing or removing a mid-conversation system message that has already been sent. Like any other change to earlier messages, editing or removing it invalidates the cache from that point forward. On Claude 5.1 and later models, it also invalidates the [thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation) in every later assistant turn. For guidance that should apply to one turn only, use a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) and leave it in place. If the instruction needs to evolve, append a new system message rather than rewriting the old one. Consecutive system messages are accepted and treated as a single system section, which follows the same placement rule as a whole.
2357Avoid editing or removing a mid-conversation system message that has already been sent. Like any other change to earlier messages, editing or removing it invalidates the cache from that point forward. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, it also invalidates the [thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation) in every later assistant turn. For guidance that should apply to one turn only, use a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) and leave it in place. If the instruction needs to evolve, append a new system message rather than rewriting the old one. Consecutive system messages are accepted and treated as a single system section, which follows the same placement rule as a whole.
23562358
23572359## Limitations
23582360
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23962398* Availability: Available
23972399* Usage: Append a message with `role` `"system"` to `messages` at the point where the instruction starts to apply.
23982400
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) |
2401| Status | Cloud platforms | Models |
2402| --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
2403| 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 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [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) |
24022404
24032405#### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
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24052407* Availability: Available
24062408* 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.
24072409
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) |
2410| Status | Cloud platforms | Models |
2411| --------- | ----------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
2412| Available | All supported platforms | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [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) |
24112413
24122414### Data handling
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