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build-with-claude/working-with-messages Changed · +1 / -1 lines

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323323 
324324### System role in messages
325325 
326On Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), Claude Opus 5.5, Claude Opus 4.8, Claude Opus 5, and Claude Sonnet 5.5, you can include messages with `"role": "system"` after a user turn (subject to [placement rules](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations)) to add a new system instruction partway through a conversation. A `system` message cannot be the first entry in `messages`. Use the top-level `system` field for instructions that apply from the start.
326On 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, and Claude Sonnet 5.5, you can include messages with `"role": "system"` after a user turn (subject to [placement rules](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations)) to add a new system instruction partway through a conversation. A `system` message cannot be the first entry in `messages`. Use the top-level `system` field for instructions that apply from the start.
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328328A mid-conversation system message has the same authority as the top-level `system` field, but because it is appended to the end of the message history, it does not invalidate any cached prefix that came before it. Use the top-level `system` field for instructions that should apply from the very first turn, and a mid-conversation system message for instructions that only become relevant later.
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7878One hook governs conversations across claude.ai, Cowork, Claude Code, and Claude Tag sessions in your Claude Enterprise organization, whether they run on the web, in the desktop or mobile apps, in the CLI, or in Slack. Inference hooks are not available on Amazon Bedrock or Google Cloud.
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80Governed requests are the inference requests behind the user's conversation. Ancillary requests aren't sent to your endpoint. These include conversation title generation and a measurement that Anthropic runs on Claude's reply after Claude Tag has posted it in Slack. System prompts and tool definitions are never included in what is sent. Voice mode is not covered.
80Governed requests are the inference requests behind the user's conversation. Ancillary requests aren't sent to your endpoint. These include conversation title generation and a measurement that Anthropic runs on Claude's reply after Claude Tag has posted it in Slack. System prompts and tool definitions are never included in what is sent. In voice mode, prompts and tool calls are sent to your endpoint too, though a small share of tool calls isn't, so don't rely on inference hooks as your only control for voice. If your endpoint denies a request in voice mode, the voice call usually ends with a general error message, and your deny reason isn't shown.
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82With **Validate tool calls** on, `tool_call` events may leave out calls to some of claude.ai's own tools, such as tools that suggest connectors or skills, give Claude the user's local time, or read the user's own past chats and saved memory. These calls and their results stay in the conversation, so the next `prompt` event from that conversation, if there is one, includes them.
82With **Validate tool calls** on, `tool_call` events may leave out calls to some of claude.ai's own tools, such as tools that list, search for, or suggest connectors, plugins, or skills, suggest starting research or turning on web search, search for other tools, switch to a larger model, end the chat, give Claude the user's local time, or read the user's own past chats and saved memory. These calls and their results stay in the conversation, so the next `prompt` event from that conversation, if there is one, includes them.
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8484Some features that Anthropic runs for your organization make model calls of their own. Your organization sees the results of those calls but not their transcripts. These calls aren't governed requests and aren't sent to your endpoint. They include the following:
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1717## Overview
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19Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5.5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still fall short. Claude Mythos 5.1 offers the same capabilities to [Project Glasswing](https://anthropic.com/glasswing) participants only.
19Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5.5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still fall short. Claude Mythos 5.1 offers the same capabilities only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
2020 
2121If you already call Claude Fable 5, three changes are breaking: [forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported), [earlier models can't read its thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them), and [editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks). Five are additive: [per-message effort](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) (beta), [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) (beta), [readable progress updates between tool calls](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#progress-updates-between-tool-calls-beta) (`display: "updates"`, beta), a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing), and [content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance).
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2424 
2525## Claude Fable 5.1 and Claude Mythos 5.1
2626 
27[Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) offers the same capabilities by invitation only, as part of [Project Glasswing](https://anthropic.com/glasswing). It shares Claude Fable 5.1's specifications and pricing. For access, contact your Anthropic, AWS, or Google Cloud account team.
27[Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) offers the same capabilities only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). It shares Claude Fable 5.1's specifications and pricing. To request access, apply to the program that covers your use case, or contact your Anthropic, AWS, or Google Cloud account team.
2828 
2929## How it compares
3030 

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1717## Fable vs. Mythos
1818 
19[Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview) is offered separately, by invitation only, for defensive cybersecurity workflows as part of [Project Glasswing](https://anthropic.com/glasswing). It shares Claude Fable 5's specifications and pricing. For access, contact your Anthropic, AWS, or Google Cloud account team.
19[Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview) is offered separately for defensive cybersecurity workflows and is available only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). It shares Claude Fable 5's specifications and pricing.
2020 
2121## How it compares to the current lineup
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2323### September 30, 2026
2424 
25* In Python SDK 1.10.0, TypeScript SDK 0.130.0, C# SDK 12.52.0, Go SDK 1.77.0, Java SDK 2.67.0, PHP SDK 0.53.0, Ruby SDK 1.75.0, and version 1.37.0 of the `ant` CLI, the [Admin API](https://platform.claude.com/docs/en/manage-claude/admin-api) methods for organization info, members, invites, workspaces, API keys, rate limits, service accounts, workload identity federation, customer-managed encryption keys, and compliance settings are out of beta: call them under `client.organization` and `ant organization`. These releases also keep `client.beta.organization` and `ant beta:organization`.
2526* We announced the deprecation of the Claude Sonnet 4.5 model (`claude-sonnet-4-5-20250929`), with retirement on the Claude API scheduled for November 30, 2026. We recommend migrating to [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/migration-guide#migrating-from-sonnet-45). Read more in [Model deprecations](https://platform.claude.com/docs/en/about-claude/model-deprecations).
2627 
2728### September 28, 2026
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