Claude Developer Platform Updates & Release Notes

We’ve launched Claude Fable 5.2 (claude-fable-5-2), built for demanding reasoning and long-horizon agentic work. Claude Mythos 5.2 (claude-mythos-5-2) offers the same capabilities only to organizations verified through Anthropic’s verification programs, such as the Cyber Verification Program. Both models have a 1M token context window, 128k max output tokens, and always-on adaptive thinking. They cost $8 USD per MTok of input and $40 USD per MTok of output (Claude Fable 5.1 and Claude Mythos 5.1 cost $10 USD per MTok of input and $50 USD per MTok of output). Claude Fable 5.2 is available on the Claude API, Amazon Bedrock (bedrock-runtime), Claude Platform on AWS, Claude Models on Google Cloud (Anthropic managed), Claude Models on Google Cloud (Google managed), and Claude in Microsoft Foundry. See What’s new in Claude Fable 5.2 for API changes and migration guidance. For model-specific prompting patterns, see Prompting Claude Fable 5.2.

Claude Fable 5.2 and Claude Mythos 5.2 enforce the prefix check for every account, whatever its age. Claude Fable 5.1 returns the error by default on accounts created on or after August 31, 2026 (see When prefix_mismatch_behavior is unset). If your code edits the system prompt, tools, or an earlier message and then sends back a Claude Fable 5.2 or Claude Mythos 5.2 thinking block from a later turn, the request returns a 400 error. Before you switch, check whether your code edits the prefix.

Claude Fable 5.2 and Claude Mythos 5.2 read other models’ thinking blocks on the Claude API and Claude Models on Google Cloud (Google managed). So on those platforms, a conversation that moves onto either model keeps its reasoning. The other models include Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5.

No other model reads thinking blocks from Claude Fable 5.2 or Claude Mythos 5.2. When a conversation moves from either one to a model outside this pair, the API drops those blocks. That includes a move to Claude Opus 5.5 or Claude Fable 5.1. The request still succeeds. See Switching models mid-conversation.

Claude Fable 5.2 and Claude Mythos 5.2 support computer use through the computer_toolset_20260801 toolset. A request that declares the earlier computer_20251124 computer use tool returns a 400 error on every platform except Amazon Bedrock, where the earlier tool still works. See Migrate from computer_20251124.

With the advisor tool, which is in beta, a Claude Fable 5.2 or Claude Mythos 5.2 executor accepts only Claude Fable 5.2 or Claude Mythos 5.2 as its advisor. Any other advisor returns a 400 error, so set the advisor’s model to one of these two models when you move an executor to them. Every executor model accepts Claude Fable 5.2 as its advisor. See Model compatibility.

Both models require 30-day data retention, like Claude Fable 5.1 and Claude Mythos 5.1. They aren’t available under zero data retention unless expressly authorized by Anthropic. See Model-specific data retention requirements.

Thinking blocks that Claude Fable 5.2 and Claude Mythos 5.2 produce work only in the account that produced them, or in an account linked to it. See Preserved thinking.

Dreams (research preview) now supports Claude Fable 5.2. See Supported models.

We’ve launched fast mode for Claude Fable 5.2 in Messages API requests on the Claude API. Fast mode delivers up to 2.5x higher output tokens per second, at $16 USD per MTok of input and $80 USD per MTok of output. Set speed: "fast" on your request, with no beta header.

The Compliance API now includes Security posture data, in beta for Claude Enterprise organizations. The Activity Feed records changes to attestations on the Security posture page in claude.ai Organization settings. The GET /v1/compliance/organizations/{organization_id}/security_posture endpoint returns an organization’s current Security posture results. See Security posture in the Compliance API.

You can now set up an IP allowlist for your organization in the Claude Console, in beta. Requests to covered services from addresses not on your list are refused, which limits where a leaked API key can be used. Add IPv4 and IPv6 addresses or CIDR ranges, up to 1,000 per organization. See IP allowlisting for what’s covered and how to set it up.

You can now sign a member of your Claude Enterprise (claude.ai) organization out of Claude everywhere with the Admin API. This capability is in beta. See the Admin API reference. The member stays in the organization and can sign in again. See Sign a member out everywhere for the beta header and what the request doesn’t end.

We’ve launched the Usage and Cost Analytics API in beta with the analytics-usage-cost-2026-08-24 beta header. Its /v1/organizations/analytics/usage and /v1/organizations/analytics/cost endpoints report token usage and cost for Claude Console and Claude Enterprise organizations. You can filter and group results by dimensions such as model, workspace, API key, and the user or service account behind each request.

The Usage and Cost API is now a legacy API, and so are the usage_report, cost_report, user_usage_report, and user_cost_report endpoints of the Claude Enterprise Analytics API. All of them remain supported. To move an existing integration, see Migrate from the Usage and Cost API or Migrate from the Claude Enterprise usage and cost reports.

We've released API version 2026-10-21. Migrating to API version 2026-10-21 shows how to upgrade.

We've released new versions of the client SDKs and the ant CLI that send API version 2026-10-21 by default: Python SDK 2.0.0, TypeScript SDK 1.0.0, C# SDK 13.0.0, Go SDK 2.0.0, Java SDK 3.0.0, PHP SDK 1.0.0, Ruby SDK 2.0.0, and ant CLI 2.0.0. See Switch to 2026-10-21.

The new SDK and CLI versions also have breaking changes that don't depend on the API version, and they are licensed under Apache 2.0 instead of the MIT License. See SDK and CLI changes.

We've added the enforcement_status and enforcement_actions fields to user profiles, so you can see whether Anthropic has restricted or banned one of your end customers. To find the profiles that have an open restriction or ban, list your profiles with the new enforcement_status[] filter. See Check whether Anthropic has restricted or banned a profile.

We've added capabilities.thinking.types.between_tools to the Models API. GET /v1/models and GET /v1/models/{model_id} now report whether each model accepts thinking: {type: "between_tools"}, which turns off up-front thinking. See Using the Models API.

The Compliance API now returns the text each organization member saved as Instructions for Claude in claude.ai and as Global instructions in Cowork, in beta for Claude Enterprise organizations. Call GET /v1/compliance/organizations/{org_uuid}/user_preferences with your existing Compliance Access Key and the read:compliance_user_data scope. See List user preferences.

Fast mode is out of research preview for Claude Opus 5.5, and you no longer need to request access. Requests to Claude Opus 5.5 no longer require the fast-mode-2026-02-01 beta header, and requests that still send it keep working. Claude Opus 5.5 now has a default fast mode rate limit in the Start, Build, and Scale usage tiers. Fast mode for Claude Opus 5 and Claude Opus 4.8 remains in research preview. Keep sending the beta header with fast mode requests to those models.

Fast mode for Claude Opus 5.5 is now available on Claude in Microsoft Foundry for deployments hosted on Azure.

We've launched OpenTelemetry export in beta, so you can trace Messages API requests and Claude Managed Agents sessions in your observability backend. The Claude SDK emits spans in Python, TypeScript, C#, Go, Java, PHP, and Ruby. To get spans from the API, add an OpenTelemetry destination in the Claude Console.

Claude Managed Agents scheduled deployments are now available on Claude Platform on AWS.