Claude Developer Platform Updates & Release Notes

• The code execution tool now supports code_execution_20260521, which discloses the 90-second per-cell execution time limit in the tool description so Claude can budget long-running cells. No beta header is required.

• The web search tool and web fetch tool now support web_search_20260318 and web_fetch_20260318, adding a response_inclusion parameter to drop consumed result blocks from the API response for agentic workflows. No beta header is required.

• We've retired the Claude Sonnet 4 model (claude-sonnet-4-20250514) and the Claude Opus 4 model (claude-opus-4-20250514). All requests to these models will now return an error. We recommend upgrading to Claude Sonnet 4.6 and Claude Opus 4.8 respectively. Researchers can request ongoing access through the External Researcher Access Program.

• The GET /v1/environments/{id}/work endpoint, which lists pending work for a self-hosted sandbox, is now available on Claude Platform on AWS. See IAM actions for Claude Platform on AWS for the GetEnvironment action that authorizes it.

• We've launched Claude Fable 5 (claude-fable-5), our most capable widely released model, alongside Claude Mythos 5 (claude-mythos-5) for Project Glasswing participants. Both models support a 1M token context window by default, 128k max output tokens, and always-on adaptive thinking. See Introducing Claude Fable 5 and Claude Mythos 5 for capabilities, API changes, and availability.
• Claude Fable 5 and Claude Mythos 5 use the tokenizer introduced with Claude Opus 4.7. Compared to models before Claude Opus 4.7, the same text produces roughly 30% more tokens. Use the token counting API with model: "claude-fable-5" to measure your prompts under the new tokenizer.
• Claude Fable 5 runs safety classifiers on requests and during response generation. When a classifier declines a request, the Messages API returns stop_reason: "refusal". You are not billed for a request refused before any output is generated. An opt-in fallbacks parameter (in beta on the Claude API and Claude Platform on AWS; not supported on the Message Batches API) re-runs refused requests on another model, billed at the fallback model's rates. See Handling stop reasons.
• The stop_details.category field on refusal responses now includes "reasoning_extraction" on Claude Fable 5, returned when a request is blocked under Anthropic's Terms of Service restrictions on reverse engineering or duplicating model outputs. The existing "cyber" and "bio" categories are unchanged. No beta header is required.
• On Claude Fable 5 and Claude Mythos 5, adaptive thinking is the only thinking mode: thinking: {"type": "disabled"} is not supported, and manual extended thinking budgets and assistant prefill are not supported (both return a 400 error). See Migrating from Claude Mythos Preview to Claude Mythos 5.
• On Claude Fable 5 and Claude Mythos 5, thinking.display defaults to "omitted", the same as Claude Opus 4.8, Claude Opus 4.7, and Claude Mythos Preview; set display: "summarized" to receive readable thinking summaries. The raw chain of thought is never returned; pass thinking blocks back unchanged in multi-turn conversations on the same model. See Thinking output on Claude Fable 5 and Claude Mythos 5.
• Claude Fable 5 requires 30-day data retention on the Claude API and is not available under zero data retention. See Model-specific data retention requirements.
• Claude Managed Agents now supports scheduled deployments, letting you run sessions on a cron schedule without managing your own scheduler.
• Claude Managed Agents vaults now support environment variable credentials, so you can securely inject secrets into the agent's sandbox for CLIs, SDKs, and other services that authenticate through environment variables.
• The session.thread_* webhook events now include a session_thread_id field identifying the multi-agent thread that triggered the event.

• We announced the deprecation of the Claude Opus 4.1 model (claude-opus-4-1-20250805), with retirement on the Claude API scheduled for August 5, 2026. We recommend migrating to Claude Opus 4.8. Read more in model deprecations.

• The advisor tool now supports a max_tokens parameter to cap the advisor model's output per call, reducing latency and output token cost for workloads that don't need full-length advisor responses. Set tools[].max_tokens on the advisor tool definition; see Capping advisor output.

• On the Claude API, you are no longer billed for a request when it returns stop_reason: "refusal" without Claude having generated any output. See Streaming refusals for detecting and handling refusals.

• Claude Managed Agents webhooks, multiagent orchestration, and self-hosted sandboxes are now available on Claude Platform on AWS. See IAM actions for Claude Platform on AWS for the new IAM actions and the AnthropicSelfHostedEnvironmentAccess managed policy.

Release notes

• We've launched Claude Opus 4.8 (claude-opus-4-8), our most capable generally available model. Claude Opus 4.8 supports a 1M token context window by default on the Claude API, Amazon Bedrock, and Vertex AI (200k on Microsoft Foundry), 128k max output tokens, and the same set of tools and platform features as Claude Opus 4.7. See What's new in Claude Opus 4.8 for capability improvements, new features, and migration guidance.
• We've launched mid-conversation system messages. On Claude Opus 4.8, you can send role: "system" messages after a user turn (subject to placement rules) in the messages array, preserving prompt cache hits when instructions change during a long-running session. No beta header is required.
• The stop_details field on refusal responses is now publicly documented; it returns a category (cyber, bio, or null) and a human-readable explanation, so your application can route different classes of refusal to the right next step. No beta header is required.
• On Claude Opus 4.8, the effort parameter defaults to high across all surfaces, including Claude Code and the Messages API.
• On Claude Opus 4.8, the minimum cacheable prompt length for prompt caching is 1,024 tokens, lower than on Claude Opus 4.7.
• With adaptive thinking enabled, Claude Opus 4.8 triggers reasoning only when a turn needs it, reducing wasted thinking tokens compared to Claude Opus 4.7 at the same effort level.
• Claude Opus 4.8 supports high-resolution image input (up to 2576 pixels on the long edge), same as Claude Opus 4.7.
• Task budgets now support Claude Opus 4.8.
• The advisor tool now supports Claude Opus 4.8.
• Computer use now supports Claude Opus 4.8.
• Fast mode for Claude Opus 4.8 is available as a research preview on the Claude API only.
• Setting the sampling parameters temperature, top_p, or top_k to a non-default value returns a 400 error on Claude Opus 4.8, same as on Claude Opus 4.7. See the migration guide for details.
• In Claude Code, we've expanded Auto mode to more users for long-running tasks. See the Claude Code documentation.
• In Claude Code, Max plan users now default to fast mode on Claude Opus 4.8. See the Claude Code documentation.
• In Claude Code, Workflows are available as a research preview, letting you define and run multi-step agentic plans. See the Claude Code documentation.
• We've deprecated fast mode for Claude Opus 4.6, with removal approximately 30 days after launch. Migrate to fast mode for Claude Opus 4.8 or Claude Opus 4.7. Read more in Fast mode.
• For updates to claude.ai, Cowork, Claude for Microsoft 365, and other Claude apps in this release, see the release notes for Claude Apps.

• MCP tunnels is now available as a Research Preview, so you can connect to MCP servers in your private network.
• Self-hosted sandboxes are now available for Claude Managed Agents, as an alternative to running tool execution in Anthropic's infrastructure. See Self-hosted sandboxes.
• With Claude Managed Agents, you can now update the agent's MCP server and tool configurations associated with an active session.
• With Claude Managed Agents, large outputs from agent_toolset and MCP tools exceeding 100K tokens are now automatically spilled to a file in the sandbox. The model receives a truncated preview with the file path and can read the full content from there.

• The web search tool now returns richer SEC filing data, making it easier to ground financial research agents, earnings analysis, and due-diligence workflows in primary sources with citations.

• We've launched cache diagnostics in public beta. Pass diagnostics.previous_message_id on a Messages request and the API reports a cache_miss_reason explaining where the prompt cache prefix diverged from the previous turn. Include the cache-diagnosis-2026-04-07 beta header in your requests.

• Fast mode (research preview) now supports Claude Opus 4.7. Set speed: "fast" with model: "claude-opus-4-7" and the fast-mode-2026-02-01 beta header for significantly faster output token generation at premium pricing. Pricing, rate limits, and access are the same as for Opus 4.6 fast mode; interested customers should join the waitlist.

• We've launched Claude Platform on AWS, bringing the Claude API to Anthropic-managed infrastructure accessible through AWS, with AWS billing and IAM authentication. Access the full Messages API, Files API, Message Batches API, Claude Managed Agents, Agent Skills, code execution, and tool use through native AWS endpoints. Learn more in Claude Platform on AWS.

• Multiagent sessions and Outcomes are now in public beta under the standard managed-agents-2026-04-01 beta header.

• Vault credential background refresh is now supported for mcp_oauth credentials. See Authenticate with vaults.

• Webhooks for Claude Managed Agents are now supported. Webhook event types include session and vault lifecycle events. See Subscribe to webhooks.

• Additional filtering and sorting options are now supported. Sessions can be filtered by status, and events can be filtered by type. Events can now be filtered by creation time.

• We've retired the 1M token context window beta (context-1m-2025-08-07) for Claude Sonnet 4.5 and Claude Sonnet 4. The beta header now has no effect on these models, and requests exceeding the standard 200k-token context window return an error. To use the 1M context window, migrate to Claude Sonnet 4.6 or Claude Opus 4.6, where it's generally available at standard pricing with no beta header required.