2026-09-05 08:35 UTC · anthropic/api-release-notes · live page
Summary by fallback. It may be wrong; the diff below is what actually changed. Raw: e1cf982ca6-anthropic-api-release-notes.diff · commit e1cf982ca6
diff --git a/snapshots/anthropic/api-release-notes.md b/snapshots/anthropic/api-release-notes.mdindex deb87e3..6bdd6cc 100644--- a/snapshots/anthropic/api-release-notes.md+++ b/snapshots/anthropic/api-release-notes.md@@ -9,4 +9,21 @@ For release notes on Claude Apps, see the Release notes for Claude Apps in the C For updates to Claude Code, see the complete CHANGELOG.md in the `claude-code` repository. +### September 3, 2026++- Version 1.30.0 of the `ant` CLI adds `ant apply`, which creates and updates agents, environments, skills, memory stores, and deployments from files in your repository. Describe each resource in a file, run `ant apply`, and approve the plan it prints. Commit the `claude-lock.json` lockfile it writes so that later runs, on your machine or in CI, update the same resources instead of creating new ones. See Manage resources as code with ant apply.++### September 1, 2026++- We've launched Claude Fable 5.1 (`claude-fable-5-1`), the successor to Claude Fable 5 for long-running agentic coding, knowledge work, and research, alongside Claude Mythos 5.1 (`claude-mythos-5-1`) for Project Glasswing participants. Both models support a 1M token context window by default, 128k max output tokens, and always-on adaptive thinking, at $10 / $50 USD per MTok, the same as Claude Fable 5, with cache reads cut to $0.25 per MTok. Claude Fable 5.1 is available on the Claude API, Claude in Amazon Bedrock, Claude Platform on AWS, Claude on Google Cloud, and Claude in Microsoft Foundry. See What's new in Claude Fable 5.1 for capabilities, API changes, and migration guidance.+- Prompt cache reads on Claude Fable 5.1 and Claude Mythos 5.1 cost $0.25 USD per million tokens: 0.025x the base input price, compared with 0.1x on other models. Cache writes are unchanged. See Prompt caching pricing.+- On Claude Fable 5.1 and Claude Mythos 5.1, `tool_choice` types `any` and `tool` aren't supported and return a 400 error. `auto` and `none` are unchanged. To guarantee schema-conformant tool inputs, use strict tool use or structured outputs.+- Thinking blocks produced by Claude Fable 5.1 and Claude Mythos 5.1 are preserved only for the model that produced them or a newer one: earlier models can't read them, and the API drops one replayed to an earlier model. Claude Fable 5.1 accepts thinking blocks from Claude Opus 5, Claude Fable 5, Claude Mythos 5, and earlier Claude models. On Claude Fable 5.1, the API also checks that nothing before a block has changed: for new accounts created on or after August 31, 2026, replaying one after the `system` prompt, `tools`, or an earlier message changed returns a 400 error. With the `thinking-binding-controls-2026-08-01` beta header, dropped blocks are reported in an `input_transformations` response field, and `thinking.block_binding.prefix_mismatch_behavior` chooses between rejecting and dropping blocks whose history changed. See Preserved thinking.+- Per-message effort changes are in beta on Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5 on the Claude API. Add a `role: "system"` message with `output_config.effort` inside `messages` to change effort for later turns while preserving the prompt cache. Include the `mid-conversation-output-config-2026-07-01` beta header in your requests. See Per-message effort.+- Turn-scoped system messages are in beta (`mid-conversation-system-clear-at-2026-08-21` header). Set `clear_at: "next_user_message"` on a mid-conversation `role: "system"` message and it renders for the current turn only, then stays in the history at no token cost. Per-turn reminders don't accumulate and don't invalidate the prompt cache or later thinking blocks.+- `thinking.display` accepts a third value, `"updates"`, in beta (`thinking-display-updates-2026-08-18` header). Reasoning comes back with an empty `thinking` field, as under `"omitted"`, and the short progress updates that Claude Fable 5.1, Claude Mythos 5.1, and Claude Fable 5 write between tool calls come back as text, at most one `thinking` block before a tool call. See Progress updates between tool calls.+- Text generated by Claude Fable 5.1 and Claude Mythos 5.1 carries Anthropic's text watermark, and supported image, video, and audio files that Claude produces through the code execution tool carry C2PA Content Credentials when you retrieve them through the Files API on the Claude API. Marking requires no changes to your requests or response handling.+- Like Claude Fable 5, both models require 30-day data retention and aren't available under zero data retention unless expressly authorized by Anthropic. See Model-specific data retention requirements.+- The guides for the Claude Enterprise endpoints of the Admin API (user management and spend limits), the Claude Enterprise Analytics API, and the Compliance API now show the `anthropic-version` header; send it on every request to these endpoints, as in the rest of the Claude API. See API versions.+ ### August 27, 2026 @@ -147,5 +164,5 @@ For updates to Claude Code, see the complete CHANGELOG.md in the `claude-code` r ### June 18, 2026 -- The Python, TypeScript, Go, Java, Ruby, PHP, and C# SDKs now include support for `code_execution_20260120`, the code execution tool version that adds REPL state persistence and is the minimum version for programmatic tool calling. To adopt it, set the tool's `type` to `code_execution_20260120`; no beta header is required. It's available on Claude Fable 5, Claude Mythos 5, Claude Opus 4.5 and newer, and Claude Sonnet 4.5 and newer; see the model compatibility table.+- The Python, TypeScript, Go, Java, Ruby, PHP, and C# SDKs now include support for `code_execution_20260120`, the code execution tool version that adds REPL state persistence and is the minimum version for programmatic tool calling. To adopt it, set the tool's `type` to `code_execution_20260120`; no beta header is required. It's available on Claude Fable 5, Claude Mythos 5, Claude Opus 4.5 and newer, and Claude Sonnet 4.5 and newer; see the code execution tool's Compatibility section. ### June 15, 2026