Mikayla Maki
cd0241b7e3
Claude Fable 5.1 introduces two breaking Messages API changes that produce 400 errors with request shapes Zed currently sends. This change adapts the shared Anthropic request path so the model works out of the box, without restructuring how requests are built. The first change is thinking-block prefix binding. Fable 5.1 binds each thinking block to the exact conversation prefix that produced it, and replaying a block after anything earlier changed (the system prompt, the tools array, or an edited turn) fails the whole request with a 400 reading "The block is bound to a different conversation". Zed routinely rebuilds the system prompt and tool list between requests (rules file changes, profile switches, MCP servers coming online), so this would surface as a hard error mid-thread. Requests to Fable 5.1 now send the `thinking-binding-controls-2026-08-01` beta header with `thinking.block_binding.prefix_mismatch_behavior: "drop_block"`, which makes the API drop invalidated blocks and continue instead of rejecting the request. Claude Mythos 5.1 doesn't run this check and is excluded. The cloud and Copilot Chat paths pass `block_binding: None` since their proxies control the upstream beta headers; opting in there belongs server-side. The second change is that forced tool use (`tool_choice: "any"` or `"tool"`) now returns a 400. Zed sends `"any"` from inline assist's streaming-tools path, which is on by default. The Anthropic and Zed-cloud providers now report `tool_choice: any` as unsupported for Fable/Mythos 5.1, and inline assist already degrades gracefully to prompt-guided auto tool choice, which is Anthropic's recommended migration. Finally, `claude-fable-5-1` and `claude-mythos-5-1` are added to the compaction model list. This matters more than a routine table update: with prefix binding, server-side compaction is the sanctioned way to trim history without invalidating thinking blocks. Everything else already carries over via the existing `claude-fable-5` id prefix logic (data-retention consent, Opus 4.8 refusal fallback, which remains a permitted fallback target for 5.1). Bedrock is intentionally untouched. Deeper follow-up work is deferred: keeping conversation history append-only so blocks aren't invalidated in the first place (turn-scoped system messages, mid-conversation tool changes) and monitoring `input_transformations` for drop reporting. Testing: - New unit tests covering the `block_binding` wire format (sent for `claude-fable-5-1` only) and beta-header wiring in `Model::from_listed`. - `cargo nextest run` for `anthropic`, `language_models`, `language_models_cloud`, and `copilot_chat`; `cargo fmt --check`. Release Notes: - Improved support for Claude Fable 5.1
Zed
Welcome to Zed, a high-performance, multiplayer code editor from the creators of Atom and Tree-sitter.
Installation
On macOS, Linux, and Windows you can download Zed directly or install Zed via your local package manager (macOS/Linux/Windows).
Other platforms are not yet available:
- Web (tracking discussion)
Developing Zed
Contributing
See CONTRIBUTING.md for ways you can contribute to Zed.
Also... we're hiring! Check out our jobs page for open roles.
Licensing
Zed source code is licensed primarily under GPL-3.0-or-later, with Apache-2.0 components where marked.
License information for third party dependencies must be correctly provided for CI to pass.
We use cargo-about to automatically comply with open source licenses. If CI is failing, check the following:
- Is it showing a
no license specifiederror for a crate you've created? If so, addpublish = falseunder[package]in your crate's Cargo.toml. - Is the error
failed to satisfy license requirementsfor a dependency? If so, first determine what license the project has and whether this system is sufficient to comply with this license's requirements. If you're unsure, ask a lawyer. Once you've verified that this system is acceptable add the license's SPDX identifier to theacceptedarray inscript/licenses/zed-licenses.toml. - Is
cargo-aboutunable to find the license for a dependency? If so, add a clarification field at the end ofscript/licenses/zed-licenses.toml, as specified in the cargo-about book.
Sponsorship
Zed is developed by Zed Industries, Inc., a for-profit company.
If you’d like to financially support the project, you can do so via GitHub Sponsors. Sponsorships go directly to Zed Industries and are used as general company revenue. There are no perks or entitlements associated with sponsorship.