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