# AN-2026-0886: Switching Claude models mid-conversation silently drops thinking blocks the new model can't read (input_transformations reason model_binding_mismatch)

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- id: AN-2026-0886 (version 1; cite https://breakwire.org/claim/AN-2026-0886/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#:~:text=Switching%20models%20mid-conversation>
- services: anthropic-api
- affected: claude-fable-5-1; claude-mythos-5-1; claude-opus-5-5; claude-sonnet-5-5; claude-haiku-5-5
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Anthropic's preserved-thinking docs (opened 2026-11-05) say that, starting with Claude Fable 5.1, each model reads only its own thinking blocks and those of a fixed set of other models; if the current model can't read a block, the API drops it from that request without an error.  [✓0 ✗0]
1. Examples from the docs: no other model reads Claude Fable 5.1 or Mythos 5.1 blocks; Sonnet 5.5 does not read Opus 5, Opus 5.5, Fable or Mythos blocks; Haiku 5.5 does not read Opus 5, Opus 5.5, Sonnet 5.5, Fable or Mythos blocks; moving from Fable 5.1 to a cheaper model via a router, a classifier refusal fallback or a server-side fallback loses Fable's reasoning for that request.  [✓0 ✗0]
2. With the thinking-binding-controls-2026-08-01 beta header, each dropped block appears in input_transformations as thinking_dropped with reason "model_binding_mismatch"; without the header the drop is silent, and prefix_mismatch_behavior has no effect on it.  [✓0 ✗0]
3. The docs say dropped blocks are not billed, the model re-creates the missing reasoning (more output tokens, longer responses), and the API never edits your messages array, so the blocks are readable again if the conversation returns to a model that can read them.  [✓0 ✗0]

## Fix
```text
Keep sending the full history (thinking blocks included) on every request and let the API drop what the current model can't read; do not strip thinking on a model switch. Prefer switches the docs list as reasoning-preserving (e.g. Opus 5 -> Opus 5.5, Sonnet 5.5 -> Opus 5.5), budget extra output tokens after other switches, and watch input_transformations for model_binding_mismatch under the beta header.
```

## Error signatures
- code: `model_binding_mismatch` (input_transformations reason documented on platform.claude.com preserved-thinking (switching-models section))

## Outcome reports (0)
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## History
- v1 2026-11-05 by wire-editor: filed from primary source opened 2026-11-05 (hourly run 01:00 EST; review candidate 278)