@luan.sh/pi-thinking-binding

Pi gallery

A Pi extension that drops Anthropic thinking blocks a system prompt or tool change has already invalidated, so the prefix_binding_mismatch warning appears once instead of on every turn.

Install

pi install npm:@luan.sh/pi-thinking-binding

Nothing else is needed. The extension registers no tools, commands, settings, or keybindings, and it has no companion packages.

The problem

Anthropic binds every thinking block signature to the request prefix that produced it: the system prompt, the tools set, and the messages before the block. When that prefix changes, typically because an extension's tool description or the system prompt changed and Pi was restarted into an existing session, every earlier block is invalid. The API then drops each one on every later turn and Pi reports Anthropic dropped N thinking blocks: prefix_binding_mismatch for the rest of the session.

What it does

The extension does the drop itself, once. It only acts on models whose API is anthropic-messages; other providers are untouched.

1. On each before_provider_request, it fingerprints the request's system and tools (a SHA-256 over their JSON) and compares it with the fingerprint recorded on the current session branch.
2. If the fingerprint is new for this branch, it records it. The first sighting in a session invalidates nothing.
3. If the fingerprint differs from the recorded one, it strips thinking and redacted_thinking blocks from the assistant messages in that request body and records the newest assistant message timestamp as the stale cutoff.
4. On later turns, the context hook removes thinking from assistant messages at or before the cutoff before they reach the provider. Newer reasoning is left alone.
5. For sessions that were already broken before a fingerprint was recorded, the message_end hook trusts Anthropic's own thinking_dropped / prefix_binding_mismatch diagnostic once and records the cutoff from it.

A message whose only content is thinking is never emptied; it is left as is.

State

State lives in the session as custom entries of type @luan.sh/pi-thinking-binding, one per prefix change on the branch:

{ "version": 1, "fingerprint": "<sha256 hex>", "staleThrough": 1712345678901 }

staleThrough is the assistant message timestamp at or before which thinking is dead; it is absent until the first prefix change. The newest valid entry on the branch wins.

The session file keeps the original thinking blocks. Only the messages handed to the provider are changed, which is what the API would have done anyway.

Configuration

There is none. Behaviour is fixed in source; there are no settings, keybindings, or environment variables.

Programmatic use

@luan.sh/pi-thinking-binding exports the core functions for reuse or testing: BINDING_ENTRY_TYPE, BindingState, fingerprintPrefix, newestAssistantTimestamp, parseAnthropicPayload, readBindingState, reportsPrefixMismatch, stripPayloadThinking, stripStaleThinking.

Layout

Responsibility	File
Pi registration; context, before_provider_request, message_end hooks	src/extension.ts
Payload narrowing, fingerprint, state read, stale cutoff, stripping	src/core/binding.ts
Public exports	src/index.ts

Develop

Source: https://github.com/luan/agents, directory harnesses/pi/agent/packages/pi-thinking-binding. Run bun run typecheck and bun test test in that directory.