Engineering-quality cleanups from the SDK review (no behavior change):
- Return-type discoverability: add pageindex/types.py with TypedDicts
(DocumentInfo, DocumentDetail, PageContent) and annotate Collection /
Backend methods with them; add docstrings to every public Collection
method (including the get_page_content `pages` spec). Exported from the
package. Zero runtime cost — these are plain dicts.
- Backend protocol as a real contract:
* query_stream is an async generator, so the protocol now declares it
as `def ... -> AsyncIterator[QueryEvent]` (not `async def`, which
typed it as a coroutine and never matched the implementations).
* custom-parser support is expressed as a runtime_checkable
SupportsParserRegistration capability protocol; the client uses
isinstance(...) instead of hasattr(...) duck-typing.
- Parser layering: move count_tokens into a leaf module pageindex/tokens.py
so parser/* imports it from there instead of reaching back into
pageindex.index (a reverse dependency). index.utils re-exports it for
backward compatibility.
Adds tests/test_architecture.py enforcing: parser never imports index,
count_tokens is a single shared leaf, the capability protocol works,
both backends satisfy Backend, and the TypedDicts are exported.
Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS
