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retrieval.backend decides how searching your brain works. There are two options, and the default is genuinely usable - the second is an upgrade, not a fix for something broken.

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The local backend

Keyword matching, embedded in px0, no server and no models. It is fast, it works offline, and it needs zero setup. Its one real limitation is worth stating plainly: it only matches words that are actually in the text. A search for “connection pooling” will not surface a passage that says “reusing database connections” - the same idea with no shared keywords. For a library you wrote or curated yourself, where you tend to remember the words used, this matters less than you might expect.

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Reranking

With retrieval.rerank on (the default), a search asks the backend for more candidates than k and then reorders them: how much of the query each passage covers first, how close the matched terms sit to each other second, the backend’s own score as the tie-break. BM25 rewards a passage that repeats one query term; a question with three terms in it usually wants the passage that mentions all three. The rescoring is local arithmetic - no model call. Turn it off and k rows in are k rows out, ranked by the backend alone:

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The qmd backend

qmd adds vector search and reranking, so meaning matches even when vocabulary does not.
If qmd is not on your PATH, px0 says so and points you back at local rather than failing obscurely mid-search.

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The model download is your call

The first reindex after switching prints the download sizes and waits for an explicit y:
Decline and px0 keeps indexing keywords only. It degrades rather than breaks, and it will not ask again until you consent - nothing about px0 downloads two gigabytes onto your machine without asking. Your answer is recorded in the store, and px0 doctor reports both the consent state and whether your installed qmd matches the version px0 is pinned to:
If the versions drift, doctor tells you the exact install command for the pinned build - and reminds you that dropping back to local is a legitimate answer, not a defeat.

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Pointing at a specific binary

Useful when qmd is not the right command - a wrapper script, a pinned path, a version manager shim.

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Switching back

Nothing is lost. The index is rebuilt from brain/ either way, because the Markdown files are the source of truth and the index is derived.

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Settings

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What is never indexed

The private folder (brain/work/ by default) is excluded from every retrieval on both backends - a hard exclusion, not a ranking penalty. See The brain.