Summary

palace._DEFAULT_BACKEND is hard-coded to ChromaBackend(), so the MEMPALACE_BACKEND environment variable that backends.registry.resolve_backend_for_palace is documented to honour (RFC 001 §3.3 priority 3) has no effect on the miner, search, or the MCP server.

As a result, third-party backends declared via the mempalace.backends entry point are discoverable via available_backends() but never actually used in practice.

This PR wires palace._DEFAULT_BACKEND through the registry so an opt-in env var selects the backend, and falls back to ChromaBackend when no override is set — existing installs behave exactly as before.

Change

def _resolve_default_backend():
    name = resolve_backend_for_palace(
        env_value=os.environ.get("MEMPALACE_BACKEND"),
    )
    if name == "chroma":
        return ChromaBackend()
    return get_backend(name)


_DEFAULT_BACKEND = _resolve_default_backend()

+17 / -1 in one file.

Why this matters

With this single hook in place, a backend distributed as a separate package (e.g. the in-development mempalace-postgres) can be selected simply with:

pip install mempalace-postgres
export MEMPALACE_BACKEND=postgres
export MEMPALACE_PG_DSN=postgres://…
mempalace mine ./my-project

No fork, no monkey-patch, no touching the Chroma default.

This unlocks the contract already described in backends/registry.py and aligns the default-selection path with the RFC 001 priority order.

Scope

Deliberately minimal:

• No behavioural change for existing Chroma users (when MEMPALACE_BACKEND is unset the resolution lands on "chroma" and we still instantiate ChromaBackend() directly).
• No new dependencies.
• No changes to the get_collection call site — only the resolution of _DEFAULT_BACKEND.

Test plan

• Unset MEMPALACE_BACKEND → mempalace mine still uses Chroma (no regression).
• Export MEMPALACE_BACKEND=chroma → identical behaviour.
• Export MEMPALACE_BACKEND=<bogus> → early KeyError: unknown backend from the registry (fail fast, same as existing get_backend behaviour).
• With an entry-point backend installed (e.g. mempalace-postgres) and MEMPALACE_BACKEND=postgres → miner writes to that backend.