Module turn_cache 

Per-turn memoization of turn-stable host capability reads.

Context assembly repeatedly reads the same host facts per agent-loop iteration. harn#5190 measured ~20 identical runtime.pipeline_input round-trips per turn; Burin H-063 later measured roughly 164 repeated project.metadata_get calls in an ordinary context build. This module front-runs the thread-local HOST_CALL_BRIDGE with a per-turn memo so those reads collapse to one host round-trip per stable fact. Project metadata is fetched once as a typed directory snapshot and namespace reads project from it locally. The allowlist ([is_turn_stable]) is deliberately narrow, and metadata mutations invalidate the whole memo before and after dispatch so inherited read-after-write values cannot be stale.

The memoized value is stable only within a turn: the host re-projects runtime.pipeline_input each turn (e.g. so a mid-session model switch is observed on the next prompt). The memo is therefore cleared at each agent-loop iteration boundary (iteration_start, wired in __host_agent_emit_event) and at run/embedder boundaries via [reset].

Functions

lookup

Read a turn-stable host fact from the current turn’s memo.

lookup_by_name

Split a dotted capability.operation host-call name and lookup it. Convenience for embedder host_call implementations, which receive the dotted wire name rather than a split pair.

store

Memoize a turn-stable host fact for the remainder of the current turn. Non-allowlisted (capability, operation) pairs are ignored, so a caller cannot widen the allowlist by calling this directly. See lookup.

store_by_name

Dotted-name counterpart to store. See lookup_by_name.