What it means
A cache is declared on the descriptor (CachePolicy::Process { max_entries, ttl }) and lives in the one executor every transport uses, so MCP, HTTP and the command line share it and none has a cache of its own. For every cached capability and every case its input type provides, the value answered with the cache off, with an empty cache and with a warm cache is the same; a warm answer runs no handler; an error is computed every time; a command is never cached and the registry refuses a descriptor that asks; the cache is bounded per capability and evicts the oldest; the same input with its keys in another order is the same key; and the key carries the registry fingerprint, a hash of every object's content and every descriptor, so two repository states never share an entry.
How it works
apps/majordomus-cli/src/capability/executor.rs: CapabilityExecutor::execute looks the capability up, normalises the input (canonical_json), consults the cache under the descriptor's policy, runs the handler through registry.dispatch, and stores the value; index.rs and registry.rs compute the fingerprints with sha-256; perf.rs counts hits, misses and evictions.
How to see it
apps/majordomus-cli/target/debug/majordomus capabilities describe objects.search --format json | jq .cache
apps/majordomus-cli/target/debug/majordomus bench objects.search --transport direct --profile quick # cold and warm rows, the direct one with handler invocationsWhat it does not cover
Nothing is persisted: the cache is one process's memory and ends with it. Only what a measurement justified is cached (objects.search, capabilities.list); the rest answers from the immutable index in microseconds and a cache there would be a second copy of nothing. A TTL is supported by the policy and used by no builtin today.
Why it exists
A cache that is not proved equivalent is a lie waiting for an input. project.rust-hot-path is the rule, ADR 0004 the decision. apps/majordomus-cli/tests/executor.rs iterates every cached capability of the registry with every case; tests/properties.rs does the same over generated inputs with proptest, including shuffled key order; test/cases/91_canonical_architecture.sh sees the hits in the counters after repeated searches through the built binary.