The negotiated-capability PREDICATE: given a Connection's snapshotted capability tree, decide whether a wire method is negotiated and therefore permitted.

This is the security-relevant half of the capability gate. MethodTable owns the :capability column (which method is gated, by which struct-field path — see its capability typedoc and the ORACLE-DIVERGENCE note); this module resolves a path against a live snapshot.

Which snapshot

The gate lives on the receiver and checks the receiver's OWN advertised capabilities (Connection.capability_denied?/2, before Router.decode/4):

• an agent rejects an inbound client→agent method it never advertised (session/load unless it advertised loadSession, …) — the snapshot is the agent's own AgentCapabilities (from the initialize RESPONSE it sent);
• a client rejects an inbound agent→client callback it never advertised (terminal/* unless it advertised terminal, fs/write_text_file unless it advertised fileSystem.writeTextFile, …) — the snapshot is the client's own ClientCapabilities (from the initialize REQUEST it sent).

Because every gated wire method is single-direction and its MethodTable capability tag (:agent / :client) matches the side that HANDLES it, negotiated?/2 needs only the receiver's snapshot and the method string — the tag is not consulted for resolution (it is a compile-time assertion, invariant 7).

Fail closed

Resolution walks the field path from the snapshot struct. A truthy leaf (true, or a present nested capability struct/map) ⇒ negotiated. Anything else — a missing field, a nil/false leaf, a nil snapshot, or an :never-gated method — is NOT negotiated. Absent capability is denial, by design: the Connection maps a non-negotiated gated method to -32601 BEFORE decode (so -32601 beats -32602). Baseline methods (capability: nil in the table — initialize, session/new, session/prompt, session/cancel, …) are always available. An unknown method is NOT a capability denial (decode/4 answers it with -32601 on its own).