The salvaged observer contract gated the API-request hot path on has_hook()
but left the per-tool emit ungated: every tool call ran result-field
derivation + payload dict build + invoke_hook dispatch even with zero
plugins registered.
- _emit_post_tool_call_hook now short-circuits on has_hook("post_tool_call")
and derives status/error fields lazily (after the gate, only when a
listener will consume them). status defaults to None -> derived; explicit
blocked/cancelled callers still pass status through.
- transform_tool_result emit (pre-existing hook) likewise gated on
has_hook(); skips _tool_result_observer_fields when no listener.
- Removed the now-redundant _tool_result_observer_fields pre-computation at
the three ok-path call sites (model_tools, agent_runtime_helpers,
tool_executor) — the helper derives them, so the no-listener path costs
one dict lookup and the call sites shrink.
- Tests: stub has_hook=True where payload correctness is asserted; add a
no-listener regression proving post_tool_call/transform_tool_result emit
is skipped when nothing is registered.
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