AIAgent.__init__ calls set_current_session_id(self.session_id), which
mutated both the task-local ContextVar and the process-global os.environ.
Because _build_child_agent wraps construction in delegated_child_context(),
the ContextVar write is harmless (task-local), but the os.environ write
clobbered the parent's HERMES_SESSION_ID for the rest of the process —
leaking the child id into parent tools and subprocesses spawned after
the child was built.
Root cause of HermesPRDelegationSessionContext: parent
20260729_212118_5d797e dispatched child 20260730_160515_736ea1; later
parent terminal inherited HERMES_SESSION_ID=the child.
Fix: set_current_session_id() skips the process-global os.environ write
when called from within a delegated_child_context(). The child's own
tools and subprocesses still resolve their id through the ContextVar
(task-local), while the parent's process-wide env keeps the parent's
session identity. Root agents (CLI, gateway, cron) retain both paths.
Adds 7 regression tests covering single child, concurrent children (8
parallel), parent-tool observation after construction, and root-agent
session rotation backward compatibility. All pass; ruff clean.
Xipong
0af8fb05bf