What 100% Test Coverage Missed: State Across Google ADK A2A Boundaries

I created this article for the purpose of entering the All Things Agentic Hackathon. TL;DR — An ADK output_key writes into the session of the agent that declares it. In-process that session is shared, so it looks like state flows. Across a RemoteA2aAgent hop it is the worker's session, and it never comes back. Nothing raises. Nothing warns. Every local run and every CI job exercises the working topology, so the failure is invisible to an offline test suite by construction — including at 100% coverage. The system that passed Bastion is a three-agent access-governance fleet built with Google ADK and A2A. An Orchestrator owns investigation state, an Access Auditor reads production IAM through a read-only identity, and a model-free Escalation Agent delivers validated count-only reviews. The local graph passed its configured core statement and branch coverage gate. Every branch, every seam. Then the same graph was split across deployed A2A workers, and an assumption that looked natural in-process became false. The boundary we had not modeled In-process, the previous step's result is simply there : # The Auditor declares output_key; the Orchestrator reads it back. report = ctx . session . state . get ( AUDIT_FINDINGS_KEY ) Deploy the same sequence and only the construction changes. The graph is identical: RemoteA2aAgent ( name = " access_auditor " , agent_card = card_url ( auditor , " access_auditor " ), description = " Reads the live IAM policy and flags anomalies. Read-only. " , httpx_client = private_a2a_client ( auditor ), a2a_request_meta_provider = _forward_investigation , ) output_key still writes. It writes into the worker's session, which never crosses back. The deployed Orchestrator saw an empty state key while every local run and every test saw a populated one. Observed 2026-08-22: the Auditor completed a full sub-trail, and the next step then refused with "returned no structured report." No exception at the boundary. No warning at construction. The run still r