Multi-Agent Does Not Mean Parallel: Safe Workflows with Google ADK

“Let’s split it into agents” has become the AI equivalent of “let’s make it a microservice.” Sometimes the boundary is useful. Sometimes it only creates more state, more coordination, and a harder failure to explain. The most dangerous assumption is that separate agents should run in parallel. Parallelism is safe only when the branches are genuinely independent. If one branch changes the world while another is evaluating it, both agents can make locally reasonable decisions that are unsafe together. Google ADK 2.0 makes workflow topology explicit through graph-based Workflow objects. That is valuable because sequences, branches, and joins become part of the program instead of an agreement hidden in a supervisor prompt. Series note: This is Part 5 of Reliable Google AI Agents in TypeScript . The examples were checked against @google/adk 2.0.0 in September 2026. Start with the dependency, not the agent count Imagine a system preparing a hotel recommendation. It needs live inventory, company travel policy, and a final recommendation. Inventory lookup and policy evaluation can run concurrently because both observe the same request and neither changes shared state. The final decision must wait for both. Now consider a different pair of operations: one agent changes the reservation; another calculates an upgrade using the current reservation. Those branches are not independent. Running them concurrently can make the upgrade decision depend on state that no longer exists. Before drawing a parallel branch, ask: Do both operations only read the same starting state? Can either operation change data the other consumes? Can either produce an irreversible side effect? Is there a deterministic way to combine their results? What happens when one succeeds and the other times out? If those answers are unclear, parallel is an optimization you have not earned yet. Encode safe parallelism as fan-out and join ADK’s TypeScript Workflow graph can express two independent branches and a joi