Multi-Agent Orchestration With the ADK

SequentialAgent runs its sub-agents one after another. Output flows forward through shared session state. Use it when step B genuinely depends on step A's result: research, then draft, then edit.

Parallel: fan out independent work

ParallelAgent runs its sub-agents concurrently. Each writes to its own key in session state. Use it when tasks are independent and you want them to finish in the time of the slowest one, not the sum of all of them. Think: summarize three documents at once, or query three data sources in parallel.

Coordinator: dispatch by decision

A coordinator is a normal LlmAgent that has other agents registered as sub_agents. Instead of running everything, the coordinator *reads the request and decides* which specialist should handle it, then transfers control. This is the dynamic pattern: the model routes at runtime rather than following a fixed order.

The key distinction: sequential and parallel are *deterministic* (you decided the flow at build time). A coordinator is *model-driven* (the LLMLLMA Large Language Model is an AI system trained on vast text data to predict and generate language, enabling tasks like writing, summarizing, and answering questions.View full definition → decides the flow per request). Real systems mix all three.

When multi-agent beats a single agent

Splitting has real costs, so justify it. A multi-agent design wins when:

• Roles need different tools or instructions. A research agent needs Google Search grounding; a drafting agent needs a house style guide and no web access. Separate agents keep each prompt short and each toolset scoped.
• You want parallelism. One agent cannot truly do two things at once. Fanning out cuts wall-clock time.
• You want isolation and testability. You can unit-test the "reviewer" agent alone, swap its model from Gemini Pro to Flash, and never touch the rest.
• The context is getting crowded. Even with Gemini's long context, stuffing research notes, style rules, and draft history into one window degrades focus. Specialists keep each context lean.

It costs you: more moving parts, more places for latency to accumulate, harder debugging (which agent produced the bad output?), and more tokenstokensA token is the basic unit of text that language models process, often a word fragment, whole word, or punctuation mark rather than a single character.View full definition → overall because agents re-establish context when they hand off. If one clear prompt with two tools does the job, use one agent. ReachReachThe number of unique people exposed to your message in a given period. Unlike impressions, reach counts each person once, no matter how often they see it.View full definition → for orchestration when the seams above start to hurt.

A concrete example: coordinator fans out to research and drafting

Let's build the classic content pipelinepipelineAll active sales opportunities across the stages of the sales process, together with their combined potential value and probability of closing.View full definition →. A coordinator receives a topic. It fans out to a research agent (which uses Google Search grounding) and a drafting agent in parallel, then a final stage stitches the results.

Here the fan-out is genuinely parallel: research and an outline can start simultaneously, then a writer combines them. We compose a ParallelAgent inside a SequentialAgent.

Read the data flowdata flowAn automated sequence of steps that moves data from source to destination: ingestion, transformation, validation, and loading, so it arrives clean and ready to use.View full definition →, because that is where orchestration actually lives:

• output_key="research_notes" tells ADK to write the researcher's final output into session state under that key.
• The outliner writes to outline, running concurrently inside gather.
• The writer reads both with the {research_notes} and {outline} template placeholders in its instruction. ADK substitutes state values before the prompt is sent.
• SequentialAgent guarantees gather fully completes before writer starts, so both keys exist by the time the writer runs.

Notice the model choices. Research and outlining go to Gemini Flash: fast, cheap, high volume. The final draft, where quality matters most, goes to Gemini Pro. Mixing tiers per agent is one of the biggest practical wins of multi-agent design, and something a single agent cannot do.

Running it

ADK gives you a local dev loop. From your project directory:

adk web launches a browser UI where you send input, watch each sub-agent fire, and inspect session state key by key. When the writer produces nonsense, you look at research_notes and outline first: if they were bad, the fault is upstream, not in the writer. That traceability is the practical payoff of explicit orchestration.

Coordinator-as-router: the dynamic variant

The example above hard-codes the flow. Sometimes you want the model to decide. Say some requests need research and others are pure rewrites. Make the coordinator an LlmAgent with specialists as sub_agents:

Now the LLMLLMA Large Language Model is an AI system trained on vast text data to predict and generate language, enabling tasks like writing, summarizing, and answering questions.View full definition → reads intent and *transfers* to the right sub-agent. This is called agent transfer in ADK: control (and the conversation) moves to the chosen sub-agent, which can transfer back or to a sibling. Powerful, but the tradeoff is real: routing is now a model decision, so it can be wrong. For high-stakes flows, prefer deterministic SequentialAgent/ParallelAgent and keep model-driven routing for genuinely branching intent.

Tools, grounding, and where this runs

Two things make these agents useful rather than academic.

Grounding with Google Search. The google_search tool gives the researcher live, cited results instead of stale training data. That is the difference between "facts as of the model's training cutoff" and "facts as of this morning." Scope it to only the agent that needs it: your drafting agent should not silently invent citations, so give it no search tool at all and force it to rely on state.

Sub-agents can be tools too. Beyond sub_agents (which hand off control), ADK lets you wrap an agent as a callable tool via AgentTool. The difference matters: a sub-agent takes over the turn, while an agent-as-tool returns a result to the caller, like any function. Use AgentTool when the coordinator should stay in charge and just "consult" a specialist mid-reasoning; use sub_agents when you want a clean handoff.

Where it deploys. You prototype locally with adk web or adk run. For production, the same agent definition deploys to Vertex AI Agent Engine, Google's managed runtime that handles sessions, scaling, and tracing, so you are not running the orchestration loop on your own server. You can also containerize and run it on Cloud Run. The point: the code above does not change between your laptop and production. You configure credentials and target, not architecture.

Practical guardrails

A few things that separate a demo from something you would ship:

• Name and describe every agent clearly. When a coordinator routes by model decision, it uses each sub-agent's name and description to choose. Vague descriptions cause misrouting. Write them like you are writing tool docs for the model, because that is exactly what they are.
• Keep state keys explicit and few. Every output_key is a contract between a producer and a consumer. Fewer, well-named keys mean fewer silent mismatches.
• Watch token cost at the seams. Each handoff can replay context. If your parallel branches are large, they multiply. Send bulky research to Flash, reserve Pro for the synthesis step, and trim what each agent passes downstream.
• Test agents in isolation first. Run the researcher alone with a fixed topic until its output is reliable. Only then wire it into the pipelinepipelineAll active sales opportunities across the stages of the sales process, together with their combined potential value and probability of closing.View full definition →. Debugging a five-agent system end-to-end is miserable; debugging one agent is easy.
• Fail loudly. Decide what the writer does if

Key Takeaways

• Use `SequentialAgent` for dependent stages, `ParallelAgent` for independent work, and an `LlmAgent` coordinator for runtime routing. Deterministic flow is more reliable; save model-driven routing for genuinely branching intent.
• State keys are the wiring. output_key writes to session state and {placeholder} reads it back. Keep keys few, named, and treated as contracts between agents.
• Mix model tiers per agent. Send high-volume research and outlining to Gemini Flash, reserve Gemini Pro for the quality-critical synthesis step. This per-agent choice is a core reason to split at all.
• Scope tools to the agent that needs them. Give Google Search grounding only to the researcher; deny it to the drafter so it cannot invent citations.
• Prototype with `adk web`, deploy the same code to Vertex AI Agent Engine. Split into multiple agents only when roles, tools, parallelism, or context pressure justify the extra debugging surface.