Google ADK Agent Development

Critical Architecture Insight

SequentialAgent runs ALL sub-agents unconditionally. There is NO native conditional branching. The pipeline does not stop if an agent outputs "REJECT" or any other signal.

# THIS RUNS ALL 4 AGENTS regardless of gatekeeper output
research_cycle = SequentialAgent(
    sub_agents=[theorist, critic, gatekeeper, architect]
)

To skip agents, use transfer_to_agent tool.

Data Flow: output_key -> session.state -> {placeholder}

# Agent A stores output
agent_a = Agent(
    instruction="Generate hypothesis",
    output_key="hypothesis"  # Stored in session.state["hypothesis"]
)

# Agent B reads via placeholder
agent_b = Agent(
    instruction="""
    Critique this hypothesis:
    {hypothesis}
    """,  # Resolved from session.state at runtime
    output_key="critique"
)

Placeholders support optional syntax: {variable?} (no error if missing).

Conditional Branching with transfer_to_agent

from google.adk.tools import transfer_to_agent

gatekeeper = Agent(
    name="gatekeeper",
    instruction="""
    Decide: PROCEED, REVISE, or REJECT

    - PROCEED: Continue normally (don't call transfer_to_agent)
    - REVISE: Call transfer_to_agent("theorist")
    - REJECT: Call transfer_to_agent("reporter") to skip experiment
    """,
    tools=[transfer_to_agent],
    output_key="gate_decision"
)

The runner sees event.actions.transfer_to_agent and jumps to that agent.

FunctionTool Design

Automatic tool_context injection

def my_tool(
    query: str,                    # From LLM function call
    limit: int = 10,               # Optional parameter
    tool_context: ToolContext      # Auto-injected, never passed by LLM
) -> str:
    # Access session state
    session = tool_context.invocation_context.session
    previous = session.state.get("previous_result")
    return f"Result for {query}"

my_tool_wrapped = FunctionTool(func=my_tool)

Docstring becomes tool description

def search_papers(query: str, tool_context: ToolContext) -> str:
    """
    Search academic papers on arXiv.

    Args:
        query: Search terms for paper lookup

    Returns:
        Formatted list of matching papers
    """

The docstring is sent to the LLM as the tool description.

Agent Types

Type	Behavior
Agent (LlmAgent)	Single LLM agent with tools
SequentialAgent	Runs sub_agents in order, ALL of them
LoopAgent	Repeats until escalate=True or max_iterations
ParallelAgent	Runs sub_agents concurrently with branch isolation

Common Pitfalls

1. Expecting SequentialAgent to stop on rejection

# WRONG: Architect runs even if gatekeeper rejects
SequentialAgent(sub_agents=[theorist, gatekeeper, architect])

# RIGHT: Gatekeeper uses transfer_to_agent to skip
gatekeeper = Agent(
    tools=[transfer_to_agent],
    instruction="If REJECT, call transfer_to_agent('reporter')"
)

2. Missing output_key breaks data flow

# WRONG: No output_key, next agent can't access result
theorist = Agent(instruction="Generate hypothesis")

# RIGHT: Output stored in session.state
theorist = Agent(instruction="...", output_key="hypothesis")

3. Placeholder without matching output_key

# WRONG: {analysis} referenced but no agent sets output_key="analysis"
editor = Agent(instruction="Review: {analysis}")

# Debug: Check which agents set output_key

4. tool_context as required parameter

# WRONG: tool_context has no default, LLM can't call it
def bad_tool(query: str, tool_context: ToolContext) -> str: ...

# RIGHT: Default allows LLM to omit it (auto-injected anyway)
def good_tool(query: str, tool_context: ToolContext = None) -> str: ...

5. launch_persistent_context returns BrowserContext

# Type hint should be BrowserContext, not Browser
self._browser: Optional[BrowserContext] = None
self._browser = playwright.chromium.launch_persistent_context(...)

6. Context overflow from session history (token limit exceeded)

ADK stores all conversation history in session.db which gets sent to the LLM. This causes "input token count exceeds maximum" errors.

# WRONG: Agent loads full conversation history (can exceed 128K+ tokens)
root_agent = Agent(name="my_agent", model=MODEL)

# RIGHT: Prevent loading conversation history
root_agent = Agent(
    name="my_agent",
    model=MODEL,
    include_contents='none',  # Stateless - no prior context
)

Also clear .adk/session.db files when they grow too large:

rm -rf ika_agent/.adk/session.db

Control Flow Actions

# From tool or callback
tool_context.actions.transfer_to_agent = "agent_name"  # Jump to agent
tool_context.actions.escalate = True                    # Exit loop
tool_context.actions.skip_summarization = True          # Skip post-tool summary

# State updates
event.actions.state_delta = {"key": "value"}            # Update session.state

Thread Safety for Shared State

When tools share global state:

import threading

memory_lock = threading.Lock()

def save_result(data: str, tool_context: ToolContext) -> str:
    global shared_state
    with memory_lock:
        shared_state = reload_from_disk()  # Get fresh state
        shared_state.append(data)
        save_to_disk(shared_state)
    return "Saved"

Sub-Agent Hierarchy

root_agent = Agent(
    name="orchestrator",
    sub_agents=[
        SequentialAgent(
            name="pipeline",
            sub_agents=[agent_a, agent_b, agent_c]
        ),
        standalone_agent,
    ],
    instruction="""
    Commands:
    - "run pipeline" -> Transfer to pipeline
    - "analyze" -> Transfer to standalone_agent
    """,
    tools=[transfer_to_agent]
)

All agents in SequentialAgent share the same session.state.

Computer Use Model

Computer use requires dedicated model - no Gemini 3 version exists yet:

COMPUTER_USE_MODEL = "gemini-2.5-computer-use-preview-10-2025"

Standard agents can use gemini-3-flash-preview, but browser automation agents must use the 2.5 computer use model.

Skill Maintenance

When discovering new ADK patterns through actual code execution, add them to this skill file at C:\Users\cuba6\.claude\skills\google-adk\SKILL.md. Only add 100% verified behaviors - no assumptions.

Resources

• GitHub: https://github.com/google/adk-python
• Docs: https://google.github.io/adk-docs/
• Source: Explore installed package at google/adk/agents/ and google/adk/tools/