pydantic-ai-common-pitfalls

PydanticAI Common Pitfalls and Debugging

Tool Decorator Errors

Wrong: RunContext in tool_plain

# ERROR: RunContext not allowed in tool_plain
@agent.tool_plain
async def bad_tool(ctx: RunContext[MyDeps]) -> str:
    return "oops"
# UserError: RunContext annotations can only be used with tools that take context

Fix: Use @agent.tool if you need context:

@agent.tool
async def good_tool(ctx: RunContext[MyDeps]) -> str:
    return "works"

Wrong: Missing RunContext in tool

# ERROR: First param must be RunContext
@agent.tool
def bad_tool(user_id: int) -> str:
    return "oops"
# UserError: First parameter of tools that take context must be annotated with RunContext[...]

Fix: Add RunContext as first parameter:

@agent.tool
def good_tool(ctx: RunContext[MyDeps], user_id: int) -> str:
    return "works"

Wrong: RunContext not first

# ERROR: RunContext must be first parameter
@agent.tool
def bad_tool(user_id: int, ctx: RunContext[MyDeps]) -> str:
    return "oops"

Fix: RunContext must always be the first parameter.

Valid Patterns (Not Errors)

Raw Function Tool Registration

The following pattern IS valid and supported by pydantic-ai:

from pydantic_ai import Agent, RunContext

async def search_db(ctx: RunContext[MyDeps], query: str) -> list[dict]:
    """Search the database."""
    return await ctx.deps.db.search(query)

async def get_user(ctx: RunContext[MyDeps], user_id: int) -> dict:
    """Get user by ID."""
    return await ctx.deps.db.get_user(user_id)

# Valid: Pass raw functions to Agent(tools=[...])
agent = Agent(
    'openai:gpt-4o',
    deps_type=MyDeps,
    tools=[search_db, get_user]  # RunContext detected from signature
)

Why this works: PydanticAI inspects function signatures. If the first parameter is RunContext[T], it's treated as a context-aware tool. No decorator required.

Reference: https://ai.pydantic.dev/agents/#registering-tools-via-the-tools-argument