feat(a2a): add agent_card_url property to A2AServer for customizable url in AgentCard

[waitasecant]

Description

A2AServer currently sets the url field in the AgentCard to http_url, which always appends a trailing slash. In load-balanced or reverse-proxy deployments, the advertised URL may need to differ from the internal http_url - for example, to omit the trailing slash or use a completely different base URL.

This PR adds an agent_card_url property to A2AServer that defaults to http_url but can be overridden via a setter, giving users control over the URL advertised in the AgentCard.

Related Issues

No related issues.

Documentation PR

To be created after approval

Type of Change

New feature

Testing

Yes, I have tested the change and verified it doesn't break functionality.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

Use Cases

1. Trailing slashes are not preferred.

# The agent will be accessible at http://my-alb.amazonaws.com/calculator/
a2a_server = A2AServer(
    agent=agent,
    http_url="http://my-alb.amazonaws.com/calculator"
)

# The agent will be accessible at http://my-alb.amazonaws.com/calculator
a2a_server = A2AServer(
    agent=agent,
    http_url="http://my-alb.amazonaws.com/calculator"
)
a2a_server.agent_card_url("http://my-alb.amazonaws.com/calculator")

2. In load-balanced or reverse-proxy deployments, the advertised URL may need to differ.

from unittest.mock import MagicMock
from strands import Agent
from strands.models.model import Model
from strands.multiagent.a2a import A2AServer
from a2a.types import AgentSkill


INTERNAL_HOST = "0.0.0.0"
INTERNAL_PORT = 8001
INTERNAL_URL = f"http://{INTERNAL_HOST}:{INTERNAL_PORT}/chat"
PUBLIC_URL = "https://api.example.com"

SKILLS = [
    AgentSkill(
        id="skill-id",
        name="Agent Name",
        description="Agent Description",
        tags=["tag"],
        examples=["Example."],
    )
]

mock_model = MagicMock(spec=Model)

agent = Agent(
    model=mock_model,
    name="Agent Name",
    description="Agent Description",
    tools=[],
    callback_handler=None,
)


# Without agent_card_url
server = A2AServer(
    agent=agent,
    http_url=INTERNAL_URL,
    skills=SKILLS,
)

# The agent card advertises `http://0.0.0.0:8001/chat/`. External clients cannot reach this address.
# Any orchestrator or peer agent that reads the card will fail to connect.
card = server.public_agent_card

"""
One workaround could be to pass public url as `http_url`. The card.url would be correct, but
- The server derives its internal mount_path from http_url.
- Starlette routes are built around that external hostname / path.
- In the container, the process still binds to {INTERNAL_HOST}:{INTERNAL_PORT}.
- The mismatch between what A2AServer thinks is its base URL and
  what uvicorn actually serves on leads to routing confusion.
- You'd also have to manually reconcile the  /agent-card mount that live outside the A2A server.
"""

A2AServer(agent=agent, http_url="{PUBLIC_URL}/chat")

# With agent_card_url
server_with = A2AServer(
    agent=agent,
    http_url=INTERNAL_HTTP_URL,  # container-internal address
    skills=SKILLS,
)

# One-liner override - the card now advertises the gateway URL
server.agent_card_url = f"{PUBLIC_URL}/chat"

card = server.public_agent_card

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