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If the application calls A2A agents, the same key finds them. The SDK does not speak A2A: like agent.mcp, it hands over an address and the headers that authenticate it, and an A2A client (a2a-sdk, @a2a-js/sdk) reads the card and sends the calls, both through the gateway.

What an agent carries

With an A2A client

The a2a_agent.py and a2a-agent.ts examples in the SDK repository run as they are.

For an end user

Name the person the calls are for, as with for_end_user(). The headers then carry X-NeuralTrust-End-User, and the call runs as that user: their tasks are theirs, and an agent that verifies TrustGate’s token sees them as sub.

When it fails

Errors from the agent and from the gateway’s agent plane reach the A2A client, not the SDK: see Connect to an agent.