User identity at the tool boundary answers whose authority applies when an agent acts on someone’s behalf. Agent identity in A2A answers which autonomous system is delegating to another autonomous system. The first is about scoped authorization for tool use. The second is about how agents authenticate and trust each other across a coordination chain.
How the Two Identities Differ at the Boundary
User identity at the tool boundary is about whose authority is being exercised when a tool call is made on a person’s behalf. Agent identity in A2A is about which autonomous system is one agent talking to, delegating to, or trusting in another autonomous system. That difference matters because the first scopes permission to an action, while the second establishes the machine-to-machine trust relationship that makes the whole coordination chain work.
At the tool boundary, the key question is whether the agent is allowed to use a specific capability under a user’s delegated authority. In A2A, the key question is whether the receiving agent can verify the caller as a legitimate peer agent, not just a relay for a human request. Those are related, but they are not the same control point.
The practical separation is important when an agent performs multiple hops. A tool call may still be acting under a user’s scope even if the agent-to-agent relationship behind it is different, and the trust checks for A2A do not automatically answer whether the downstream tool invocation is authorized for that user. The architecture has to answer both questions cleanly.
Why Authorization and Trust Move to Different Control Points
The user-facing side of the boundary is mainly about scoped authorization, consent, and least privilege for tool use. If the agent overreaches there, the failure is that a user-authorized action becomes broader than intended. That is the same basic pattern as any delegated access model, only with the added complexity that the actor is autonomous and may chain tools quickly.
The A2A side is mainly about peer authentication, message trust, and delegated coordination between agents. Here the risk is not whether a tool can be used, but whether the recipient should believe that another agent is who it claims to be, should accept its instructions, and should preserve the intended delegation chain. That trust decision is what keeps inter-agent workflows from becoming an open relay for untrusted automation.
When teams blur these layers, they often use one identity system to prove both things and assume the rest is handled. In practice, a valid user token does not automatically establish a trustworthy agent peer relationship, and a valid agent-to-agent trust channel does not automatically authorize the final tool action. The two controls should be designed to complement each other, not substitute for each other.
What Practitioners Should Check Before Treating Them as Equivalent
Tool-boundary identity should answer: who is the human principal, what scope was delegated, and what tool action is allowed under that scope. A2A identity should answer: which agent is speaking, how was it authenticated, what delegation proof is attached, and whether the receiving agent should continue the chain. If either answer is vague, the system is relying on implicit trust instead of explicit control.
For broader agent systems, the boundary between these identities is easiest to lose in logs and policy. Teams should be able to distinguish a user acting through an agent, an agent acting as a peer, and an agent acting with its own standing authority. Those are different accountability states, and they need different audit and approval logic.
One useful test is whether you can revoke one without breaking the other. If you can remove a user’s permission to invoke a tool but still allow the agent to participate in A2A coordination, the design is probably separating authorization from peer trust correctly. If revoking one also silently breaks the other, the trust model is likely too entangled.
Risk and Threat Considerations
Collapsing user authority and agent peer identity creates a confused-deputy problem: a system may accept a request because the user is legitimate, while the receiving agent treats it as if it came from a trusted autonomous peer. That can widen blast radius, hide delegation boundaries, and make abuse harder to detect across a multi-hop chain.
Failure mechanism: An attacker, or a mis-scoped automation path, can exploit the gap between delegated user scope and autonomous peer trust by replaying, forwarding, or overextending authority across agents and tools.
Impact: The result can be unauthorized tool use, privilege amplification, weak attribution, and accidental trust propagation across agents that were never meant to share the same authority model.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Agentic AI Top 10 addresses the attack and risk surface, while NIST SP 800-53 Rev 5 sets the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP Agentic AI Top 10 | ASI03 — Identity & Privilege Abuse | Covers delegated authority and privilege boundaries between users and agents. |
| ASI07 — Insecure Inter-Agent Communication | A2A depends on authenticated, trustworthy agent-to-agent exchanges. | |
| ASI09 — Human-Agent Trust Exploitation | The question centers on how user authority can be confused with autonomous agent trust. | |
| Recommendation — Separate user-scoped authorization from agent peer trust at every hop. Authenticate peer agents and sign inter-agent messages before trusting delegation. Prevent human authority from being implicitly extended across autonomous agent boundaries. | ||
| NIST SP 800-53 Rev 5 | IA-2 — Identification and Authentication (Organizational Users) | User identity at the tool boundary requires strong user authentication before delegated actions. |
| IA-9 — Identification and Authentication (Service and Non-Organizational Users) | Agent-to-agent trust is a service-style authentication problem between autonomous systems. | |
| AC-6 — Least Privilege | Tool-boundary identity should limit delegated actions to the minimum necessary scope. | |
| Recommendation — Authenticate the human principal before allowing scoped tool use. Use service authentication and peer credentials for agent-to-agent trust. Restrict delegated tool permissions to the minimum required for the user task. | ||
Practitioner Guidance
What to verify: Confirm that the system can show both the originating user scope and the receiving agent’s authenticated identity for every hop. If audit records only show “an agent did it,” the boundary is too coarse to support safe delegation.
Decision rule: If the action changes external state, treat user authorization and agent peer trust as separate checks. If the action only routes or enriches context, the A2A trust layer can be lighter, but it still needs explicit peer authentication.
Common mistake: Designing the agent graph as if a valid upstream user token is enough to trust every downstream autonomous hop. That shortcut usually works in demos and fails when delegation chains become longer or cross organizational boundaries.
Practitioner takeaway: User identity answers “who may authorize the tool action,” while agent identity in A2A answers “which autonomous peer may be trusted in the chain,” and secure systems must keep those decisions distinct even when they are linked.
Related resources from NHI Mgmt Group
- What is the difference between human identity governance and AI agent governance?
- What is the difference between workload identity and API keys for AI agents?
- What is the difference between governing human access and governing AI agent access?
- What is the difference between agent identity policy and tool policy?
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 30, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
