Security teams should treat MCP as the execution layer and A2A as the coordination layer. MCP standardizes how an agent discovers and calls tools, while A2A standardizes how agents find each other and delegate work. Keep the tool layer strictly separate so coordination choices can change later without forcing a rewrite of every integration or access path.
Why MCP and A2A need different security boundaries
MCP and A2A solve different control problems, so they should not share the same security assumptions. MCP is about how an agent reaches tools and services. A2A is about how agents discover one another, exchange intent, and delegate work. If those layers blur together, teams usually end up with brittle trust paths, unclear authorization points, and harder incident containment.
The practical separation is architectural as much as it is protocol-level. Tool access should be governed where the action occurs, while inter-agent coordination should be governed where delegation and task handoff occur. That distinction helps teams reason about auditability, blast radius, and which policy decisions must remain stable even if the coordination model changes.
This is why teams should avoid letting a coordination mechanism become a hidden shortcut into tool execution. If the same path is used for both tool invocation and agent-to-agent delegation, it becomes much harder to tell whether a request came from a directly authorized agent action or from a delegated chain that should have been constrained differently. The MCP Security Guide is useful here because it frames MCP as an authorization and token-handling problem, not just a transport choice.
What belongs in the MCP layer, and what belongs in A2A
MCP should own the rules for discovering tools, invoking them, and enforcing the authorization model around those calls. That means the security team should care about token audience, passthrough behavior, gateway policy, local server exposure, and which tools can be called under which conditions. The object is to keep tool access explicit and bounded so that a tool change does not silently change agent permissions.
A2A should own the rules for how one agent finds another, exchanges context, and delegates work across boundaries. The security questions here are different: who may delegate, what evidence is attached to the delegation, whether a task can be re-delegated, and how trust is established between agents that may belong to different teams or organisations. For a deeper protocol-level treatment, the Multi-Agent and A2A Security Guide covers authentication, signed Agent Cards, and multi-hop delegation.
Keeping the layers separate also makes lifecycle management more realistic. MCP integrations often change as tools are added, replaced, or retired. A2A relationships may change as workflows are decomposed or consolidated. If the two are mixed, a change in agent coordination can force a rewrite of tool integrations, and a tool migration can unintentionally alter inter-agent trust. The cleanest design is to let each layer evolve on its own contract, with narrow interfaces between them. The Agentic AI Identity Guide is useful for understanding how delegation, registration, and retirement fit into that contract.
How to separate the control planes without breaking the architecture
The most reliable pattern is to make MCP the execution boundary and A2A the coordination boundary. In practice, that means tool permissions should be decided where the tool is called, while agent-to-agent requests should be authenticated and scoped before any work is handed off. Do not let an A2A relationship imply automatic tool authority unless the tool layer has its own explicit decision point.
Security teams should also design for least privilege at both layers, but not in the same way. MCP needs task-scoped access to specific tools, resources, or operations. A2A needs delegated authority that is narrow enough to preserve accountability across handoffs. The AI Agent Authorisation Guide helps with the tool-side decision, while the AI Agent Observability, Audit and Incident Response Guide helps teams verify that those boundaries remain visible in logs and attribution data.
At scale, the main design mistake is allowing the coordination layer to become the policy layer for execution. Once that happens, every new delegation pattern becomes a new access path, and every access path becomes an incident-response problem. A cleaner design is to make delegation request intent, but not inherit unlimited execution power.
Risk and Threat Considerations
When MCP and A2A are not separated, the most likely failure mode is privilege collapse: a coordination relationship starts acting like an execution credential. That creates a larger blast radius, makes revocation harder, and increases the chance that a delegated agent can reach tools it should never directly control.
Failure mechanism: A shared or loosely coupled trust path allows an A2A delegation to pass through to MCP tool execution without an independent authorization check, so a benign coordination change turns into an unintended access grant.
Impact: Attackers or misconfigured agents can expand from one task into many tools, which increases lateral movement, data exposure, and the difficulty of tracing who actually caused the action.
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 and OWASP API Security Top 10 address the attack and risk surface, while NIST SP 800-53 Rev 5 and NIST Zero Trust (SP 800-207) set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP Agentic AI Top 10 | ASI03 — Identity & Privilege Abuse | MCP/A2A separation is about preventing delegation from becoming execution privilege. |
| ASI07 — Insecure Inter-Agent Communication | A2A is the inter-agent trust path, so its authentication and handoff rules matter directly. | |
| ASI02 — Tool Misuse | MCP governs tool invocation, where mis-scoped or overbroad tool use creates risk. | |
| Recommendation — Enforce separate authorization for agent delegation and tool execution to prevent privilege collapse. Authenticate inter-agent messages and bind delegation to the intended agent and task. Constrain tool access at the MCP layer so coordination changes cannot widen execution rights. | ||
| NIST SP 800-53 Rev 5 | AC-6 — Least Privilege | Separate MCP and A2A to keep tool access and delegation narrowly scoped. |
| IA-5 — Authenticator Management | MCP and A2A both depend on controlled credentials, tokens, and rotation behavior. | |
| AU-2 — Audit Events | Distinct MCP and A2A layers need separate auditability for attribution and response. | |
| Recommendation — Apply least privilege so delegated agents can reach only the tools required for the task. Manage and rotate the credentials that authorize tool use and inter-agent delegation. Log delegation and tool execution as separate event classes for attribution and review. | ||
| OWASP API Security Top 10 | API5 — Broken Function Level Authorization | Tool calls in MCP need function-level checks so coordination cannot imply execution rights. |
| Recommendation — Authorize each tool function explicitly instead of inheriting permissions from agent coordination. | ||
| NIST Zero Trust (SP 800-207) | AC-6 — Least Privilege Principle | Zero trust supports separate verification for request, principal, and action across both layers. |
| Recommendation — Verify each request at the point of action and do not trust delegation alone. | ||
Practitioner Guidance
What to prioritise: Define the enforcement point for MCP separately from the enforcement point for A2A before you scale either protocol. If the team cannot point to the exact place where tool access is approved, the boundary is too soft.
What to verify: Check that delegated A2A work does not inherit standing tool access by default, and confirm that every MCP call can be traced back to a bounded, attributable request. If your logs cannot distinguish coordination from execution, your model is already too coupled.
Practitioner takeaway: The goal is not to make agents “more connected”, it is to keep delegation flexible while forcing execution to remain explicitly controlled, observable, and revocable.
Related resources from NHI Mgmt Group
- How should security teams handle AI agent visibility?
- How should security teams monitor AI agent activity without disrupting developers?
- How should security teams separate gateway responsibilities in an MCP platform architecture?
- How should security teams limit agent authority in MCP and A2A workflows to reduce confused deputy risk?
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
