OAuth 2.1.
A consolidation of OAuth 2.0 plus the security best-practice RFCs into a single specification, making PKCE mandatory and removing the Implicit and ROPC grants.
OAuth 2.1 reached IETF draft status in 2020 and has progressed through iterations as the consolidated reference for OAuth practitioners. By 2026 it is the de-facto baseline for new OAuth deployments, every major CIAM platform ships OAuth 2.1-compliant defaults for new clients, with backwards-compatibility shims for legacy clients still on OAuth 2.0 patterns.
Common questions
Is OAuth 2.1 backwards-compatible with OAuth 2.0?
Why was the Implicit grant removed?
Do I need to migrate my OAuth 2.0 code?
Related terms
In the guides
AI Agent Identity and MCP: Authenticating Non-Human Identities
How CIAM evolves for AI agents in 2026: MCP, OAuth 2.1 Dynamic Client Registration, scoped agent tokens, and patterns separating agent from human identity.
OAuth 2.1 Explained: What Changed and Why It Matters
OAuth 2.1 consolidates fifteen years of OAuth 2.0 practice into a single coherent specification. What it deprecates, what it requires, and how to migrate existing OAuth 2.0 code.