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OAuth PKCE in Python: Generate code_verifier and code_challenge (S256), Common Mistakes and a Known-Answer Test

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Self-Hosting & Infra intermediate 5 min read $0.01 Updated 2026-10-20

Executed on Python 3.10: a 64-character verifier giving a 43-character challenge with no padding, the RFC 7636 appendix vector matching E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM, and what the challenge looks like when padding is left in.

Build the PKCE values correctly for public clients, agents and MCP servers: random verifier, SHA-256, base64url without padding. Verify against the RFC 7636 test vector and avoid padding and encoding bugs.

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