selectBetaFlags now gates the heavy-agent betas (context-1m, effort,
advanced-tool-use) on Opus/Sonnet only; Haiku with OAuth was rejecting
context-1m with 400 'incompatible with the long context beta header'. base.ts
stops deleting Haiku's thinking config (real Claude Desktop keeps it). chatCore
passthrough converts historical thinking/redacted_thinking blocks to
redacted_thinking with a synthetic signature, fixing 400 'Invalid signature in
thinking block' on mid-session model switches. Co-authored analysis by havockdev.