[Bug] Thinking blocks in latest assistant message are read-only

Bug Description
thinking or redacted_thinking blocks in the latest assistant message cannot be modified.

Environment Info

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.12
• Feedback ID: 6251738d-4309-4606-a190-05b19de3477e

Errors

[{"error":"Error\n    at nb (/$bunfs/root/claude:42:1143)\n    at <anonymous> (/$bunfs/root/claude:47:10085)\n    at emit (node:events:92:22)\n    at endReadableNT (internal:streams/readable:861:50)\n    at processTicksAndRejections (native:7:39)\n    at request (/$bunfs/root/claude:49:2147)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-01-20T18:21:20.606Z"},{"error":"Error\n    at nb (/$bunfs/root/claude:42:1143)\n    at <anonymous> (/$bunfs/root/claude:47:10085)\n    at emit (node:events:92:22)\n    at endReadableNT (internal:streams/readable:861:50)\n    at processTicksAndRejections (native:7:39)\n    at request (/$bunfs/root/claude:49:2147)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-01-20T18:21:20.830Z"},{"error":"Error: 400 {\"type\":\"error\",\"error\":{\"type\":\"invalid_request_error\",\"message\":\"messages.5.content.1: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.\"},\"request_id\":\"req_011CXK75TSiZXiAMYLmLN91R\"}\n    at generate (/$bunfs/root/claude:867:2085)\n    at makeRequest (/$bunfs/root/claude:884:5435)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-01-20T18:30:07.600Z"},{"error":"Error: 400 {\"type\":\"error\",\"error\":{\"type\":\"invalid_request_error\",\"message\":\"messages.5.content.1: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.\"},\"request_id\":\"req_011CXK79UuspUvniQQGtYTtK\"}\n    at generate (/$bunfs/root/claude:867:2085)\n    at makeRequest (/$bunfs/root/claude:884:5435)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-01-20T18:31:02.037Z"}]