Pi chat falls back to the terminal once a session file passes 64 MiB (branch_unreadable) #868

What happened

A long-lived Pi session stops reading as a conversation once its session file passes 64 MiB. The chat falls back to the terminal's text, and GET /api/pane/conversation answers branch_unreadable. The session file itself is intact, and Pi keeps working in it.

piBranchSegments() refuses the active branch when its cumulative byte count crosses 64 MiB, before the paged reader that would only read a window of it gets to run. Claude and Codex transcripts of the same size read fine, because they page from the tail.

Reproduced on 0.4.8 with two real Pi sessions (177 MB and 71 MB): both return branch_unreadable.

How often

Every time, once a Pi session file is over 64 MiB.

herdr web ui version

0.4.8 (also 0.4.0 to 0.4.7)

How it runs

herdr plugin

Server PC

Windows 11 x64, herdr 0.9.3

Thank you for herdr web ui. I use it every day across several PCs. I have a fix that has been running locally for a while, and I will open a PR for it so you can see whether the approach fits. Please feel free to close or redirect this if you would rather handle it another way.