[BUG] Forking a session re-creates context cache

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When using /branch, claude code seems to re-create the cache instead of reading from it, is it expected that context cache might not be easily shared between sessions?

What Should Happen?

Sessions with shared prefixes should share the context cache.

Error Messages/Logs

Steps to Reproduce

1) Create a warmed session in claude code (choose any large-ish file)

❯ head /var/log/syslog -1000 | claude

2) Fork/ branch the session in claude code

/branch

3) Ask something in the new session,

What are we doing here

4) check usage stats of the warmed and the branched sessions

❯ npx ccusage session -O -b -i 97aae588-7518-45ff-b710-5a69e6bdf358

...

┌───────────┬─────────────────┬───────────┬───────────┬───────────────┬─────────────┬─────────────┐
│ Timestamp │ Model           │     Input │    Output │  Cache Create │  Cache Read │  Cost (USD) │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         2 │        28 │        71,867 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         2 │       127 │        71,867 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
└───────────┴─────────────────┴───────────┴───────────┴───────────────┴─────────────┴─────────────┘


❯ npx ccusage session -O -b -i 76781672-ed1a-4483-bf60-ceaa10b605ad

....
┌───────────┬─────────────────┬───────────┬───────────┬───────────────┬─────────────┬─────────────┐
│ Timestamp │ Model           │     Input │    Output │  Cache Create │  Cache Read │  Cost (USD) │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         2 │        28 │        71,867 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         2 │       127 │        71,867 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         3 │        29 │        69,946 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         3 │       592 │        69,946 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         3 │        52 │        72,229 │      12,463 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
├───────────┼─────────────────┼───────────┼───────────┼───────────────┼─────────────┼─────────────┤
│ 2026      │ claude-opus-4-6 │         3 │       243 │            59 │      84,692 │       $0.00 │
│ 03-30     │                 │           │           │               │             │             │
└───────────┴─────────────────┴───────────┴───────────┴───────────────┴─────────────┴─────────────┘

The Cache gets re-created in the branch and we do not hit a cache read immediately in the branched session.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

v2.1.88

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_

user_id: jsonStringify({
  device_id: getOrCreateUserID(),
  account_uuid: ...,
  session_id: getSessionId(),   // ← fresh per forked subprocess
})