[BUG] Input history should be scoped to session, not shared across all sessions started in the same directory

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?

Summary

When running multiple Claude Code instances in the same directory, each instance gets its own session (separate session ID, separate context), yet up-arrow input history is shared in real-time across all instances. This is confusing and undermines the mental model users need to develop for effective LLM interaction.

Current Behavior

• Running claude twice in the same directory creates two separate sessions (different session IDs)
• However, up-arrow history is shared in real-time between them
• Pressing up-arrow in one terminal shows commands typed in the other terminal's session

Expected Behavior

Input history should be scoped to the session, not the directory. Each session should have its own history that:

• Only shows commands typed in that session
• Is restored when using /resume to continue that session

Why This Matters

1. Contradicts shell conventions

Standard shell behavior (bash, zsh) maintains per-terminal history during a session. You don't see another terminal's commands via up-arrow in real-time. Claude Code's behavior is more confusing than standard shells, not less.

2. Breaks resume semantics

If history were per-session, /resume would restore both the conversation context and the relevant input history—you'd get back exactly where you left off. Currently, sessions are separate but history is shared, which is the worst of both worlds.

3. Undermines the LLM mental model

Effective LLM use requires understanding "context"—the conversation history that Claude can see and reference. Up-arrow reflecting session-scoped history would reinforce this: "these are the things I said in this context."

Showing commands from another session (that this Claude instance has no knowledge of) suggests a shared memory that doesn't exist, teaching users the wrong mental model about what Claude can reference.

Common Use Case Affected

Running a second Claude instance to ask questions about the codebase while another instance is doing work. This is read-only and doesn't require a separate checkout/worktree, but the shared history makes it confusing to work with both sessions.

What Should Happen?

Two separate claudes run in the same directory, but with different sessions, should not share real time history.

Up arrow in session 1 in terminal 1 should not see a prompt just types in session 2 in terminal 2

Error Messages/Logs

Steps to Reproduce

cd ~/project
claude
hello1
/status
will see that in session 1

--

cd ~/project
claude
hit up arrow - will see hello1
/status
will see in session 2

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.15

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_