history_previous drops older session history and mixes prompt history across projects

Description

Desc:

This looks related to #25811 and #19536, both of which were closed as not planned, but I think there is a little more going on here than just whether history should be project-scoped.

Two symptoms:

1. Pressing up-arrow eventually shows prompts from completely unrelated projects.
2. After restarting OpenCode, some of the prompts I previously entered in the current session/project are no longer available in history.

This is still happening in 1.18.27 and it is most obvious when resuming a session with opencode -c or opencode -s because one up-arrow to see the previous prompt usually pulls in a random prompt from a random project and session.

I dug into it because at first I thought I was somehow switching sessions without realizing it. It looks like prompt history is actually stored in one global file:

~/.local/state/opencode/prompt-history.jsonl
On my machine that file currently has exactly 50 lines:

wc -l ~/.local/state/opencode/prompt-history.jsonl
and those 50 entries contain prompts from at least three unrelated repos.

The JSONL records don't appear to contain any session or project identifier:

{"input":"...","parts":[],"mode":"normal"}
I also found the PromptHistory code in the bundled code reading prompt-history.jsonl, and the same code path appears to cap the history with a trailing slice(-N).

So unless I'm missing something, this is one global fixed-size prompt-history buffer shared by every OpenCode instance/project on the machine.

That would explain both things I'm seeing:

• history from project B appears when I press up-arrow in project A
• prompts from project A can disappear from history simply because enough prompts were entered elsewhere

The session itself is still fine. This is only the up-arrow prompt recall that gets lost.

One other possibly relevant detail: current OpenCode storage on this install is SQLite:

~/.local/share/opencode/opencode.db
and the DB already has a session_input table with session_id and prompt columns. Mine currently has no rows:

SELECT COUNT(*) FROM session_input;
-- 0

So it doesn't look like history_previous is using that table.

Expected behavior:

At minimum, pressing up-arrow in one project/session should not show prompts entered in a completely different project.

I would also expect prompts from the current session not to disappear from history because I used OpenCode somewhere else.

Steps to reproduce

1. Run OpenCode in repo A and enter several recognizable prompts.
2. Run OpenCode in unrelated repo B and enter several different prompts.
3. Go back to repo A.
4. Press up-arrow repeatedly in the prompt box.

Eventually prompts entered in repo B show up in repo A.

To see the eviction behavior, enter enough prompts across other projects to roll the global history forward, then restart OpenCode in repo A and check its up-arrow history again.

You can also inspect the backing file directly:

wc -l ~/.local/state/opencode/prompt-history.jsonl
cat ~/.local/state/opencode/prompt-history.jsonl

OpenCode version: 1.18.27
OS: Linux
Plugins: None that should affect prompt history.

Screenshot / share link

I don't have a useful screenshot for this, and I don't want to create a public share containing prompts from unrelated/private projects.

Plugins

No response

OpenCode version

No response

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response