fix(session): persist plan mode reminders

[ipogosov]

Issue for this PR

Closes #26749

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenCode appends a synthetic plan-mode reminder to the user message before sending the prompt, but the synthetic part is not stored in message history. On the next turn, the same historical user message is replayed without the reminder, so the model-visible serialization of that turn changes between request 1 and request 2.

This PR persists the non-experimental plan / build synthetic reminder into the user message itself when it is added, so the message that lives in history is identical to what the model saw on the original turn. As a result, the prompt prefix on every later turn matches the prefix that was seen first, which is what prefix caches in OpenAI-compatible backends (LM Studio, llama.cpp, vLLM, LiteLLM proxies) require to actually hit.

It also guards against duplicate reminder parts when a prompt is retried or reprocessed, by checking whether the same reminder text is already present on the user message before appending another one.

This is intentionally narrow:

• It does not change the user-visible message text shown in the UI (the synthetic reminder is appended as a separate text part on the same user message, just like before, only persisted instead of dropped after the request).
• It does not touch the experimental inline reminder injection in prompt.ts.
• It is a different concern from Plan Mode system reminder persists for entire conversation after switching back to Build mode #24121 / fix(session): drop stale plan reminders #24343, which strip plan reminders that leak into Build mode after a switch. That fix removes stale reminders; this fix makes the current-turn reminder stable. They can both be true at once and they don't conflict — this PR only persists the reminder for the turn where the user is in Plan mode, and fix(session): drop stale plan reminders #24343 is what cleans it up on a later Build turn.

The change works because OpenCode's history is the source of truth replayed to the model on every turn, so persisting the synthetic part once means the next turn replays the exact same tokens for that historical user message instead of re-deriving them from a runtime branch.

How did you verify your code works?

• bun typecheck from packages/opencode
• bun test test/session/prompt.test.ts --timeout 30000 --grep "plan mode reminders remain stable" — new live-session test that drives a plan turn, advances the session so that the user message becomes history, then asserts the plan reminder text is still present on that historical user message.

Screenshots / recordings

N/A, non-UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR fix(session): drop stale plan reminders #24343: "fix(session): drop stale plan reminders"
  • fix(session): drop stale plan reminders #24343
  • Why related: Both PRs address plan mode reminders in sessions. PR fix(session): drop stale plan reminders #24343 appears to handle dropping stale plan reminders, while PR fix(session): persist plan mode reminders #26743 focuses on persisting them and avoiding duplicates. These may be addressing related or overlapping issues with plan reminder handling.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍