feat(core): make system prompt immutable after session creation

[0byte-coding]

Issue for this PR

Closes #29672

Type of change

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

What does this PR do?

Caches the system prompt in-memory per session ID so it becomes immutable after the session is created. This prevents unnecessary cache misses when the same session makes multiple requests.

The implementation stores system prompts in a HashMap keyed by session ID. This is very minimal and focused - just enough to lock the prompt after initialization.

How did you verify your code works?

I tested locally that:

• Session initialization caches the system prompt correctly
• Subsequent requests retrieve the cached prompt instead of regenerating it
• Cache lookups return the exact same prompt object

Screenshots / recordings

N/A - This is not a UI change

Checklist

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

[github-actions]

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

Based on the search results, I found several related PRs that involve system prompt and caching optimizations:

Potentially Related PRs:

1. feat: improve deepseek prompt cache reuse #31867 - feat: improve deepseek prompt cache reuse

   • Related to prompt caching optimization for better cache hit rates

2. fix(cache): improve Anthropic prompt cache hit rate with system split and tool stability #14743 - fix(cache): improve Anthropic prompt cache hit rate with system split and tool stability

   • Focuses on cache stability for system prompts with provider-specific optimization

3. fix(session): move env block to tail of system prompt for cache stability #29949 - fix(session): move env block to tail of system prompt for cache stability

   • Another system prompt optimization for cache stability

4. feat(provider): add provider-specific cache configuration system (significant token usage reduction) #5422 - feat(provider): add provider-specific cache configuration system (significant token usage reduction)

   • Related to caching systems at a broader level

5. fix(session): fire system.transform before messages.transform #19961 - fix(session): fire system.transform before messages.transform

   • Related to system prompt handling in sessions

These PRs address similar domains of system prompt caching and session initialization, though none appear to be exact duplicates of PR #33246's specific implementation (in-memory HashMap caching per session ID). However, PRs #31867, #14743, and #29949 are closest in addressing cache efficiency and system prompt immutability concepts.

[Capicua25x]

Bumping this — it's still the right fix, and it's more urgent than when it was filed. Credit for the approach goes to @0byte-coding (original #33246); it had gone dirty against dev, so I ported it forward and validated it in production.

Why it's urgent. The system prompt is rebuilt on every request — instruction.system() re-reads the instruction files (AGENTS.md, memory.md), and system.ts injects Today's date. Any change to that prefix — editing an instruction file mid-session, or the date rolling over at midnight — makes the cached prefix diverge at token ~0 and forces a full re-prefill of the entire conversation. On a long agent session (we run a 1M-context DeepSeek V4 Flash via an OpenAI-compatible local server) that's ~450K tokens re-prefilled, ~4 minutes, with no signal in the UI — it just looks like the model is thinking. This is not Anthropic-specific; it hits every provider. The date variant is guaranteed once a day for any session that spans midnight.

Evidence: two consecutive requests with byte-identical tools and identical messages but a different system-message hash → 0 cached tokens → ~450K re-prefill. #29584 reports this exactly (auto-closed as stale); see also #36483 and #43722.

Fix + validation. Freeze the environment + instruction + MCP system parts once per session (on the first LLM call) and reuse them for the session's lifetime, keeping skills fresh. Rebased onto current dev, now open as #48777:

• PR: fix(session): freeze system prompt per session to preserve prefix cache #48777
• branch fix/system-prompt-immutable on Capicua25x/opencode; commit Capicua25x@00d0a710e — co-authored by @0byte-coding, touches only packages/opencode/src/session/prompt.ts (+26/−10)
• deployed and dogfooded: after the freeze, editing memory.md mid-session no longer causes a prefill, and the midnight rollover no longer does either

(#47816 handles the date-only case; this is the general fix.)