Per-turn ISO timestamp in early system prompt busts prompt cache #46

Problem

renderMemoryMetadata injects wall-clock time into the system prompt on every turn:

// src/prompt.ts
return `<memory_metadata>
- The current system date is: ${now.toISOString()}
- Memory blocks were last modified: ${lastModified.toISOString()}
...
`;

That chunk is then spliced early in the system array (index 1, right after the provider header):

// src/plugin.ts
const insertAt = output.system.length > 0 ? 1 : 0;
output.system.splice(insertAt, 0, xml);

Prompt caching (OpenAI / Anthropic-style) is prefix-based. Changing one character early invalidates everything after it. A fresh toISOString() every request means the conversation/tool history never gets a cache hit past that point.

Observed impact

On OpenCode Go with this plugin enabled, token-tracker logs showed:

• cacheRead stuck around ~22–23k tokens (stable prefix before the memory inject)
• uncached input growing to 300k–450k/turn in long sessions
• worst session: ~92% uncached over 376 turns

When the system prefix stayed stable (no per-turn mutation), the same setup hit ~97% cache read. So the provider cache path works; the early volatile metadata is what breaks it.

Empty seeded blocks (human / persona) still emit metadata + timestamp, so this hits even when memory values are unused.

Suggested fix

1. Drop now.toISOString() from the system prompt (OpenCode already exposes calendar date elsewhere). Keep lastModified only, or omit metadata entirely until a block changes.
2. Append memory after the stable system prefix (output.system.push(xml)) instead of splicing at index 1, so memory edits only invalidate the trailing chunk — not tools + core instructions.

Happy to send a PR if useful. For now I removed the plugin from my config to stop burning subscription quota.

Refs

• OpenAI prompt caching: https://developers.openai.com/api/docs/guides/prompt-caching
• Anthropic prompt caching: https://platform.claude.com/docs/en/build-with-claude/prompt-caching
• Related pattern (memory in cache-sensitive prefix): [FEATURE] Auto-memory writes invalidate entire prompt cache chain — MEMORY.md should not sit in cache-sensitive prefix anthropics/claude-code#46644