[FEATURE]: Inject per-message timestamps as AI-visible context #15876

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Describe the enhancement you want to request

Problem

The AI model has no awareness of how much time passes between user messages within a conversation. The static <omo-env> block provides the session start time, but this is set once at session initialization and never updates.

This causes a concrete usability problem: when a user asks a question, leaves for hours or days, then returns and replies, the AI treats it as if seconds have passed. It says things like "wait a few minutes, I'm still working on it" when in reality the user has been gone for 3 days.

Current Workaround

Using a UserPromptSubmit hook that:

1. Reads current timestamp
2. Reads last message timestamp from a state file
3. Calculates human-readable elapsed time
4. Outputs to stdout (which gets injected as AI-visible context per the hooks spec)
5. Updates the state file

This produces context like:

[Time Context] Current time: 2026-03-03 17:58 CET | Last user message: 3 days 2 hours ago

Proposed Native Feature

Per-message timestamps should be a first-class feature in OpenCode, not a hook workaround. Specifically:

1. Inject current datetime into each user message's context (updating the stale <omo-env> block, or adding a separate per-message timestamp)
2. Track elapsed time between user messages and make it visible to the AI
3. Per-session state: Track the last AI response time as well, so the model knows if it was the one that went silent vs. the user

Why This Should Be Native

• Every user benefits — time awareness is universally needed, not niche
• Hook workaround is fragile — relies on file I/O, shell execution, and stdout injection semantics
• Model can't distinguish between "user is typing slowly" (30 sec gap) and "user left for the weekend" (3 day gap) without this
• The <omo-env> block already exists — extending it with a per-message Current time field is minimal effort

Related Issues

• [FEATURE]: Add timestamp next to messages in chat (both agent and user messages) #8634 — Timestamp display in chat UI (visual only, not AI-visible)
• feat: Add !\command\`` dynamic context syntax in skills #10892 — Dynamic context via !command syntax in skills
• Native Claude Code hooks compatibility (PreToolUse, PostToolUse, Stop) #12472 — Native Claude Code hooks compatibility