diff --git a/.agents/AGENTS.md b/.agents/AGENTS.md
index 44d8139..2a026ef 100644
--- a/.agents/AGENTS.md
+++ b/.agents/AGENTS.md
@@ -151,6 +151,12 @@ calling the script. See `agent-infrastructure.md` item 22 for the refactor plan.
 - Fires once per session. Good for: current branch, active investigations, dead
   ends.
 - Not good for: precise rule reminders (use PostToolUse or nested AGENTS.md).
+- **OpenCode delivery:** injected as a synthetic `text` part via
+  `output.parts.unshift()` on the first `chat.message` turn. **Not** via
+  `experimental.chat.system.transform` — that hook fires for task-spawned
+  subagent sessions after a user message is already in context, which causes
+  Qwen-family GGUF models to abort with a Jinja "System message must be at the
+  beginning" error. See Forbidden Patterns below.
 
 ### `stop.sh` — End-of-session reflection
 
@@ -237,6 +243,15 @@ Some things cannot be unified and live in tool-specific locations:
   model may be correct. Applies to: OpenCode tool names (`read`/`edit`/`task`),
   parameter names (`offset`/`limit` not `startLine`/`endLine`), and plugin guard
   logic.
+- ❌ Using `experimental.chat.system.transform` to inject session-start content
+  in OpenCode. That hook fires for every model call — including task-spawned
+  subagent sessions — **after** the task prompt (a user message) is already in
+  the conversation. Pushing to `output.system` at that point places a system
+  message at a non-zero position, which Qwen-family GGUF models reject with
+  _"System message must be at the beginning"_ (Jinja chat template guard).
+  Fix: inject session-start as a synthetic `text` part via `output.parts.unshift()`
+  on the first `chat.message` turn (guarded by an `initializedSessions` set).
+  Text parts have no position constraint. Committed `f0d21e9` in dotfiles.
 - ❌ Asserting that a third-party tool does **not** support a feature (config
   mechanism, directory, option) without fetching the tool's current docs first.
   Training data is frequently stale. Negative claims ("X doesn't have Y") must