new file mode 100644
index 0000000..40aff1d
@@ -0,0 +1,75 @@
+---
+visibility: public
+---
+
+# Attention gating
+
+## Why
+
+In a group chat with four humans and a bot, the bot should not respond every 3 seconds. But it should respond when the conversation is about a bug it can help with, even if no one @mentioned it. That's the "discriminator" lane.
+
+## The ladder
+
+```
+ silent ──▶ discriminate-quiet ──▶ discriminate ──▶ mentions-only ──▶ always
+ (record) (LLM) (LLM) (strict match) (no filter)
+```
+
+Going right = more responsive. Going left = quieter.
+
+## Rule before LLM
+
+The LLM discriminator is slow and costs money. Hard rules short-circuit:
+
+1. `/picortex <cmd>` slash command → always respond
+2. Reply-to a bot message → respond
+3. `@picortex` mention → respond
+4. Admin override from Jacob's DM → respond
+5. Non-text payload → mode-dependent
+
+## Discriminator prompt
+
+Lives at `$CHAT_HOME/.picortex/prompts/discriminator.md`, git-tracked inside the chat's home. Seeded from a default that can be tuned live.
+
+Default (sketched):
+
+> You are deciding whether a chat bot should respond to the most recent message. Only respond if the message:
+> - asks a question that a capable coding / research assistant could help with, OR
+> - is reacting to a previous message from you, OR
+> - contains a task/request that the assistant should pick up.
+>
+> Do NOT respond if the message is:
+> - small talk between humans
+> - private conversation you weren't addressed in
+> - off-topic or tangential
+>
+> Return JSON: `{"should_respond": bool, "reason": "short"}`.
+
+Prompt is git-versioned so Jacob can tune per-chat without redeploying.
+
+## Model choice
+
+Default: `claude-3-5-haiku` (cheap, fast). Swap via `chat_config.discriminator_model`.
+
+Cost: ~200 input tokens + 30 output tokens per classification ≈ $0.0005. 1,000 classifications = $0.50. Cheap.
+
+## Failure modes
+
+- LLM returns non-JSON → 1 retry with reformat prompt; if still bad, fail-open for 1:1, fail-closed for groups.
+- LLM times out (> 5 s) → same fail policy.
+- Cost spike → per-day per-chat budget cap in `chat_config`. Hitting the cap disables `discriminate` until midnight.
+
+## Observability
+
+Every discriminator decision is logged as an `event_type: "discriminator.decision"` with: `chat_id`, `message_id`, `should_respond`, `reason`, `model`, `latency_ms`, `cost_usd`.
+
+## Future
+
+- Personalized discriminator (fine-tune or few-shot on Jacob's own "respond / don't respond" label history). v0.3.
+- Hierarchical gating: per-user overrides inside a group. v0.2.
+
+## References
+
+- [Spec 005](../specs/005-attention-gating.md)
+- Cortex R4 — the pattern we adopted
+- Default template: `/usr/local/share/picortex/discriminator.default.md` (created by installer in S4)
\ No newline at end of file