diff --git a/README.md b/README.md
index 3a67eaa..ee3a3d7 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,22 @@ Anthropic-compatible HTTP endpoint.
 **Deferred**: OAuth (Code Plan portal), MiniMax Portal provider, image /
 TTS / video / music providers (separate plugins later if wanted).
 
+## API key + model matrix
+
+MiniMax issues two key types from the same Anthropic-compatible endpoint;
+this plugin sends both with `Authorization: Bearer <key>` and lets the
+server decide. The key prefix tells you which billing model you're on:
+
+| key prefix | billing | `MiniMax-M2.7` | `MiniMax-M2.7-highspeed` |
+|---|---|---|---|
+| `sk-cp-...` | **Token Plan** (per-plan quota) | ✅ included | ❌ `Token Plan Starter (0/0 used)` — highspeed is NOT in the base Token Plan |
+| `sk-api-...` | **pay-per-token** (per-request billed) | ✅ if balance > 0; else `insufficient_balance (1008)` | ✅ if balance > 0 |
+
+**Takeaway**: if you only have a Token Plan key, set the agent's model
+to `MiniMax-M2.7` (base). For the highspeed variant you need a funded
+`sk-api-` key. The plugin doesn't reject highspeed at startup — the
+error surfaces in the agent's response when the model is invoked.
+
 ## Install
 
 ```bash