Add Synthetic provider support

docs/concepts/model-providers.md

@@ -143,6 +143,34 @@ Moonshot uses OpenAI-compatible endpoints, so configure it as a custom provider:
 }
 ```
 
+### Synthetic
+
+Synthetic provides Anthropic-compatible models behind the `synthetic` provider:
+
+- Provider: `synthetic`
+- Auth: `SYNTHETIC_API_KEY`
+- Example model: `synthetic/hf:MiniMaxAI/MiniMax-M2.1`
+- CLI: `clawdbot onboard --auth-choice synthetic-api-key`
+
+```json5
+{
+  agents: {
+    defaults: { model: { primary: "synthetic/hf:MiniMaxAI/MiniMax-M2.1" } }
+  },
+  models: {
+    mode: "merge",
+    providers: {
+      synthetic: {
+        baseUrl: "https://api.synthetic.new/anthropic",
+        apiKey: "${SYNTHETIC_API_KEY}",
+        api: "anthropic-messages",
+        models: [{ id: "hf:MiniMaxAI/MiniMax-M2.1", name: "MiniMax M2.1" }]
+      }
+    }
+  }
+}
+```
+
 ### MiniMax
 
 MiniMax is configured via `models.providers` because it uses custom endpoints: