feat: add Anthropic fast mode support

docs/providers/anthropic.md

@@ -44,6 +44,34 @@ openclaw onboard --anthropic-api-key "$ANTHROPIC_API_KEY"
   - [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/adaptive-thinking)
   - [Extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking)
 
+## Fast mode (Anthropic API)
+
+OpenClaw's shared `/fast` toggle also supports direct Anthropic API-key traffic.
+
+- `/fast on` maps to `service_tier: "auto"`
+- `/fast off` maps to `service_tier: "standard_only"`
+- Config default:
+
+```json5
+{
+  agents: {
+    defaults: {
+      models: {
+        "anthropic/claude-sonnet-4-5": {
+          params: { fastMode: true },
+        },
+      },
+    },
+  },
+}
+```
+
+Important limits:
+
+- This is **API-key only**. Anthropic setup-token / OAuth auth does not honor OpenClaw fast-mode tier injection.
+- OpenClaw only injects Anthropic service tiers for direct `api.anthropic.com` requests. If you route `anthropic/*` through a proxy or gateway, `/fast` leaves `service_tier` untouched.
+- Anthropic reports the effective tier on the response under `usage.service_tier`. On accounts without Priority Tier capacity, `service_tier: "auto"` may still resolve to `standard`.
+
 ## Prompt caching (Anthropic API)
 
 OpenClaw supports Anthropic's prompt caching feature. This is **API-only**; subscription auth does not honor cache settings.