fix: clamp variant "max" to "high" for non-Opus Claude models on fallback

When an agent configured with variant: "max" falls back from Opus to
Sonnet (or Haiku), the "max" variant was passed through unchanged.
OpenCode sends this as level: "max" to the Anthropic API, which rejects
it with: level "max" not supported, valid levels: low, medium, high

The anthropic-effort hook previously only handled Opus (inject effort=max)
and skipped all other Claude models. Now it actively clamps "max" → "high"
for non-Opus Claude models and mutates message.variant so OpenCode
doesn't pass the unsupported level to the API.

src/hooks/anthropic-effort/hook.ts

@@ -28,6 +28,20 @@ interface ChatParamsOutput {
   options: Record<string, unknown>
 }
 
+/**
+ * Valid thinking budget levels per model tier.
+ * Opus supports "max"; all other Claude models cap at "high".
+ */
+const MAX_VARIANT_BY_TIER: Record<string, string> = {
+  opus: "max",
+  default: "high",
+}
+
+function clampVariant(variant: string, isOpus: boolean): string {
+  if (variant !== "max") return variant
+  return isOpus ? MAX_VARIANT_BY_TIER.opus : MAX_VARIANT_BY_TIER.default
+}
+
 export function createAnthropicEffortHook() {
   return {
     "chat.params": async (
@@ -38,15 +52,27 @@ export function createAnthropicEffortHook() {
       if (!model?.modelID || !model?.providerID) return
       if (message.variant !== "max") return
       if (!isClaudeProvider(model.providerID, model.modelID)) return
-      if (!isOpus46(model.modelID)) return
       if (output.options.effort !== undefined) return
 
-      output.options.effort = "max"
-      log("anthropic-effort: injected effort=max", {
-        sessionID: input.sessionID,
-        provider: model.providerID,
-        model: model.modelID,
-      })
+      const opus = isOpus46(model.modelID)
+      const clamped = clampVariant(message.variant, opus)
+      output.options.effort = clamped
+
+      if (!opus) {
+        // Override the variant so OpenCode doesn't pass "max" to the API
+        ;(message as { variant?: string }).variant = clamped
+        log("anthropic-effort: clamped variant max→high for non-Opus model", {
+          sessionID: input.sessionID,
+          provider: model.providerID,
+          model: model.modelID,
+        })
+      } else {
+        log("anthropic-effort: injected effort=max", {
+          sessionID: input.sessionID,
+          provider: model.providerID,
+          model: model.modelID,
+        })
+      }
     },
   }
 }

src/hooks/anthropic-effort/index.test.ts

@@ -143,8 +143,8 @@ describe("createAnthropicEffortHook", () => {
       expect(output.options.effort).toBeUndefined()
     })
 
-    it("should NOT inject effort for non-opus model", async () => {
-      //#given claude-sonnet-4-6 (not opus)
+    it("should clamp effort to high for non-opus claude model with variant max", async () => {
+      //#given claude-sonnet-4-6 (not opus) with variant max
       const hook = createAnthropicEffortHook()
       const { input, output } = createMockParams({
         modelID: "claude-sonnet-4-6",
@@ -153,8 +153,9 @@ describe("createAnthropicEffortHook", () => {
       //#when chat.params hook is called
       await hook["chat.params"](input, output)
 
-      //#then effort should NOT be injected
-      expect(output.options.effort).toBeUndefined()
+      //#then effort should be clamped to high (not max)
+      expect(output.options.effort).toBe("high")
+      expect(input.message.variant).toBe("high")
     })
 
     it("should NOT inject effort for non-anthropic provider with non-claude model", async () => {