diff --git a/src/hooks/anthropic-effort/hook.ts b/src/hooks/anthropic-effort/hook.ts
index a545768d6..7d4eca99b 100644
--- a/src/hooks/anthropic-effort/hook.ts
+++ b/src/hooks/anthropic-effort/hook.ts
@@ -76,23 +76,35 @@ export function createAnthropicEffortHook() {
       const { agent, model, message } = input
       if (!model?.modelID || !model?.providerID) return
       if (isEffortUnsupportedModel(model.modelID)) return
-      if (message.variant !== "max") return
       if (!isClaudeProvider(model.providerID, model.modelID)) return
       if (shouldSkipForInternalAgent(agent?.name)) return
-      if (output.options.effort !== undefined) return
 
       const opus = isOpusModel(model.modelID)
       const constrained = isConstrainedProvider(model.providerID)
+
+      if (output.options.effort !== undefined) {
+        if (output.options.effort === "max" && constrained) {
+          const clamped = clampVariant("max", opus, constrained)
+          output.options.effort = clamped
+          ;(message as { variant?: string }).variant = clamped
+          log("anthropic-effort: clamped pre-set effort max→high", {
+            sessionID: input.sessionID,
+            provider: model.providerID,
+            model: model.modelID,
+            reason: "constrained-provider",
+          })
+        }
+        return
+      }
+
+      if (message.variant !== "max") return
+
       const clamped = clampVariant(message.variant, opus, constrained)
       output.options.effort = clamped
 
       const shouldOverrideMessageVariant = !opus || constrained
 
       if (shouldOverrideMessageVariant) {
-        // Override the variant so OpenCode doesn't pass "max" to the API.
-        // Non-Opus models cap at high; Anthropic OAuth (Claude Pro/Max) also
-        // caps at high even on Opus because the OAuth API only accepts
-        // low | medium | high.
         ;(message as { variant?: string }).variant = clamped
         log("anthropic-effort: clamped variant max→high", {
           sessionID: input.sessionID,
diff --git a/src/hooks/anthropic-effort/index.test.ts b/src/hooks/anthropic-effort/index.test.ts
index 129b6513e..c4ea7a297 100644
--- a/src/hooks/anthropic-effort/index.test.ts
+++ b/src/hooks/anthropic-effort/index.test.ts
@@ -221,6 +221,80 @@ describe("createAnthropicEffortHook", () => {
     })
   })
 
+  describe("#given pre-set effort via session params — regression for #3563", () => {
+    it("#given pre-set effort=max + variant=max + github-copilot Opus #when hook runs #then effort and variant are both clamped to high", async () => {
+      // given
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "github-copilot",
+        modelID: "claude-opus-4-7",
+        variant: "max",
+        existingOptions: { effort: "max" },
+      })
+
+      // when
+      await hook["chat.params"](input, output)
+
+      // then
+      expect(output.options.effort).toBe("high")
+      expect(input.message.variant).toBe("high")
+    })
+
+    it("#given pre-set effort=max + variant=high + github-copilot Opus #when hook runs #then effort and variant are both clamped to high", async () => {
+      // given — this is the cubic violation case from PR #3608:
+      // message.variant is NOT "max", but pre-set effort IS "max"
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "github-copilot",
+        modelID: "claude-opus-4-7",
+        variant: "high",
+        existingOptions: { effort: "max" },
+      })
+
+      // when
+      await hook["chat.params"](input, output)
+
+      // then — constrained provider must clamp pre-set effort=max regardless of variant
+      expect(output.options.effort).toBe("high")
+      expect(input.message.variant).toBe("high")
+    })
+
+    it("#given pre-set effort=max + non-constrained Opus #when hook runs #then effort stays max", async () => {
+      // given — anthropic (non-OAuth, non-constrained) Opus with pre-set effort=max
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "anthropic",
+        modelID: "claude-opus-4-7",
+        variant: "max",
+        existingOptions: { effort: "max" },
+      })
+
+      // when
+      await hook["chat.params"](input, output)
+
+      // then — non-constrained Opus keeps max
+      expect(output.options.effort).toBe("max")
+      expect(input.message.variant).toBe("max")
+    })
+
+    it("#given pre-set effort=high + github-copilot Opus #when hook runs #then effort stays high", async () => {
+      // given — legitimate pre-set effort=high should not be overwritten
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "github-copilot",
+        modelID: "claude-opus-4-7",
+        variant: "max",
+        existingOptions: { effort: "high" },
+      })
+
+      // when
+      await hook["chat.params"](input, output)
+
+      // then — high is already valid for constrained providers, don't touch it
+      expect(output.options.effort).toBe("high")
+    })
+  })
+
   describe("#given anthropic OAuth auth (Claude Pro/Max) — regression for #3429", () => {
     let tempDataDir: string
     const originalXdgDataHome = process.env.XDG_DATA_HOME