diff --git a/src/hooks/anthropic-effort/index.test.ts b/src/hooks/anthropic-effort/index.test.ts
new file mode 100644
index 000000000..84168fbb3
--- /dev/null
+++ b/src/hooks/anthropic-effort/index.test.ts
@@ -0,0 +1,196 @@
+import { describe, expect, it } from "bun:test"
+import { createAnthropicEffortHook } from "./index"
+
+interface ChatParamsInput {
+  sessionID: string
+  agent: { name?: string }
+  model: { providerID: string; modelID: string; id?: string; api?: { npm?: string } }
+  provider: { id: string }
+  message: { variant?: string }
+}
+
+interface ChatParamsOutput {
+  temperature?: number
+  topP?: number
+  topK?: number
+  options: Record<string, unknown>
+}
+
+function createMockParams(overrides: {
+  providerID?: string
+  modelID?: string
+  variant?: string
+  agentName?: string
+  existingOptions?: Record<string, unknown>
+}): { input: ChatParamsInput; output: ChatParamsOutput } {
+  const providerID = overrides.providerID ?? "anthropic"
+  const modelID = overrides.modelID ?? "claude-opus-4-6"
+  const variant = "variant" in overrides ? overrides.variant : "max"
+  const agentName = overrides.agentName ?? "sisyphus"
+  const existingOptions = overrides.existingOptions ?? {}
+
+  return {
+    input: {
+      sessionID: "test-session",
+      agent: { name: agentName },
+      model: { providerID, modelID },
+      provider: { id: providerID },
+      message: { variant },
+    },
+    output: {
+      temperature: 0.1,
+      options: { ...existingOptions },
+    },
+  }
+}
+
+describe("createAnthropicEffortHook", () => {
+  describe("opus 4-6 with variant max", () => {
+    it("should inject effort max for anthropic opus-4-6 with variant max", async () => {
+      //#given anthropic opus-4-6 model with variant max
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({})
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should be injected into options
+      expect(output.options.effort).toBe("max")
+    })
+
+    it("should inject effort max for github-copilot claude-opus-4-6", async () => {
+      //#given github-copilot provider with claude-opus-4-6
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "github-copilot",
+        modelID: "claude-opus-4-6",
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should be injected (github-copilot resolves to anthropic)
+      expect(output.options.effort).toBe("max")
+    })
+
+    it("should inject effort max for opencode provider with claude-opus-4-6", async () => {
+      //#given opencode provider with claude-opus-4-6
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "opencode",
+        modelID: "claude-opus-4-6",
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should be injected
+      expect(output.options.effort).toBe("max")
+    })
+
+    it("should handle normalized model ID with dots (opus-4.6)", async () => {
+      //#given model ID with dots instead of hyphens
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        modelID: "claude-opus-4.6",
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then should normalize and inject effort
+      expect(output.options.effort).toBe("max")
+    })
+  })
+
+  describe("conditions NOT met - should skip", () => {
+    it("should NOT inject effort when variant is not max", async () => {
+      //#given opus-4-6 with variant high (not max)
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({ variant: "high" })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should NOT be injected
+      expect(output.options.effort).toBeUndefined()
+    })
+
+    it("should NOT inject effort when variant is undefined", async () => {
+      //#given opus-4-6 with no variant
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({ variant: undefined })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should NOT be injected
+      expect(output.options.effort).toBeUndefined()
+    })
+
+    it("should NOT inject effort for non-opus model", async () => {
+      //#given claude-sonnet-4-5 (not opus)
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        modelID: "claude-sonnet-4-5",
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should NOT be injected
+      expect(output.options.effort).toBeUndefined()
+    })
+
+    it("should NOT inject effort for non-anthropic provider with non-claude model", async () => {
+      //#given openai provider with gpt model
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        providerID: "openai",
+        modelID: "gpt-5.2",
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should NOT be injected
+      expect(output.options.effort).toBeUndefined()
+    })
+  })
+
+  describe("preserves existing options", () => {
+    it("should NOT overwrite existing effort if already set", async () => {
+      //#given options already have effort set
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        existingOptions: { effort: "high" },
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then existing effort should be preserved
+      expect(output.options.effort).toBe("high")
+    })
+
+    it("should preserve other existing options when injecting effort", async () => {
+      //#given options with existing thinking config
+      const hook = createAnthropicEffortHook()
+      const { input, output } = createMockParams({
+        existingOptions: {
+          thinking: { type: "enabled", budgetTokens: 31999 },
+        },
+      })
+
+      //#when chat.params hook is called
+      await hook["chat.params"](input, output)
+
+      //#then effort should be added without affecting thinking
+      expect(output.options.effort).toBe("max")
+      expect(output.options.thinking).toEqual({
+        type: "enabled",
+        budgetTokens: 31999,
+      })
+    })
+  })
+})
diff --git a/src/hooks/anthropic-effort/index.ts b/src/hooks/anthropic-effort/index.ts
new file mode 100644
index 000000000..b52fc32ad
--- /dev/null
+++ b/src/hooks/anthropic-effort/index.ts
@@ -0,0 +1,55 @@
+import { log } from "../../shared"
+
+const OPUS_4_6_PATTERN = /claude-opus-4[-.]6/i
+
+function normalizeModelID(modelID: string): string {
+  return modelID.replace(/\.(\d+)/g, "-$1")
+}
+
+function isClaudeProvider(providerID: string, modelID: string): boolean {
+  if (["anthropic", "opencode"].includes(providerID)) return true
+  if (providerID === "github-copilot" && modelID.toLowerCase().includes("claude")) return true
+  return false
+}
+
+function isOpus46(modelID: string): boolean {
+  const normalized = normalizeModelID(modelID)
+  return OPUS_4_6_PATTERN.test(normalized)
+}
+
+interface ChatParamsInput {
+  sessionID: string
+  agent: { name?: string }
+  model: { providerID: string; modelID: string }
+  provider: { id: string }
+  message: { variant?: string }
+}
+
+interface ChatParamsOutput {
+  temperature?: number
+  topP?: number
+  topK?: number
+  options: Record<string, unknown>
+}
+
+export function createAnthropicEffortHook() {
+  return {
+    "chat.params": async (
+      input: ChatParamsInput,
+      output: ChatParamsOutput
+    ): Promise<void> => {
+      const { model, message } = input
+      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,
+      })
+    },
+  }
+}