diff --git a/src/hooks/no-sisyphus-gpt/hook.ts b/src/hooks/no-sisyphus-gpt/hook.ts
index d5ed53c34..f62a5d37e 100644
--- a/src/hooks/no-sisyphus-gpt/hook.ts
+++ b/src/hooks/no-sisyphus-gpt/hook.ts
@@ -5,7 +5,7 @@ import {
   resolveRegisteredAgentName,
   updateSessionAgent,
 } from "../../features/claude-code-session-state"
-import { log } from "../../shared"
+import { AGENT_MODEL_REQUIREMENTS, log } from "../../shared"
 import { getAgentConfigKey } from "../../shared/agent-display-names"
 
 const TOAST_TITLE = "NEVER Use Sisyphus with GPT"
@@ -30,6 +30,18 @@ function showToast(ctx: PluginInput, sessionID: string): void {
   })
 }
 
+function getNativeSisyphusGptVariant(model: { providerID: string; modelID: string }): string | undefined {
+  const chain = AGENT_MODEL_REQUIREMENTS["sisyphus"]?.fallbackChain ?? []
+  const exactMatch = chain.find((entry) =>
+    entry.providers.includes(model.providerID) && entry.model === model.modelID
+  )
+  if (exactMatch?.variant !== undefined) {
+    return exactMatch.variant
+  }
+
+  return chain.find((entry) => entry.model === model.modelID)?.variant
+}
+
 export function createNoSisyphusGptHook(ctx: PluginInput) {
   return {
     "chat.message": async (input: {
@@ -43,6 +55,20 @@ export function createNoSisyphusGptHook(ctx: PluginInput) {
       const agentKey = getAgentConfigKey(rawAgent)
       const modelID = input.model?.modelID
 
+      if (
+        agentKey === "sisyphus"
+        && input.model
+        && modelID
+        && isGptNativeSisyphusModel(modelID)
+        && output?.message
+        && output.message.variant === undefined
+      ) {
+        const variant = getNativeSisyphusGptVariant(input.model)
+        if (variant !== undefined) {
+          output.message.variant = variant
+        }
+      }
+
       if (agentKey === "sisyphus" && modelID && isGptModel(modelID) && !isGptNativeSisyphusModel(modelID)) {
         showToast(ctx, input.sessionID)
         input.agent = resolveRegisteredAgentName("hephaestus") ?? "hephaestus"
diff --git a/src/hooks/no-sisyphus-gpt/index.test.ts b/src/hooks/no-sisyphus-gpt/index.test.ts
index 5dd7616b4..125c57432 100644
--- a/src/hooks/no-sisyphus-gpt/index.test.ts
+++ b/src/hooks/no-sisyphus-gpt/index.test.ts
@@ -1,4 +1,7 @@
+/// <reference types="bun-types" />
+
 import { describe, expect, spyOn, test } from "bun:test"
+import type { PluginInput } from "@opencode-ai/plugin"
 import { _resetForTesting, updateSessionAgent } from "../../features/claude-code-session-state"
 import { getAgentDisplayName } from "../../shared/agent-display-names"
 import { createNoSisyphusGptHook } from "./index"
@@ -6,20 +9,29 @@ import { createNoSisyphusGptHook } from "./index"
 const SISYPHUS_DISPLAY = getAgentDisplayName("sisyphus")
 const HEPHAESTUS_DISPLAY = getAgentDisplayName("hephaestus")
 
-function createOutput() {
+type HookOutput = {
+  message: { agent?: string; variant?: string; [key: string]: unknown }
+  parts: unknown[]
+}
+
+function createOutput(): HookOutput {
   return {
     message: {},
     parts: [],
   }
 }
 
+function createHookContext(showToast: (input: unknown) => Promise<unknown>): PluginInput {
+  return {
+    client: { tui: { showToast } },
+  } as unknown as PluginInput
+}
+
 describe("no-sisyphus-gpt hook", () => {
   test("shows toast on every chat.message when sisyphus uses gpt model", async () => {
     // given - sisyphus (display name) with gpt model
     const showToast = spyOn({ fn: async () => ({}) }, "fn")
-    const hook = createNoSisyphusGptHook({
-      client: { tui: { showToast } },
-    } as any)
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
 
     const output1 = createOutput()
     const output2 = createOutput()
@@ -40,7 +52,8 @@ describe("no-sisyphus-gpt hook", () => {
     expect(showToast).toHaveBeenCalledTimes(2)
     expect(output1.message.agent).toBe("hephaestus")
     expect(output2.message.agent).toBe("hephaestus")
-    expect(showToast.mock.calls[0]?.[0]).toMatchObject({
+    const firstToastCall = (showToast.mock.calls as Array<Array<unknown>>)[0]?.[0]
+    expect(firstToastCall).toMatchObject({
       body: {
         title: "NEVER Use Sisyphus with GPT",
         message: expect.stringContaining("For other GPT models, always use Hephaestus."),
@@ -52,9 +65,7 @@ describe("no-sisyphus-gpt hook", () => {
   test("does not show toast for gpt-5.4 model (Sisyphus has specialized support)", async () => {
     // given - sisyphus with gpt-5.4 model (should be allowed)
     const showToast = spyOn({ fn: async () => ({}) }, "fn")
-    const hook = createNoSisyphusGptHook({
-      client: { tui: { showToast } },
-    } as any)
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
 
     const output = createOutput()
 
@@ -73,9 +84,7 @@ describe("no-sisyphus-gpt hook", () => {
   test("does not show toast for gpt-5.5 model (native Sisyphus support)", async () => {
     // given - sisyphus with gpt-5.5 model (should be allowed)
     const showToast = spyOn({ fn: async () => ({}) }, "fn")
-    const hook = createNoSisyphusGptHook({
-      client: { tui: { showToast } },
-    } as any)
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
 
     const output = createOutput()
 
@@ -91,12 +100,50 @@ describe("no-sisyphus-gpt hook", () => {
     expect(output.message.agent).toBeUndefined()
   })
 
+  test("sets medium variant for gpt-5.5 model when native Sisyphus support is used", async () => {
+    // given - sisyphus with gpt-5.5 model and no selected variant
+    const showToast = spyOn({ fn: async () => ({}) }, "fn")
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
+
+    const output = createOutput()
+
+    // when - chat.message runs with gpt-5.5
+    await hook["chat.message"]?.({
+      sessionID: "ses_gpt55_medium",
+      agent: SISYPHUS_DISPLAY,
+      model: { providerID: "openai", modelID: "gpt-5.5" },
+    }, output)
+
+    // then - Sisyphus stays active and receives its configured GPT-5.5 variant
+    expect(showToast).toHaveBeenCalledTimes(0)
+    expect(output.message.agent).toBeUndefined()
+    expect(output.message.variant).toBe("medium")
+  })
+
+  test("preserves selected variant for gpt-5.5 model when native Sisyphus support is used", async () => {
+    // given - sisyphus with gpt-5.5 model and a selected variant
+    const showToast = spyOn({ fn: async () => ({}) }, "fn")
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
+
+    const output: HookOutput = { message: { variant: "high" }, parts: [] }
+
+    // when - chat.message runs with gpt-5.5
+    await hook["chat.message"]?.({
+      sessionID: "ses_gpt55_high",
+      agent: SISYPHUS_DISPLAY,
+      model: { providerID: "openai", modelID: "gpt-5.5" },
+    }, output)
+
+    // then - user-selected variant is not overwritten
+    expect(showToast).toHaveBeenCalledTimes(0)
+    expect(output.message.agent).toBeUndefined()
+    expect(output.message.variant).toBe("high")
+  })
+
   test("does not show toast for non-gpt model", async () => {
     // given - sisyphus with claude model
     const showToast = spyOn({ fn: async () => ({}) }, "fn")
-    const hook = createNoSisyphusGptHook({
-      client: { tui: { showToast } },
-    } as any)
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
 
     const output = createOutput()
 
@@ -115,9 +162,7 @@ describe("no-sisyphus-gpt hook", () => {
   test("does not show toast for non-sisyphus agent", async () => {
     // given - hephaestus with gpt model
     const showToast = spyOn({ fn: async () => ({}) }, "fn")
-    const hook = createNoSisyphusGptHook({
-      client: { tui: { showToast } },
-    } as any)
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
 
     const output = createOutput()
 
@@ -138,9 +183,7 @@ describe("no-sisyphus-gpt hook", () => {
     _resetForTesting()
     updateSessionAgent("ses_4", SISYPHUS_DISPLAY)
     const showToast = spyOn({ fn: async () => ({}) }, "fn")
-    const hook = createNoSisyphusGptHook({
-      client: { tui: { showToast } },
-    } as any)
+    const hook = createNoSisyphusGptHook(createHookContext(showToast))
 
     const output = createOutput()