diff --git a/src/plugin-handlers/config-handler.test.ts b/src/plugin-handlers/config-handler.test.ts
index 62a4b0d0..6f6fa3ae 100644
--- a/src/plugin-handlers/config-handler.test.ts
+++ b/src/plugin-handlers/config-handler.test.ts
@@ -145,8 +145,8 @@ describe("Plan agent demote behavior", () => {
     expect(ordered).toEqual(coreAgents)
   })
 
-  test("plan agent should be demoted to subagent mode when replacePlan is true", async () => {
-    // given
+  test("plan agent should be demoted to subagent without inheriting prometheus prompt", async () => {
+    // #given
     const pluginConfig: OhMyOpenCodeConfig = {
       sisyphus_agent: {
         planner_enabled: true,
@@ -172,62 +172,22 @@ describe("Plan agent demote behavior", () => {
       },
     })
 
-    // when
+    // #when
     await handler(config)
 
-    // then
+    // #then - plan is demoted to subagent but does NOT inherit prometheus prompt
     const agents = config.agent as Record<string, { mode?: string; name?: string; prompt?: string }>
     expect(agents.plan).toBeDefined()
     expect(agents.plan.mode).toBe("subagent")
-    expect(agents.plan.name).toBe("plan")
-    expect(agents.plan.prompt).toBe("original plan prompt")
-    expect(agents.plan.prompt).not.toBe(agents.prometheus?.prompt)
+    expect(agents.plan.prompt).toBeUndefined()
+    expect(agents.prometheus?.prompt).toBeDefined()
   })
 
-  test("plan agent should not be demoted when replacePlan is false", async () => {
-    // given
-    const pluginConfig: OhMyOpenCodeConfig = {
-      sisyphus_agent: {
-        planner_enabled: true,
-        replace_plan: false,
-      },
-    }
-    const config: Record<string, unknown> = {
-      model: "anthropic/claude-opus-4-5",
-      agent: {
-        plan: {
-          name: "plan",
-          mode: "primary",
-          prompt: "original plan prompt",
-        },
-      },
-    }
-    const handler = createConfigHandler({
-      ctx: { directory: "/tmp" },
-      pluginConfig,
-      modelCacheState: {
-        anthropicContext1MEnabled: false,
-        modelContextLimitsCache: new Map(),
-      },
-    })
-
-    // when
-    await handler(config)
-
-    // then
-    const agents = config.agent as Record<string, { mode?: string; name?: string; prompt?: string }>
-    expect(agents.plan).toBeDefined()
-    expect(agents.plan.mode).toBe("primary")
-    expect(agents.plan.name).toBe("plan")
-    expect(agents.plan.prompt).toBe("original plan prompt")
-  })
-
-  test("plan agent should not be demoted when planner is disabled", async () => {
-    // given
+  test("plan agent remains unchanged when planner is disabled", async () => {
+    // #given
     const pluginConfig: OhMyOpenCodeConfig = {
       sisyphus_agent: {
         planner_enabled: false,
-        replace_plan: true,
       },
     }
     const config: Record<string, unknown> = {
@@ -249,50 +209,15 @@ describe("Plan agent demote behavior", () => {
       },
     })
 
-    // when
+    // #when
     await handler(config)
 
-    // then
+    // #then - plan is not touched, prometheus is not created
     const agents = config.agent as Record<string, { mode?: string; name?: string; prompt?: string }>
     expect(agents.prometheus).toBeUndefined()
     expect(agents.plan).toBeDefined()
     expect(agents.plan.mode).toBe("primary")
-  })
-
-  test("preserves empty plan prompt when demoting", async () => {
-    // given
-    const pluginConfig: OhMyOpenCodeConfig = {
-      sisyphus_agent: {
-        planner_enabled: true,
-        replace_plan: true,
-      },
-    }
-    const config: Record<string, unknown> = {
-      model: "anthropic/claude-opus-4-5",
-      agent: {
-        plan: {
-          name: "plan",
-          mode: "primary",
-          prompt: "",
-        },
-      },
-    }
-    const handler = createConfigHandler({
-      ctx: { directory: "/tmp" },
-      pluginConfig,
-      modelCacheState: {
-        anthropicContext1MEnabled: false,
-        modelContextLimitsCache: new Map(),
-      },
-    })
-
-    // when
-    await handler(config)
-
-    // then
-    const agents = config.agent as Record<string, { prompt?: string }>
-    expect(agents.plan).toBeDefined()
-    expect(agents.plan.prompt).toBe("")
+    expect(agents.plan.prompt).toBe("original plan prompt")
   })
 
   test("prometheus should have mode 'all' to be callable via delegate_task", async () => {
diff --git a/src/plugin-handlers/config-handler.ts b/src/plugin-handlers/config-handler.ts
index ce0390ce..457e8069 100644
--- a/src/plugin-handlers/config-handler.ts
+++ b/src/plugin-handlers/config-handler.ts
@@ -195,7 +195,7 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
     const plannerEnabled =
       pluginConfig.sisyphus_agent?.planner_enabled ?? true;
     const replacePlan = pluginConfig.sisyphus_agent?.replace_plan ?? true;
-    const shouldReplacePlan = plannerEnabled && replacePlan;
+    const shouldDemotePlan = plannerEnabled && replacePlan;
 
     type AgentConfig = Record<
       string,
@@ -211,12 +211,6 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
     };
     const configAgent = config.agent as AgentConfig | undefined;
 
-    const { name: _planName, mode: _planMode, ...planConfigWithoutName } =
-      configAgent?.plan ?? {};
-    const planPrompt = (migrateAgentConfig(
-      planConfigWithoutName as Record<string, unknown>
-    ) as { prompt?: string }).prompt;
-
     if (isSisyphusEnabled && builtinAgents.sisyphus) {
       (config as { default_agent?: string }).default_agent = "sisyphus";
 
@@ -345,7 +339,7 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
           Object.entries(configAgent)
             .filter(([key]) => {
               if (key === "build") return false;
-              if (key === "plan" && shouldReplacePlan) return false;
+              if (key === "plan" && shouldDemotePlan) return false;
               // Filter out agents that oh-my-opencode provides to prevent
               // OpenCode defaults from overwriting user config in oh-my-opencode.json
               // See: https://github.com/code-yeongyu/oh-my-opencode/issues/472
@@ -363,13 +357,8 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
         ? migrateAgentConfig(configAgent.build as Record<string, unknown>)
         : {};
 
-      const planDemoteConfig = shouldReplacePlan && agentConfig["prometheus"]
-        ? {
-            ...agentConfig["prometheus"],
-            name: "plan",
-            mode: "subagent" as const,
-            ...(typeof planPrompt === "string" ? { prompt: planPrompt } : {}),
-          }
+      const planDemoteConfig = shouldDemotePlan
+        ? { mode: "subagent" as const }
         : undefined;
 
       config.agent = {