fix(config): plan agent inherits model settings from prometheus when not explicitly configured

Previously, demoted plan agent only received { mode: 'subagent' } with no
model settings, causing fallback to step-3.5-flash. Now inherits all
model-related settings (model, variant, temperature, top_p, maxTokens,
thinking, reasoningEffort, textVerbosity, providerOptions) from the
resolved prometheus config. User overrides via agents.plan.* take priority.

Prompt, permission, description, and color are intentionally NOT inherited.

src/plugin-handlers/config-handler.test.ts

@@ -600,6 +600,187 @@ describe("Prometheus direct override priority over category", () => {
   })
 })
 
+describe("Plan agent model inheritance from prometheus", () => {
+  test("plan agent inherits all model-related settings from resolved prometheus config", async () => {
+    //#given - prometheus resolves to claude-opus-4-6 with model settings
+    spyOn(shared, "resolveModelPipeline" as any).mockReturnValue({
+      model: "anthropic/claude-opus-4-6",
+      provenance: "provider-fallback",
+      variant: "max",
+    })
+    const pluginConfig: OhMyOpenCodeConfig = {
+      sisyphus_agent: {
+        planner_enabled: true,
+        replace_plan: true,
+      },
+    }
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      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 - plan inherits model and variant from prometheus, but NOT prompt
+    const agents = config.agent as Record<string, { mode?: string; model?: string; variant?: string; prompt?: string }>
+    expect(agents.plan).toBeDefined()
+    expect(agents.plan.mode).toBe("subagent")
+    expect(agents.plan.model).toBe("anthropic/claude-opus-4-6")
+    expect(agents.plan.variant).toBe("max")
+    expect(agents.plan.prompt).toBeUndefined()
+  })
+
+  test("plan agent inherits temperature, reasoningEffort, and other model settings from prometheus", async () => {
+    //#given - prometheus configured with category that has temperature and reasoningEffort
+    spyOn(shared, "resolveModelPipeline" as any).mockReturnValue({
+      model: "openai/gpt-5.2",
+      provenance: "override",
+      variant: "high",
+    })
+    const pluginConfig: OhMyOpenCodeConfig = {
+      sisyphus_agent: {
+        planner_enabled: true,
+        replace_plan: true,
+      },
+      agents: {
+        prometheus: {
+          model: "openai/gpt-5.2",
+          variant: "high",
+          temperature: 0.3,
+          top_p: 0.9,
+          maxTokens: 16000,
+          reasoningEffort: "high",
+          textVerbosity: "medium",
+          thinking: { type: "enabled", budgetTokens: 8000 },
+        },
+      },
+    }
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    //#when
+    await handler(config)
+
+    //#then - plan inherits ALL model-related settings from resolved prometheus
+    const agents = config.agent as Record<string, Record<string, unknown>>
+    expect(agents.plan).toBeDefined()
+    expect(agents.plan.mode).toBe("subagent")
+    expect(agents.plan.model).toBe("openai/gpt-5.2")
+    expect(agents.plan.variant).toBe("high")
+    expect(agents.plan.temperature).toBe(0.3)
+    expect(agents.plan.top_p).toBe(0.9)
+    expect(agents.plan.maxTokens).toBe(16000)
+    expect(agents.plan.reasoningEffort).toBe("high")
+    expect(agents.plan.textVerbosity).toBe("medium")
+    expect(agents.plan.thinking).toEqual({ type: "enabled", budgetTokens: 8000 })
+  })
+
+  test("plan agent user override takes priority over prometheus inherited settings", async () => {
+    //#given - prometheus resolves to opus, but user has plan override for gpt-5.2
+    spyOn(shared, "resolveModelPipeline" as any).mockReturnValue({
+      model: "anthropic/claude-opus-4-6",
+      provenance: "provider-fallback",
+      variant: "max",
+    })
+    const pluginConfig: OhMyOpenCodeConfig = {
+      sisyphus_agent: {
+        planner_enabled: true,
+        replace_plan: true,
+      },
+      agents: {
+        plan: {
+          model: "openai/gpt-5.2",
+          variant: "high",
+          temperature: 0.5,
+        },
+      },
+    }
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    //#when
+    await handler(config)
+
+    //#then - plan uses its own override, not prometheus settings
+    const agents = config.agent as Record<string, Record<string, unknown>>
+    expect(agents.plan.model).toBe("openai/gpt-5.2")
+    expect(agents.plan.variant).toBe("high")
+    expect(agents.plan.temperature).toBe(0.5)
+  })
+
+  test("plan agent does NOT inherit prompt, description, or color from prometheus", async () => {
+    //#given
+    spyOn(shared, "resolveModelPipeline" as any).mockReturnValue({
+      model: "anthropic/claude-opus-4-6",
+      provenance: "provider-fallback",
+      variant: "max",
+    })
+    const pluginConfig: OhMyOpenCodeConfig = {
+      sisyphus_agent: {
+        planner_enabled: true,
+        replace_plan: true,
+      },
+    }
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    //#when
+    await handler(config)
+
+    //#then - plan has model settings but NOT prompt/description/color
+    const agents = config.agent as Record<string, Record<string, unknown>>
+    expect(agents.plan.model).toBe("anthropic/claude-opus-4-6")
+    expect(agents.plan.prompt).toBeUndefined()
+    expect(agents.plan.description).toBeUndefined()
+    expect(agents.plan.color).toBeUndefined()
+  })
+})
+
 describe("Deadlock prevention - fetchAvailableModels must not receive client", () => {
   test("fetchAvailableModels should be called with undefined client to prevent deadlock during plugin init", async () => {
     // given - This test ensures we don't regress on issue #1301

src/plugin-handlers/config-handler.ts

@@ -32,6 +32,7 @@ import { AGENT_NAME_MAP } from "../shared/migration";
 import { AGENT_MODEL_REQUIREMENTS } from "../shared/model-requirements";
 import { PROMETHEUS_SYSTEM_PROMPT, PROMETHEUS_PERMISSION } from "../agents/prometheus";
 import { DEFAULT_CATEGORIES } from "../tools/delegate-task/constants";
+import { buildPlanDemoteConfig } from "./plan-model-inheritance";
 import type { ModelCacheState } from "../plugin-state";
 import type { CategoryConfig } from "../config/schema";
 
@@ -385,8 +386,10 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
         : {};
 
       const planDemoteConfig = shouldDemotePlan
-           ? { mode: "subagent" as const
+        ? buildPlanDemoteConfig(
-          }
+            agentConfig["prometheus"] as Record<string, unknown> | undefined,
+            pluginConfig.agents?.plan as Record<string, unknown> | undefined,
+          )
         : undefined;
 
       config.agent = {

src/plugin-handlers/plan-model-inheritance.test.ts

@@ -0,0 +1,118 @@
+import { describe, test, expect } from "bun:test"
+import { buildPlanDemoteConfig } from "./plan-model-inheritance"
+
+describe("buildPlanDemoteConfig", () => {
+  test("returns only mode when prometheus and plan override are both undefined", () => {
+    //#given
+    const prometheusConfig = undefined
+    const planOverride = undefined
+
+    //#when
+    const result = buildPlanDemoteConfig(prometheusConfig, planOverride)
+
+    //#then
+    expect(result).toEqual({ mode: "subagent" })
+  })
+
+  test("extracts all model settings from prometheus config", () => {
+    //#given
+    const prometheusConfig = {
+      name: "prometheus",
+      model: "anthropic/claude-opus-4-6",
+      variant: "max",
+      mode: "all",
+      prompt: "You are Prometheus...",
+      permission: { edit: "allow" },
+      description: "Plan agent (Prometheus)",
+      color: "#FF5722",
+      temperature: 0.1,
+      top_p: 0.95,
+      maxTokens: 32000,
+      thinking: { type: "enabled", budgetTokens: 10000 },
+      reasoningEffort: "high",
+      textVerbosity: "medium",
+      providerOptions: { key: "value" },
+    }
+
+    //#when
+    const result = buildPlanDemoteConfig(prometheusConfig, undefined)
+
+    //#then - picks model settings, NOT prompt/permission/description/color/name/mode
+    expect(result.mode).toBe("subagent")
+    expect(result.model).toBe("anthropic/claude-opus-4-6")
+    expect(result.variant).toBe("max")
+    expect(result.temperature).toBe(0.1)
+    expect(result.top_p).toBe(0.95)
+    expect(result.maxTokens).toBe(32000)
+    expect(result.thinking).toEqual({ type: "enabled", budgetTokens: 10000 })
+    expect(result.reasoningEffort).toBe("high")
+    expect(result.textVerbosity).toBe("medium")
+    expect(result.providerOptions).toEqual({ key: "value" })
+    expect(result.prompt).toBeUndefined()
+    expect(result.permission).toBeUndefined()
+    expect(result.description).toBeUndefined()
+    expect(result.color).toBeUndefined()
+    expect(result.name).toBeUndefined()
+  })
+
+  test("plan override takes priority over prometheus for all model settings", () => {
+    //#given
+    const prometheusConfig = {
+      model: "anthropic/claude-opus-4-6",
+      variant: "max",
+      temperature: 0.1,
+      reasoningEffort: "high",
+    }
+    const planOverride = {
+      model: "openai/gpt-5.2",
+      variant: "high",
+      temperature: 0.5,
+      reasoningEffort: "low",
+    }
+
+    //#when
+    const result = buildPlanDemoteConfig(prometheusConfig, planOverride)
+
+    //#then
+    expect(result.model).toBe("openai/gpt-5.2")
+    expect(result.variant).toBe("high")
+    expect(result.temperature).toBe(0.5)
+    expect(result.reasoningEffort).toBe("low")
+  })
+
+  test("falls back to prometheus when plan override has partial settings", () => {
+    //#given
+    const prometheusConfig = {
+      model: "anthropic/claude-opus-4-6",
+      variant: "max",
+      temperature: 0.1,
+      reasoningEffort: "high",
+    }
+    const planOverride = {
+      model: "openai/gpt-5.2",
+    }
+
+    //#when
+    const result = buildPlanDemoteConfig(prometheusConfig, planOverride)
+
+    //#then - plan model wins, rest inherits from prometheus
+    expect(result.model).toBe("openai/gpt-5.2")
+    expect(result.variant).toBe("max")
+    expect(result.temperature).toBe(0.1)
+    expect(result.reasoningEffort).toBe("high")
+  })
+
+  test("skips undefined values from both sources", () => {
+    //#given
+    const prometheusConfig = {
+      model: "anthropic/claude-opus-4-6",
+    }
+
+    //#when
+    const result = buildPlanDemoteConfig(prometheusConfig, undefined)
+
+    //#then
+    expect(result).toEqual({ mode: "subagent", model: "anthropic/claude-opus-4-6" })
+    expect(Object.keys(result)).toEqual(["mode", "model"])
+  })
+})

src/plugin-handlers/plan-model-inheritance.ts

@@ -0,0 +1,27 @@
+const MODEL_SETTINGS_KEYS = [
+  "model",
+  "variant",
+  "temperature",
+  "top_p",
+  "maxTokens",
+  "thinking",
+  "reasoningEffort",
+  "textVerbosity",
+  "providerOptions",
+] as const
+
+export function buildPlanDemoteConfig(
+  prometheusConfig: Record<string, unknown> | undefined,
+  planOverride: Record<string, unknown> | undefined,
+): Record<string, unknown> {
+  const modelSettings: Record<string, unknown> = {}
+
+  for (const key of MODEL_SETTINGS_KEYS) {
+    const value = planOverride?.[key] ?? prometheusConfig?.[key]
+    if (value !== undefined) {
+      modelSettings[key] = value
+    }
+  }
+
+  return { mode: "subagent" as const, ...modelSettings }
+}