diff --git a/src/agents/dynamic-agent-core-sections.ts b/src/agents/dynamic-agent-core-sections.ts
index 69742ff16..1f91c7a78 100644
--- a/src/agents/dynamic-agent-core-sections.ts
+++ b/src/agents/dynamic-agent-core-sections.ts
@@ -196,7 +196,7 @@ export function buildNonClaudePlannerSection(model: string): string {
 Multi-step task? **ALWAYS consult Plan Agent first.** Do NOT start implementation without a plan.
 
 - Single-file fix or trivial change → proceed directly
-- Anything else (2+ steps, unclear scope, architecture) → \`task(subagent_type="prometheus", ...)\` FIRST
+- Anything else (2+ steps, unclear scope, architecture) → \`task(subagent_type="plan", ...)\` FIRST
 - Use \`task_id\` to resume the same Plan Agent - ask follow-up questions aggressively
 - If ANY part of the task is ambiguous, ask Plan Agent before guessing
 
diff --git a/src/agents/sisyphus/gpt-5-4.ts b/src/agents/sisyphus/gpt-5-4.ts
index 5f5e5bc2c..964087f13 100644
--- a/src/agents/sisyphus/gpt-5-4.ts
+++ b/src/agents/sisyphus/gpt-5-4.ts
@@ -288,7 +288,7 @@ Every implementation task follows this cycle. No exceptions.
    Follow \`<explore>\` protocol for tool usage and agent prompts.
 
 2. PLAN - List files to modify, specific changes, dependencies, complexity estimate.
-   Multi-step (2+) → consult Plan Agent via \`task(subagent_type="prometheus", ...)\`.
+   Multi-step (2+) → consult Plan Agent via \`task(subagent_type="plan", ...)\`.
    Single-step → mental plan is sufficient.
 
    <dependency_checks>
diff --git a/src/features/team-mode/team-registry/validator.test.ts b/src/features/team-mode/team-registry/validator.test.ts
index ffc9ad263..12d64015c 100644
--- a/src/features/team-mode/team-registry/validator.test.ts
+++ b/src/features/team-mode/team-registry/validator.test.ts
@@ -13,7 +13,7 @@ import {
 } from "./validator"
 
 const PROMETHEUS_REJECTION_MESSAGE =
-  "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use category: 'plan' instead."
+  "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use delegate-task with subagent_type: 'plan' instead."
 
 function createCategoryMember(name: string): Member {
   return {
diff --git a/src/features/team-mode/types.test.ts b/src/features/team-mode/types.test.ts
index 1d944eec0..0a047a930 100644
--- a/src/features/team-mode/types.test.ts
+++ b/src/features/team-mode/types.test.ts
@@ -136,7 +136,7 @@ describe("team-mode types", () => {
       ],
       [
         "prometheus",
-        "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use category: 'plan' instead.",
+        "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use delegate-task with subagent_type: 'plan' instead.",
       ],
     ] as const
 
@@ -286,7 +286,7 @@ describe("team-mode types", () => {
       "Agent 'momus' is read-only (plan reviewer). Cannot write to mailbox as team member. Use delegate-task for plan review instead.",
     )
     expect(AGENT_ELIGIBILITY_REGISTRY.prometheus.rejectionMessage).toBe(
-      "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use category: 'plan' instead.",
+      "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use delegate-task with subagent_type: 'plan' instead.",
     )
     expect(CategoryMemberSchema).toBeDefined()
     expect(SubagentMemberSchema).toBeDefined()
diff --git a/src/features/team-mode/types.ts b/src/features/team-mode/types.ts
index 21f7a0d6a..a218cf3c0 100644
--- a/src/features/team-mode/types.ts
+++ b/src/features/team-mode/types.ts
@@ -229,7 +229,7 @@ export const AGENT_ELIGIBILITY_REGISTRY: Readonly<Record<string, {
   prometheus: {
     verdict: "hard-reject",
     rejectionMessage:
-      "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use category: 'plan' instead.",
+      "Agent 'prometheus' is plan-mode-only; can only write to .omo/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use delegate-task with subagent_type: 'plan' instead.",
   },
   "sisyphus-junior": { verdict: "eligible" },
 } as const
diff --git a/src/tools/delegate-task/subagent-discovery.ts b/src/tools/delegate-task/subagent-discovery.ts
index 46e2f00e4..9512bb565 100644
--- a/src/tools/delegate-task/subagent-discovery.ts
+++ b/src/tools/delegate-task/subagent-discovery.ts
@@ -1,10 +1,9 @@
+import { loadProjectAgents, loadUserAgents } from "../../features/claude-code-agent-loader"
 import {
   getAgentConfigKey,
   getAgentDisplayName,
   stripAgentListSortPrefix,
-  stripInvisibleAgentCharacters,
 } from "../../shared/agent-display-names"
-import { loadUserAgents, loadProjectAgents } from "../../features/claude-code-agent-loader"
 
 export type AgentMode = "subagent" | "primary" | "all" | undefined
 
@@ -16,7 +15,7 @@ export type AgentInfo = {
 }
 
 export function sanitizeSubagentType(subagentType: string): string {
-  return subagentType.trim().replace(/^[\\\/"']+|[\\\/"']+$/g, "").trim()
+  return subagentType.trim().replace(/^[\\/"']+|[\\/"']+$/g, "").trim()
 }
 
 export function mergeWithClaudeCodeAgents(
@@ -69,10 +68,10 @@ export function isTaskCallableAgentMode(mode: AgentMode): boolean {
   return mode === "all" || mode === "subagent"
 }
 
-function isDemotedPlanAgent(agent: AgentInfo): boolean {
+export function isDemotedPlanAgent(agent: AgentInfo): boolean {
   return agent.hidden === true
     && agent.mode === "subagent"
-    && stripInvisibleAgentCharacters(agent.name).trim().toLowerCase() === "plan"
+    && stripAgentListSortPrefix(agent.name).trim().toLowerCase() === "plan"
 }
 
 function isVisibleToTask(agent: AgentInfo): boolean {
diff --git a/src/tools/delegate-task/subagent-resolver.ts b/src/tools/delegate-task/subagent-resolver.ts
index 44e789f70..31127a052 100644
--- a/src/tools/delegate-task/subagent-resolver.ts
+++ b/src/tools/delegate-task/subagent-resolver.ts
@@ -1,30 +1,65 @@
 import type { DelegateTaskArgs } from "./types"
 import type { ExecutorContext } from "./executor-types"
 import type { DelegatedModelConfig } from "./types"
-import { isPlanFamily } from "./constants"
+import { isPlanAgent, isPlanFamily } from "./constants"
 import { SISYPHUS_JUNIOR_AGENT } from "./sisyphus-junior-agent"
 import { applyCategoryParams } from "./delegated-model-config"
+import { getAvailableModelsForDelegateTask } from "./available-models"
 import { resolveEffectiveFallbackEntry } from "./fallback-entry-resolution"
 import { applyFallbackEntrySettings } from "./fallback-entry-settings"
+import type { AgentInfo } from "./subagent-discovery"
 import {
-  type AgentInfo,
-  sanitizeSubagentType,
-  mergeWithClaudeCodeAgents,
   findPrimaryAgentMatch,
   findCallableAgentMatch,
+  sanitizeSubagentType,
   listCallableAgentNames,
+  mergeWithClaudeCodeAgents,
+  isDemotedPlanAgent,
 } from "./subagent-discovery"
-import { normalizeModelFormat } from "../../shared/model-format-normalizer"
-import { AGENT_MODEL_REQUIREMENTS } from "../../shared/model-requirements"
-import { normalizeFallbackModels, flattenToFallbackModelStrings } from "../../shared/model-resolver"
-import { buildFallbackChainFromModels } from "../../shared/fallback-chain-from-models"
-import { getAgentConfigKey, stripAgentListSortPrefix } from "../../shared/agent-display-names"
-import { normalizeSDKResponse } from "../../shared"
-import { log } from "../../shared/logger"
-import { getAvailableModelsForDelegateTask } from "./available-models"
 import type { FallbackEntry } from "../../shared/model-requirements"
+import { AGENT_MODEL_REQUIREMENTS } from "../../shared/model-requirements"
 import { resolveModelForDelegateTask } from "./model-selection"
 import { fuzzyMatchModel } from "../../shared/model-availability"
+import { getAgentConfigKey, stripAgentListSortPrefix } from "../../shared/agent-display-names"
+import { buildFallbackChainFromModels } from "../../shared/fallback-chain-from-models"
+import { normalizeSDKResponse } from "../../shared"
+import { normalizeModelFormat } from "../../shared/model-format-normalizer"
+import { flattenToFallbackModelStrings, normalizeFallbackModels } from "../../shared/model-resolver"
+import { log } from "../../shared/logger"
+
+const DEFAULT_PLAN_FALLBACK_AGENT = "plan"
+const RESERVED_HIDDEN_NATIVE_AGENTS = new Set(["build"])
+
+function isReservedHiddenNativeAgent(agentName: string): boolean {
+  return RESERVED_HIDDEN_NATIVE_AGENTS.has(getAgentConfigKey(agentName))
+}
+
+function shouldUseHiddenPlanAgent(
+  requestedAgent: string,
+  serverPrimaryAgent: AgentInfo | undefined,
+  serverMatchedAgent: AgentInfo | undefined,
+  sisyphusAgentConfig: ExecutorContext["sisyphusAgentConfig"],
+  hasDemotedPlan: boolean,
+): boolean {
+  if (serverPrimaryAgent) {
+    return false
+  }
+
+  if (hasDemotedPlan) {
+    return false
+  }
+
+  if (serverMatchedAgent) {
+    return false
+  }
+
+  if (!isPlanAgent(requestedAgent)) {
+    return false
+  }
+
+  return sisyphusAgentConfig?.planner_enabled !== false
+    && sisyphusAgentConfig?.replace_plan !== false
+}
 
 export interface ResolveSubagentExecutionOptions {
   allowSisyphusJuniorDirect?: boolean
@@ -74,18 +109,36 @@ Create the work plan directly - that's your job as the planning agent.`,
 
   let agentToUse = agentName
   let categoryModel: DelegatedModelConfig | undefined
-  let fallbackChain: FallbackEntry[] | undefined = undefined
+  let fallbackChain: FallbackEntry[] | undefined
 
   try {
     const agentsResult = await client.app.agents()
     const agents = normalizeSDKResponse(agentsResult, [] as AgentInfo[], {
       preferResponseOnMissingData: true,
     })
+    const hasDemotedPlan = agents.some(isDemotedPlanAgent)
+    const serverPrimaryAgent = findPrimaryAgentMatch(agents, agentToUse)
+    const serverMatchedAgent = findCallableAgentMatch(agents, agentToUse)
 
     const mergedAgents = mergeWithClaudeCodeAgents(agents, executorCtx.directory)
     const matchedPrimaryAgent = findPrimaryAgentMatch(mergedAgents, agentToUse)
+    const useHiddenPlanFallback = shouldUseHiddenPlanAgent(
+      agentToUse,
+      serverPrimaryAgent,
+      serverMatchedAgent,
+      executorCtx.sisyphusAgentConfig,
+      hasDemotedPlan,
+    )
 
-    if (matchedPrimaryAgent && !options.allowPrimaryAgentDelegation) {
+    if (isReservedHiddenNativeAgent(agentToUse) && !serverPrimaryAgent && !serverMatchedAgent) {
+      return {
+        agentToUse: "",
+        categoryModel: undefined,
+        error: `Unknown agent: "${agentToUse}". Available agents: ${listCallableAgentNames(agents)}`,
+      }
+    }
+
+    if (matchedPrimaryAgent && !options.allowPrimaryAgentDelegation && !useHiddenPlanFallback) {
       return {
         agentToUse: "",
         categoryModel: undefined,
@@ -94,10 +147,17 @@ Create the work plan directly - that's your job as the planning agent.`,
     }
 
     const usePrimary = options.allowPrimaryAgentDelegation && matchedPrimaryAgent !== undefined
-    const matchedAgent = usePrimary
+    let matchedAgent = usePrimary
       ? matchedPrimaryAgent
       : findCallableAgentMatch(mergedAgents, agentToUse)
 
+    if (useHiddenPlanFallback) {
+      matchedAgent = {
+        name: DEFAULT_PLAN_FALLBACK_AGENT,
+        mode: "subagent",
+      }
+    }
+
     if (!matchedAgent) {
       return {
         agentToUse: "",
@@ -153,7 +213,8 @@ Create the work plan directly - that's your job as the planning agent.`,
           categoryModel = applyCategoryParams(resolvedModel, agentCategoryConfig)
         }
       } else if (resolutionSkipped && (agentOverride?.model ?? agentCategoryModel)) {
-        const normalized = normalizeModelFormat((agentOverride?.model ?? agentCategoryModel)!)
+        const explicitModel = agentOverride?.model ?? agentCategoryModel
+        const normalized = explicitModel ? normalizeModelFormat(explicitModel) : undefined
         if (normalized) {
           const variantToUse = agentOverride?.variant ?? agentCategoryConfig?.variant
           const resolvedModel = variantToUse ? { ...normalized, variant: variantToUse } : normalized
diff --git a/src/tools/delegate-task/zauc-mocks-subagent-resolver/subagent-resolver.test.ts b/src/tools/delegate-task/zauc-mocks-subagent-resolver/subagent-resolver.test.ts
index 6a9fe65e6..7aa4c0509 100644
--- a/src/tools/delegate-task/zauc-mocks-subagent-resolver/subagent-resolver.test.ts
+++ b/src/tools/delegate-task/zauc-mocks-subagent-resolver/subagent-resolver.test.ts
@@ -1,4 +1,4 @@
-import { describe, test, expect, beforeEach, afterEach, mock } from "bun:test"
+import { afterEach, beforeEach, describe, expect, mock, test } from "bun:test"
 import type { DelegateTaskArgs } from "../types"
 import type { ExecutorContext } from "../executor-types"
 
@@ -282,6 +282,270 @@ describe("resolveSubagentExecution", () => {
     expect(result.categoryModel).toBeUndefined()
   })
 
+  test("preserves hidden sort-prefixed plan agent model instead of using fallback", async () => {
+    //#given
+    readProviderModelsCacheMock.mockReturnValue({
+      models: { anthropic: ["claude-opus-4-7"] },
+      connected: ["anthropic"],
+      updatedAt: "2026-03-03T00:00:00.000Z",
+    })
+    const args = createBaseArgs({ subagent_type: "plan" })
+    const executorCtx = createExecutorContext(async () => ([
+      { name: "1|plan", mode: "subagent", hidden: true, model: "anthropic/claude-opus-4-7" },
+      { name: "oracle", mode: "subagent" },
+    ]))
+
+    //#when
+    const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+    //#then
+    expect(result.error).toBeUndefined()
+    expect(result.agentToUse).toBe("plan")
+    expect(result.categoryModel).toEqual({ providerID: "anthropic", modelID: "claude-opus-4-7" })
+  })
+
+  test("allows OpenCode-hidden-list plan fallback when planner_enabled and replace_plan are true", async () => {
+    //#given
+    const args = createBaseArgs({ subagent_type: "plan" })
+    const executorCtx = createExecutorContext(async () => ([
+      { name: "oracle", mode: "subagent" },
+    ]), {
+      sisyphusAgentConfig: {
+        planner_enabled: true,
+        replace_plan: true,
+      },
+    })
+
+    //#when
+    const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+    //#then
+    expect(result.error).toBeUndefined()
+    expect(result.agentToUse).toBe("plan")
+    expect(result.categoryModel).toBeUndefined()
+  })
+
+  test.each([
+    { loader: "user", aliasName: "plan" },
+    { loader: "user", aliasName: '"plan"' },
+    { loader: "project", aliasName: "plan" },
+    { loader: "project", aliasName: '"plan"' },
+  ])(
+    "uses built-in hidden plan fallback when a $loader $aliasName alias exists",
+    async ({ loader, aliasName }) => {
+      //#given
+      readProviderModelsCacheMock.mockReturnValue({
+        models: { openai: ["gpt-5.3-codex"] },
+        connected: ["openai"],
+        updatedAt: "2026-03-03T00:00:00.000Z",
+      })
+
+      loadUserAgentsMock.mockImplementation(() => {
+        if (loader === "user") {
+          return {
+            [aliasName]: {
+              description: "Colliding plan alias from user agents",
+              mode: "subagent",
+              model: "openai/gpt-5.3-codex",
+            },
+          } satisfies ClaudeCodeAgentRecord
+        }
+        return {}
+      })
+
+      loadProjectAgentsMock.mockImplementation(() => {
+        if (loader === "project") {
+          return {
+            [aliasName]: {
+              description: "Colliding plan alias from project agents",
+              mode: "subagent",
+              model: "openai/gpt-5.3-codex",
+            },
+          } satisfies ClaudeCodeAgentRecord
+        }
+        return {}
+      })
+
+      const args = createBaseArgs({ subagent_type: "plan" })
+      const executorCtx = createExecutorContext(async () => ([
+        { name: "oracle", mode: "subagent" },
+      ]), {
+        sisyphusAgentConfig: {
+          planner_enabled: true,
+          replace_plan: true,
+        },
+      })
+
+      //#when
+      const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+      //#then
+      expect(result.error).toBeUndefined()
+      expect(result.agentToUse).toBe("plan")
+      expect(result.categoryModel).toBeUndefined()
+    },
+  )
+
+  test.each([
+    { loader: "user", aliasName: "plan" },
+    { loader: "user", aliasName: '"plan"' },
+    { loader: "project", aliasName: "plan" },
+    { loader: "project", aliasName: '"plan"' },
+  ])(
+    "uses built-in hidden plan fallback when a $loader primary $aliasName alias exists",
+    async ({ loader, aliasName }) => {
+      //#given
+      loadUserAgentsMock.mockImplementation(() => {
+        if (loader === "user") {
+          return {
+            [aliasName]: {
+              description: "Colliding primary plan alias from user agents",
+              mode: "primary",
+              model: "openai/gpt-5.3-codex",
+            },
+          } satisfies ClaudeCodeAgentRecord
+        }
+        return {}
+      })
+
+      loadProjectAgentsMock.mockImplementation(() => {
+        if (loader === "project") {
+          return {
+            [aliasName]: {
+              description: "Colliding primary plan alias from project agents",
+              mode: "primary",
+              model: "openai/gpt-5.3-codex",
+            },
+          } satisfies ClaudeCodeAgentRecord
+        }
+        return {}
+      })
+
+      const args = createBaseArgs({ subagent_type: "plan" })
+      const executorCtx = createExecutorContext(async () => ([
+        { name: "oracle", mode: "subagent" },
+      ]), {
+        sisyphusAgentConfig: {
+          planner_enabled: true,
+          replace_plan: true,
+        },
+      })
+
+      //#when
+      const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+      //#then
+      expect(result.error).toBeUndefined()
+      expect(result.agentToUse).toBe("plan")
+      expect(result.categoryModel).toBeUndefined()
+    },
+  )
+
+  test.each([
+    { loader: "user", aliasName: "build" },
+    { loader: "user", aliasName: '"build"' },
+    { loader: "user", aliasName: "1|build" },
+    { loader: "user", aliasName: "\u200Bbuild" },
+    { loader: "project", aliasName: "build" },
+    { loader: "project", aliasName: '"build"' },
+    { loader: "project", aliasName: "1|build" },
+    { loader: "project", aliasName: "\u200Bbuild" },
+  ])(
+    "rejects omitted hidden build when a $loader $aliasName alias exists",
+    async ({ loader, aliasName }) => {
+      //#given
+      loadUserAgentsMock.mockImplementation(() => {
+        if (loader === "user") {
+          return {
+            [aliasName]: {
+              description: "Colliding hidden build alias from user agents",
+              mode: "subagent",
+              model: "openai/gpt-5.3-codex",
+            },
+          } satisfies ClaudeCodeAgentRecord
+        }
+        return {}
+      })
+
+      loadProjectAgentsMock.mockImplementation(() => {
+        if (loader === "project") {
+          return {
+            [aliasName]: {
+              description: "Colliding hidden build alias from project agents",
+              mode: "subagent",
+              model: "openai/gpt-5.3-codex",
+            },
+          } satisfies ClaudeCodeAgentRecord
+        }
+        return {}
+      })
+
+      const args = createBaseArgs({ subagent_type: "build" })
+      const executorCtx = createExecutorContext(async () => ([
+        { name: "oracle", mode: "subagent" },
+      ]))
+
+      //#when
+      const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+      //#then
+      expect(result.agentToUse).toBe("")
+      expect(result.categoryModel).toBeUndefined()
+      expect(result.error).toBe('Unknown agent: "build". Available agents: oracle')
+    },
+  )
+
+  test("preserves a visible server plan agent instead of using fallback", async () => {
+    //#given
+    readProviderModelsCacheMock.mockReturnValue({
+      models: { openai: ["gpt-5.3-codex"] },
+      connected: ["openai"],
+      updatedAt: "2026-03-03T00:00:00.000Z",
+    })
+    const args = createBaseArgs({ subagent_type: "plan" })
+    const executorCtx = createExecutorContext(async () => ([
+      { name: "plan", mode: "subagent", model: "openai/gpt-5.3-codex" },
+      { name: "oracle", mode: "subagent" },
+    ]), {
+      sisyphusAgentConfig: {
+        planner_enabled: true,
+        replace_plan: true,
+      },
+    })
+
+    //#when
+    const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+    //#then
+    expect(result.error).toBeUndefined()
+    expect(result.agentToUse).toBe("plan")
+    expect(result.categoryModel).toEqual({ providerID: "openai", modelID: "gpt-5.3-codex" })
+  })
+
+  test.each([
+    [{ planner_enabled: false, replace_plan: true }],
+    [{ planner_enabled: true, replace_plan: false }],
+  ])(
+    "does not allow hidden plan fallback when planner config blocks replacement (%j)",
+    async (sisyphusAgentConfig) => {
+      //#given
+      const args = createBaseArgs({ subagent_type: "plan" })
+      const executorCtx = createExecutorContext(async () => ([
+        { name: "oracle", mode: "subagent" },
+      ]), {
+        sisyphusAgentConfig,
+      })
+
+      //#when
+      const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
+
+      //#then
+      expect(result.agentToUse).toBe("")
+      expect(result.categoryModel).toBeUndefined()
+      expect(result.error).toBe('Unknown agent: "plan". Available agents: oracle')
+    },
+  )
+
   test("hidden agents are excluded from error hints except callable demoted plan", async () => {
     //#given
     const args = createBaseArgs({ subagent_type: "nonexistent" })