fix(delegate-task): allow hidden plan task

src/tools/delegate-task/subagent-discovery.ts

@@ -1,4 +1,9 @@
-import { getAgentConfigKey, getAgentDisplayName, stripAgentListSortPrefix } from "../../shared/agent-display-names"
+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
@@ -64,6 +69,16 @@ export function isTaskCallableAgentMode(mode: AgentMode): boolean {
   return mode === "all" || mode === "subagent"
 }
 
+function isDemotedPlanAgent(agent: AgentInfo): boolean {
+  return agent.hidden === true
+    && agent.mode === "subagent"
+    && stripInvisibleAgentCharacters(agent.name).trim().toLowerCase() === "plan"
+}
+
+function isVisibleToTask(agent: AgentInfo): boolean {
+  return agent.hidden !== true || isDemotedPlanAgent(agent)
+}
+
 export function findPrimaryAgentMatch(
   agents: AgentInfo[],
   requestedAgentName: string,
@@ -75,12 +90,12 @@ export function findCallableAgentMatch(
   agents: AgentInfo[],
   requestedAgentName: string,
 ): AgentInfo | undefined {
-  return agents.find(agent => isTaskCallableAgentMode(agent.mode) && agent.hidden !== true && matchesRequestedAgent(agent, requestedAgentName))
+  return agents.find(agent => isTaskCallableAgentMode(agent.mode) && isVisibleToTask(agent) && matchesRequestedAgent(agent, requestedAgentName))
 }
 
 export function listCallableAgentNames(agents: AgentInfo[]): string {
   return agents
-    .filter(agent => isTaskCallableAgentMode(agent.mode) && agent.hidden !== true)
+    .filter(agent => isTaskCallableAgentMode(agent.mode) && isVisibleToTask(agent))
     .map(agent => stripAgentListSortPrefix(agent.name))
     .sort()
     .join(", ")

src/tools/delegate-task/zauc-mocks-subagent-resolver/subagent-resolver.test.ts

@@ -265,7 +265,7 @@ describe("resolveSubagentExecution", () => {
     expect(result.error).toBe('Unknown agent: "build". Available agents: oracle')
   })
 
-  test("rejects delegation to hidden plan agent demoted to subagent (regression #3957)", async () => {
+  test("allows delegation to hidden plan agent demoted to subagent", async () => {
     //#given
     const args = createBaseArgs({ subagent_type: "plan" })
     const executorCtx = createExecutorContext(async () => ([
@@ -277,12 +277,12 @@ describe("resolveSubagentExecution", () => {
     const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
 
     //#then
-    expect(result.agentToUse).toBe("")
+    expect(result.error).toBeUndefined()
+    expect(result.agentToUse).toBe("plan")
     expect(result.categoryModel).toBeUndefined()
-    expect(result.error).toBe('Unknown agent: "plan". Available agents: oracle')
   })
 
-  test("hidden agents are excluded from listCallableAgentNames in error messages (regression #3957)", async () => {
+  test("hidden agents are excluded from error hints except callable demoted plan", async () => {
     //#given
     const args = createBaseArgs({ subagent_type: "nonexistent" })
     const executorCtx = createExecutorContext(async () => ([
@@ -298,9 +298,8 @@ describe("resolveSubagentExecution", () => {
     //#then
     expect(result.agentToUse).toBe("")
     expect(result.error).toBeDefined()
-    expect(result.error).toContain('Available agents: explore, oracle')
+    expect(result.error).toContain('Available agents: explore, oracle, plan')
     expect(result.error).not.toContain("build")
-    expect(result.error).not.toContain("plan")
   })
 
   test("rejects ZWSP-prefixed project agent that canonicalizes to hidden build (regression #3957 canonical-key bypass)", async () => {
@@ -327,7 +326,7 @@ describe("resolveSubagentExecution", () => {
     expect(result.error).toBe('Unknown agent: "build". Available agents: oracle')
   })
 
-  test("rejects quoted user agent that canonicalizes to hidden plan (regression #3957 canonical-key bypass)", async () => {
+  test("uses built-in hidden plan instead of quoted user agent alias", async () => {
     //#given
     loadUserAgentsMock.mockImplementation(() => ({
       '"plan"': {
@@ -346,9 +345,9 @@ describe("resolveSubagentExecution", () => {
     const result = await resolveSubagentExecution(args, executorCtx, "sisyphus", "deep")
 
     //#then
-    expect(result.agentToUse).toBe("")
+    expect(result.error).toBeUndefined()
+    expect(result.agentToUse).toBe("plan")
     expect(result.categoryModel).toBeUndefined()
-    expect(result.error).toBe('Unknown agent: "plan". Available agents: oracle')
   })
 
   test("rejects sort-prefixed project agent that canonicalizes to hidden build (regression #3957 canonical-key bypass)", async () => {