diff --git a/src/tools/delegate-task/constants.ts b/src/tools/delegate-task/constants.ts
index d6718762b..f2e4f33be 100644
--- a/src/tools/delegate-task/constants.ts
+++ b/src/tools/delegate-task/constants.ts
@@ -346,3 +346,28 @@ export function isPlanFamily(category: string | undefined): boolean {
   const lowerCategory = getAgentConfigKey(category).toLowerCase().trim()
   return PLAN_FAMILY_NAMES.some((name) => lowerCategory === name)
 }
+
+/**
+ * Coordinator/meta agents that own the orchestration loop and must not be used as
+ * arbitrary subagent targets via task(). Delegating to these creates duplicate
+ * orchestration and conflicting team state (issue #4027).
+ *
+ * Scoped to AGENT_ELIGIBILITY_REGISTRY hard-reject entries only — sisyphus and atlas
+ * are explicitly marked `verdict: "eligible"` for team membership in the registry
+ * (src/features/team-mode/types.ts), so they are NOT included here. Adding them would
+ * conflict with the team-mode resolver's intentional `allowPrimaryAgentDelegation: true`
+ * opt-in.
+ *
+ * Symmetric guard to the caller-eligibility check added by PR #4065 for team_create.
+ */
+export const COORDINATOR_AGENT_NAMES = ["prometheus"]
+
+/**
+ * Returns true when the given agent name refers to a coordinator/meta agent that
+ * should not be reachable as a subagent_type target via task().
+ */
+export function isCoordinatorAgent(agentName: string | undefined): boolean {
+  if (!agentName) return false
+  const normalized = getAgentConfigKey(agentName).toLowerCase().trim()
+  return COORDINATOR_AGENT_NAMES.some((name) => normalized === name)
+}
diff --git a/src/tools/delegate-task/coordinator-subagent-guard.test.ts b/src/tools/delegate-task/coordinator-subagent-guard.test.ts
new file mode 100644
index 000000000..12099bce3
--- /dev/null
+++ b/src/tools/delegate-task/coordinator-subagent-guard.test.ts
@@ -0,0 +1,144 @@
+/**
+ * Regression test for issue #4027: coordinator agents must not be selectable as
+ * subagent targets via task(). Symmetric guard to PR #4065 (team_create caller
+ * eligibility) — this covers the TARGET side of delegation.
+ */
+const { describe, test, expect } = require("bun:test")
+
+import { resolveSubagentExecution } from "./subagent-resolver"
+import { COORDINATOR_AGENT_NAMES } from "./constants"
+import type { ExecutorContext } from "./executor-types"
+
+function makeCtx(): ExecutorContext {
+  return {
+    client: {
+      app: { agents: async () => ({ data: [] }) },
+      config: { get: async () => ({ data: {} }) },
+    } as unknown as ExecutorContext["client"],
+    manager: {} as unknown as ExecutorContext["manager"],
+    directory: "/tmp/test",
+  }
+}
+
+describe("coordinator subagent guard (#4027)", () => {
+  for (const coordinatorName of COORDINATOR_AGENT_NAMES) {
+    test(`#given subagent_type="${coordinatorName}" #when resolveSubagentExecution is called #then it is rejected before spawning`, async () => {
+      //#given
+      const ctx = makeCtx()
+      const args = {
+        subagent_type: coordinatorName,
+        prompt: "do something",
+        load_skills: [],
+        run_in_background: false,
+        description: "test delegation",
+      }
+
+      //#when
+      const result = await resolveSubagentExecution(args, ctx, "sisyphus", "")
+
+      //#then
+      expect(result.error).toBeDefined()
+      expect(result.agentToUse).toBe("")
+      expect(result.error).toContain(coordinatorName)
+      expect(result.error).toContain("coordinator agent")
+    })
+  }
+
+  test("#given subagent_type=prometheus #when resolveSubagentExecution is called #then error names the agent and explains the conflict", async () => {
+    //#given
+    const ctx = makeCtx()
+    const args = {
+      subagent_type: "prometheus",
+      prompt: "plan something",
+      load_skills: [],
+      run_in_background: false,
+      description: "test delegation",
+    }
+
+    //#when
+    const result = await resolveSubagentExecution(args, ctx, "sisyphus", "")
+
+    //#then
+    expect(result.error).toContain("prometheus")
+    expect(result.error).toContain("coordinator")
+    expect(result.error).toContain("duplicate")
+    expect(result.agentToUse).toBe("")
+    expect(result.categoryModel).toBeUndefined()
+  })
+
+  test("#given subagent_type=hephaestus #when resolveSubagentExecution is called #then it is not blocked by coordinator guard", async () => {
+    //#given
+    const ctx = makeCtx()
+    const args = {
+      subagent_type: "hephaestus",
+      prompt: "write some code",
+      load_skills: [],
+      run_in_background: false,
+      description: "test delegation",
+    }
+
+    //#when
+    const result = await resolveSubagentExecution(args, ctx, "sisyphus", "")
+
+    //#then — hephaestus may fail for other reasons (API call), but NOT the coordinator guard
+    expect(result.error).not.toContain("coordinator agent")
+  })
+
+  test("#given subagent_type=sisyphus #when resolveSubagentExecution is called #then sisyphus is NOT blocked by coordinator guard (registry: eligible)", async () => {
+    //#given — sisyphus is verdict:'eligible' in AGENT_ELIGIBILITY_REGISTRY; it must not be rejected by the coordinator guard
+    const ctx = makeCtx()
+    const args = {
+      subagent_type: "sisyphus",
+      prompt: "do team-mode work",
+      load_skills: [],
+      run_in_background: false,
+      description: "test delegation",
+    }
+
+    //#when
+    const result = await resolveSubagentExecution(args, ctx, "sisyphus", "")
+
+    //#then — sisyphus may fail for primary-agent reasons (separate guard), but NOT the coordinator guard
+    expect(result.error).not.toContain("coordinator agent")
+  })
+
+  test("#given subagent_type=atlas #when resolveSubagentExecution is called #then atlas is NOT blocked by coordinator guard (registry: eligible)", async () => {
+    //#given — atlas is verdict:'eligible' in AGENT_ELIGIBILITY_REGISTRY; it must not be rejected by the coordinator guard
+    const ctx = makeCtx()
+    const args = {
+      subagent_type: "atlas",
+      prompt: "do team-mode work",
+      load_skills: [],
+      run_in_background: false,
+      description: "test delegation",
+    }
+
+    //#when
+    const result = await resolveSubagentExecution(args, ctx, "sisyphus", "")
+
+    //#then — atlas may fail for primary-agent reasons (separate guard), but NOT the coordinator guard
+    expect(result.error).not.toContain("coordinator agent")
+  })
+
+  test("#given subagent_type=prometheus AND allowPrimaryAgentDelegation=true #when resolveSubagentExecution is called #then prometheus is STILL rejected (registry hard-reject is authoritative)", async () => {
+    //#given — prometheus is verdict:'hard-reject' in AGENT_ELIGIBILITY_REGISTRY; the coordinator guard must fire even when the team-mode resolver opts into primary-agent delegation
+    const ctx = makeCtx()
+    const args = {
+      subagent_type: "prometheus",
+      prompt: "plan something",
+      load_skills: [],
+      run_in_background: false,
+      description: "test delegation",
+    }
+
+    //#when
+    const result = await resolveSubagentExecution(args, ctx, "sisyphus", "", { allowPrimaryAgentDelegation: true })
+
+    //#then
+    expect(result.error).toContain("prometheus")
+    expect(result.error).toContain("coordinator agent")
+    expect(result.agentToUse).toBe("")
+  })
+})
+
+export {}
diff --git a/src/tools/delegate-task/subagent-resolver.ts b/src/tools/delegate-task/subagent-resolver.ts
index 31127a052..3d2fd92bb 100644
--- a/src/tools/delegate-task/subagent-resolver.ts
+++ b/src/tools/delegate-task/subagent-resolver.ts
@@ -1,7 +1,7 @@
 import type { DelegateTaskArgs } from "./types"
 import type { ExecutorContext } from "./executor-types"
 import type { DelegatedModelConfig } from "./types"
-import { isPlanAgent, isPlanFamily } from "./constants"
+import { isPlanAgent, isPlanFamily, isCoordinatorAgent, COORDINATOR_AGENT_NAMES } from "./constants"
 import { SISYPHUS_JUNIOR_AGENT } from "./sisyphus-junior-agent"
 import { applyCategoryParams } from "./delegated-model-config"
 import { getAvailableModelsForDelegateTask } from "./available-models"
@@ -107,6 +107,14 @@ Create the work plan directly - that's your job as the planning agent.`,
     }
   }
 
+  if (isCoordinatorAgent(agentName)) {
+    return {
+      agentToUse: "",
+      categoryModel: undefined,
+      error: `Cannot delegate to coordinator agent "${agentName}" via task(). Coordinator agents (${COORDINATOR_AGENT_NAMES.join(", ")}) own the orchestration loop and must not be used as subagent targets — doing so creates duplicate coordinators and conflicting team state. Select a worker agent (e.g., sisyphus-junior via category, hephaestus, oracle) instead.`,
+    }
+  }
+
   let agentToUse = agentName
   let categoryModel: DelegatedModelConfig | undefined
   let fallbackChain: FallbackEntry[] | undefined
diff --git a/src/tools/delegate-task/tools.test.ts b/src/tools/delegate-task/tools.test.ts
index 242a567b6..d5667a548 100644
--- a/src/tools/delegate-task/tools.test.ts
+++ b/src/tools/delegate-task/tools.test.ts
@@ -4430,8 +4430,8 @@ describe("sisyphus-task", () => {
         { sessionID: "p", messageID: "m", agent: "sisyphus", abort: new AbortController().signal }
       )
       
-      //#then
-      expect(result).toContain('Cannot delegate to primary agent "prometheus" via task. Select that agent directly instead.')
+      //#then — coordinator guard fires before primary-agent check; message names the agent and explains the conflict
+      expect(result).toContain('Cannot delegate to coordinator agent "prometheus" via task()')
     }, { timeout: 20000 })
 
     test("non-plan subagent should NOT have task permission", async () => {
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 7aa4c0509..ca1e8aeba 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
@@ -165,7 +165,10 @@ describe("resolveSubagentExecution", () => {
     //#then
     expect(result.agentToUse).toBe("")
     expect(result.categoryModel).toBeUndefined()
-    expect(result.error).toBe('Cannot delegate to primary agent "Prometheus - Plan Builder" via task. Select that agent directly instead.')
+    // Prometheus is registry-hard-reject (AGENT_ELIGIBILITY_REGISTRY); the coordinator guard (#4027 / #4071) fires before
+    // the primary-agent guard. Either rejection message is acceptable as long as prometheus is blocked from delegation.
+    expect(result.error).toContain('"Prometheus - Plan Builder"')
+    expect(result.error).toMatch(/Cannot delegate to (coordinator agent|primary agent)/)
   })
 
   test("allows delegating to a primary agent when allowPrimaryAgentDelegation is enabled (team-mode path)", async () => {