feat(delegate-task): prepend system prompt for plan agent invocations

When plan agent (plan/prometheus/planner) is invoked via delegate_task,
automatically prepend a <system> prompt instructing the agent to:
- Launch explore/librarian agents in background to gather context
- Summarize user request and list uncertainties
- Ask clarifying questions until requirements are 100% clear

src/tools/delegate-task/constants.ts

@@ -185,4 +185,49 @@ export const CATEGORY_DESCRIPTIONS: Record<string, string> = {
   writing: "Documentation, prose, technical writing",
 }
 
+/**
+ * System prompt prepended to plan agent invocations.
+ * Instructs the plan agent to first gather context via explore/librarian agents,
+ * then summarize user requirements and clarify uncertainties before proceeding.
+ */
+export const PLAN_AGENT_SYSTEM_PREPEND = `<system>
+BEFORE you begin planning, you MUST first understand the user's request deeply.
+
+MANDATORY CONTEXT GATHERING PROTOCOL:
+1. Launch background agents to gather context:
+   - call_omo_agent(agent="explore", background=true, prompt="<search for relevant patterns, files, and implementations in the codebase related to user's request>")
+   - call_omo_agent(agent="librarian", background=true, prompt="<search for external documentation, examples, and best practices related to user's request>")
+
+2. After gathering context, ALWAYS present:
+   - **User Request Summary**: Concise restatement of what the user is asking for
+   - **Uncertainties**: List of unclear points, ambiguities, or assumptions you're making
+   - **Clarifying Questions**: Specific questions to resolve the uncertainties
+
+3. ITERATE until ALL requirements are crystal clear:
+   - Do NOT proceed to planning until you have 100% clarity
+   - Ask the user to confirm your understanding
+   - Resolve every ambiguity before generating the work plan
+
+REMEMBER: Vague requirements lead to failed implementations. Take the time to understand thoroughly.
+</system>
+
+`
+
+/**
+ * List of agent names that should be treated as plan agents.
+ * Case-insensitive matching is used.
+ */
+export const PLAN_AGENT_NAMES = ["plan", "prometheus", "planner"]
+
+/**
+ * Check if the given agent name is a plan agent.
+ * @param agentName - The agent name to check
+ * @returns true if the agent is a plan agent
+ */
+export function isPlanAgent(agentName: string | undefined): boolean {
+  if (!agentName) return false
+  const lowerName = agentName.toLowerCase().trim()
+  return PLAN_AGENT_NAMES.some(name => lowerName === name || lowerName.includes(name))
+}
+
 

src/tools/delegate-task/tools.test.ts

@@ -1,5 +1,5 @@
 import { describe, test, expect, beforeEach } from "bun:test"
-import { DEFAULT_CATEGORIES, CATEGORY_PROMPT_APPENDS, CATEGORY_DESCRIPTIONS } from "./constants"
+import { DEFAULT_CATEGORIES, CATEGORY_PROMPT_APPENDS, CATEGORY_DESCRIPTIONS, isPlanAgent, PLAN_AGENT_NAMES } from "./constants"
 import { resolveCategoryConfig } from "./tools"
 import type { CategoryConfig } from "../../config/schema"
 import { __resetModelCache } from "../../shared/model-availability"
@@ -77,6 +77,87 @@ describe("sisyphus-task", () => {
     })
   })
 
+  describe("isPlanAgent", () => {
+    test("returns true for 'plan'", () => {
+      // #given / #when
+      const result = isPlanAgent("plan")
+
+      // #then
+      expect(result).toBe(true)
+    })
+
+    test("returns true for 'prometheus'", () => {
+      // #given / #when
+      const result = isPlanAgent("prometheus")
+
+      // #then
+      expect(result).toBe(true)
+    })
+
+    test("returns true for 'planner'", () => {
+      // #given / #when
+      const result = isPlanAgent("planner")
+
+      // #then
+      expect(result).toBe(true)
+    })
+
+    test("returns true for case-insensitive match 'PLAN'", () => {
+      // #given / #when
+      const result = isPlanAgent("PLAN")
+
+      // #then
+      expect(result).toBe(true)
+    })
+
+    test("returns true for case-insensitive match 'Prometheus'", () => {
+      // #given / #when
+      const result = isPlanAgent("Prometheus")
+
+      // #then
+      expect(result).toBe(true)
+    })
+
+    test("returns false for 'oracle'", () => {
+      // #given / #when
+      const result = isPlanAgent("oracle")
+
+      // #then
+      expect(result).toBe(false)
+    })
+
+    test("returns false for 'explore'", () => {
+      // #given / #when
+      const result = isPlanAgent("explore")
+
+      // #then
+      expect(result).toBe(false)
+    })
+
+    test("returns false for undefined", () => {
+      // #given / #when
+      const result = isPlanAgent(undefined)
+
+      // #then
+      expect(result).toBe(false)
+    })
+
+    test("returns false for empty string", () => {
+      // #given / #when
+      const result = isPlanAgent("")
+
+      // #then
+      expect(result).toBe(false)
+    })
+
+    test("PLAN_AGENT_NAMES contains expected values", () => {
+      // #given / #when / #then
+      expect(PLAN_AGENT_NAMES).toContain("plan")
+      expect(PLAN_AGENT_NAMES).toContain("prometheus")
+      expect(PLAN_AGENT_NAMES).toContain("planner")
+    })
+  })
+
   describe("category delegation config validation", () => {
     test("proceeds without error when systemDefaultModel is undefined", async () => {
       // #given a mock client with no model in config
@@ -1481,6 +1562,87 @@ describe("sisyphus-task", () => {
       expect(result).toContain(categoryPromptAppend)
       expect(result).toContain("\n\n")
     })
+
+    test("prepends plan agent system prompt when agentName is 'plan'", () => {
+      // #given
+      const { buildSystemContent } = require("./tools")
+      const { PLAN_AGENT_SYSTEM_PREPEND } = require("./constants")
+
+      // #when
+      const result = buildSystemContent({ agentName: "plan" })
+
+      // #then
+      expect(result).toContain("<system>")
+      expect(result).toContain("MANDATORY CONTEXT GATHERING PROTOCOL")
+      expect(result).toBe(PLAN_AGENT_SYSTEM_PREPEND)
+    })
+
+    test("prepends plan agent system prompt when agentName is 'prometheus'", () => {
+      // #given
+      const { buildSystemContent } = require("./tools")
+      const { PLAN_AGENT_SYSTEM_PREPEND } = require("./constants")
+
+      // #when
+      const result = buildSystemContent({ agentName: "prometheus" })
+
+      // #then
+      expect(result).toContain("<system>")
+      expect(result).toBe(PLAN_AGENT_SYSTEM_PREPEND)
+    })
+
+    test("prepends plan agent system prompt when agentName is 'Prometheus' (case insensitive)", () => {
+      // #given
+      const { buildSystemContent } = require("./tools")
+      const { PLAN_AGENT_SYSTEM_PREPEND } = require("./constants")
+
+      // #when
+      const result = buildSystemContent({ agentName: "Prometheus" })
+
+      // #then
+      expect(result).toContain("<system>")
+      expect(result).toBe(PLAN_AGENT_SYSTEM_PREPEND)
+    })
+
+    test("combines plan agent prepend with skill content", () => {
+      // #given
+      const { buildSystemContent } = require("./tools")
+      const { PLAN_AGENT_SYSTEM_PREPEND } = require("./constants")
+      const skillContent = "You are a planning expert"
+
+      // #when
+      const result = buildSystemContent({ skillContent, agentName: "plan" })
+
+      // #then
+      expect(result).toContain(PLAN_AGENT_SYSTEM_PREPEND)
+      expect(result).toContain(skillContent)
+      expect(result!.indexOf(PLAN_AGENT_SYSTEM_PREPEND)).toBeLessThan(result!.indexOf(skillContent))
+    })
+
+    test("does not prepend plan agent prompt for non-plan agents", () => {
+      // #given
+      const { buildSystemContent } = require("./tools")
+      const skillContent = "You are an expert"
+
+      // #when
+      const result = buildSystemContent({ skillContent, agentName: "oracle" })
+
+      // #then
+      expect(result).toBe(skillContent)
+      expect(result).not.toContain("<system>")
+    })
+
+    test("does not prepend plan agent prompt when agentName is undefined", () => {
+      // #given
+      const { buildSystemContent } = require("./tools")
+      const skillContent = "You are an expert"
+
+      // #when
+      const result = buildSystemContent({ skillContent, agentName: undefined })
+
+      // #then
+      expect(result).toBe(skillContent)
+      expect(result).not.toContain("<system>")
+    })
   })
 
   describe("modelInfo detection via resolveCategoryConfig", () => {

src/tools/delegate-task/tools.ts

@@ -4,7 +4,7 @@ import { join } from "node:path"
 import type { BackgroundManager } from "../../features/background-agent"
 import type { DelegateTaskArgs } from "./types"
 import type { CategoryConfig, CategoriesConfig, GitMasterConfig, BrowserAutomationProvider } from "../../config/schema"
-import { DEFAULT_CATEGORIES, CATEGORY_PROMPT_APPENDS, CATEGORY_DESCRIPTIONS } from "./constants"
+import { DEFAULT_CATEGORIES, CATEGORY_PROMPT_APPENDS, CATEGORY_DESCRIPTIONS, PLAN_AGENT_SYSTEM_PREPEND, isPlanAgent } from "./constants"
 import { findNearestMessageWithFields, findFirstMessageWithAgent, MESSAGE_STORAGE } from "../../features/hook-message-injector"
 import { resolveMultipleSkillsAsync } from "../../features/opencode-skill-loader/skill-content"
 import { discoverSkills } from "../../features/opencode-skill-loader"
@@ -171,20 +171,33 @@ export interface DelegateTaskToolOptions {
 export interface BuildSystemContentInput {
   skillContent?: string
   categoryPromptAppend?: string
+  agentName?: string
 }
 
 export function buildSystemContent(input: BuildSystemContentInput): string | undefined {
-  const { skillContent, categoryPromptAppend } = input
+  const { skillContent, categoryPromptAppend, agentName } = input
 
-  if (!skillContent && !categoryPromptAppend) {
+  const planAgentPrepend = isPlanAgent(agentName) ? PLAN_AGENT_SYSTEM_PREPEND : ""
+
+  if (!skillContent && !categoryPromptAppend && !planAgentPrepend) {
     return undefined
   }
 
-  if (skillContent && categoryPromptAppend) {
-    return `${skillContent}\n\n${categoryPromptAppend}`
+  const parts: string[] = []
+
+  if (planAgentPrepend) {
+    parts.push(planAgentPrepend)
   }
 
-  return skillContent || categoryPromptAppend
+  if (skillContent) {
+    parts.push(skillContent)
+  }
+
+  if (categoryPromptAppend) {
+    parts.push(categoryPromptAppend)
+  }
+
+  return parts.join("\n\n") || undefined
 }
 
 export function createDelegateTask(options: DelegateTaskToolOptions): ToolDefinition {
@@ -382,6 +395,7 @@ Use \`background_output\` with task_id="${task.id}" to check progress.`
                 task: false,
                 delegate_task: false,
                 call_omo_agent: true,
+                question: false,
               },
               parts: [{ type: "text", text: args.prompt }],
             },
@@ -580,7 +594,7 @@ To continue this session: session_id="${args.session_id}"`
         })
 
         if (isUnstableAgent && isRunInBackgroundExplicitlyFalse) {
-          const systemContent = buildSystemContent({ skillContent, categoryPromptAppend })
+          const systemContent = buildSystemContent({ skillContent, categoryPromptAppend, agentName: agentToUse })
 
           try {
             const task = await manager.launch({
@@ -772,7 +786,7 @@ Sisyphus-Junior is spawned automatically when you specify a category. Pick the a
         }
       }
 
-      const systemContent = buildSystemContent({ skillContent, categoryPromptAppend })
+      const systemContent = buildSystemContent({ skillContent, categoryPromptAppend, agentName: agentToUse })
 
       if (runInBackground) {
         try {
@@ -903,6 +917,7 @@ To continue this session: session_id="${task.sessionID}"`
                 task: false,
                 delegate_task: false,
                 call_omo_agent: true,
+                question: false,
               },
               parts: [{ type: "text", text: args.prompt }],
               ...(categoryModel ? { model: categoryModel } : {}),