refactor(delegate-task): use empty array instead of null for skills parameter

- Change skills type from string[] | null to string[]
- Allow skills=[] for no skills, reject skills=null
- Remove emojis from error messages and prompts
- Update tests accordingly

src/tools/delegate-task/constants.ts

@@ -63,7 +63,7 @@ Approach:
 </Category_Context>
 
 <Caller_Warning>
-⚠️ THIS CATEGORY USES A LESS CAPABLE MODEL (claude-haiku-4-5).
+THIS CATEGORY USES A LESS CAPABLE MODEL (claude-haiku-4-5).
 
 The model executing this task has LIMITED reasoning capacity. Your prompt MUST be:
 
@@ -146,7 +146,7 @@ Approach:
 </Category_Context>
 
 <Caller_Warning>
-⚠️ THIS CATEGORY USES A MID-TIER MODEL (claude-sonnet-4-5).
+THIS CATEGORY USES A MID-TIER MODEL (claude-sonnet-4-5).
 
 While capable, this model benefits significantly from EXPLICIT instructions.
 
@@ -244,7 +244,7 @@ MUTUALLY EXCLUSIVE: Provide EITHER category OR agent, not both (unless resuming)
 - agent: Use specific agent directly (e.g., "oracle", "explore")
 - background: true=async (returns task_id), false=sync (waits for result). Default: false. Use background=true ONLY for parallel exploration with 5+ independent queries.
 - resume: Session ID to resume (from previous task output). Continues agent with FULL CONTEXT PRESERVED - saves tokens, maintains continuity.
-- skills: Array of skill names to prepend to prompt (e.g., ["playwright", "frontend-ui-ux"]). Skills will be resolved and their content prepended with a separator. Empty array [] is NOT allowed - use null if no skills needed.
+- skills: Array of skill names to prepend to prompt (e.g., ["playwright", "frontend-ui-ux"]). Use [] (empty array) if no skills needed.
 
 **WHEN TO USE resume:**
 - Task failed/incomplete → resume with "fix: [specific issue]"

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

@@ -319,7 +319,7 @@ describe("sisyphus-task", () => {
           prompt: "Do something",
           category: "ultrabrain",
           run_in_background: true,
-          skills: null,
+          skills: [],
         },
         toolContext
       )
@@ -334,12 +334,11 @@ describe("sisyphus-task", () => {
   })
 
   describe("skills parameter", () => {
-    test("DELEGATE_TASK_DESCRIPTION documents skills parameter with null option", () => {
+    test("DELEGATE_TASK_DESCRIPTION documents skills parameter with empty array option", () => {
       // #given / #when / #then
       expect(DELEGATE_TASK_DESCRIPTION).toContain("skills")
       expect(DELEGATE_TASK_DESCRIPTION).toContain("Array of skill names")
-      expect(DELEGATE_TASK_DESCRIPTION).toContain("Empty array [] is NOT allowed")
+      expect(DELEGATE_TASK_DESCRIPTION).toContain("[] (empty array) if no skills needed")
-      expect(DELEGATE_TASK_DESCRIPTION).toContain("null if no skills needed")
     })
 
     test("skills parameter is required - returns error when not provided", async () => {
@@ -385,7 +384,7 @@ describe("sisyphus-task", () => {
       expect(result).toContain("REQUIRED")
     })
 
-    test("empty array [] returns error with available skills list", async () => {
+    test("null skills returns error", async () => {
       // #given
       const { createDelegateTask } = require("./tools")
       
@@ -412,26 +411,26 @@ describe("sisyphus-task", () => {
         abort: new AbortController().signal,
       }
       
-      // #when - empty array passed
+      // #when - null passed
       const result = await tool.execute(
         {
           description: "Test task",
           prompt: "Do something",
           category: "ultrabrain",
           run_in_background: false,
-          skills: [],
+          skills: null,
         },
         toolContext
       )
       
-      // #then - should return error about empty array with guidance
+      // #then - should return error about null
-      expect(result).toContain("❌")
+      expect(result).toContain("Invalid arguments")
-      expect(result).toContain("Empty array []")
-      expect(result).toContain("not allowed")
       expect(result).toContain("skills=null")
+      expect(result).toContain("not allowed")
+      expect(result).toContain("skills=[]")
     })
 
-    test("null skills is allowed and proceeds without skill content", async () => {
+    test("empty array [] is allowed and proceeds without skill content", async () => {
       // #given
       const { createDelegateTask } = require("./tools")
       let promptBody: any
@@ -466,21 +465,20 @@ describe("sisyphus-task", () => {
         abort: new AbortController().signal,
       }
       
-      // #when - null skills passed
+      // #when - empty array skills passed
       await tool.execute(
         {
           description: "Test task",
           prompt: "Do something",
           category: "ultrabrain",
           run_in_background: false,
-          skills: null,
+          skills: [],
         },
         toolContext
       )
       
       // #then - should proceed without system content from skills
       expect(promptBody).toBeDefined()
-      // system should not contain skill content (only category prompt append if any)
     }, { timeout: 20000 })
   })
 
@@ -540,7 +538,7 @@ describe("sisyphus-task", () => {
         prompt: "Continue the task",
         resume: "ses_resume_test",
         run_in_background: false,
-        skills: null,
+        skills: [],
       },
       toolContext
     )
@@ -595,7 +593,7 @@ describe("sisyphus-task", () => {
         prompt: "Continue in background",
         resume: "ses_bg_resume",
         run_in_background: true,
-        skills: null,
+        skills: [],
       },
       toolContext
     )
@@ -650,13 +648,12 @@ describe("sisyphus-task", () => {
           prompt: "Do something",
           category: "ultrabrain",
           run_in_background: false,
-          skills: null,
+          skills: [],
         },
         toolContext
       )
       
       // #then - should return detailed error message with args and stack trace
-      expect(result).toContain("❌")
       expect(result).toContain("Send prompt failed")
       expect(result).toContain("JSON Parse error")
       expect(result).toContain("**Arguments**:")
@@ -711,7 +708,7 @@ describe("sisyphus-task", () => {
           prompt: "Do something",
           category: "ultrabrain",
           run_in_background: false,
-          skills: null,
+          skills: [],
         },
         toolContext
       )
@@ -764,13 +761,12 @@ describe("sisyphus-task", () => {
           prompt: "Do something",
           category: "ultrabrain",
           run_in_background: false,
-          skills: null,
+          skills: [],
         },
         toolContext
       )
       
       // #then - should return agent not found error
-      expect(result).toContain("❌")
       expect(result).toContain("not found")
       expect(result).toContain("registered")
     })
@@ -819,7 +815,7 @@ describe("sisyphus-task", () => {
         prompt: "test",
         category: "custom-cat",
         run_in_background: false,
-        skills: null
+        skills: []
       }, toolContext)
 
       // #then

src/tools/delegate-task/tools.ts

@@ -64,7 +64,7 @@ function formatDetailedError(error: unknown, ctx: ErrorContext): string {
   const stack = error instanceof Error ? error.stack : undefined
 
   const lines: string[] = [
-    `❌ ${ctx.operation} failed`,
+    `${ctx.operation} failed`,
     "",
     `**Error**: ${message}`,
   ]
@@ -181,37 +181,28 @@ export function createDelegateTask(options: DelegateTaskToolOptions): ToolDefini
       subagent_type: tool.schema.string().optional().describe("Agent name directly (e.g., 'oracle', 'explore'). Mutually exclusive with category."),
       run_in_background: tool.schema.boolean().describe("Run in background. MUST be explicitly set. Use false for task delegation, true only for parallel exploration."),
       resume: tool.schema.string().optional().describe("Session ID to resume - continues previous agent session with full context"),
-      skills: tool.schema.array(tool.schema.string()).nullable().describe("Array of skill names to prepend to the prompt. Use null if no skills needed. Empty array [] is NOT allowed."),
+      skills: tool.schema.array(tool.schema.string()).describe("Array of skill names to prepend to the prompt. Use [] (empty array) if no skills needed."),
     },
     async execute(args: DelegateTaskArgs, toolContext) {
       const ctx = toolContext as ToolContextWithMetadata
       if (args.run_in_background === undefined) {
-        return `❌ Invalid arguments: 'run_in_background' parameter is REQUIRED. Use run_in_background=false for task delegation, run_in_background=true only for parallel exploration.`
+        return `Invalid arguments: 'run_in_background' parameter is REQUIRED. Use run_in_background=false for task delegation, run_in_background=true only for parallel exploration.`
       }
       if (args.skills === undefined) {
-        return `❌ Invalid arguments: 'skills' parameter is REQUIRED. Use skills=null if no skills are needed, or provide an array of skill names.`
+        return `Invalid arguments: 'skills' parameter is REQUIRED. Use skills=[] if no skills are needed, or provide an array of skill names.`
       }
-      if (Array.isArray(args.skills) && args.skills.length === 0) {
+      if (args.skills === null) {
-        const allSkills = await discoverSkills({ includeClaudeCodePaths: true })
+        return `Invalid arguments: skills=null is not allowed. Use skills=[] (empty array) if no skills are needed.`
-        const availableSkillsList = allSkills.map(s => `  - ${s.name}`).slice(0, 15).join("\n")
-        return `❌ Invalid arguments: Empty array [] is not allowed for 'skills' parameter.
-
-Use skills=null if this task genuinely requires no specialized skills.
-Otherwise, select appropriate skills from available options:
-
-${availableSkillsList}${allSkills.length > 15 ? `\n  ... and ${allSkills.length - 15} more` : ""}
-
-If you believe no skills are needed, you MUST explicitly explain why to the user before using skills=null.`
       }
       const runInBackground = args.run_in_background === true
 
       let skillContent: string | undefined
-      if (args.skills !== null && args.skills.length > 0) {
+      if (args.skills.length > 0) {
         const { resolved, notFound } = await resolveMultipleSkillsAsync(args.skills, { gitMasterConfig })
         if (notFound.length > 0) {
           const allSkills = await discoverSkills({ includeClaudeCodePaths: true })
           const available = allSkills.map(s => s.name).join(", ")
-          return `❌ Skills not found: ${notFound.join(", ")}. Available: ${available}`
+          return `Skills not found: ${notFound.join(", ")}. Available: ${available}`
         }
         skillContent = Array.from(resolved.values()).join("\n\n")
       }
@@ -334,7 +325,7 @@ Use \`background_output\` with task_id="${task.id}" to check progress.`
             toastManager.removeTask(taskId)
           }
           const errorMessage = promptError instanceof Error ? promptError.message : String(promptError)
-          return `❌ Failed to send resume prompt: ${errorMessage}\n\nSession ID: ${args.resume}`
+          return `Failed to send resume prompt: ${errorMessage}\n\nSession ID: ${args.resume}`
         }
 
         // Wait for message stability after prompt completes
@@ -372,7 +363,7 @@ Use \`background_output\` with task_id="${task.id}" to check progress.`
           if (toastManager) {
             toastManager.removeTask(taskId)
           }
-          return `❌ Error fetching result: ${messagesResult.error}\n\nSession ID: ${args.resume}`
+          return `Error fetching result: ${messagesResult.error}\n\nSession ID: ${args.resume}`
         }
 
         const messages = ((messagesResult as { data?: unknown }).data ?? messagesResult) as Array<{
@@ -390,7 +381,7 @@ Use \`background_output\` with task_id="${task.id}" to check progress.`
         }
 
         if (!lastMessage) {
-          return `❌ No assistant response found.\n\nSession ID: ${args.resume}`
+          return `No assistant response found.\n\nSession ID: ${args.resume}`
         }
 
         // Extract text from both "text" and "reasoning" parts (thinking models use "reasoning")
@@ -409,11 +400,11 @@ ${textContent || "(No text output)"}`
       }
 
       if (args.category && args.subagent_type) {
-        return `❌ Invalid arguments: Provide EITHER category OR subagent_type, not both.`
+        return `Invalid arguments: Provide EITHER category OR subagent_type, not both.`
       }
 
       if (!args.category && !args.subagent_type) {
-        return `❌ Invalid arguments: Must provide either category or subagent_type.`
+        return `Invalid arguments: Must provide either category or subagent_type.`
       }
 
       // Fetch OpenCode config at boundary to get system default model
@@ -443,7 +434,7 @@ ${textContent || "(No text output)"}`
           systemDefaultModel,
         })
         if (!resolved) {
-          return `❌ Unknown category: "${args.category}". Available: ${Object.keys({ ...DEFAULT_CATEGORIES, ...userCategories }).join(", ")}`
+          return `Unknown category: "${args.category}". Available: ${Object.keys({ ...DEFAULT_CATEGORIES, ...userCategories }).join(", ")}`
         }
 
         // Determine model source by comparing against the actual resolved model
@@ -452,11 +443,11 @@ ${textContent || "(No text output)"}`
         const categoryDefaultModel = DEFAULT_CATEGORIES[args.category]?.model
 
         if (!actualModel) {
-          return `❌ No model configured. Set a model in your OpenCode config, plugin config, or use a category with a default model.`
+          return `No model configured. Set a model in your OpenCode config, plugin config, or use a category with a default model.`
         }
 
         if (!parseModelString(actualModel)) {
-          return `❌ Invalid model format "${actualModel}". Expected "provider/model" format (e.g., "anthropic/claude-sonnet-4-5").`
+          return `Invalid model format "${actualModel}". Expected "provider/model" format (e.g., "anthropic/claude-sonnet-4-5").`
         }
 
         switch (actualModel) {
@@ -484,7 +475,7 @@ ${textContent || "(No text output)"}`
         categoryPromptAppend = resolved.promptAppend || undefined
       } else {
         if (!args.subagent_type?.trim()) {
-          return `❌ Agent name cannot be empty.`
+          return `Agent name cannot be empty.`
         }
         const agentName = args.subagent_type.trim()
         agentToUse = agentName
@@ -501,13 +492,13 @@ ${textContent || "(No text output)"}`
           if (!callableNames.includes(agentToUse)) {
             const isPrimaryAgent = agents.some((a) => a.name === agentToUse && a.mode === "primary")
             if (isPrimaryAgent) {
-              return `❌ Cannot call primary agent "${agentToUse}" via delegate_task. Primary agents are top-level orchestrators.`
+              return `Cannot call primary agent "${agentToUse}" via delegate_task. Primary agents are top-level orchestrators.`
             }
 
             const availableAgents = callableNames
               .sort()
               .join(", ")
-            return `❌ Unknown agent: "${agentToUse}". Available agents: ${availableAgents}`
+            return `Unknown agent: "${agentToUse}". Available agents: ${availableAgents}`
           }
         } catch {
           // If we can't fetch agents, proceed anyway - the session.prompt will fail with a clearer error
@@ -527,7 +518,7 @@ ${textContent || "(No text output)"}`
             parentModel,
             parentAgent,
             model: categoryModel,
-            skills: args.skills ?? undefined,
+            skills: args.skills.length > 0 ? args.skills : undefined,
             skillContent: systemContent,
           })
 
@@ -576,7 +567,7 @@ System notifies on completion. Use \`background_output\` with task_id="${task.id
         })
 
         if (createResult.error) {
-          return `❌ Failed to create session: ${createResult.error}`
+          return `Failed to create session: ${createResult.error}`
         }
 
         const sessionID = createResult.data.id
@@ -591,7 +582,7 @@ System notifies on completion. Use \`background_output\` with task_id="${task.id
             description: args.description,
             agent: agentToUse,
             isBackground: false,
-            skills: args.skills ?? undefined,
+            skills: args.skills.length > 0 ? args.skills : undefined,
             modelInfo,
           })
         }
@@ -713,7 +704,7 @@ System notifies on completion. Use \`background_output\` with task_id="${task.id
         })
 
         if (messagesResult.error) {
-          return `❌ Error fetching result: ${messagesResult.error}\n\nSession ID: ${sessionID}`
+          return `Error fetching result: ${messagesResult.error}\n\nSession ID: ${sessionID}`
         }
 
         const messages = ((messagesResult as { data?: unknown }).data ?? messagesResult) as Array<{
@@ -727,7 +718,7 @@ System notifies on completion. Use \`background_output\` with task_id="${task.id
         const lastMessage = assistantMessages[0]
         
         if (!lastMessage) {
-          return `❌ No assistant response found.\n\nSession ID: ${sessionID}`
+          return `No assistant response found.\n\nSession ID: ${sessionID}`
         }
         
         // Extract text from both "text" and "reasoning" parts (thinking models use "reasoning")

src/tools/delegate-task/types.ts

@@ -5,5 +5,5 @@ export interface DelegateTaskArgs {
   subagent_type?: string
   run_in_background: boolean
   resume?: string
-  skills: string[] | null
+  skills: string[]
 }