diff --git a/src/features/task-toast-manager/manager.test.ts b/src/features/task-toast-manager/manager.test.ts
index 9558fe8d..069f1851 100644
--- a/src/features/task-toast-manager/manager.test.ts
+++ b/src/features/task-toast-manager/manager.test.ts
@@ -144,14 +144,35 @@ describe("TaskToastManager", () => {
   })
 
   describe("model fallback info in toast message", () => {
-    test("should display warning when model falls back to default", () => {
-      // #given - a task with model fallback to default
+    test("should display warning when model falls back to category-default", () => {
+      // #given - a task with model fallback to category-default
       const task = {
         id: "task_1",
-        description: "Task with default model",
+        description: "Task with category default model",
         agent: "Sisyphus-Junior",
         isBackground: false,
-        modelInfo: { model: "anthropic/claude-sonnet-4-5", type: "default" as const },
+        modelInfo: { model: "google/gemini-3-pro-preview", type: "category-default" as const },
+      }
+
+      // #when - addTask is called
+      toastManager.addTask(task)
+
+      // #then - toast should show warning with model info
+      expect(mockClient.tui.showToast).toHaveBeenCalled()
+      const call = mockClient.tui.showToast.mock.calls[0][0]
+      expect(call.body.message).toContain("⚠️")
+      expect(call.body.message).toContain("google/gemini-3-pro-preview")
+      expect(call.body.message).toContain("(category default)")
+    })
+
+    test("should display warning when model falls back to system-default", () => {
+      // #given - a task with model fallback to system-default
+      const task = {
+        id: "task_1b",
+        description: "Task with system default model",
+        agent: "Sisyphus-Junior",
+        isBackground: false,
+        modelInfo: { model: "anthropic/claude-sonnet-4-5", type: "system-default" as const },
       }
 
       // #when - addTask is called
@@ -162,7 +183,7 @@ describe("TaskToastManager", () => {
       const call = mockClient.tui.showToast.mock.calls[0][0]
       expect(call.body.message).toContain("⚠️")
       expect(call.body.message).toContain("anthropic/claude-sonnet-4-5")
-      expect(call.body.message).toContain("(default)")
+      expect(call.body.message).toContain("(system default)")
     })
 
     test("should display warning when model is inherited from parent", () => {
@@ -204,7 +225,8 @@ describe("TaskToastManager", () => {
       const call = mockClient.tui.showToast.mock.calls[0][0]
       expect(call.body.message).not.toContain("⚠️ Model:")
       expect(call.body.message).not.toContain("(inherited)")
-      expect(call.body.message).not.toContain("(default)")
+      expect(call.body.message).not.toContain("(category default)")
+      expect(call.body.message).not.toContain("(system default)")
     })
 
     test("should not display model info when not provided", () => {
diff --git a/src/features/task-toast-manager/manager.ts b/src/features/task-toast-manager/manager.ts
index 20086c6a..5cb5a7b1 100644
--- a/src/features/task-toast-manager/manager.ts
+++ b/src/features/task-toast-manager/manager.ts
@@ -110,7 +110,12 @@ export class TaskToastManager {
     // Show model fallback warning for the new task if applicable
     if (newTask.modelInfo && newTask.modelInfo.type !== "user-defined") {
       const icon = "⚠️"
-      const suffix = newTask.modelInfo.type === "inherited" ? " (inherited)" : " (default)"
+      const suffixMap: Partial<Record<ModelFallbackInfo["type"], string>> = {
+        inherited: " (inherited)",
+        "category-default": " (category default)",
+        "system-default": " (system default)",
+      }
+      const suffix = suffixMap[newTask.modelInfo.type] ?? ""
       lines.push(`${icon} Model: ${newTask.modelInfo.model}${suffix}`)
       lines.push("")
     }
diff --git a/src/features/task-toast-manager/types.ts b/src/features/task-toast-manager/types.ts
index 5132e147..33d6f451 100644
--- a/src/features/task-toast-manager/types.ts
+++ b/src/features/task-toast-manager/types.ts
@@ -2,7 +2,7 @@ export type TaskStatus = "running" | "queued" | "completed" | "error"
 
 export interface ModelFallbackInfo {
   model: string
-  type: "user-defined" | "inherited" | "default"
+  type: "user-defined" | "inherited" | "category-default" | "system-default"
 }
 
 export interface TrackedTask {
diff --git a/src/tools/sisyphus-task/tools.test.ts b/src/tools/sisyphus-task/tools.test.ts
index 87fddc39..7b3cae68 100644
--- a/src/tools/sisyphus-task/tools.test.ts
+++ b/src/tools/sisyphus-task/tools.test.ts
@@ -4,9 +4,13 @@ import type { CategoryConfig } from "../../config/schema"
 
 function resolveCategoryConfig(
   categoryName: string,
-  userCategories?: Record<string, CategoryConfig>,
-  parentModelString?: string
-): { config: CategoryConfig; promptAppend: string } | null {
+  options: {
+    userCategories?: Record<string, CategoryConfig>
+    parentModelString?: string
+    systemDefaultModel?: string
+  }
+): { config: CategoryConfig; promptAppend: string; model: string | undefined } | null {
+  const { userCategories, parentModelString, systemDefaultModel } = options
   const defaultConfig = DEFAULT_CATEGORIES[categoryName]
   const userConfig = userCategories?.[categoryName]
   const defaultPromptAppend = CATEGORY_PROMPT_APPENDS[categoryName] ?? ""
@@ -15,10 +19,11 @@ function resolveCategoryConfig(
     return null
   }
 
+  const model = userConfig?.model ?? parentModelString ?? defaultConfig?.model ?? systemDefaultModel
   const config: CategoryConfig = {
     ...defaultConfig,
     ...userConfig,
-    model: userConfig?.model ?? parentModelString ?? defaultConfig?.model ?? "anthropic/claude-sonnet-4-5",
+    model,
   }
 
   let promptAppend = defaultPromptAppend
@@ -28,7 +33,7 @@ function resolveCategoryConfig(
       : userConfig.prompt_append
   }
 
-  return { config, promptAppend }
+  return { config, promptAppend, model }
 }
 
 describe("sisyphus-task", () => {
@@ -115,7 +120,7 @@ describe("sisyphus-task", () => {
       const categoryName = "unknown-category"
 
       // #when
-      const result = resolveCategoryConfig(categoryName)
+      const result = resolveCategoryConfig(categoryName, {})
 
       // #then
       expect(result).toBeNull()
@@ -126,7 +131,7 @@ describe("sisyphus-task", () => {
       const categoryName = "visual-engineering"
 
       // #when
-      const result = resolveCategoryConfig(categoryName)
+      const result = resolveCategoryConfig(categoryName, {})
 
       // #then
       expect(result).not.toBeNull()
@@ -142,7 +147,7 @@ describe("sisyphus-task", () => {
       }
 
       // #when
-      const result = resolveCategoryConfig(categoryName, userCategories)
+      const result = resolveCategoryConfig(categoryName, { userCategories })
 
       // #then
       expect(result).not.toBeNull()
@@ -160,7 +165,7 @@ describe("sisyphus-task", () => {
       }
 
       // #when
-      const result = resolveCategoryConfig(categoryName, userCategories)
+      const result = resolveCategoryConfig(categoryName, { userCategories })
 
       // #then
       expect(result).not.toBeNull()
@@ -180,7 +185,7 @@ describe("sisyphus-task", () => {
       }
 
       // #when
-      const result = resolveCategoryConfig(categoryName, userCategories)
+      const result = resolveCategoryConfig(categoryName, { userCategories })
 
       // #then
       expect(result).not.toBeNull()
@@ -200,7 +205,7 @@ describe("sisyphus-task", () => {
       }
 
       // #when
-      const result = resolveCategoryConfig(categoryName, userCategories)
+      const result = resolveCategoryConfig(categoryName, { userCategories })
 
       // #then
       expect(result).not.toBeNull()
@@ -213,7 +218,7 @@ describe("sisyphus-task", () => {
       const parentModelString = "cliproxy/claude-opus-4-5"
 
       // #when
-      const result = resolveCategoryConfig(categoryName, undefined, parentModelString)
+      const result = resolveCategoryConfig(categoryName, { parentModelString })
 
       // #then
       expect(result).not.toBeNull()
@@ -229,7 +234,7 @@ describe("sisyphus-task", () => {
       const parentModelString = "cliproxy/claude-opus-4-5"
 
       // #when
-      const result = resolveCategoryConfig(categoryName, userCategories, parentModelString)
+      const result = resolveCategoryConfig(categoryName, { userCategories, parentModelString })
 
       // #then
       expect(result).not.toBeNull()
@@ -241,7 +246,7 @@ describe("sisyphus-task", () => {
       const categoryName = "visual-engineering"
 
       // #when
-      const result = resolveCategoryConfig(categoryName, undefined, undefined)
+      const result = resolveCategoryConfig(categoryName, {})
 
       // #then
       expect(result).not.toBeNull()
@@ -270,6 +275,7 @@ describe("sisyphus-task", () => {
 
       const mockClient = {
         app: { agents: async () => ({ data: [] }) },
+        config: { get: async () => ({}) },
         session: {
           create: async () => ({ data: { id: "test-session" } }),
           prompt: async () => ({ data: {} }),
@@ -327,6 +333,7 @@ describe("sisyphus-task", () => {
       const mockManager = { launch: async () => ({}) }
       const mockClient = {
         app: { agents: async () => ({ data: [] }) },
+        config: { get: async () => ({}) },
         session: {
           create: async () => ({ data: { id: "test-session" } }),
           prompt: async () => ({ data: {} }),
@@ -394,6 +401,7 @@ describe("sisyphus-task", () => {
           ],
         }),
       },
+      config: { get: async () => ({}) },
       app: {
         agents: async () => ({ data: [] }),
       },
@@ -451,6 +459,7 @@ describe("sisyphus-task", () => {
           data: [],
         }),
       },
+      config: { get: async () => ({}) },
     }
     
     const tool = createSisyphusTask({
@@ -502,6 +511,7 @@ describe("sisyphus-task", () => {
           messages: async () => ({ data: [] }),
           status: async () => ({ data: {} }),
         },
+        config: { get: async () => ({}) },
         app: {
           agents: async () => ({ data: [{ name: "ultrabrain", mode: "subagent" }] }),
         },
@@ -560,6 +570,7 @@ describe("sisyphus-task", () => {
           }),
           status: async () => ({ data: { "ses_sync_success": { type: "idle" } } }),
         },
+        config: { get: async () => ({}) },
         app: {
           agents: async () => ({ data: [{ name: "ultrabrain", mode: "subagent" }] }),
         },
@@ -612,6 +623,7 @@ describe("sisyphus-task", () => {
           messages: async () => ({ data: [] }),
           status: async () => ({ data: {} }),
         },
+        config: { get: async () => ({}) },
         app: {
           agents: async () => ({ data: [{ name: "ultrabrain", mode: "subagent" }] }),
         },
@@ -666,6 +678,7 @@ describe("sisyphus-task", () => {
           }),
           status: async () => ({ data: {} }),
         },
+        config: { get: async () => ({}) },
         app: { agents: async () => ({ data: [] }) },
       }
 
@@ -707,7 +720,7 @@ describe("sisyphus-task", () => {
       const { buildSystemContent } = require("./tools")
 
       // #when
-      const result = buildSystemContent({ skills: undefined, categoryPromptAppend: undefined })
+      const result = buildSystemContent({ skillContent: undefined, categoryPromptAppend: undefined })
 
       // #then
       expect(result).toBeUndefined()
@@ -754,18 +767,18 @@ describe("sisyphus-task", () => {
   })
 
   describe("modelInfo detection via resolveCategoryConfig", () => {
-    test("when parentModelString exists but default model wins - modelInfo should report default", () => {
+    test("when parentModelString exists but default model wins - modelInfo should report category-default", () => {
       // #given - Bug scenario: parentModelString is passed but userModel is undefined,
       // and the resolution order is: userModel ?? parentModelString ?? defaultModel
       // If parentModelString matches the resolved model, it's "inherited"
-      // If defaultModel matches, it's "default"
+      // If defaultModel matches, it's "category-default"
       const categoryName = "ultrabrain"
       const parentModelString = undefined
       
       // #when
-      const resolved = resolveCategoryConfig(categoryName, undefined, parentModelString)
+      const resolved = resolveCategoryConfig(categoryName, { parentModelString })
       
-      // #then - actualModel should be defaultModel, type should be "default"
+      // #then - actualModel should be defaultModel, type should be "category-default"
       expect(resolved).not.toBeNull()
       const actualModel = resolved!.config.model
       const defaultModel = DEFAULT_CATEGORIES[categoryName]?.model
@@ -779,7 +792,7 @@ describe("sisyphus-task", () => {
       const parentModelString = "cliproxy/claude-opus-4-5"
       
       // #when
-      const resolved = resolveCategoryConfig(categoryName, undefined, parentModelString)
+      const resolved = resolveCategoryConfig(categoryName, { parentModelString })
       
       // #then - actualModel should be parentModelString, type should be "inherited"
       expect(resolved).not.toBeNull()
@@ -794,7 +807,7 @@ describe("sisyphus-task", () => {
       const parentModelString = "cliproxy/claude-opus-4-5"
       
       // #when
-      const resolved = resolveCategoryConfig(categoryName, userCategories, parentModelString)
+      const resolved = resolveCategoryConfig(categoryName, { userCategories, parentModelString })
       
       // #then - actualModel should be userModel, type should be "user-defined"
       expect(resolved).not.toBeNull()
@@ -812,7 +825,7 @@ describe("sisyphus-task", () => {
       const userCategories = { "ultrabrain": { model: "user/model" } }
       
       // #when - user model wins
-      const resolved = resolveCategoryConfig(categoryName, userCategories, parentModelString)
+      const resolved = resolveCategoryConfig(categoryName, { userCategories, parentModelString })
       const actualModel = resolved!.config.model
       const userDefinedModel = userCategories[categoryName]?.model
       const defaultModel = DEFAULT_CATEGORIES[categoryName]?.model
@@ -823,11 +836,40 @@ describe("sisyphus-task", () => {
         : actualModel === parentModelString 
         ? "inherited" 
         : actualModel === defaultModel 
-        ? "default" 
+        ? "category-default" 
         : undefined
       
       expect(detectedType).toBe("user-defined")
       expect(actualModel).not.toBe(parentModelString)
     })
+
+    test("systemDefaultModel is used when no other model is available", () => {
+      // #given - custom category with no model, but systemDefaultModel is set
+      const categoryName = "my-custom"
+      // Using type assertion since we're testing fallback behavior for categories without model
+      const userCategories = { "my-custom": { temperature: 0.5 } } as unknown as Record<string, CategoryConfig>
+      const systemDefaultModel = "anthropic/claude-sonnet-4-5"
+      
+      // #when
+      const resolved = resolveCategoryConfig(categoryName, { userCategories, systemDefaultModel })
+      
+      // #then - actualModel should be systemDefaultModel
+      expect(resolved).not.toBeNull()
+      expect(resolved!.model).toBe(systemDefaultModel)
+    })
+
+    test("model is undefined when no model available anywhere", () => {
+      // #given - custom category with no model, no systemDefaultModel
+      const categoryName = "my-custom"
+      // Using type assertion since we're testing fallback behavior for categories without model
+      const userCategories = { "my-custom": { temperature: 0.5 } } as unknown as Record<string, CategoryConfig>
+      
+      // #when
+      const resolved = resolveCategoryConfig(categoryName, { userCategories })
+      
+      // #then - model should be undefined
+      expect(resolved).not.toBeNull()
+      expect(resolved!.model).toBeUndefined()
+    })
   })
 })
diff --git a/src/tools/sisyphus-task/tools.ts b/src/tools/sisyphus-task/tools.ts
index 413d27b0..b8a519ef 100644
--- a/src/tools/sisyphus-task/tools.ts
+++ b/src/tools/sisyphus-task/tools.ts
@@ -61,9 +61,13 @@ type ToolContextWithMetadata = {
 
 function resolveCategoryConfig(
   categoryName: string,
-  userCategories?: CategoriesConfig,
-  parentModelString?: string
-): { config: CategoryConfig; promptAppend: string } | null {
+  options: {
+    userCategories?: CategoriesConfig
+    parentModelString?: string
+    systemDefaultModel?: string
+  }
+): { config: CategoryConfig; promptAppend: string; model: string | undefined } | null {
+  const { userCategories, parentModelString, systemDefaultModel } = options
   const defaultConfig = DEFAULT_CATEGORIES[categoryName]
   const userConfig = userCategories?.[categoryName]
   const defaultPromptAppend = CATEGORY_PROMPT_APPENDS[categoryName] ?? ""
@@ -72,12 +76,13 @@ function resolveCategoryConfig(
     return null
   }
 
-  // Model priority: user override > parent model (inherit) > category default > hardcoded fallback
+  // Model priority: user override > parent model (inherit) > category default > system default
   // Parent model takes precedence over category default so custom providers work out-of-box
+  const model = userConfig?.model ?? parentModelString ?? defaultConfig?.model ?? systemDefaultModel
   const config: CategoryConfig = {
     ...defaultConfig,
     ...userConfig,
-    model: userConfig?.model ?? parentModelString ?? defaultConfig?.model ?? "anthropic/claude-sonnet-4-5",
+    model,
   }
 
   let promptAppend = defaultPromptAppend
@@ -87,7 +92,7 @@ function resolveCategoryConfig(
       : userConfig.prompt_append
   }
 
-  return { config, promptAppend }
+  return { config, promptAppend, model }
 }
 
 export interface SisyphusTaskToolOptions {
@@ -329,6 +334,16 @@ ${textContent || "(No text output)"}`
         return `❌ Invalid arguments: Must provide either category or subagent_type.`
       }
 
+      // Fetch OpenCode config at boundary to get system default model
+      let systemDefaultModel: string | undefined
+      try {
+        const openCodeConfig = await client.config.get()
+        systemDefaultModel = (openCodeConfig as { model?: string })?.model
+      } catch {
+        // Config fetch failed, proceed without system default
+        systemDefaultModel = undefined
+      }
+
       let agentToUse: string
       let categoryModel: { providerID: string; modelID: string; variant?: string } | undefined
       let categoryPromptAppend: string | undefined
@@ -340,26 +355,45 @@ ${textContent || "(No text output)"}`
       let modelInfo: ModelFallbackInfo | undefined
 
       if (args.category) {
-        const resolved = resolveCategoryConfig(args.category, userCategories, parentModelString)
+        const resolved = resolveCategoryConfig(args.category, {
+          userCategories,
+          parentModelString,
+          systemDefaultModel,
+        })
         if (!resolved) {
           return `❌ Unknown category: "${args.category}". Available: ${Object.keys({ ...DEFAULT_CATEGORIES, ...userCategories }).join(", ")}`
         }
 
         // Determine model source by comparing against the actual resolved model
-        const actualModel = resolved.config.model
+        const actualModel = resolved.model
         const userDefinedModel = userCategories?.[args.category]?.model
-        const defaultModel = DEFAULT_CATEGORIES[args.category]?.model
+        const categoryDefaultModel = DEFAULT_CATEGORIES[args.category]?.model
 
-        if (actualModel === userDefinedModel) {
-          modelInfo = { model: actualModel, type: "user-defined" }
-        } else if (actualModel === parentModelString) {
-          modelInfo = { model: actualModel, type: "inherited" }
-        } else if (actualModel === defaultModel) {
-          modelInfo = { model: actualModel, type: "default" }
+        if (!actualModel) {
+          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").`
+        }
+
+        switch (actualModel) {
+          case userDefinedModel:
+            modelInfo = { model: actualModel, type: "user-defined" }
+            break
+          case parentModelString:
+            modelInfo = { model: actualModel, type: "inherited" }
+            break
+          case categoryDefaultModel:
+            modelInfo = { model: actualModel, type: "category-default" }
+            break
+          case systemDefaultModel:
+            modelInfo = { model: actualModel, type: "system-default" }
+            break
         }
 
         agentToUse = SISYPHUS_JUNIOR_AGENT
-        const parsedModel = parseModelString(resolved.config.model)
+        const parsedModel = parseModelString(actualModel)
         categoryModel = parsedModel
           ? (resolved.config.variant
             ? { ...parsedModel, variant: resolved.config.variant }
@@ -367,10 +401,11 @@ ${textContent || "(No text output)"}`
           : undefined
         categoryPromptAppend = resolved.promptAppend || undefined
       } else {
-        agentToUse = args.subagent_type!.trim()
-        if (!agentToUse) {
+        if (!args.subagent_type?.trim()) {
           return `❌ Agent name cannot be empty.`
         }
+        const agentName = args.subagent_type.trim()
+        agentToUse = agentName
 
         // Validate agent exists and is callable (not a primary agent)
         try {