feat(KB-119): add configurable thinking effort level

- Add defaultThinkingLevel to core Settings types and exports
- Thread thinking level through engine executor, merger, reviewer, triage, and pi
- Add effort dropdown to Model settings UI in SettingsModal
- Add tests for thinking effort dropdown in SettingsModal

packages/core/src/index.ts

@@ -1,4 +1,4 @@
-export { COLUMNS, COLUMN_LABELS, COLUMN_DESCRIPTIONS, VALID_TRANSITIONS, DEFAULT_SETTINGS } from "./types.js";
-export type { Column, Task, TaskAttachment, TaskCreateInput, TaskDetail, AgentLogEntry, BoardConfig, MergeResult, Settings, TaskStep, StepStatus, TaskLogEntry } from "./types.js";
+export { COLUMNS, COLUMN_LABELS, COLUMN_DESCRIPTIONS, VALID_TRANSITIONS, DEFAULT_SETTINGS, THINKING_LEVELS } from "./types.js";
+export type { Column, Task, TaskAttachment, TaskCreateInput, TaskDetail, AgentLogEntry, BoardConfig, MergeResult, Settings, TaskStep, StepStatus, TaskLogEntry, ThinkingLevel } from "./types.js";
 export { TaskStore } from "./store.js";
 export { canTransition, getValidTransitions, resolveDependencyOrder } from "./board.js";

packages/core/src/types.ts

@@ -1,3 +1,7 @@
+/** Valid thinking effort levels for AI agent sessions, controlling the cost/quality tradeoff of reasoning. */
+export const THINKING_LEVELS = ["off", "minimal", "low", "medium", "high"] as const;
+export type ThinkingLevel = (typeof THINKING_LEVELS)[number];
+
 export const COLUMNS = ["triage", "todo", "in-progress", "in-review", "done"] as const;
 export type Column = (typeof COLUMNS)[number];
 
@@ -110,6 +114,11 @@ export interface Settings {
    *  Must be set together with `defaultProvider`. When both are undefined,
    *  the engine uses pi's automatic model resolution. */
   defaultModelId?: string;
+  /** Default thinking effort level for AI agent sessions.
+   *  Controls how much reasoning effort the model uses — higher levels
+   *  produce better results but cost more. When undefined, the engine
+   *  uses the model's default thinking level. */
+  defaultThinkingLevel?: ThinkingLevel;
 }
 
 export const DEFAULT_SETTINGS: Settings = {
@@ -124,6 +133,7 @@ export const DEFAULT_SETTINGS: Settings = {
   includeTaskIdInCommit: true,
   defaultProvider: undefined,
   defaultModelId: undefined,
+  defaultThinkingLevel: undefined,
 };
 
 export interface BoardConfig {

packages/dashboard/app/components/SettingsModal.tsx

@@ -1,4 +1,5 @@
 import { useState, useEffect, useCallback, useRef } from "react";
+import { THINKING_LEVELS } from "@kb/core";
 import type { Settings } from "@kb/core";
 import { fetchSettings, updateSettings, fetchAuthStatus, loginProvider, logoutProvider, fetchModels } from "../api";
 import type { AuthProvider, ModelInfo } from "../api";
@@ -254,6 +255,33 @@ export function SettingsModal({ onClose, addToast }: SettingsModalProps) {
                 <small>Select the AI model used for agent sessions. "Use default" lets the engine choose automatically.</small>
               </div>
             )}
+            {(() => {
+              const selectedModel = availableModels.find(
+                (m) => m.provider === form.defaultProvider && m.id === form.defaultModelId,
+              );
+              if (selectedModel && !selectedModel.reasoning) return null;
+              return (
+                <div className="form-group">
+                  <label htmlFor="defaultThinkingLevel">Thinking Effort</label>
+                  <select
+                    id="defaultThinkingLevel"
+                    value={form.defaultThinkingLevel || ""}
+                    onChange={(e) => {
+                      const val = e.target.value;
+                      setForm((f) => ({ ...f, defaultThinkingLevel: val || undefined } as any));
+                    }}
+                  >
+                    <option value="">Default</option>
+                    {THINKING_LEVELS.map((level) => (
+                      <option key={level} value={level}>
+                        {level.charAt(0).toUpperCase() + level.slice(1)}
+                      </option>
+                    ))}
+                  </select>
+                  <small>Controls how much reasoning effort the AI model uses. Higher levels produce better results but cost more.</small>
+                </div>
+              );
+            })()}
           </>
         );
       }

packages/dashboard/app/components/__tests__/SettingsModal.test.tsx

@@ -436,6 +436,53 @@ describe("SettingsModal", () => {
     expect(elementsWithStyle.length).toBe(0);
   });
 
+  it("shows Thinking Effort dropdown with correct options in Model section", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Model"));
+    await waitFor(() => expect(fetchModels).toHaveBeenCalled());
+
+    const select = screen.getByLabelText("Thinking Effort") as HTMLSelectElement;
+    expect(select.tagName).toBe("SELECT");
+
+    const options = Array.from(select.options).map((o) => o.textContent);
+    expect(options).toEqual(["Default", "Off", "Minimal", "Low", "Medium", "High"]);
+  });
+
+  it("changing Thinking Effort dropdown updates form and is included in save payload", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Model"));
+    await waitFor(() => expect(fetchModels).toHaveBeenCalled());
+
+    const select = screen.getByLabelText("Thinking Effort") as HTMLSelectElement;
+    fireEvent.change(select, { target: { value: "high" } });
+
+    fireEvent.click(screen.getByText("Save"));
+    await waitFor(() => expect(updateSettings).toHaveBeenCalledTimes(1));
+
+    const payload = (updateSettings as ReturnType<typeof vi.fn>).mock.calls[0][0];
+    expect(payload.defaultThinkingLevel).toBe("high");
+  });
+
+  it("Thinking Effort dropdown is hidden when selected model does not support reasoning", async () => {
+    (fetchSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ...defaultSettings,
+      defaultProvider: "openai",
+      defaultModelId: "gpt-4o",
+    });
+
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Model"));
+    await waitFor(() => expect(fetchModels).toHaveBeenCalled());
+
+    expect(screen.queryByLabelText("Thinking Effort")).toBeNull();
+  });
+
   it("shows loading state during login", async () => {
     // Make loginProvider hang
     (loginProvider as ReturnType<typeof vi.fn>).mockReturnValue(new Promise(() => {}));

packages/engine/src/executor.ts

@@ -352,6 +352,7 @@ export class TaskExecutor {
           onToolStart: agentLogger.onToolStart,
           defaultProvider: settings.defaultProvider,
           defaultModelId: settings.defaultModelId,
+          defaultThinkingLevel: settings.defaultThinkingLevel,
         });
 
         // Register session so the pause listener can terminate it
@@ -542,6 +543,7 @@ export class TaskExecutor {
               onText: (delta) => options.onAgentText?.(taskId, delta),
               defaultProvider: settings.defaultProvider,
               defaultModelId: settings.defaultModelId,
+              defaultThinkingLevel: settings.defaultThinkingLevel,
             },
           );
 

packages/engine/src/merger.ts

@@ -239,6 +239,7 @@ export async function aiMergeTask(
     onToolStart: agentLogger.onToolStart,
     defaultProvider: settings.defaultProvider,
     defaultModelId: settings.defaultModelId,
+    defaultThinkingLevel: settings.defaultThinkingLevel,
   });
 
   try {

packages/engine/src/pi.ts

@@ -34,6 +34,8 @@ export interface AgentOptions {
   defaultProvider?: string;
   /** Default model ID within the provider (e.g. "claude-sonnet-4-5"). Used with `defaultProvider`. */
   defaultModelId?: string;
+  /** Default thinking effort level (e.g. "medium", "high"). When provided, sets the session's thinking level after creation. */
+  defaultThinkingLevel?: string;
 }
 
 /**
@@ -79,6 +81,11 @@ export async function createKbAgent(options: AgentOptions): Promise<AgentResult>
     ...(selectedModel ? { model: selectedModel } : {}),
   });
 
+  // Apply thinking level if specified
+  if (options.defaultThinkingLevel) {
+    session.setThinkingLevel(options.defaultThinkingLevel as any);
+  }
+
   // Wire up event listeners
   session.subscribe((event) => {
     if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {

packages/engine/src/reviewer.ts

@@ -115,6 +115,8 @@ export interface ReviewOptions {
   defaultProvider?: string;
   /** Default model ID within the provider (e.g. "claude-sonnet-4-5"). When set with `defaultProvider`, overrides the reviewer's model selection. */
   defaultModelId?: string;
+  /** Default thinking effort level for the reviewer agent session. */
+  defaultThinkingLevel?: string;
 }
 
 /**
@@ -143,6 +145,7 @@ export async function reviewStep(
     onText: (delta) => options.onText?.(delta),
     defaultProvider: options.defaultProvider,
     defaultModelId: options.defaultModelId,
+    defaultThinkingLevel: options.defaultThinkingLevel,
   });
 
   let reviewText = "";

packages/engine/src/triage.ts

@@ -273,6 +273,7 @@ export class TriageProcessor {
           onToolStart: agentLogger.onToolStart,
           defaultProvider: settings.defaultProvider,
           defaultModelId: settings.defaultModelId,
+          defaultThinkingLevel: settings.defaultThinkingLevel,
         });
 
         try {