diff --git a/src/agent-modes.ts b/src/agent-modes.ts
new file mode 100644
index 0000000..b2be302
--- /dev/null
+++ b/src/agent-modes.ts
@@ -0,0 +1,161 @@
+// src/agent-modes.ts
+
+import { AgentMode, OllamaTool } from './types';
+export { AgentMode };
+
+/**
+ * Supported agent modes that change the assistant's behavior,
+ * available tools, and system prompt.
+ */
+
+export const ALL_AGENT_MODES: AgentMode[] = ['ask', 'edit', 'organize', 'research', 'workflow'];
+
+export const DEFAULT_AGENT_MODE: AgentMode = 'ask';
+
+export interface AgentModeConfig {
+  label: string;
+  description: string;
+  systemPrompt: string;
+  toolFilter: (tools: OllamaTool[]) => OllamaTool[];
+  requiresPreview: boolean;
+  showModeIndicator: boolean;
+}
+
+function filterToolsByName(tools: OllamaTool[], allowed: Set<string>): OllamaTool[] {
+  return tools.filter((t) => allowed.has(t.function.name));
+}
+
+const READ_TOOLS = new Set([
+  'read_vault_file',
+  'search_vault_files',
+]);
+
+const ORGANIZE_TOOLS = new Set([
+  'read_vault_file',
+  'search_vault_files',
+  'update_frontmatter',
+  'rename_note',
+  'move_note',
+  'insert_link',
+]);
+
+const EDIT_TOOLS = new Set([
+  'read_vault_file',
+  'search_vault_files',
+  'create_note',
+  'append_to_note',
+  'replace_note_section',
+  'update_frontmatter',
+  'rename_note',
+  'move_note',
+  'delete_note',
+  'insert_link',
+]);
+
+const RESEARCH_TOOLS = new Set([
+  'read_vault_file',
+  'search_vault_files',
+]);
+
+export const AGENT_MODE_CONFIGS: Record<AgentMode, AgentModeConfig> = {
+  ask: {
+    label: 'Ask',
+    description: 'Answer questions using vault context. Read-only mode.',
+    systemPrompt: `You are a helpful assistant that answers questions using the contents of the user's Obsidian vault.
+You have access to search and read tools to find relevant information.
+Always base your answers on vault content when possible.
+If you cannot find relevant information, say so clearly.
+Do not make up facts.`,
+    toolFilter: (tools) => filterToolsByName(tools, READ_TOOLS),
+    requiresPreview: false,
+    showModeIndicator: true,
+  },
+
+  edit: {
+    label: 'Edit',
+    description: 'Create, modify, and organize notes with full editing tools.',
+    systemPrompt: `You are an assistant that helps edit and manage notes in the user's Obsidian vault.
+You have full access to reading, searching, creating, appending, renaming, moving, and deleting notes.
+When editing notes:
+- Prefer modifying existing content over creating duplicates.
+- Use the replace_note_section tool to update specific sections.
+- Use update_frontmatter to manage metadata.
+- Always confirm destructive actions (deletes, moves) with the user when possible.
+- Preview changes when the system supports it.`,
+    toolFilter: (tools) => filterToolsByName(tools, EDIT_TOOLS),
+    requiresPreview: true,
+    showModeIndicator: true,
+  },
+
+  organize: {
+    label: 'Organize',
+    description: 'Tag, rename, move, and link notes to keep the vault tidy.',
+    systemPrompt: `You are an assistant that helps organize the user's Obsidian vault.
+You can search notes, read them, update frontmatter tags, rename files, move files to folders, and insert wiki-links.
+When organizing:
+- Suggest consistent tag vocabularies.
+- Group related notes by linking them.
+- Propose folder structures that match the user's existing patterns.
+- Avoid destructive changes unless explicitly requested.`,
+    toolFilter: (tools) => filterToolsByName(tools, ORGANIZE_TOOLS),
+    requiresPreview: true,
+    showModeIndicator: true,
+  },
+
+  research: {
+    label: 'Research',
+    description: 'Deep vault search and synthesis across multiple notes.',
+    systemPrompt: `You are a research assistant that dives deep into the user's Obsidian vault.
+Your job is to synthesize information across multiple notes, find connections, and produce comprehensive summaries.
+Search broadly, read key sources, and cross-reference information.
+Cite specific notes and quotes where possible.
+If information is incomplete or contradictory, note it explicitly.`,
+    toolFilter: (tools) => filterToolsByName(tools, RESEARCH_TOOLS),
+    requiresPreview: false,
+    showModeIndicator: true,
+  },
+
+  workflow: {
+    label: 'Workflow',
+    description: 'Execute multi-step workflows via the /workflow command.',
+    systemPrompt: `You are a workflow orchestrator. Users can trigger workflows with the /workflow command.
+When a user describes a multi-step task, you can suggest using /workflow.
+Workflows can chain vault searches, LLM calls, tool executions, and formatting steps together.
+You do not have direct tool access in this mode — workflows handle tool use.`,
+    toolFilter: () => [],
+    requiresPreview: false,
+    showModeIndicator: true,
+  },
+};
+
+/**
+ * Get the display label for an agent mode.
+ */
+export function getAgentModeLabel(mode: AgentMode): string {
+  return AGENT_MODE_CONFIGS[mode]?.label ?? mode;
+}
+
+/**
+ * Check if a mode requires action previews for write tools.
+ */
+export function modeRequiresPreview(mode: AgentMode): boolean {
+  return AGENT_MODE_CONFIGS[mode]?.requiresPreview ?? false;
+}
+
+/**
+ * Get the system prompt for an agent mode.
+ */
+export function getSystemPromptForMode(mode: AgentMode): string {
+  return AGENT_MODE_CONFIGS[mode]?.systemPrompt ?? AGENT_MODE_CONFIGS.ask.systemPrompt;
+}
+
+/**
+ * Filter tools based on the current agent mode.
+ */
+export function filterToolsForMode(tools: OllamaTool[], mode: AgentMode): OllamaTool[] {
+  const config = AGENT_MODE_CONFIGS[mode];
+  if (!config) {
+    return tools;
+  }
+  return config.toolFilter(tools);
+}
diff --git a/src/chat-view.ts b/src/chat-view.ts
index 674b483..1aed40c 100755
--- a/src/chat-view.ts
+++ b/src/chat-view.ts
@@ -1,4 +1,11 @@
 import { ItemView, Notice, WorkspaceLeaf } from 'obsidian';
+import {
+  ALL_AGENT_MODES,
+  getAgentModeLabel,
+  getSystemPromptForMode,
+  filterToolsForMode,
+  modeRequiresPreview,
+} from './agent-modes';
 import { OllamaClient } from './ollama-client';
 import { VaultIndexer } from './vault-indexer';
 import { VaultVectorStore } from './vault-vector-store';
@@ -7,6 +14,7 @@ import { ActionPreviewBuilder, isWriteTool } from './action-preview-builder';
 import { WorkflowEngine } from './workflow-engine';
 import { NoteContextBuilder } from './note-context-builder';
 import {
+  AgentMode,
   PluginSettings,
   OllamaMessage,
   OllamaTool,
@@ -50,6 +58,7 @@ export class ChatView extends ItemView {
     this.newChatButtonClickWrapper = null;
     this.listenersAttached = false;
     this.settings = settings;
+    this.currentAgentMode = settings.agentMode ?? 'ask';
     this.ollamaClient = new OllamaClient(
       settings.ollamaUrl,
       settings.model,
@@ -72,6 +81,10 @@ export class ChatView extends ItemView {
 
   updateSettings(newSettings: PluginSettings) {
     this.settings = newSettings;
+    this.currentAgentMode = newSettings.agentMode ?? 'ask';
+    if (this.modeSelectorEl) {
+      this.modeSelectorEl.value = this.currentAgentMode;
+    }
     this.ollamaClient = new OllamaClient(
       newSettings.ollamaUrl,
       newSettings.model,
@@ -214,6 +227,27 @@ export class ChatView extends ItemView {
       }
     }
 
+    // Setup mode selector
+    if (!this.modeSelectorEl) {
+      this.modeSelectorEl = newChatContainer.createEl('select', {
+        cls: 'ollama-mode-selector',
+      });
+      for (const mode of ALL_AGENT_MODES) {
+        const option = this.modeSelectorEl.createEl('option', {
+          text: getAgentModeLabel(mode),
+          attr: { value: mode },
+        });
+        if (mode === this.currentAgentMode) {
+          option.setAttribute('selected', 'selected');
+        }
+      }
+      this.modeSelectorEl.addEventListener('change', () => {
+        this.currentAgentMode = this.modeSelectorEl!.value as AgentMode;
+      });
+    } else {
+      newChatContainer.appendChild(this.modeSelectorEl);
+    }
+
     // Setup new chat button
     if (!this.newChatButton) {
       this.newChatButton = newChatContainer.createEl('button', {
@@ -308,6 +342,17 @@ export class ChatView extends ItemView {
     this.listenersAttached = false;
   }
 
+  getAgentMode(): AgentMode {
+    return this.currentAgentMode;
+  }
+
+  setAgentMode(mode: AgentMode): void {
+    this.currentAgentMode = mode;
+    if (this.modeSelectorEl) {
+      this.modeSelectorEl.value = mode;
+    }
+  }
+
   clearConversation(): void {
     this.messages = [];
     this.conversationStateManager.clear();
@@ -334,7 +379,8 @@ export class ChatView extends ItemView {
   }
 
   getTools(): OllamaTool[] {
-    return [
+    const allTools: OllamaTool[] = [
+
       {
         type: 'function',
         function: {
@@ -549,15 +595,11 @@ export class ChatView extends ItemView {
         },
       },
     ];
+    return filterToolsForMode(allTools, this.currentAgentMode);
   }
 
   buildMessages(userMessageContent: string, tools?: OllamaTool[]): OllamaMessage[] {
-    const systemContent = `You are an assistant that can help answer questions using the contents of a vault.
-        The user can ask questions about their vault contents, and you should provide helpful responses based on the files.
-        Vault context includes note titles, content, and any tags (shown as "Tags: ..." at the top of a note entry).
-        When organizing or categorizing notes, pay attention to tags as they reflect the note's topics and categories.
-        When a user asks for information, try to find relevant files using the search_vault_files tool and read their contents with the read_vault_file tool.
-        Only use the tools if you need to access vault content that is not already in the context.`;
+    const systemContent = getSystemPromptForMode(this.currentAgentMode);
     const systemMessage: OllamaMessage = {
       role: 'system',
       content: systemContent,
@@ -616,7 +658,7 @@ export class ChatView extends ItemView {
       }
     }
 
-    if (writePreviews.length > 0) {
+    if (writePreviews.length > 0 && modeRequiresPreview(this.currentAgentMode)) {
       // Store pending state for apply/cancel
       this.pendingActions = writePreviews;
       this.pendingReadResults = readResults;
@@ -632,8 +674,26 @@ export class ChatView extends ItemView {
       return;
     }
 
-    // No write tools — proceed with follow-up as before
-    const allResults = readResults;
+    // If mode does not require preview, execute write tools immediately
+    let writeResults: (ToolResult & { id?: string })[] = [];
+    if (writePreviews.length > 0 && !modeRequiresPreview(this.currentAgentMode)) {
+      writeResults = (
+        await Promise.all(
+          writePreviews.map(async (action) => {
+            try {
+              const toolResult = await this.toolExecutor.handleToolCall(action.toolCall);
+              return { ...toolResult, id: action.toolCall.id };
+            } catch (error) {
+              ErrorHandler.handleError(error, 'ChatView.processToolCalls');
+              return null;
+            }
+          })
+        )
+      ).filter((result): result is NonNullable<typeof result> => result !== null);
+    }
+
+    // No write tools (or they were already executed) — proceed with follow-up
+    const allResults = [...readResults, ...writeResults];
     const followUpMessages: OllamaMessage[] = allResults.map((result) => ({
       role: 'tool',
       content: JSON.stringify(result),
@@ -1042,6 +1102,9 @@ export class ChatView extends ItemView {
   private conversationStateManager: ConversationStateManager;
   private vectorStore?: VaultVectorStore;
 
+  private modeSelectorEl: HTMLSelectElement | null = null;
+  private currentAgentMode: AgentMode;
+
   // Pending action state
   private pendingActions: ProposedAction[] = [];
   private pendingReadResults: (ToolResult & { id?: string })[] = [];
diff --git a/src/constants.ts b/src/constants.ts
index b15c6e3..5b23190 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -5,6 +5,7 @@ export const DEFAULT_SETTINGS = {
   maxMessageHistory: 50,
   maxContextLength: 8000,
   lastIndexTime: 0,
+  agentMode: 'ask' as const,
   cacheConfig: {
     enabled: false,
     similarityThreshold: 0.85,
diff --git a/src/main.ts b/src/main.ts
index 1aa75f7..519e3c3 100755
--- a/src/main.ts
+++ b/src/main.ts
@@ -7,6 +7,8 @@ import { VaultIndexer } from './vault-indexer';
 import { AutoTagger, AutoLinker } from './auto-organizer';
 import { PluginSettings } from './types';
 import { Logger } from './utils';
+import { ALL_AGENT_MODES, getAgentModeLabel } from './agent-modes';
+import { AgentMode } from './types';
 
 export default class OllamaPlugin extends Plugin {
   settings: PluginSettings = DEFAULT_SETTINGS;
@@ -443,6 +445,27 @@ class OllamaSettingTab extends PluginSettingTab {
         })
       );
 
+    // Agent Mode Setting
+    containerEl.createEl('h3', { text: 'Agent Mode' });
+    containerEl.createEl('p', {
+      text: 'Default chat mode that controls available tools and system behavior.',
+    });
+
+    new Setting(containerEl)
+      .setName('Default Agent Mode')
+      .setDesc('Select the default mode for new chat sessions.')
+      .addDropdown((dropdown) => {
+        for (const mode of ALL_AGENT_MODES) {
+          dropdown.addOption(mode, getAgentModeLabel(mode));
+        }
+        dropdown.setValue(this.plugin.settings.agentMode ?? 'ask');
+        dropdown.onChange(async (value) => {
+          this.plugin.settings.agentMode = value as AgentMode;
+          await this.plugin.saveSettings();
+          this.plugin.notifyChatViews();
+        });
+      });
+
     // Vault Index Settings
     containerEl.createEl('h3', { text: 'Vault Semantic Index' });
 
diff --git a/src/types.ts b/src/types.ts
index b532637..f129a8f 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -197,6 +197,12 @@ export interface DependencyGraph {
   }[];
 }
 
+// ============================================================
+// Agent Modes
+// ============================================================
+
+export type AgentMode = 'ask' | 'edit' | 'organize' | 'research' | 'workflow';
+
 // ============================================================
 // Plugin Configuration
 // ============================================================
@@ -224,6 +230,7 @@ export interface PluginSettings {
   maxMessageHistory: number;
   maxContextLength: number;
   lastIndexTime: number;
+  agentMode: AgentMode;
   cacheConfig: CacheConfig;
   vaultIndexConfig: VaultIndexConfig;
   autoTagConfig: {
diff --git a/styles.css b/styles.css
index 62f7034..61772cc 100644
--- a/styles.css
+++ b/styles.css
@@ -273,3 +273,27 @@
 .ollama-cancel-button:hover {
   background-color: var(--background-modifier-hover);
 }
+
+/* Agent Mode Selector */
+.ollama-new-chat-container {
+  display: flex;
+  justify-content: flex-end;
+  align-items: center;
+  gap: var(--size-4-1);
+  padding: var(--size-4-1) var(--size-4-2);
+}
+
+.ollama-mode-selector {
+  padding: var(--size-4-1) var(--size-4-2);
+  border-radius: var(--ollama-radius);
+  border: 1px solid var(--ollama-border);
+  background-color: var(--background-modifier-form-field);
+  color: var(--text-normal);
+  font-size: var(--font-ui-small);
+  cursor: pointer;
+}
+
+.ollama-mode-selector:focus {
+  outline: none;
+  border-color: var(--interactive-accent);
+}
diff --git a/tests/agent-modes.test.ts b/tests/agent-modes.test.ts
new file mode 100644
index 0000000..e868c5e
--- /dev/null
+++ b/tests/agent-modes.test.ts
@@ -0,0 +1,167 @@
+import {
+  AgentMode,
+  ALL_AGENT_MODES,
+  DEFAULT_AGENT_MODE,
+  AGENT_MODE_CONFIGS,
+  getAgentModeLabel,
+  modeRequiresPreview,
+  getSystemPromptForMode,
+  filterToolsForMode,
+} from '../src/agent-modes';
+import { OllamaTool } from '../src/types';
+
+describe('Agent Modes', () => {
+  const allTools: OllamaTool[] = [
+    {
+      type: 'function',
+      function: {
+        name: 'read_vault_file',
+        description: 'Read',
+        parameters: { type: 'object', properties: {} },
+      },
+    },
+    {
+      type: 'function',
+      function: {
+        name: 'search_vault_files',
+        description: 'Search',
+        parameters: { type: 'object', properties: {} },
+      },
+    },
+    {
+      type: 'function',
+      function: {
+        name: 'create_note',
+        description: 'Create',
+        parameters: { type: 'object', properties: {} },
+      },
+    },
+    {
+      type: 'function',
+      function: {
+        name: 'update_frontmatter',
+        description: 'Update frontmatter',
+        parameters: { type: 'object', properties: {} },
+      },
+    },
+    {
+      type: 'function',
+      function: {
+        name: 'delete_note',
+        description: 'Delete',
+        parameters: { type: 'object', properties: {} },
+      },
+    },
+  ];
+
+  describe('constants', () => {
+    it('should define all expected modes', () => {
+      expect(ALL_AGENT_MODES).toEqual(['ask', 'edit', 'organize', 'research', 'workflow']);
+    });
+
+    it('should have default mode ask', () => {
+      expect(DEFAULT_AGENT_MODE).toBe('ask');
+    });
+  });
+
+  describe('getAgentModeLabel', () => {
+    it('should return labels for known modes', () => {
+      expect(getAgentModeLabel('ask')).toBe('Ask');
+      expect(getAgentModeLabel('edit')).toBe('Edit');
+      expect(getAgentModeLabel('organize')).toBe('Organize');
+      expect(getAgentModeLabel('research')).toBe('Research');
+      expect(getAgentModeLabel('workflow')).toBe('Workflow');
+    });
+
+    it('should fallback to raw mode name for unknown modes', () => {
+      expect(getAgentModeLabel('unknown' as AgentMode)).toBe('unknown');
+    });
+  });
+
+  describe('modeRequiresPreview', () => {
+    it('should require preview for edit and organize', () => {
+      expect(modeRequiresPreview('edit')).toBe(true);
+      expect(modeRequiresPreview('organize')).toBe(true);
+    });
+
+    it('should not require preview for ask, research, workflow', () => {
+      expect(modeRequiresPreview('ask')).toBe(false);
+      expect(modeRequiresPreview('research')).toBe(false);
+      expect(modeRequiresPreview('workflow')).toBe(false);
+    });
+  });
+
+  describe('getSystemPromptForMode', () => {
+    it('should return a non-empty prompt for each mode', () => {
+      for (const mode of ALL_AGENT_MODES) {
+        const prompt = getSystemPromptForMode(mode);
+        expect(typeof prompt).toBe('string');
+        expect(prompt.length).toBeGreaterThan(0);
+      }
+    });
+
+    it('should fallback to ask prompt for unknown mode', () => {
+      const askPrompt = getSystemPromptForMode('ask');
+      const fallback = getSystemPromptForMode('unknown' as AgentMode);
+      expect(fallback).toBe(askPrompt);
+    });
+  });
+
+  describe('filterToolsForMode', () => {
+    it('ask mode should only allow read/search tools', () => {
+      const filtered = filterToolsForMode(allTools, 'ask');
+      const names = filtered.map((t) => t.function.name);
+      expect(names).toContain('read_vault_file');
+      expect(names).toContain('search_vault_files');
+      expect(names).not.toContain('create_note');
+      expect(names).not.toContain('delete_note');
+    });
+
+    it('edit mode should allow all tools', () => {
+      const filtered = filterToolsForMode(allTools, 'edit');
+      const names = filtered.map((t) => t.function.name);
+      expect(names).toContain('read_vault_file');
+      expect(names).toContain('create_note');
+      expect(names).toContain('delete_note');
+    });
+
+    it('organize mode should allow organize tools but not delete', () => {
+      const filtered = filterToolsForMode(allTools, 'organize');
+      const names = filtered.map((t) => t.function.name);
+      expect(names).toContain('read_vault_file');
+      expect(names).toContain('search_vault_files');
+      expect(names).toContain('update_frontmatter');
+      expect(names).not.toContain('delete_note');
+      expect(names).not.toContain('create_note');
+    });
+
+    it('research mode should only allow read/search tools', () => {
+      const filtered = filterToolsForMode(allTools, 'research');
+      const names = filtered.map((t) => t.function.name);
+      expect(names).toContain('read_vault_file');
+      expect(names).toContain('search_vault_files');
+      expect(names).not.toContain('create_note');
+    });
+
+    it('workflow mode should return no tools', () => {
+      const filtered = filterToolsForMode(allTools, 'workflow');
+      expect(filtered).toHaveLength(0);
+    });
+
+    it('unknown mode should return all tools', () => {
+      const filtered = filterToolsForMode(allTools, 'unknown' as AgentMode);
+      expect(filtered).toEqual(allTools);
+    });
+  });
+
+  describe('AGENT_MODE_CONFIGS', () => {
+    it('should have a config for every mode', () => {
+      for (const mode of ALL_AGENT_MODES) {
+        expect(AGENT_MODE_CONFIGS[mode]).toBeDefined();
+        expect(AGENT_MODE_CONFIGS[mode].label).toBeDefined();
+        expect(AGENT_MODE_CONFIGS[mode].description).toBeDefined();
+        expect(AGENT_MODE_CONFIGS[mode].systemPrompt).toBeDefined();
+      }
+    });
+  });
+});
diff --git a/tests/chat-view.test.ts b/tests/chat-view.test.ts
index 0aa26c5..629906e 100755
--- a/tests/chat-view.test.ts
+++ b/tests/chat-view.test.ts
@@ -38,6 +38,7 @@ const mockSettings: PluginSettings = {
   maxMessageHistory: 50,
   maxContextLength: 8000,
   lastIndexTime: 0,
+  agentMode: 'ask',
   cacheConfig: {
     enabled: false,
     similarityThreshold: 0.9,
@@ -412,6 +413,7 @@ describe('ChatView', () => {
     });
 
     it('should show preview for write tool calls and defer follow-up', async () => {
+      view.setAgentMode('edit');
       view['sendButton'] = document.createElement('button');
       view['inputEl'] = document.createElement('textarea');
       (view['inputEl'] as HTMLTextAreaElement).value = 'test';
@@ -464,6 +466,7 @@ describe('ChatView', () => {
     });
 
     it('should apply pending actions and trigger follow-up', async () => {
+      view.setAgentMode('edit');
       view['sendButton'] = document.createElement('button');
       view['inputEl'] = document.createElement('textarea');
       (view['inputEl'] as HTMLTextAreaElement).value = 'test';
@@ -524,6 +527,7 @@ describe('ChatView', () => {
     });
 
     it('should cancel pending actions', async () => {
+      view.setAgentMode('edit');
       view['sendButton'] = document.createElement('button');
       view['inputEl'] = document.createElement('textarea');
       (view['inputEl'] as HTMLTextAreaElement).value = 'test';
@@ -567,6 +571,7 @@ describe('ChatView', () => {
     });
 
     it('should handle tool call errors gracefully and continue with partial results', async () => {
+      view.setAgentMode('edit');
       view['sendButton'] = document.createElement('button');
       view['inputEl'] = document.createElement('textarea');
       (view['inputEl'] as HTMLTextAreaElement).value = 'test';