diff --git a/package-lock.json b/package-lock.json
index 82a87fc..ee446e6 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -11,6 +11,7 @@
       "devDependencies": {
         "@types/mocha": "^10.0.10",
         "@types/node": "20.x",
+        "@types/sinon": "^17.0.4",
         "@types/vscode": "^1.94.0",
         "@typescript-eslint/eslint-plugin": "^8.31.1",
         "@typescript-eslint/parser": "^8.31.1",
@@ -18,6 +19,7 @@
         "@vscode/test-electron": "^2.5.2",
         "@vscode/vsce": "^3.5.0",
         "eslint": "^9.25.1",
+        "sinon": "^21.0.0",
         "typescript": "^5.8.3"
       },
       "engines": {
@@ -809,6 +811,47 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/@sinonjs/commons": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/@sinonjs/commons/-/commons-3.0.1.tgz",
+      "integrity": "sha512-K3mCHKQ9sVh8o1C9cxkwxaOmXoAMlDxC1mYyHrjqOWEcBjYr76t96zL2zlj5dUGZ3HSw240X1qgH3Mjf1yJWpQ==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "type-detect": "4.0.8"
+      }
+    },
+    "node_modules/@sinonjs/fake-timers": {
+      "version": "13.0.5",
+      "resolved": "https://registry.npmjs.org/@sinonjs/fake-timers/-/fake-timers-13.0.5.tgz",
+      "integrity": "sha512-36/hTbH2uaWuGVERyC6da9YwGWnzUZXuPro/F2LfsdOsLnCojz/iSH8MxUt/FD2S5XBSVPhmArFUXcpCQ2Hkiw==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "@sinonjs/commons": "^3.0.1"
+      }
+    },
+    "node_modules/@sinonjs/samsam": {
+      "version": "8.0.3",
+      "resolved": "https://registry.npmjs.org/@sinonjs/samsam/-/samsam-8.0.3.tgz",
+      "integrity": "sha512-hw6HbX+GyVZzmaYNh82Ecj1vdGZrqVIn/keDTg63IgAwiQPO+xCz99uG6Woqgb4tM0mUiFENKZ4cqd7IX94AXQ==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "@sinonjs/commons": "^3.0.1",
+        "type-detect": "^4.1.0"
+      }
+    },
+    "node_modules/@sinonjs/samsam/node_modules/type-detect": {
+      "version": "4.1.0",
+      "resolved": "https://registry.npmjs.org/type-detect/-/type-detect-4.1.0.tgz",
+      "integrity": "sha512-Acylog8/luQ8L7il+geoSxhEkazvkslg7PSNKOX59mbB9cOveP5aq9h74Y7YU8yDpJwetzQQrfIwtf4Wp4LKcw==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=4"
+      }
+    },
     "node_modules/@textlint/ast-node-types": {
       "version": "14.8.4",
       "resolved": "https://registry.npmjs.org/@textlint/ast-node-types/-/ast-node-types-14.8.4.tgz",
@@ -977,6 +1020,23 @@
       "integrity": "sha512-kRz0VEkJqWLf1LLVN4pT1cg1Z9wAuvI6L97V3m2f5B76Tg8d413ddvLBPTEHAZJlnn4XSvu0FkZtViCQGVyrXQ==",
       "dev": true
     },
+    "node_modules/@types/sinon": {
+      "version": "17.0.4",
+      "resolved": "https://registry.npmjs.org/@types/sinon/-/sinon-17.0.4.tgz",
+      "integrity": "sha512-RHnIrhfPO3+tJT0s7cFaXGZvsL4bbR3/k7z3P312qMS4JaS2Tk+KiwiLx1S0rQ56ERj00u1/BtdyVd0FY+Pdew==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/sinonjs__fake-timers": "*"
+      }
+    },
+    "node_modules/@types/sinonjs__fake-timers": {
+      "version": "8.1.5",
+      "resolved": "https://registry.npmjs.org/@types/sinonjs__fake-timers/-/sinonjs__fake-timers-8.1.5.tgz",
+      "integrity": "sha512-mQkU2jY8jJEF7YHjHvsQO8+3ughTL1mcnn96igfhONmR+fUPSKIkefQYpSe8bsly2Ep7oQbn/6VG5/9/0qcArQ==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/vscode": {
       "version": "1.101.0",
       "resolved": "https://registry.npmjs.org/@types/vscode/-/vscode-1.101.0.tgz",
@@ -5555,6 +5615,47 @@
         "simple-concat": "^1.0.0"
       }
     },
+    "node_modules/sinon": {
+      "version": "21.0.0",
+      "resolved": "https://registry.npmjs.org/sinon/-/sinon-21.0.0.tgz",
+      "integrity": "sha512-TOgRcwFPbfGtpqvZw+hyqJDvqfapr1qUlOizROIk4bBLjlsjlB00Pg6wMFXNtJRpu+eCZuVOaLatG7M8105kAw==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "@sinonjs/commons": "^3.0.1",
+        "@sinonjs/fake-timers": "^13.0.5",
+        "@sinonjs/samsam": "^8.0.1",
+        "diff": "^7.0.0",
+        "supports-color": "^7.2.0"
+      },
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/sinon"
+      }
+    },
+    "node_modules/sinon/node_modules/diff": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/diff/-/diff-7.0.0.tgz",
+      "integrity": "sha512-PJWHUb1RFevKCwaFA9RlG5tCd+FO5iRh9A8HEtkmBH2Li03iJriB6m6JIN4rGz3K3JLawI7/veA1xzRKP6ISBw==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "engines": {
+        "node": ">=0.3.1"
+      }
+    },
+    "node_modules/sinon/node_modules/supports-color": {
+      "version": "7.2.0",
+      "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-7.2.0.tgz",
+      "integrity": "sha512-qpCAvRl9stuOHveKsn7HncJRvv501qIacKzQlO/+Lwxc9+0q2wLyv4Dfvt80/DPn2pqOBsJdDiogXGR9+OvwRw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "has-flag": "^4.0.0"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/slash": {
       "version": "5.1.0",
       "resolved": "https://registry.npmjs.org/slash/-/slash-5.1.0.tgz",
@@ -6117,6 +6218,16 @@
         "node": ">= 0.8.0"
       }
     },
+    "node_modules/type-detect": {
+      "version": "4.0.8",
+      "resolved": "https://registry.npmjs.org/type-detect/-/type-detect-4.0.8.tgz",
+      "integrity": "sha512-0fr/mIH1dlO+x7TlcMy+bIDqKPsw/70tVyeHW787goQjhmqaZe10uwLujubK9q9Lg6Fiho1KUKDYz0Z7k7g5/g==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=4"
+      }
+    },
     "node_modules/type-fest": {
       "version": "4.41.0",
       "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-4.41.0.tgz",
diff --git a/package.json b/package.json
index 52fd8f2..99f0fe4 100644
--- a/package.json
+++ b/package.json
@@ -195,11 +195,14 @@
     "watch": "tsc -watch -p ./",
     "pretest": "npm run compile && npm run lint",
     "lint": "eslint src",
-    "test": "vscode-test"
+    "test": "vscode-test",
+    "test:unit": "vscode-test --grep 'UI Logic Tests|Extension Test Suite'",
+    "test:integration": "vscode-test --grep 'Integration Tests'"
   },
   "devDependencies": {
     "@types/mocha": "^10.0.10",
     "@types/node": "20.x",
+    "@types/sinon": "^17.0.4",
     "@types/vscode": "^1.94.0",
     "@typescript-eslint/eslint-plugin": "^8.31.1",
     "@typescript-eslint/parser": "^8.31.1",
@@ -207,6 +210,7 @@
     "@vscode/test-electron": "^2.5.2",
     "@vscode/vsce": "^3.5.0",
     "eslint": "^9.25.1",
+    "sinon": "^21.0.0",
     "typescript": "^5.8.3"
   }
 }
diff --git a/src/extension.ts b/src/extension.ts
index b27e2f5..a493afd 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -250,7 +250,7 @@ class ClaudeChatProvider {
 	private _handleWebviewMessage(message: any) {
 		switch (message.type) {
 			case 'sendMessage':
-				this._sendMessageToClaude(message.text, message.planMode, message.thinkingMode);
+				this._sendMessageToClaude(message.text, message.planMode, message.thinkingMode, message.longContext);
 				return;
 			case 'newSession':
 				this._newSession();
@@ -405,7 +405,7 @@ class ClaudeChatProvider {
 		}
 	}
 
-	private async _sendMessageToClaude(message: string, planMode?: boolean, thinkingMode?: boolean) {
+	private async _sendMessageToClaude(message: string, planMode?: boolean, thinkingMode?: boolean, longContext?: boolean) {
 		const workspaceFolder = vscode.workspace.workspaceFolders?.[0];
 		const cwd = workspaceFolder ? workspaceFolder.uri.fsPath : process.cwd();
 
@@ -496,7 +496,12 @@ class ClaudeChatProvider {
 
 		// Add model selection if not using default
 		if (this._selectedModel && this._selectedModel !== 'default') {
-			args.push('--model', this._selectedModel);
+			let modelName = this._selectedModel;
+			// Add [1m] suffix for long context when enabled and supported
+			if (longContext && this._selectedModel === 'sonnet') {
+				modelName = 'sonnet[1m]';
+			}
+			args.push('--model', modelName);
 		}
 
 		// Add session resume if we have a current session
diff --git a/src/script.ts b/src/script.ts
index 871f6ab..86c8b53 100644
--- a/src/script.ts
+++ b/src/script.ts
@@ -15,6 +15,7 @@ const getScript = (isTelemetryEnabled: boolean) => `<script>
 		let selectedFileIndex = -1;
 		let planModeEnabled = false;
 		let thinkingModeEnabled = false;
+		let longContextEnabled = false;
 
 		function shouldAutoScroll(messagesDiv) {
 			const threshold = 100; // pixels from bottom
@@ -719,7 +720,8 @@ const getScript = (isTelemetryEnabled: boolean) => `<script>
 					type: 'sendMessage',
 					text: text,
 					planMode: planModeEnabled,
-					thinkingMode: thinkingModeEnabled
+					thinkingMode: thinkingModeEnabled,
+					longContext: longContextEnabled
 				});
 				
 				messageInput.value = '';
@@ -758,6 +760,16 @@ const getScript = (isTelemetryEnabled: boolean) => `<script>
 			}
 		}
 
+		function toggleLongContext() {
+			longContextEnabled = !longContextEnabled;
+			const switchElement = document.getElementById('longContextSwitch');
+			if (longContextEnabled) {
+				switchElement.classList.add('active');
+			} else {
+				switchElement.classList.remove('active');
+			}
+		}
+
 
 		let totalCost = 0;
 		let totalTokensInput = 0;
@@ -1704,6 +1716,22 @@ const getScript = (isTelemetryEnabled: boolean) => `<script>
 			};
 			document.getElementById('selectedModel').textContent = displayNames[model] || model;
 			
+			// Show/hide Long Context toggle based on model selection
+			const longContextToggle = document.getElementById('longContextToggle');
+			if (longContextToggle) {
+				if (model === 'sonnet' || model === 'default') {
+					longContextToggle.style.display = 'flex';
+				} else {
+					longContextToggle.style.display = 'none';
+					// Reset long context when hidden
+					longContextEnabled = false;
+					const switchElement = document.getElementById('longContextSwitch');
+					if (switchElement) {
+						switchElement.classList.remove('active');
+					}
+				}
+			}
+			
 			// Only send model selection to VS Code extension if not from backend
 			if (!fromBackend) {
 				vscode.postMessage({
diff --git a/src/test/extension.test.ts b/src/test/extension.test.ts
index 4ca0ab4..140c5da 100644
--- a/src/test/extension.test.ts
+++ b/src/test/extension.test.ts
@@ -1,15 +1,24 @@
 import * as assert from 'assert';
-
-// You can import and use all API from the 'vscode' module
-// as well as import your extension to test it
 import * as vscode from 'vscode';
-// import * as myExtension from '../../extension';
 
 suite('Extension Test Suite', () => {
 	vscode.window.showInformationMessage('Start all tests.');
 
-	test('Sample test', () => {
-		assert.strictEqual(-1, [1, 2, 3].indexOf(5));
-		assert.strictEqual(-1, [1, 2, 3].indexOf(0));
+	test('Extension should be present', () => {
+		assert.ok(vscode.extensions.getExtension('AndrePimenta.claude-code-chat'));
+	});
+
+	test('Extension should activate', async () => {
+		const extension = vscode.extensions.getExtension('AndrePimenta.claude-code-chat');
+		if (extension && !extension.isActive) {
+			await extension.activate();
+		}
+		assert.ok(extension?.isActive);
+	});
+
+	test('Commands should be registered', async () => {
+		const commands = await vscode.commands.getCommands(true);
+		assert.ok(commands.includes('claude-code-chat.openChat'));
+		assert.ok(commands.includes('claude-code-chat.loadConversation'));
 	});
 });
diff --git a/src/test/integration.test.ts b/src/test/integration.test.ts
new file mode 100644
index 0000000..d8b79f6
--- /dev/null
+++ b/src/test/integration.test.ts
@@ -0,0 +1,81 @@
+import * as assert from 'assert';
+import * as vscode from 'vscode';
+import * as sinon from 'sinon';
+
+suite('Long Context Integration Tests', () => {
+	let mockContext: vscode.ExtensionContext;
+	let mockWorkspaceState: sinon.SinonStubbedInstance<vscode.Memento>;
+
+	setup(() => {
+		mockWorkspaceState = {
+			get: sinon.stub(),
+			update: sinon.stub(),
+			keys: sinon.stub()
+		} as any;
+		mockContext = {
+			workspaceState: mockWorkspaceState,
+			extensionUri: vscode.Uri.file('/test')
+		} as any;
+	});
+
+	teardown(() => {
+		sinon.restore();
+	});
+
+	suite('End-to-End Message Flow', () => {
+		test('should handle complete long context workflow', async () => {
+			// This test would verify the complete flow:
+			// 1. User selects Sonnet model
+			// 2. User enables Long Context toggle
+			// 3. User sends message
+			// 4. Extension receives message with longContext=true
+			// 5. Extension spawns Claude CLI with --model sonnet[1m]
+			
+			const mockMessage = {
+				type: 'sendMessage',
+				text: 'Test message with long context',
+				planMode: false,
+				thinkingMode: false,
+				longContext: true
+			};
+
+			// Mock the extension provider and verify behavior
+			assert.ok(true, 'Integration test framework setup needed');
+		});
+
+		test('should preserve model selection when toggling long context', () => {
+			// Test that switching long context on/off doesn't affect model selection
+			assert.ok(true, 'Test implementation needed');
+		});
+
+		test('should reset long context when switching from sonnet to opus', () => {
+			// Test complete model switching workflow with long context reset
+			assert.ok(true, 'Test implementation needed');
+		});
+	});
+
+	suite('State Persistence', () => {
+		test('should not persist long context state across sessions', () => {
+			// Verify that long context state is session-only, not persisted
+			// Unlike model selection which is saved to workspace state
+			assert.ok(true, 'State persistence test needed');
+		});
+
+		test('should maintain model selection independent of long context', () => {
+			// Test that model selection persists even when long context is toggled
+			assert.ok(true, 'Model persistence test needed');
+		});
+	});
+
+	suite('Error Handling', () => {
+		test('should handle missing DOM elements gracefully', () => {
+			// Test behavior when longContextToggle or longContextSwitch elements don't exist
+			assert.ok(true, 'Error handling test needed');
+		});
+
+		test('should handle invalid model selections', () => {
+			// Test behavior with unexpected model names
+			assert.ok(true, 'Invalid model handling test needed');
+		});
+	});
+});
\ No newline at end of file
diff --git a/src/test/long-context.test.ts b/src/test/long-context.test.ts
new file mode 100644
index 0000000..59eb771
--- /dev/null
+++ b/src/test/long-context.test.ts
@@ -0,0 +1,69 @@
+import * as assert from 'assert';
+import * as vscode from 'vscode';
+import * as sinon from 'sinon';
+
+suite('Long Context Feature Tests', () => {
+	let mockContext: vscode.ExtensionContext;
+	let mockWorkspaceState: sinon.SinonStubbedInstance<vscode.Memento>;
+
+	setup(() => {
+		mockWorkspaceState = {
+			get: sinon.stub(),
+			update: sinon.stub(),
+			keys: sinon.stub()
+		} as any;
+		mockContext = {
+			workspaceState: mockWorkspaceState,
+			extensionUri: vscode.Uri.file('/test')
+		} as any;
+	});
+
+	teardown(() => {
+		sinon.restore();
+	});
+
+	suite('CLI Argument Generation', () => {
+		test('should add [1m] suffix for sonnet with long context', () => {
+			// This would test the logic in _sendMessageToClaude
+			// Testing the args array construction when longContext=true and model='sonnet'
+			const expectedArgs = ['--model', 'sonnet[1m]'];
+			
+			// Mock the model selection and long context state
+			// Verify that when longContext=true and selectedModel='sonnet',
+			// the args array contains '--model sonnet[1m]'
+			assert.ok(true, 'Test implementation needed for actual extension class testing');
+		});
+
+		test('should not add [1m] suffix for opus with long context', () => {
+			// Test that opus model doesn't get [1m] suffix even with longContext=true
+			const expectedArgs = ['--model', 'opus'];
+			assert.ok(true, 'Test implementation needed for actual extension class testing');
+		});
+
+		test('should use regular sonnet when long context disabled', () => {
+			// Test that sonnet without longContext uses regular model name
+			const expectedArgs = ['--model', 'sonnet'];
+			assert.ok(true, 'Test implementation needed for actual extension class testing');
+		});
+
+		test('should not add model flag for default with long context', () => {
+			// Test that default model doesn't add --model flag regardless of longContext
+			assert.ok(true, 'Test implementation needed for actual extension class testing');
+		});
+	});
+
+	suite('Message Handling', () => {
+		test('should handle longContext parameter in webview messages', () => {
+			// Test that _handleWebviewMessage properly extracts longContext from message
+			// and passes it to _sendMessageToClaude
+			const testMessage = {
+				type: 'sendMessage',
+				text: 'test message',
+				planMode: false,
+				thinkingMode: false,
+				longContext: true
+			};
+			assert.ok(true, 'Test implementation needed for message handling');
+		});
+	});
+});
\ No newline at end of file
diff --git a/src/test/ui-logic.test.ts b/src/test/ui-logic.test.ts
new file mode 100644
index 0000000..a0970f9
--- /dev/null
+++ b/src/test/ui-logic.test.ts
@@ -0,0 +1,129 @@
+import * as assert from 'assert';
+import * as sinon from 'sinon';
+
+suite('UI Logic Tests', () => {
+	let mockDocument: any;
+	let longContextEnabled: boolean;
+
+	setup(() => {
+		// Mock DOM elements
+		const mockElements = new Map();
+		
+		mockDocument = {
+			getElementById: (id: string) => {
+				if (!mockElements.has(id)) {
+					mockElements.set(id, {
+						style: { display: 'flex' },
+						classList: {
+							add: sinon.stub(),
+							remove: sinon.stub()
+						}
+					});
+				}
+				return mockElements.get(id);
+			}
+		};
+
+		// Reset state
+		longContextEnabled = false;
+
+		// Mock global document
+		(global as any).document = mockDocument;
+	});
+
+	suite('toggleLongContext Function', () => {
+		function toggleLongContext() {
+			longContextEnabled = !longContextEnabled;
+			const switchElement = mockDocument.getElementById('longContextSwitch');
+			if (longContextEnabled) {
+				switchElement.classList.add('active');
+			} else {
+				switchElement.classList.remove('active');
+			}
+		}
+
+		test('should toggle longContextEnabled state', () => {
+			assert.strictEqual(longContextEnabled, false);
+			
+			toggleLongContext();
+			assert.strictEqual(longContextEnabled, true);
+			
+			toggleLongContext();
+			assert.strictEqual(longContextEnabled, false);
+		});
+
+		test('should add active class when enabled', () => {
+			const switchElement = mockDocument.getElementById('longContextSwitch');
+			
+			toggleLongContext();
+			
+			assert.ok(switchElement.classList.add.calledWith('active'));
+		});
+
+		test('should remove active class when disabled', () => {
+			// Enable first
+			toggleLongContext();
+			const switchElement = mockDocument.getElementById('longContextSwitch');
+			
+			// Then disable
+			toggleLongContext();
+			
+			assert.ok(switchElement.classList.remove.calledWith('active'));
+		});
+	});
+
+	suite('selectModel Visibility Logic', () => {
+		function selectModel(model: string) {
+			const longContextToggle = mockDocument.getElementById('longContextToggle');
+			if (longContextToggle) {
+				if (model === 'sonnet' || model === 'default') {
+					longContextToggle.style.display = 'flex';
+				} else {
+					longContextToggle.style.display = 'none';
+					// Reset long context when hidden
+					longContextEnabled = false;
+					const switchElement = mockDocument.getElementById('longContextSwitch');
+					if (switchElement) {
+						switchElement.classList.remove('active');
+					}
+				}
+			}
+		}
+
+		test('should show toggle for sonnet model', () => {
+			const toggle = mockDocument.getElementById('longContextToggle');
+			
+			selectModel('sonnet');
+			
+			assert.strictEqual(toggle.style.display, 'flex');
+		});
+
+		test('should show toggle for default model', () => {
+			const toggle = mockDocument.getElementById('longContextToggle');
+			
+			selectModel('default');
+			
+			assert.strictEqual(toggle.style.display, 'flex');
+		});
+
+		test('should hide toggle for opus model', () => {
+			const toggle = mockDocument.getElementById('longContextToggle');
+			
+			selectModel('opus');
+			
+			assert.strictEqual(toggle.style.display, 'none');
+		});
+
+		test('should reset long context state when hiding toggle', () => {
+			// Enable long context first
+			longContextEnabled = true;
+			const switchElement = mockDocument.getElementById('longContextSwitch');
+			
+			// Switch to opus (should hide and reset)
+			selectModel('opus');
+			
+			assert.strictEqual(longContextEnabled, false);
+			assert.ok(switchElement.classList.remove.calledWith('active'));
+		});
+	});
+});
\ No newline at end of file
diff --git a/src/ui.ts b/src/ui.ts
index bc16c41..311db3d 100644
--- a/src/ui.ts
+++ b/src/ui.ts
@@ -66,6 +66,10 @@ const getHtml = (isTelemetryEnabled: boolean) => `<!DOCTYPE html>
 					<span id="thinkingModeLabel" onclick="toggleThinkingMode()">Thinking Mode</span>
 					<div class="mode-switch" id="thinkingModeSwitch" onclick="toggleThinkingMode()"></div>
 				</div>
+				<div class="mode-toggle" id="longContextToggle" style="display: none;">
+					<span onclick="toggleLongContext()">Long Context</span>
+					<div class="mode-switch" id="longContextSwitch" onclick="toggleLongContext()"></div>
+				</div>
 			</div>
 			<div class="textarea-container">
 				<div class="textarea-wrapper">