diff --git a/packages/coding-agent/src/core/extensions/runner.ts b/packages/coding-agent/src/core/extensions/runner.ts
index bcdd9fa5..a813b70e 100644
--- a/packages/coding-agent/src/core/extensions/runner.ts
+++ b/packages/coding-agent/src/core/extensions/runner.ts
@@ -170,6 +170,7 @@ export class ExtensionRunner {
 	private switchSessionHandler: SwitchSessionHandler = async () => ({ cancelled: false });
 	private shutdownHandler: ShutdownHandler = () => {};
 	private shortcutDiagnostics: ResourceDiagnostic[] = [];
+	private commandDiagnostics: ResourceDiagnostic[] = [];
 
 	constructor(
 		extensions: Extension[],
@@ -371,16 +372,31 @@ export class ExtensionRunner {
 		return undefined;
 	}
 
-	getRegisteredCommands(): RegisteredCommand[] {
+	getRegisteredCommands(reserved?: Set<string>): RegisteredCommand[] {
+		this.commandDiagnostics = [];
+
 		const commands: RegisteredCommand[] = [];
 		for (const ext of this.extensions) {
 			for (const command of ext.commands.values()) {
+				if (reserved?.has(command.name)) {
+					const message = `Extension command '${command.name}' from ${ext.path} conflicts with built-in commands. Skipping.`;
+					this.commandDiagnostics.push({ type: "warning", message, path: ext.path });
+					if (!this.hasUI()) {
+						console.warn(message);
+					}
+					continue;
+				}
+
 				commands.push(command);
 			}
 		}
 		return commands;
 	}
 
+	getCommandDiagnostics(): ResourceDiagnostic[] {
+		return this.commandDiagnostics;
+	}
+
 	getRegisteredCommandsWithPaths(): Array<{ command: RegisteredCommand; extensionPath: string }> {
 		const result: Array<{ command: RegisteredCommand; extensionPath: string }> = [];
 		for (const ext of this.extensions) {
diff --git a/packages/coding-agent/src/modes/interactive/interactive-mode.ts b/packages/coding-agent/src/modes/interactive/interactive-mode.ts
index 300a7516..91451d1b 100644
--- a/packages/coding-agent/src/modes/interactive/interactive-mode.ts
+++ b/packages/coding-agent/src/modes/interactive/interactive-mode.ts
@@ -347,13 +347,14 @@ export class InteractiveMode {
 		}));
 
 		// Convert extension commands to SlashCommand format
-		const extensionCommands: SlashCommand[] = (this.session.extensionRunner?.getRegisteredCommands() ?? []).map(
-			(cmd) => ({
-				name: cmd.name,
-				description: cmd.description ?? "(extension command)",
-				getArgumentCompletions: cmd.getArgumentCompletions,
-			}),
-		);
+		const builtinCommandNames = new Set(slashCommands.map((c) => c.name));
+		const extensionCommands: SlashCommand[] = (
+			this.session.extensionRunner?.getRegisteredCommands(builtinCommandNames) ?? []
+		).map((cmd) => ({
+			name: cmd.name,
+			description: cmd.description ?? "(extension command)",
+			getArgumentCompletions: cmd.getArgumentCompletions,
+		}));
 
 		// Build skill commands from session.skills (if enabled)
 		this.skillCommands.clear();
@@ -959,6 +960,9 @@ export class InteractiveMode {
 			}
 		}
 
+		const commandDiagnostics = this.session.extensionRunner?.getCommandDiagnostics() ?? [];
+		extensionDiagnostics.push(...commandDiagnostics);
+
 		const shortcutDiagnostics = this.session.extensionRunner?.getShortcutDiagnostics() ?? [];
 		extensionDiagnostics.push(...shortcutDiagnostics);
 
diff --git a/packages/coding-agent/test/extensions-runner.test.ts b/packages/coding-agent/test/extensions-runner.test.ts
index 6e139ff1..e9747c1d 100644
--- a/packages/coding-agent/test/extensions-runner.test.ts
+++ b/packages/coding-agent/test/extensions-runner.test.ts
@@ -279,6 +279,35 @@ describe("ExtensionRunner", () => {
 			const missing = runner.getCommand("not-exists");
 			expect(missing).toBeUndefined();
 		});
+
+		it("filters out commands conflict with reseved", async () => {
+			const cmdCode = (name: string) => `
+				export default function(pi) {
+					pi.registerCommand("${name}", {
+						description: "Test command",
+						handler: async () => {},
+					});
+				}
+			`;
+			fs.writeFileSync(path.join(extensionsDir, "cmd-a.ts"), cmdCode("cmd-a"));
+			fs.writeFileSync(path.join(extensionsDir, "cmd-b.ts"), cmdCode("cmd-b"));
+
+			const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+			const result = await discoverAndLoadExtensions([], tempDir, tempDir);
+			const runner = new ExtensionRunner(result.extensions, result.runtime, tempDir, sessionManager, modelRegistry);
+			const commands = runner.getRegisteredCommands(new Set(["cmd-a"]));
+			const diagnostics = runner.getCommandDiagnostics();
+
+			expect(commands.length).toBe(1);
+			expect(commands.map((c) => c.name).sort()).toEqual(["cmd-b"]);
+
+			expect(diagnostics.length).toBe(1);
+			expect(diagnostics[0].path).toEqual(path.join(extensionsDir, "cmd-a.ts"));
+
+			expect(warnSpy).toHaveBeenCalledWith(expect.stringContaining("conflicts with built-in command"));
+			warnSpy.mockRestore();
+		});
 	});
 
 	describe("error handling", () => {