diff --git a/README.md b/README.md
index 7193bd4..b747f41 100644
--- a/README.md
+++ b/README.md
@@ -262,6 +262,10 @@ termly config set defaultAI aider
 termly config get defaultAI
 ```
 
+`defaultAI` is used by `termly start` when no `--ai` flag is given. If the
+configured tool is unknown or not installed, Termly warns and falls back to
+auto-detection instead of failing.
+
 **Note:** Server URL is determined by environment and cannot be changed via config.
 
 ### Cleanup
diff --git a/lib/ai-tools/selector.js b/lib/ai-tools/selector.js
index bebb826..b6989ed 100644
--- a/lib/ai-tools/selector.js
+++ b/lib/ai-tools/selector.js
@@ -2,6 +2,7 @@ const inquirer = require('inquirer');
 const chalk = require('chalk');
 const { detectInstalledTools, isToolInstalled } = require('./detector');
 const { getToolByKey } = require('./registry');
+const { getDefaultAI } = require('../config/manager');
 const logger = require('../utils/logger');
 
 // Select AI tool based on options
@@ -12,6 +13,12 @@ async function selectAITool(options) {
     return await selectManualTool(options.ai);
   }
 
+  // Configured default (termly setup / termly config set defaultAI <tool>)
+  const configuredTool = await selectConfiguredDefault();
+  if (configuredTool) {
+    return configuredTool;
+  }
+
   // No auto-detect mode
   if (options.noAutoDetect) {
     console.error(chalk.red('❌ Please specify AI tool with --ai flag'));
@@ -27,6 +34,40 @@ async function selectAITool(options) {
   return await autoSelectTool();
 }
 
+// Configured default tool selection
+//
+// Unlike --ai, `defaultAI` is a stored preference that may have been set long
+// ago, so a stale value must never kill `termly start`: warn and let the caller
+// fall back to auto-detection.
+async function selectConfiguredDefault() {
+  const defaultAI = getDefaultAI();
+
+  if (!defaultAI) {
+    return null;
+  }
+
+  logger.debug(`Configured defaultAI: ${defaultAI}`);
+
+  const tool = getToolByKey(defaultAI);
+
+  if (!tool) {
+    logger.warn(`Configured default AI "${defaultAI}" is unknown - falling back to auto-detection`);
+    console.log(chalk.dim('   Change it with: termly config set defaultAI <tool>'));
+    return null;
+  }
+
+  const result = await isToolInstalled(tool.key);
+
+  if (!result.installed) {
+    logger.warn(`Configured default AI ${tool.displayName} is not installed - falling back to auto-detection`);
+    console.log(chalk.dim('   Change it with: termly config set defaultAI <tool>'));
+    return null;
+  }
+
+  console.log(chalk.green(`Using ${result.tool.displayName} v${result.tool.version} (configured default)`));
+  return result.tool;
+}
+
 // Manual tool selection
 async function selectManualTool(toolName) {
   const tool = getToolByKey(toolName);