diff --git a/hooks/caveman-activate.js b/hooks/caveman-activate.js
index ac080aa..0becf06 100644
--- a/hooks/caveman-activate.js
+++ b/hooks/caveman-activate.js
@@ -9,6 +9,7 @@
 const fs = require('fs');
 const path = require('path');
 const os = require('os');
+const { getDefaultMode } = require('./caveman-config');
 
 const claudeDir = path.join(os.homedir(), '.claude');
 const flagPath = path.join(claudeDir, '.caveman-active');
@@ -17,7 +18,7 @@ const settingsPath = path.join(claudeDir, 'settings.json');
 // 1. Write flag file
 try {
   fs.mkdirSync(path.dirname(flagPath), { recursive: true });
-  fs.writeFileSync(flagPath, 'full');
+  fs.writeFileSync(flagPath, getDefaultMode());
 } catch (e) {
   // Silent fail -- flag is best-effort, don't block the hook
 }
diff --git a/hooks/caveman-config.js b/hooks/caveman-config.js
new file mode 100644
index 0000000..15abdc8
--- /dev/null
+++ b/hooks/caveman-config.js
@@ -0,0 +1,61 @@
+#!/usr/bin/env node
+// caveman — shared configuration resolver
+//
+// Resolution order for default mode:
+//   1. CAVEMAN_DEFAULT_MODE environment variable
+//   2. Config file defaultMode field:
+//      - $XDG_CONFIG_HOME/caveman/config.json (any platform, if set)
+//      - ~/.config/caveman/config.json (macOS / Linux fallback)
+//      - %APPDATA%\caveman\config.json (Windows fallback)
+//   3. 'full'
+
+const fs = require('fs');
+const path = require('path');
+const os = require('os');
+
+const VALID_MODES = [
+  'lite', 'full', 'ultra',
+  'wenyan-lite', 'wenyan', 'wenyan-full', 'wenyan-ultra',
+  'commit', 'review', 'compress'
+];
+
+function getConfigDir() {
+  if (process.env.XDG_CONFIG_HOME) {
+    return path.join(process.env.XDG_CONFIG_HOME, 'caveman');
+  }
+  if (process.platform === 'win32') {
+    return path.join(
+      process.env.APPDATA || path.join(os.homedir(), 'AppData', 'Roaming'),
+      'caveman'
+    );
+  }
+  return path.join(os.homedir(), '.config', 'caveman');
+}
+
+function getConfigPath() {
+  return path.join(getConfigDir(), 'config.json');
+}
+
+function getDefaultMode() {
+  // 1. Environment variable (highest priority)
+  const envMode = process.env.CAVEMAN_DEFAULT_MODE;
+  if (envMode && VALID_MODES.includes(envMode.toLowerCase())) {
+    return envMode.toLowerCase();
+  }
+
+  // 2. Config file
+  try {
+    const configPath = getConfigPath();
+    const config = JSON.parse(fs.readFileSync(configPath, 'utf8'));
+    if (config.defaultMode && VALID_MODES.includes(config.defaultMode.toLowerCase())) {
+      return config.defaultMode.toLowerCase();
+    }
+  } catch (e) {
+    // Config file doesn't exist or is invalid — fall through
+  }
+
+  // 3. Default
+  return 'full';
+}
+
+module.exports = { getDefaultMode, getConfigDir, getConfigPath, VALID_MODES };
diff --git a/hooks/caveman-mode-tracker.js b/hooks/caveman-mode-tracker.js
index da708d7..e74491c 100644
--- a/hooks/caveman-mode-tracker.js
+++ b/hooks/caveman-mode-tracker.js
@@ -5,6 +5,7 @@
 const fs = require('fs');
 const path = require('path');
 const os = require('os');
+const { getDefaultMode } = require('./caveman-config');
 
 const flagPath = path.join(os.homedir(), '.claude', '.caveman-active');
 
@@ -35,7 +36,7 @@ process.stdin.on('end', () => {
         else if (arg === 'wenyan-lite') mode = 'wenyan-lite';
         else if (arg === 'wenyan' || arg === 'wenyan-full') mode = 'wenyan';
         else if (arg === 'wenyan-ultra') mode = 'wenyan-ultra';
-        else mode = 'full';
+        else mode = getDefaultMode();
       }
 
       if (mode) {
diff --git a/hooks/install.ps1 b/hooks/install.ps1
index 255e4fd..00ec4ea 100644
--- a/hooks/install.ps1
+++ b/hooks/install.ps1
@@ -24,7 +24,7 @@ $HooksDir = Join-Path $ClaudeDir "hooks"
 $Settings = Join-Path $ClaudeDir "settings.json"
 $RepoUrl = "https://raw.githubusercontent.com/JuliusBrussee/caveman/main/hooks"
 
-$HookFiles = @("caveman-activate.js", "caveman-mode-tracker.js", "caveman-statusline.sh", "caveman-statusline.ps1")
+$HookFiles = @("caveman-config.js", "caveman-activate.js", "caveman-mode-tracker.js", "caveman-statusline.sh", "caveman-statusline.ps1")
 
 # Resolve source — works from repo clone or remote
 $ScriptDir = if ($PSScriptRoot) { $PSScriptRoot } else { $null }
diff --git a/hooks/install.sh b/hooks/install.sh
index c7b6a66..ba235e0 100755
--- a/hooks/install.sh
+++ b/hooks/install.sh
@@ -37,7 +37,7 @@ HOOKS_DIR="$CLAUDE_DIR/hooks"
 SETTINGS="$CLAUDE_DIR/settings.json"
 REPO_URL="https://raw.githubusercontent.com/JuliusBrussee/caveman/main/hooks"
 
-HOOK_FILES=("caveman-activate.js" "caveman-mode-tracker.js" "caveman-statusline.sh")
+HOOK_FILES=("caveman-config.js" "caveman-activate.js" "caveman-mode-tracker.js" "caveman-statusline.sh")
 
 # Resolve source — works from repo clone or curl pipe
 SCRIPT_DIR=""
diff --git a/hooks/uninstall.ps1 b/hooks/uninstall.ps1
index c734ad6..a00ec83 100644
--- a/hooks/uninstall.ps1
+++ b/hooks/uninstall.ps1
@@ -11,7 +11,7 @@ $HooksDir = Join-Path $ClaudeDir "hooks"
 $Settings = Join-Path $ClaudeDir "settings.json"
 $FlagFile = Join-Path $ClaudeDir ".caveman-active"
 
-$HookFiles = @("caveman-activate.js", "caveman-mode-tracker.js", "caveman-statusline.sh", "caveman-statusline.ps1")
+$HookFiles = @("caveman-config.js", "caveman-activate.js", "caveman-mode-tracker.js", "caveman-statusline.sh", "caveman-statusline.ps1")
 
 # Detect if caveman is installed as a plugin
 $PluginInstalled = $false
diff --git a/hooks/uninstall.sh b/hooks/uninstall.sh
index b602fcf..fec9acd 100755
--- a/hooks/uninstall.sh
+++ b/hooks/uninstall.sh
@@ -10,7 +10,7 @@ HOOKS_DIR="$CLAUDE_DIR/hooks"
 SETTINGS="$CLAUDE_DIR/settings.json"
 FLAG_FILE="$CLAUDE_DIR/.caveman-active"
 
-HOOK_FILES=("caveman-activate.js" "caveman-mode-tracker.js" "caveman-statusline.sh")
+HOOK_FILES=("caveman-config.js" "caveman-activate.js" "caveman-mode-tracker.js" "caveman-statusline.sh")
 
 # Detect if caveman is installed as a plugin (check plugin cache)
 PLUGIN_INSTALLED=0
diff --git a/tests/verify_repo.py b/tests/verify_repo.py
index c3ee7ab..73def57 100644
--- a/tests/verify_repo.py
+++ b/tests/verify_repo.py
@@ -105,12 +105,19 @@ def verify_manifests_and_syntax() -> None:
     for path in manifest_paths:
         read_json(path)
 
+    run(["node", "--check", "hooks/caveman-config.js"])
     run(["node", "--check", "hooks/caveman-activate.js"])
     run(["node", "--check", "hooks/caveman-mode-tracker.js"])
     run(["bash", "-n", "hooks/install.sh"])
     run(["bash", "-n", "hooks/uninstall.sh"])
     run(["bash", "-n", "hooks/caveman-statusline.sh"])
 
+    # Ensure install/uninstall scripts include caveman-config.js
+    install_sh = (ROOT / "hooks/install.sh").read_text()
+    uninstall_sh = (ROOT / "hooks/uninstall.sh").read_text()
+    ensure("caveman-config.js" in install_sh, "install.sh missing caveman-config.js")
+    ensure("caveman-config.js" in uninstall_sh, "uninstall.sh missing caveman-config.js")
+
     print("JSON manifests and JS/bash syntax OK")
 
 
@@ -120,6 +127,8 @@ def verify_powershell_static() -> None:
     uninstall_text = (ROOT / "hooks/uninstall.ps1").read_text()
     statusline_text = (ROOT / "hooks/caveman-statusline.ps1").read_text()
 
+    ensure("caveman-config.js" in install_text, "install.ps1 missing caveman-config.js")
+    ensure("caveman-config.js" in uninstall_text, "uninstall.ps1 missing caveman-config.js")
     ensure("caveman-statusline.ps1" in install_text, "install.ps1 missing statusline.ps1")
     ensure("caveman-statusline.ps1" in uninstall_text, "uninstall.ps1 missing statusline.ps1")
     ensure("-AsHashtable" not in install_text, "install.ps1 should stay compatible with Windows PowerShell 5.1")
@@ -220,6 +229,16 @@ def verify_hook_install_flow() -> None:
         ensure("STATUSLINE SETUP NEEDED" not in activate.stdout, "activation should stay quiet when custom statusline exists")
         ensure((claude_dir / ".caveman-active").read_text() == "full", "activation flag should default to full")
 
+        # Test configurable default mode via CAVEMAN_DEFAULT_MODE env var
+        activate_custom = run(
+            ["node", "hooks/caveman-activate.js"],
+            env={"HOME": str(home), "CAVEMAN_DEFAULT_MODE": "ultra"},
+        )
+        ensure("CAVEMAN MODE ACTIVE." in activate_custom.stdout, "activation with custom default missing banner")
+        ensure((claude_dir / ".caveman-active").read_text() == "ultra", "CAVEMAN_DEFAULT_MODE=ultra should set flag to ultra")
+        # Reset back to full for subsequent tests
+        (claude_dir / ".caveman-active").write_text("full")
+
         run(
             ["node", "hooks/caveman-mode-tracker.js"],
             env={"HOME": str(home)},