diff --git a/AGENTS.md b/AGENTS.md
index 133323e..d5e9abb 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -435,3 +435,42 @@ mid-refactor). The footer is not a gate — it is a record. Other agents
 picking up this repo mid-session can read the git log and immediately
 know the state of the codebase at each commit without re-running the
 full suite to establish a baseline.
+
+---
+
+## UTF-8 compliance
+
+**Rule: always write UTF-8 to `~/.login_conf`. Never write a legacy charset.**
+
+### `~/.login_conf` format
+
+```
+me:\
+	:charset=UTF-8:\
+	:lang=sl_SI.UTF-8:
+```
+
+The `lang` tag must end in `.UTF-8`. The `charset` field must be `UTF-8`.
+Never write `ISO8859-2`, `ISO-8859-2`, `latin2`, or any other non-UTF-8
+encoding — doing so silently breaks Slovenian character rendering (č/š/ž
+appear as `_`) in every terminal session until manually corrected.
+
+### Normalisation rule for `applyHostLocale()`
+
+Strip any existing charset suffix from the detected locale tag, then
+append `.UTF-8`. For example:
+
+| Detected | Written |
+|----------|---------|
+| `sl_SI` | `sl_SI.UTF-8` |
+| `sl_SI.ISO8859-2` | `sl_SI.UTF-8` |
+| `sl_SI.UTF-8` | `sl_SI.UTF-8` |
+| `en_US.UTF-8` | `en_US.UTF-8` |
+
+### After writing `~/.login_conf`
+
+Always run `cap_mkdb ~/.login_conf` to rebuild the binary database.
+The new locale takes effect on the **next login** — a new tmux window,
+new SSH session, or `exec $SHELL -l`. **Never suggest `tmux kill-server`**
+to activate locale changes; that destroys the user's entire working
+environment.
diff --git a/setup/onboarding.ts b/setup/onboarding.ts
index 36a325e..544d893 100644
--- a/setup/onboarding.ts
+++ b/setup/onboarding.ts
@@ -274,9 +274,13 @@ function writeIdentity(envFile: string, agentName: string, assistantName: string
 
 function applyHostLocale(systemLocale: string): boolean {
   try {
+    // Always normalise to UTF-8 — strip any non-UTF-8 charset suffix and
+    // rewrite the locale tag with .UTF-8 so ~/.login_conf never locks the
+    // user into a legacy encoding like ISO8859-2.
     const dotIdx = systemLocale.indexOf('.');
-    const charset = dotIdx !== -1 ? systemLocale.slice(dotIdx + 1) : 'UTF-8';
-    const content = `me:\\\n\t:charset=${charset}:\\\n\t:lang=${systemLocale}:\n`;
+    const baseLocale = dotIdx !== -1 ? systemLocale.slice(0, dotIdx) : systemLocale;
+    const normalizedLocale = `${baseLocale}.UTF-8`;
+    const content = `me:\\\n\t:charset=UTF-8:\\\n\t:lang=${normalizedLocale}:\n`;
     const loginConfPath = path.join(os.homedir(), '.login_conf');
     fs.writeFileSync(loginConfPath, content, { encoding: 'utf-8' });
     execFileSync('cap_mkdb', [loginConfPath], { stdio: 'ignore' });
@@ -791,7 +795,7 @@ export async function run(args: string[]): Promise<void> {
       runBsddialog([
         '--title', 'Host Locale Set',
         '--msgbox',
-        `Locale set to ${systemLocale}.\n\nWritten to ~/.login_conf.\n\nRespawn tmux to activate:\n  tmux kill-server && tmux`,
+        `Locale set to ${systemLocale} (normalised to UTF-8).\n\nWritten to ~/.login_conf.\n\nOpen a new tmux window or fresh SSH login to activate.`,
         '0', '0',
       ]);
     }
@@ -891,7 +895,7 @@ export async function run(args: string[]): Promise<void> {
     const hostLocaleApplied = applyHostLocale(systemLocale);
     if (hostLocaleApplied) {
       console.log(
-        `Locale set to ${systemLocale} in ~/.login_conf. Respawn tmux to activate: tmux kill-server && tmux`,
+        `Locale set to ${systemLocale} (normalised to UTF-8) in ~/.login_conf. Open a new tmux window or fresh SSH login to activate.`,
       );
     }