diff --git a/src/openhuman/agent/harness/tool_loop_tests.rs b/src/openhuman/agent/harness/tool_loop_tests.rs
index df93325e6..6ca463471 100644
--- a/src/openhuman/agent/harness/tool_loop_tests.rs
+++ b/src/openhuman/agent/harness/tool_loop_tests.rs
@@ -878,7 +878,7 @@ async fn run_tool_call_loop_applies_per_tool_max_result_size_cap() {
     assert!(
         tool_results.content.contains("[truncated by tool cap:"),
         "expected truncation marker, got body: {}",
-        &tool_results.content[..tool_results.content.len().min(200)]
+        crate::openhuman::util::utf8_safe_prefix_at_byte_boundary(&tool_results.content, 200)
     );
     assert!(
         tool_results.content.len() < 1_000,
diff --git a/src/openhuman/integrations/seltz.rs b/src/openhuman/integrations/seltz.rs
index e3112b2ad..22c12a48e 100644
--- a/src/openhuman/integrations/seltz.rs
+++ b/src/openhuman/integrations/seltz.rs
@@ -11,6 +11,7 @@
 //! integration, this calls the Seltz API directly — no backend proxy needed.
 
 use crate::openhuman::tools::traits::{Tool, ToolCallOptions, ToolResult};
+use crate::openhuman::util::utf8_safe_prefix_at_byte_boundary;
 use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
 use serde_json::json;
@@ -290,11 +291,7 @@ impl Tool for SeltzSearchTool {
         let status = resp.status();
         if !status.is_success() {
             let body_text = resp.text().await.unwrap_or_default();
-            let detail = if body_text.len() > 500 {
-                &body_text[..500]
-            } else {
-                &body_text
-            };
+            let detail = utf8_safe_prefix_at_byte_boundary(&body_text, 500);
             tracing::warn!(
                 status = %status,
                 "[seltz] non-2xx response: {detail}"
diff --git a/src/openhuman/local_ai/service/whisper_engine.rs b/src/openhuman/local_ai/service/whisper_engine.rs
index d3c662966..d9f7d3449 100644
--- a/src/openhuman/local_ai/service/whisper_engine.rs
+++ b/src/openhuman/local_ai/service/whisper_engine.rs
@@ -12,6 +12,8 @@ use log::{debug, info, warn};
 use parking_lot::Mutex;
 use whisper_rs::{FullParams, SamplingStrategy, WhisperContext, WhisperContextParameters};
 
+use crate::openhuman::util::utf8_safe_prefix_at_byte_boundary;
+
 /// Per-segment confidence threshold: reject segments with avg log-probability below this.
 const SEGMENT_LOGPROB_REJECT: f32 = -0.7;
 
@@ -168,7 +170,7 @@ pub fn transcribe_pcm_f32(
             params.set_initial_prompt(prompt);
             debug!(
                 "{LOG_PREFIX} set initial_prompt: '{}...'",
-                &prompt[..prompt.len().min(80)]
+                utf8_safe_prefix_at_byte_boundary(prompt, 80)
             );
         }
     }
diff --git a/src/openhuman/socket/ws_loop.rs b/src/openhuman/socket/ws_loop.rs
index 1989a13a4..8e496bd41 100644
--- a/src/openhuman/socket/ws_loop.rs
+++ b/src/openhuman/socket/ws_loop.rs
@@ -12,6 +12,7 @@ use tokio_tungstenite::{
 };
 
 use crate::api::models::socket::ConnectionStatus;
+use crate::openhuman::util::utf8_safe_prefix_at_byte_boundary;
 
 use super::event_handlers::{handle_sio_event, parse_sio_event};
 use super::manager::{emit_state_change, SharedState};
@@ -371,7 +372,7 @@ async fn read_eio_open(
                 }
                 log::debug!(
                     "[socket] Skipping non-OPEN packet: {}",
-                    &s[..s.len().min(40)]
+                    utf8_safe_prefix_at_byte_boundary(s, 40)
                 );
             }
             Some(Ok(_)) => continue,
@@ -416,7 +417,7 @@ async fn read_sio_connect_ack(
                 }
                 log::debug!(
                     "[socket] Skipping packet during SIO handshake: {}",
-                    &s[..s.len().min(40)]
+                    utf8_safe_prefix_at_byte_boundary(s, 40)
                 );
             }
             Some(Ok(_)) => continue,
@@ -463,7 +464,7 @@ fn handle_eio_message(
         _ => {
             log::debug!(
                 "[socket] Unknown EIO packet: {}",
-                &text[..text.len().min(30)]
+                utf8_safe_prefix_at_byte_boundary(text, 30)
             );
         }
     }
@@ -487,7 +488,7 @@ fn handle_sio_packet(
             } else {
                 log::warn!(
                     "[socket] Failed to parse SIO EVENT: {}",
-                    &text[..text.len().min(80)]
+                    utf8_safe_prefix_at_byte_boundary(text, 80)
                 );
             }
         }
@@ -522,7 +523,7 @@ fn handle_sio_packet(
         _ => {
             log::debug!(
                 "[socket] Unknown SIO packet type: {}",
-                &text[..text.len().min(30)]
+                utf8_safe_prefix_at_byte_boundary(text, 30)
             );
         }
     }
diff --git a/src/openhuman/socket/ws_loop_tests.rs b/src/openhuman/socket/ws_loop_tests.rs
index 623a3e238..957096590 100644
--- a/src/openhuman/socket/ws_loop_tests.rs
+++ b/src/openhuman/socket/ws_loop_tests.rs
@@ -168,6 +168,19 @@ fn handle_eio_message_close_and_noop_do_not_panic() {
     handle_eio_message("9", &tx, &shared); // unknown
 }
 
+#[test]
+fn handle_eio_message_unknown_packet_is_utf8_safe_at_preview_boundary() {
+    let previous_max_level = log::max_level();
+    log::set_max_level(log::LevelFilter::Trace);
+    let shared = make_shared();
+    let (tx, _rx) = mpsc::unbounded_channel::<String>();
+    let packet = format!("9{}{}", "a".repeat(28), "魔");
+    assert!(!packet.is_char_boundary(30));
+
+    handle_eio_message(&packet, &tx, &shared);
+    log::set_max_level(previous_max_level);
+}
+
 // ── handle_sio_packet ──────────────────────────────────────────
 
 #[test]
@@ -190,6 +203,32 @@ fn handle_sio_packet_event_with_unparseable_payload_is_logged_only() {
     assert_eq!(*shared.status.read(), ConnectionStatus::Disconnected);
 }
 
+#[test]
+fn handle_sio_packet_unparseable_event_is_utf8_safe_at_preview_boundary() {
+    let previous_max_level = log::max_level();
+    log::set_max_level(log::LevelFilter::Trace);
+    let shared = make_shared();
+    let (tx, _rx) = mpsc::unbounded_channel::<String>();
+    let packet = format!("2{}{}", "a".repeat(78), "魔");
+    assert!(!packet.is_char_boundary(80));
+
+    handle_sio_packet(&packet, &tx, &shared);
+    log::set_max_level(previous_max_level);
+}
+
+#[test]
+fn handle_sio_packet_unknown_type_is_utf8_safe_at_preview_boundary() {
+    let previous_max_level = log::max_level();
+    log::set_max_level(log::LevelFilter::Trace);
+    let shared = make_shared();
+    let (tx, _rx) = mpsc::unbounded_channel::<String>();
+    let packet = format!("9{}{}", "a".repeat(28), "魔");
+    assert!(!packet.is_char_boundary(30));
+
+    handle_sio_packet(&packet, &tx, &shared);
+    log::set_max_level(previous_max_level);
+}
+
 #[test]
 fn handle_sio_packet_connect_reack_updates_sid() {
     let shared = make_shared();
diff --git a/src/openhuman/update/core.rs b/src/openhuman/update/core.rs
index aa29d7c01..d5b3be2c3 100644
--- a/src/openhuman/update/core.rs
+++ b/src/openhuman/update/core.rs
@@ -9,6 +9,7 @@ use std::os::unix::fs::PermissionsExt as _;
 
 use crate::openhuman::config::UpdateRestartStrategy;
 use crate::openhuman::update::types::{GitHubAsset, GitHubRelease, UpdateApplyResult, UpdateInfo};
+use crate::openhuman::util::utf8_safe_prefix_at_byte_boundary;
 
 /// GitHub owner/repo for the core binary releases.
 const GITHUB_OWNER: &str = "tinyhumansai";
@@ -139,7 +140,7 @@ pub async fn check_available() -> Result<UpdateInfo, String> {
         log::warn!(
             "[update] GitHub API returned {}: {}",
             status,
-            &body[..body.len().min(200)]
+            utf8_safe_prefix_at_byte_boundary(&body, 200)
         );
         let msg = format!("GitHub API error: {status}");
         if crate::core::observability::is_updater_transient_http_status(status.as_u16()) {
diff --git a/src/openhuman/util.rs b/src/openhuman/util.rs
index d7e32103f..185141151 100644
--- a/src/openhuman/util.rs
+++ b/src/openhuman/util.rs
@@ -79,6 +79,13 @@ pub fn floor_char_boundary(s: &str, index: usize) -> usize {
     end
 }
 
+/// Return a prefix of `s` whose byte length is at most `max_bytes`, backing up
+/// to the nearest UTF-8 character boundary when `max_bytes` falls in the middle
+/// of a multi-byte character.
+pub fn utf8_safe_prefix_at_byte_boundary(s: &str, max_bytes: usize) -> &str {
+    &s[..floor_char_boundary(s, max_bytes)]
+}
+
 /// Round a byte index UP to the nearest UTF-8 character boundary.
 pub fn ceil_char_boundary(s: &str, index: usize) -> usize {
     if index >= s.len() {
@@ -227,6 +234,25 @@ mod tests {
         assert_eq!(floor_char_boundary(s, 100), 6);
     }
 
+    #[test]
+    fn test_utf8_safe_prefix_at_byte_boundary() {
+        let s = format!("{}{}tail", "a".repeat(79), "魔");
+        assert_eq!(utf8_safe_prefix_at_byte_boundary(&s, 80), "a".repeat(79));
+        assert_eq!(utf8_safe_prefix_at_byte_boundary(&s, s.len()), s);
+        assert_eq!(
+            utf8_safe_prefix_at_byte_boundary("ascii preview", 5),
+            "ascii"
+        );
+        assert_eq!(utf8_safe_prefix_at_byte_boundary("short", 80), "short");
+
+        for cap in [30, 40, 80, 200, 500] {
+            let preview = format!("{}{}tail", "a".repeat(cap - 1), "界");
+            let truncated = utf8_safe_prefix_at_byte_boundary(&preview, cap);
+            assert_eq!(truncated, "a".repeat(cap - 1));
+            assert!(preview.is_char_boundary(truncated.len()));
+        }
+    }
+
     #[test]
     fn test_ceil_char_boundary() {
         let s = "A🦀C";
diff --git a/src/openhuman/voice/streaming.rs b/src/openhuman/voice/streaming.rs
index ff38da12d..df7e6861b 100644
--- a/src/openhuman/voice/streaming.rs
+++ b/src/openhuman/voice/streaming.rs
@@ -24,6 +24,7 @@ use super::postprocess;
 use crate::openhuman::config::Config;
 use crate::openhuman::local_ai;
 use crate::openhuman::local_ai::whisper_engine;
+use crate::openhuman::util::utf8_safe_prefix_at_byte_boundary;
 
 const LOG_PREFIX: &str = "[voice-stream]";
 const AUDIO_SAMPLE_RATE: usize = 16_000;
@@ -116,7 +117,7 @@ pub async fn handle_dictation_ws(mut socket: WebSocket, config: Arc<Config>) {
                                 "{LOG_PREFIX} partial transcription ({} samples, avg_logprob={:.3}): {}",
                                 samples.len(),
                                 result.avg_logprob.unwrap_or(0.0),
-                                &result.text[..result.text.len().min(80)]
+                                utf8_safe_prefix_at_byte_boundary(&result.text, 80)
                             );
                             if partial_tx.send(result.text).await.is_err() {
                                 break; // receiver dropped