diff --git a/src/main.rs b/src/main.rs
index 91d9f98..ed6b12a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1022,40 +1022,21 @@ mod tests {
     use super::*;
     use std::fs;
 
-    #[test]
-    fn mcp_proxy_stdio_accepts_hyphen_prefixed_child_args() {
-        let cli = Cli::try_parse_from([
-            "agentk",
-            "mcp-proxy-stdio",
-            "--command",
-            "sh",
-            "--arg",
-            "-c",
-            "--arg",
-            "printf ok",
-        ])
-        .expect("hyphen-prefixed child args should parse");
-
-        let Some(Command::McpProxyStdio { args, .. }) = cli.command else {
-            panic!("expected mcp-proxy-stdio command");
-        };
-        assert_eq!(args, vec!["-c".to_string(), "printf ok".to_string()]);
-    }
-
     #[cfg(unix)]
-    #[test]
-    fn mcp_proxy_stdio_trace_out_writes_verifiable_events() {
-        let trace_path = env::temp_dir().join(format!(
-            "agentk-mcp-proxy-stdio-trace-{}-{}.jsonl",
+    fn mcp_proxy_trace_out_test_path(label: &str) -> PathBuf {
+        env::temp_dir().join(format!(
+            "agentk-mcp-proxy-stdio-{label}-{}-{}.jsonl",
             std::process::id(),
             std::time::SystemTime::now()
                 .duration_since(std::time::UNIX_EPOCH)
                 .expect("system clock should be after epoch")
                 .as_nanos()
-        ));
-        let _ = fs::remove_file(&trace_path);
+        ))
+    }
 
-        let server = r#"
+    #[cfg(unix)]
+    fn mcp_proxy_trace_out_probe_server() -> String {
+        r#"
 while IFS= read -r line; do
   case "$line" in
     *'"method":"initialize"'*)
@@ -1071,14 +1052,79 @@ while IFS= read -r line; do
       ;;
   esac
 done
-"#;
-        let input = r#"
+"#
+        .to_string()
+    }
+
+    #[cfg(unix)]
+    fn mcp_proxy_trace_out_probe_input() -> &'static str {
+        r#"
 {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25"}}
 {"jsonrpc":"2.0","method":"notifications/initialized","params":{}}
 {"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}
-"#;
+"#
+    }
+
+    #[cfg(unix)]
+    #[derive(Default)]
+    struct FailOnSecondNewlineWriter {
+        bytes: Vec<u8>,
+        newline_count: usize,
+    }
+
+    #[cfg(unix)]
+    impl std::io::Write for FailOnSecondNewlineWriter {
+        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+            for byte in buf {
+                if *byte == b'\n' {
+                    self.newline_count += 1;
+                    if self.newline_count == 2 {
+                        return Err(std::io::Error::new(
+                            std::io::ErrorKind::BrokenPipe,
+                            "test writer failure after mediated event",
+                        ));
+                    }
+                }
+            }
+            self.bytes.extend_from_slice(buf);
+            Ok(buf.len())
+        }
+
+        fn flush(&mut self) -> std::io::Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn mcp_proxy_stdio_accepts_hyphen_prefixed_child_args() {
+        let cli = Cli::try_parse_from([
+            "agentk",
+            "mcp-proxy-stdio",
+            "--command",
+            "sh",
+            "--arg",
+            "-c",
+            "--arg",
+            "printf ok",
+        ])
+        .expect("hyphen-prefixed child args should parse");
+
+        let Some(Command::McpProxyStdio { args, .. }) = cli.command else {
+            panic!("expected mcp-proxy-stdio command");
+        };
+        assert_eq!(args, vec!["-c".to_string(), "printf ok".to_string()]);
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn mcp_proxy_stdio_trace_out_writes_verifiable_events() {
+        let trace_path = mcp_proxy_trace_out_test_path("trace");
+        let _ = fs::remove_file(&trace_path);
+
+        let server = mcp_proxy_trace_out_probe_server();
+        let input = mcp_proxy_trace_out_probe_input();
         let config = McpSubprocessProxyConfig::new("agent://test", "trace-out-probe", "sh")
-            .with_args(["-c".to_string(), server.to_string()]);
+            .with_args(["-c".to_string(), server]);
         let mut output = Vec::new();
 
         mcp_proxy_stdio_with_io(
@@ -1097,6 +1143,38 @@ done
         let _ = fs::remove_file(trace_path);
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn mcp_proxy_stdio_trace_out_survives_writer_failure_after_event() {
+        let trace_path = mcp_proxy_trace_out_test_path("writer-failure");
+        let _ = fs::remove_file(&trace_path);
+
+        let server = mcp_proxy_trace_out_probe_server();
+        let input = mcp_proxy_trace_out_probe_input();
+        let config = McpSubprocessProxyConfig::new("agent://test", "trace-out-probe", "sh")
+            .with_args(["-c".to_string(), server]);
+        let mut output = FailOnSecondNewlineWriter::default();
+
+        let error = mcp_proxy_stdio_with_io(
+            config,
+            Some(trace_path.clone()),
+            BufReader::new(input.as_bytes()),
+            &mut output,
+        )
+        .expect_err("client writer failure should surface");
+
+        assert!(
+            error
+                .to_string()
+                .contains("test writer failure after mediated event")
+        );
+        assert_eq!(output.newline_count, 2);
+        let verify = verify_jsonl(&trace_path).expect("trace-out should survive writer failure");
+        assert_eq!(verify.events_checked, 1);
+
+        let _ = fs::remove_file(trace_path);
+    }
+
     #[test]
     fn mcp_proxy_allow_env_collects_explicit_parent_values() {
         let names = vec!["AGENTK_PROXY_DEMO".to_string()];