diff --git a/crates/buzz-acp/src/lib.rs b/crates/buzz-acp/src/lib.rs
index ccca85e0ba..fbe6c2ba71 100644
--- a/crates/buzz-acp/src/lib.rs
+++ b/crates/buzz-acp/src/lib.rs
@@ -2488,7 +2488,10 @@ async fn tokio_main() -> Result<()> {
         return run_authenticate(args).await;
     }
 
+    // Stdout is the ACP transport when buzz-acp is launched as an agent command.
+    // Keep every harness diagnostic on stderr so logging can never corrupt NDJSON.
     tracing_subscriber::fmt()
+        .with_writer(std::io::stderr)
         .with_env_filter(
             EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("buzz_acp=info")),
         )
diff --git a/crates/buzz-acp/tests/stdio_contract.rs b/crates/buzz-acp/tests/stdio_contract.rs
new file mode 100644
index 0000000000..047607d574
--- /dev/null
+++ b/crates/buzz-acp/tests/stdio_contract.rs
@@ -0,0 +1,35 @@
+use std::process::Command;
+
+#[test]
+fn startup_and_failure_diagnostics_never_enter_acp_stdout() {
+    let output = Command::new(env!("CARGO_BIN_EXE_buzz-acp"))
+        .env("BUZZ_RELAY_URL", "ws://127.0.0.1:1")
+        .env(
+            "BUZZ_PRIVATE_KEY",
+            "0000000000000000000000000000000000000000000000000000000000000001",
+        )
+        .env("BUZZ_AGENT_COMMAND", "/definitely/missing/buzz-agent")
+        .env("RUST_LOG", "buzz_acp=info")
+        .output()
+        .expect("buzz-acp process should start");
+
+    assert!(
+        !output.status.success(),
+        "unreachable relay must fail startup"
+    );
+    assert!(
+        output.stdout.is_empty(),
+        "ACP stdout contained diagnostics: {:?}",
+        String::from_utf8_lossy(&output.stdout)
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("buzz-acp starting:"),
+        "missing startup INFO: {stderr}"
+    );
+    assert!(
+        stderr.contains("Error:") || stderr.contains("error"),
+        "missing failure diagnostic: {stderr}"
+    );
+}