diff --git a/crates/repx-runner/tests/harness.rs b/crates/repx-runner/tests/harness.rs
index a342daf..6425c96 100644
--- a/crates/repx-runner/tests/harness.rs
+++ b/crates/repx-runner/tests/harness.rs
@@ -3,6 +3,9 @@
 use assert_cmd::Command as AssertCommand;
 use repx_test_utils::harness::TestContext;
 use std::ops::{Deref, DerefMut};
+use std::path::PathBuf;
+use std::process::Command;
+use std::sync::OnceLock;
 
 pub struct TestHarness {
     pub context: TestContext,
@@ -27,10 +30,9 @@ impl TestHarness {
         }
     }
 
-    #[allow(clippy::expect_used, deprecated)]
+    #[allow(clippy::expect_used)]
     pub fn cmd(&self) -> AssertCommand {
-        let mut cmd = AssertCommand::cargo_bin("repx")
-            .expect("repx binary not found — ensure repx-cli is built");
+        let mut cmd = AssertCommand::new(repx_binary_path());
         cmd.env("XDG_CONFIG_HOME", &self.context.config_dir);
         cmd.env("RUST_BACKTRACE", "1");
         cmd.arg("--lab").arg(&self.context.lab_path);
@@ -40,6 +42,34 @@ impl TestHarness {
     }
 }
 
+#[allow(clippy::expect_used)]
+fn repx_binary_path() -> PathBuf {
+    static BINARY: OnceLock<PathBuf> = OnceLock::new();
+    BINARY
+        .get_or_init(|| {
+            let cargo = std::env::var_os("CARGO").unwrap_or_else(|| "cargo".into());
+            let status = Command::new(&cargo)
+                .args(["build", "-p", "repx-cli", "--bin", "repx"])
+                .status()
+                .expect("failed to invoke `cargo build -p repx-cli`");
+            assert!(status.success(), "`cargo build -p repx-cli` failed");
+
+            let test_exe = std::env::current_exe().expect("current_exe");
+            let target_profile = test_exe
+                .parent()
+                .and_then(|p| p.parent())
+                .expect("test exe has no grandparent");
+            let bin = target_profile.join(format!("repx{}", std::env::consts::EXE_SUFFIX));
+            assert!(
+                bin.is_file(),
+                "repx binary missing after build: {}",
+                bin.display()
+            );
+            bin
+        })
+        .clone()
+}
+
 impl Default for TestHarness {
     fn default() -> Self {
         Self::new()