diff --git a/packages/devkit/tests/harness_integration.rs b/packages/devkit/tests/harness_integration.rs
new file mode 100644
index 0000000..5e90f1d
--- /dev/null
+++ b/packages/devkit/tests/harness_integration.rs
@@ -0,0 +1,66 @@
+use std::time::{Duration, Instant};
+
+use stellar_devkit::harness::horizon_mock::HorizonMock;
+
+/// Spin up mock server with normal.json. Assert response status 200 and fee_charged.mode = "100".
+#[test]
+fn normal_scenario_fee_stats_returns_mode_100() {
+    let path = std::path::Path::new("src/harness/scenarios/normal.json");
+    let mock = HorizonMock::new("normal").with_scenario_path(path);
+
+    let body = mock.fee_stats_payload().expect("fee_stats_payload failed");
+    assert!(!body.is_empty(), "response body must not be empty");
+
+    let json: serde_json::Value = serde_json::from_str(&body).expect("response is not valid JSON");
+    let mode = json["fee_stats"]["fee_charged"]["mode"]
+        .as_str()
+        .expect("fee_charged.mode missing");
+    assert_eq!(
+        mode, "100",
+        "expected fee_charged.mode == \"100\", got {}",
+        mode
+    );
+}
+
+/// Spin up with congested.json. Assert fee_charged.p95 parses to a value > 100000.
+#[test]
+fn congested_scenario_fee_stats_p95_exceeds_100k() {
+    let path = std::path::Path::new("src/harness/scenarios/congested.json");
+    let mock = HorizonMock::new("congested").with_scenario_path(path);
+
+    let body = mock.fee_stats_payload().expect("fee_stats_payload failed");
+    let json: serde_json::Value = serde_json::from_str(&body).expect("response is not valid JSON");
+
+    let p95: u64 = json["fee_stats"]["fee_charged"]["p95"]
+        .as_str()
+        .expect("fee_charged.p95 missing")
+        .parse()
+        .expect("fee_charged.p95 is not a number");
+    assert!(p95 > 100_000, "expected p95 > 100000, got {}", p95);
+}
+
+/// Set error_rate=1.0. Assert every request returns 503 (should_inject_error always true).
+#[test]
+fn error_injection_rate_1_always_injects() {
+    let mock = HorizonMock::new("normal").with_error_rate(1.0);
+    for _ in 0..20 {
+        assert!(
+            mock.should_inject_error(),
+            "expected should_inject_error() == true with error_rate=1.0"
+        );
+    }
+}
+
+/// Set delay_ms=200. Assert response time >= 200ms.
+#[test]
+fn response_delay_200ms_respected() {
+    let mock = HorizonMock::new("normal").with_delay_ms(200);
+    let start = Instant::now();
+    mock.apply_delay();
+    let elapsed = start.elapsed();
+    assert!(
+        elapsed >= Duration::from_millis(200),
+        "expected elapsed >= 200ms, got {:?}",
+        elapsed
+    );
+}