diff --git a/scripts/setup-loopback.sh b/scripts/setup-loopback.sh
index 7870e89..60fdd80 100755
--- a/scripts/setup-loopback.sh
+++ b/scripts/setup-loopback.sh
@@ -41,6 +41,9 @@ if [ "$COUNT" -lt 1 ] || [ "$COUNT" -gt "$MAX" ]; then
 fi
 
 END=$((START + COUNT - 1))
+# Announce the range up front — also a tell when two checkouts run different
+# package versions with different pool conventions (0.3.x started at 127.0.0.1).
+echo "Configuring loopback addresses 127.0.0.$START-127.0.0.$END on lo0..."
 for i in $(seq $START $END); do
   # Use a host (/32) netmask, not the implicit class-A /8. Without an explicit netmask,
   # macOS gives each 127.0.0.x alias a 255.0.0.0 mask, so every alias claims to own the
@@ -58,4 +61,23 @@ for i in $(seq $START $END); do
   $SUDO ifconfig lo0 alias 127.0.0.$i netmask 255.255.255.255 up
 done
 
-echo "✓ Configured $COUNT loopback addresses (127.0.0.$START-127.0.0.$END)"
\ No newline at end of file
+# Verify every alias actually landed before claiming success. A partial run
+# (expired sudo credential mid-loop, interruption) previously still printed ✓,
+# leaving the test runner failing later on the missing addresses.
+MISSING=()
+LO0_STATE=$(ifconfig lo0)
+for i in $(seq $START $END); do
+  if ! echo "$LO0_STATE" | grep -q "inet 127\.0\.0\.$i "; then
+    MISSING+=("127.0.0.$i")
+  fi
+done
+
+if [ ${#MISSING[@]} -gt 0 ]; then
+  echo "✗ ${#MISSING[@]} loopback address(es) failed to configure. Fix with:"
+  for addr in "${MISSING[@]}"; do
+    echo "  sudo ifconfig lo0 alias $addr netmask 255.255.255.255 up"
+  done
+  exit 1
+fi
+
+echo "✓ Configured and verified $COUNT loopback addresses (127.0.0.$START-127.0.0.$END)"
\ No newline at end of file
diff --git a/src/harperLifecycle.ts b/src/harperLifecycle.ts
index 0fe4ed2..026ee03 100644
--- a/src/harperLifecycle.ts
+++ b/src/harperLifecycle.ts
@@ -222,7 +222,7 @@ function getHarperScript(harperBinPath?: string): string {
 	// 1. Explicit option
 	if (harperBinPath) {
 		ok(existsSync(harperBinPath), `Harper script not found at provided harperBinPath: ${harperBinPath}`);
-		return harperBinPath;
+		return logResolvedHarperScript(harperBinPath, 'harperBinPath option');
 	}
 
 	// 2. Environment variable
@@ -232,7 +232,7 @@ function getHarperScript(harperBinPath?: string): string {
 			existsSync(envPath),
 			`Harper script not found at HARPER_INTEGRATION_TEST_INSTALL_SCRIPT path: ${envPath}`
 		);
-		return envPath;
+		return logResolvedHarperScript(envPath, 'HARPER_INTEGRATION_TEST_INSTALL_SCRIPT');
 	}
 
 	// 3. Auto-resolve from node_modules
@@ -240,7 +240,7 @@ function getHarperScript(harperBinPath?: string): string {
 		const require = createRequire(import.meta.url);
 		const resolved = require.resolve('harper/dist/bin/harper.js');
 		if (existsSync(resolved)) {
-			return resolved;
+			return logResolvedHarperScript(resolved, 'node_modules');
 		}
 	} catch {
 		// harper package not found in node_modules
@@ -251,7 +251,7 @@ function getHarperScript(harperBinPath?: string): string {
 	while (true) {
 		const potentialPath = join(currentDir, 'dist/bin/harper.js');
 		if (existsSync(potentialPath)) {
-			return potentialPath;
+			return logResolvedHarperScript(potentialPath, 'ancestor dist');
 		}
 		const parentDir = dirname(currentDir);
 		if (parentDir === currentDir) {
@@ -269,6 +269,42 @@ function getHarperScript(harperBinPath?: string): string {
 	);
 }
 
+const loggedHarperScripts = new Set<string>();
+
+/**
+ * Logs which Harper binary was resolved and how, once per distinct path.
+ *
+ * Which binary runs is the single most consequential (and previously invisible)
+ * decision the harness makes: resolving a stale `harper` package from
+ * node_modules while the caller expected their local build produces confusing
+ * wholesale test failures with no hint of the cause. When the `node_modules`
+ * path wins while `./dist/bin/harper.js` also exists, warn with the override.
+ */
+function logResolvedHarperScript(scriptPath: string, source: string): string {
+	if (loggedHarperScripts.has(scriptPath)) return scriptPath;
+	loggedHarperScripts.add(scriptPath);
+	console.log(`[integration-testing] Using Harper at ${scriptPath} (via ${source})`);
+	if (source === 'node_modules') {
+		// Mirror resolution step 4's ancestor walk so the warning fires even when
+		// tests run from a subdirectory of the repo that holds the local build.
+		let currentDir = process.cwd();
+		while (true) {
+			const localDist = join(currentDir, 'dist/bin/harper.js');
+			if (existsSync(localDist) && localDist !== scriptPath) {
+				console.warn(
+					`[integration-testing] Warning: resolved the 'harper' package from node_modules, but ${localDist} also exists. ` +
+						`If you meant to test the local build, set HARPER_INTEGRATION_TEST_INSTALL_SCRIPT=${localDist}.`
+				);
+				break;
+			}
+			const parentDir = dirname(currentDir);
+			if (parentDir === currentDir) break;
+			currentDir = parentDir;
+		}
+	}
+	return scriptPath;
+}
+
 /**
  * Strips ANSI escape sequences (colors, bold, underline, cursor movement, etc.) from a string.
  */