diff --git a/etl-service/src/test/java/com/xtrmetl/etl/service/EtlServiceAmountIntegrityTest.java b/etl-service/src/test/java/com/xtrmetl/etl/service/EtlServiceAmountIntegrityTest.java
new file mode 100644
index 00000000..d905af5d
--- /dev/null
+++ b/etl-service/src/test/java/com/xtrmetl/etl/service/EtlServiceAmountIntegrityTest.java
@@ -0,0 +1,71 @@
+package com.xtrmetl.etl.service;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+import org.springframework.jdbc.core.JdbcTemplate;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+/**
+ * Proves that invalid monetary input is rejected rather than converted into a legitimate zero.
+ */
+class EtlServiceAmountIntegrityTest {
+
+    private JdbcTemplate jdbcTemplate;
+    private EtlService etlService;
+
+    @BeforeEach
+    void setUp() {
+        jdbcTemplate = mock(JdbcTemplate.class);
+        EtlBatchProperties properties = new EtlBatchProperties();
+        properties.setMaxPayloadBytes(65_536);
+        properties.setMaxBatchRecords(100);
+        etlService = new EtlService(jdbcTemplate, new ObjectMapper(), properties);
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {
+            "",
+            "not-a-number",
+            "123456789012345678901234567890123456789",
+            "0.0000000000000000001",
+            "1E+19"
+    })
+    void rejectsInvalidOrUnsupportedAmountsBeforeJdbc(String amount) {
+        EtlRequestException exception = assertThrows(
+                EtlRequestException.class,
+                () -> etlService.processData(jsonRecord("record_alpha", amount))
+        );
+
+        assertSame(EtlRequestError.INVALID_RECORD, exception.error());
+        verifyNoInteractions(jdbcTemplate);
+    }
+
+    @Test
+    void invalidAmountMakesTheWholeBatchFailBeforeAnyJdbcWrite() {
+        String payload = """
+                [
+                  {"id":"record_alpha","amount":"10.00"},
+                  {"id":"record_beta","amount":"not-a-number"}
+                ]
+                """;
+
+        EtlRequestException exception = assertThrows(
+                EtlRequestException.class,
+                () -> etlService.processData(payload)
+        );
+
+        assertSame(EtlRequestError.INVALID_RECORD, exception.error());
+        verifyNoInteractions(jdbcTemplate);
+    }
+
+    private static String jsonRecord(String id, String amount) {
+        return "[{\"id\":\"" + id + "\",\"amount\":\"" + amount + "\"}]";
+    }
+}