diff --git a/cameleer3-server-app/src/test/java/com/cameleer3/server/app/storage/DiagramLinkingIT.java b/cameleer3-server-app/src/test/java/com/cameleer3/server/app/storage/DiagramLinkingIT.java
new file mode 100644
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--- /dev/null
+++ b/cameleer3-server-app/src/test/java/com/cameleer3/server/app/storage/DiagramLinkingIT.java
@@ -0,0 +1,152 @@
+package com.cameleer3.server.app.storage;
+
+import com.cameleer3.server.app.AbstractClickHouseIT;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.web.client.TestRestTemplate;
+import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
+
+import static java.util.concurrent.TimeUnit.SECONDS;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
+
+/**
+ * Integration test proving that diagram_content_hash is populated during
+ * execution ingestion when a RouteGraph exists for the same route+agent.
+ */
+class DiagramLinkingIT extends AbstractClickHouseIT {
+
+    @Autowired
+    private TestRestTemplate restTemplate;
+
+    @Test
+    void diagramHashPopulated_whenRouteGraphExistsBeforeExecution() {
+        // 1. Ingest a RouteGraph for route "diagram-link-route" via the diagrams endpoint
+        String graphJson = """
+                {
+                  "routeId": "diagram-link-route",
+                  "description": "Linking test diagram",
+                  "version": 1,
+                  "nodes": [
+                    {"id": "n1", "type": "ENDPOINT", "label": "direct:start"},
+                    {"id": "n2", "type": "BEAN", "label": "myBean"}
+                  ],
+                  "edges": [
+                    {"source": "n1", "target": "n2", "edgeType": "FLOW"}
+                  ],
+                  "processorNodeMapping": {}
+                }
+                """;
+
+        HttpHeaders headers = new HttpHeaders();
+        headers.setContentType(MediaType.APPLICATION_JSON);
+        headers.set("X-Cameleer-Protocol-Version", "1");
+
+        ResponseEntity<String> diagramResponse = restTemplate.postForEntity(
+                "/api/v1/data/diagrams",
+                new HttpEntity<>(graphJson, headers),
+                String.class);
+        assertThat(diagramResponse.getStatusCode()).isEqualTo(HttpStatus.ACCEPTED);
+
+        // 2. Wait for diagram to be flushed to ClickHouse before ingesting execution
+        await().atMost(10, SECONDS).untilAsserted(() -> {
+            String hash = jdbcTemplate.queryForObject(
+                    "SELECT content_hash FROM route_diagrams WHERE route_id = 'diagram-link-route' LIMIT 1",
+                    String.class);
+            assertThat(hash).isNotNull().isNotEmpty();
+        });
+
+        // 3. Ingest a RouteExecution for the same routeId
+        String executionJson = """
+                {
+                  "routeId": "diagram-link-route",
+                  "exchangeId": "ex-diag-link-1",
+                  "correlationId": "corr-diag-link-1",
+                  "status": "COMPLETED",
+                  "startTime": "2026-03-11T10:00:00Z",
+                  "endTime": "2026-03-11T10:00:01Z",
+                  "durationMs": 1000,
+                  "processors": [
+                    {
+                      "processorId": "proc-1",
+                      "processorType": "bean",
+                      "status": "COMPLETED",
+                      "startTime": "2026-03-11T10:00:00Z",
+                      "endTime": "2026-03-11T10:00:00.500Z",
+                      "durationMs": 500,
+                      "children": []
+                    }
+                  ]
+                }
+                """;
+
+        ResponseEntity<String> execResponse = restTemplate.postForEntity(
+                "/api/v1/data/executions",
+                new HttpEntity<>(executionJson, headers),
+                String.class);
+        assertThat(execResponse.getStatusCode()).isEqualTo(HttpStatus.ACCEPTED);
+
+        // 4. Verify diagram_content_hash is a non-empty SHA-256 hash (64 hex chars)
+        await().atMost(10, SECONDS).untilAsserted(() -> {
+            String hash = jdbcTemplate.queryForObject(
+                    "SELECT diagram_content_hash FROM route_executions WHERE route_id = 'diagram-link-route'",
+                    String.class);
+            assertThat(hash)
+                    .isNotNull()
+                    .isNotEmpty()
+                    .hasSize(64)  // SHA-256 hex = 64 characters
+                    .matches("[a-f0-9]{64}");
+        });
+    }
+
+    @Test
+    void diagramHashEmpty_whenNoRouteGraphExists() {
+        // Ingest a RouteExecution for a route with NO prior diagram
+        String executionJson = """
+                {
+                  "routeId": "no-diagram-route",
+                  "exchangeId": "ex-no-diag-1",
+                  "correlationId": "corr-no-diag-1",
+                  "status": "COMPLETED",
+                  "startTime": "2026-03-11T10:00:00Z",
+                  "endTime": "2026-03-11T10:00:01Z",
+                  "durationMs": 1000,
+                  "processors": [
+                    {
+                      "processorId": "proc-no-diag",
+                      "processorType": "log",
+                      "status": "COMPLETED",
+                      "startTime": "2026-03-11T10:00:00Z",
+                      "endTime": "2026-03-11T10:00:00.500Z",
+                      "durationMs": 500,
+                      "children": []
+                    }
+                  ]
+                }
+                """;
+
+        HttpHeaders headers = new HttpHeaders();
+        headers.setContentType(MediaType.APPLICATION_JSON);
+        headers.set("X-Cameleer-Protocol-Version", "1");
+
+        ResponseEntity<String> response = restTemplate.postForEntity(
+                "/api/v1/data/executions",
+                new HttpEntity<>(executionJson, headers),
+                String.class);
+        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.ACCEPTED);
+
+        // Verify diagram_content_hash is empty string (graceful fallback)
+        await().atMost(10, SECONDS).untilAsserted(() -> {
+            String hash = jdbcTemplate.queryForObject(
+                    "SELECT diagram_content_hash FROM route_executions WHERE route_id = 'no-diagram-route'",
+                    String.class);
+            assertThat(hash)
+                    .isNotNull()
+                    .isEmpty();
+        });
+    }
+}