diff --git a/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnostics.java b/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnostics.java
index 0d2220462..b94d9c9b4 100644
--- a/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnostics.java
+++ b/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnostics.java
@@ -39,12 +39,16 @@ final class QueryDiagnostics {
         int statementDepth;
         MethodTiming timing(String method) { return methods.computeIfAbsent(method, MethodTiming::new); }
         String json(String endpoint, String requestId, int status, boolean failed) {
+            return json(endpoint, requestId, status, failed, "REQUEST_COMPLETE");
+        }
+        String json(String endpoint, String requestId, int status, boolean failed, String capturePoint) {
             long total = System.nanoTime() - started;
             long executor = methods.values().stream().mapToLong(t -> t.executor.nanos).sum();
             Map<String,Object> report = new LinkedHashMap<>();
             report.put("event", "SELFOP_QUERY_DIAGNOSTICS"); report.put("endpoint", endpoint);
             report.put("requestId", requestId != null && requestId.matches("[A-Za-z0-9._:-]{1,128}") ? requestId : null);
             report.put("httpStatus", status); report.put("escapedException", failed);
+            report.put("capturePoint", capturePoint);
             report.put("requestNanos", total); report.put("unattributedNanos", Math.max(0, total-executor));
             report.put("timingMeaning", "CLIENT_EXECUTOR_AND_STATEMENT_INCLUDING_MAPPING_NOT_SERVER_DB_TIME");
             report.put("poolWaitAttribution", "NOT_MEASURED_PER_REQUEST"); report.put("methods", methods);
diff --git a/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsResponseAdvice.java b/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsResponseAdvice.java
new file mode 100644
index 000000000..e02af0926
--- /dev/null
+++ b/lastleg-service/src/main/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsResponseAdvice.java
@@ -0,0 +1,35 @@
+package com.baiyue.lastleg.selfoperated.analytics.query.diagnostics;
+
+import com.baiyue.common.web.CorrelationContext;
+import org.slf4j.MDC;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
+import org.springframework.core.MethodParameter;
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.server.*;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;
+import java.util.Map;
+
+/** Opt-in diagnostic snapshot before serialization; never changes the response body. */
+@RestControllerAdvice
+@ConditionalOnExpression("'${DEPLOY_ENV:}' == 'test' && ${baiyue.self-operated-analytics.query-diagnostics.enabled:false}")
+public final class QueryDiagnosticsResponseAdvice implements ResponseBodyAdvice<Object> {
+    public static final String HEADER = "X-Selfop-Query-Diagnostics";
+    private static final String BASE = "/api/platform/v1/last-leg/self-operated-analytics/";
+    private static final Map<String,String> ENDPOINTS = Map.of(BASE+"dashboard", "DASHBOARD",
+        BASE+"sku-sales-summary", "SUMMARY", BASE+"inventory-coverage", "INVENTORY");
+    @Override public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
+        return QueryDiagnostics.CURRENT.get() != null;
+    }
+    @Override public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType contentType,
+        Class<? extends HttpMessageConverter<?>> converterType, ServerHttpRequest request, ServerHttpResponse response) {
+        var state = QueryDiagnostics.CURRENT.get();
+        String endpoint = ENDPOINTS.get(request.getURI().getPath());
+        if (state != null && endpoint != null && request.getMethod() == org.springframework.http.HttpMethod.GET) {
+            int status = response instanceof ServletServerHttpResponse servlet ? servlet.getServletResponse().getStatus() : 200;
+            response.getHeaders().set(HEADER, state.json(endpoint, MDC.get(CorrelationContext.REQUEST_ID), status, false, "BEFORE_BODY_WRITE"));
+        }
+        return body;
+    }
+}
diff --git a/lastleg-service/src/test/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsTest.java b/lastleg-service/src/test/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsTest.java
index 1e93fa2ca..8abdfe2c6 100644
--- a/lastleg-service/src/test/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsTest.java
+++ b/lastleg-service/src/test/java/com/baiyue/lastleg/selfoperated/analytics/query/diagnostics/QueryDiagnosticsTest.java
@@ -25,15 +25,47 @@ class QueryDiagnosticsTest {
     @AfterEach void cleanup() { QueryDiagnostics.CURRENT.remove(); MDC.clear(); }
 
     @Test void onlyExplicitTestAndEnabledCreatesBeans() {
-        var runner=new ApplicationContextRunner().withUserConfiguration(QueryDiagnosticsConfiguration.class);
-        runner.run(c->assertEquals(0,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size()));
+        var runner=new ApplicationContextRunner().withUserConfiguration(QueryDiagnosticsConfiguration.class,QueryDiagnosticsResponseAdvice.class);
+        runner.run(c->{assertEquals(0,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size());assertEquals(0,c.getBeansOfType(QueryDiagnosticsResponseAdvice.class).size());});
         runner.withPropertyValues("DEPLOY_ENV=test").run(c->assertEquals(0,c.getBeansOfType(FilterRegistrationBean.class).size()));
         runner.withPropertyValues("DEPLOY_ENV=production","baiyue.self-operated-analytics.query-diagnostics.enabled=true")
-            .run(c->assertEquals(0,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size()));
+            .run(c->{assertEquals(0,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size());assertEquals(0,c.getBeansOfType(QueryDiagnosticsResponseAdvice.class).size());});
         runner.withPropertyValues("baiyue.self-operated-analytics.query-diagnostics.enabled=true")
             .run(c->assertEquals(0,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size()));
         runner.withPropertyValues("DEPLOY_ENV=test","baiyue.self-operated-analytics.query-diagnostics.enabled=true")
-            .run(c->{assertEquals(1,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size());assertEquals(1,c.getBeansOfType(FilterRegistrationBean.class).size());});
+            .run(c->{assertEquals(1,c.getBeansOfType(QueryDiagnosticsInterceptor.class).size());assertEquals(1,c.getBeansOfType(FilterRegistrationBean.class).size());assertEquals(1,c.getBeansOfType(QueryDiagnosticsResponseAdvice.class).size());});
+    }
+
+    @Test void responseHeaderRequiresActiveStateAndPreservesBodyAndCapturePoint() throws Exception {
+        var advice=new QueryDiagnosticsResponseAdvice();
+        Object body=Map.of("sku","PRIVATE_SKU","token","PRIVATE_TOKEN");
+        var servletRequest=new MockHttpServletRequest("GET",BASE+"dashboard");
+        servletRequest.setQueryString("sourceAccount=PRIVATE_ACCOUNT");
+        var request=new org.springframework.http.server.ServletServerHttpRequest(servletRequest);
+        var response=new org.springframework.http.server.ServletServerHttpResponse(new MockHttpServletResponse());
+        assertFalse(advice.supports(null,null));
+        assertSame(body,advice.beforeBodyWrite(body,null,null,null,request,response));
+        assertNull(response.getHeaders().getFirst(QueryDiagnosticsResponseAdvice.HEADER));
+        var state=new QueryDiagnostics.State();QueryDiagnostics.CURRENT.set(state);MDC.put("requestId","synthetic-header-1");
+        state.timing("DashboardReadMapper.report").executor.add(123,false);
+        assertTrue(advice.supports(null,null));
+        assertSame(body,advice.beforeBodyWrite(body,null,null,null,request,response));
+        String header=response.getHeaders().getFirst(QueryDiagnosticsResponseAdvice.HEADER);
+        assertNotNull(header);assertFalse(header.contains("PRIVATE"));assertFalse(header.contains("\n"));
+        var json=new ObjectMapper().readTree(header);
+        assertEquals("BEFORE_BODY_WRITE",json.path("capturePoint").asText());
+        assertEquals("DASHBOARD",json.path("endpoint").asText());
+        assertEquals("synthetic-header-1",json.path("requestId").asText());
+        assertEquals(1,json.path("methods").path("DashboardReadMapper.report").path("executor").path("calls").asInt());
+        assertEquals("REQUEST_COMPLETE",new ObjectMapper().readTree(state.json("DASHBOARD","synthetic",200,false)).path("capturePoint").asText());
+        for(String path:List.of("other","dashboard/extra")){
+            var skipped=new org.springframework.http.server.ServletServerHttpResponse(new MockHttpServletResponse());
+            assertSame(body,advice.beforeBodyWrite(body,null,null,null,new org.springframework.http.server.ServletServerHttpRequest(new MockHttpServletRequest("GET",BASE+path)),skipped));
+            assertNull(skipped.getHeaders().getFirst(QueryDiagnosticsResponseAdvice.HEADER));
+        }
+        var post=new org.springframework.http.server.ServletServerHttpResponse(new MockHttpServletResponse());
+        advice.beforeBodyWrite(body,null,null,null,new org.springframework.http.server.ServletServerHttpRequest(new MockHttpServletRequest("POST",BASE+"dashboard")),post);
+        assertNull(post.getHeaders().getFirst(QueryDiagnosticsResponseAdvice.HEADER));
     }
 
     @Test void pathsAndMethodsAreExactAndExceptionsCleanRequestState() throws Exception {