diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 00000000..e039f463
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2026-06-29 - Missing Global API Security Headers
+**Vulnerability:** API endpoints lacking basic security headers like `X-Content-Type-Options: nosniff`.
+**Learning:** Security header filters (e.g., `ViewerSecurityHeadersWebFilter.java`) are sometimes overly specific in their path matching, leaving other critical API endpoints completely unprotected.
+**Prevention:** Always ensure standard security headers (`X-Content-Type-Options`, `Cache-Control`) are applied globally to all responses, while reserving specific headers (like strict CSPs) for HTML-serving surfaces.
diff --git a/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java b/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java
index 798246b1..09a3f67e 100644
--- a/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java
+++ b/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java
@@ -35,19 +35,21 @@ public ViewerSecurityHeadersWebFilter(
     @Override
     public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
         String path = exchange.getRequest().getPath().value();
-        if (!isViewerSurface(path)) {
-            return chain.filter(exchange);
-        }
+        final boolean isViewer = isViewerSurface(path);
 
         exchange.getResponse().beforeCommit(() -> {
             HttpHeaders headers = exchange.getResponse().getHeaders();
 
-            // Avoid caching embedded preview surfaces.
+            // Avoid caching embedded preview surfaces and API responses.
             headers.set(HttpHeaders.CACHE_CONTROL, "no-store");
 
             headers.set("X-Content-Type-Options", "nosniff");
             headers.set("Referrer-Policy", "no-referrer");
 
+            if (!isViewer) {
+                return Mono.empty();
+            }
+
             // If the response is a redirect, do not attach an error-like CSP that could confuse debugging.
             if (exchange.getResponse().getStatusCode() == HttpStatus.FOUND) {
                 return Mono.empty();
@@ -75,7 +77,7 @@ public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
         });
 
         // Ensure CSP is also applied to HEAD checks for the viewer surface.
-        if (exchange.getRequest().getMethod() == HttpMethod.HEAD) {
+        if (isViewer && exchange.getRequest().getMethod() == HttpMethod.HEAD) {
             return chain.filter(exchange).then(Mono.empty());
         }
 
diff --git a/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java b/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java
index 0e62117e..7b850e23 100644
--- a/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java
+++ b/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java
@@ -195,7 +195,8 @@ void doesNotAddHeadersForNonViewerPaths() {
 
         HttpHeaders headers = exchange.getResponse().getHeaders();
         assertNull(headers.getFirst("Content-Security-Policy"));
-        assertNull(headers.getFirst("X-Content-Type-Options"));
-        assertNull(headers.getFirst("Referrer-Policy"));
+        assertEquals("nosniff", headers.getFirst("X-Content-Type-Options"));
+        assertEquals("no-store", headers.getFirst(HttpHeaders.CACHE_CONTROL));
+        assertEquals("no-referrer", headers.getFirst("Referrer-Policy"));
     }
 }