diff --git a/pkg/server/quickproxy/quickproxy.go b/pkg/server/quickproxy/quickproxy.go
index 7593a03..036258a 100644
--- a/pkg/server/quickproxy/quickproxy.go
+++ b/pkg/server/quickproxy/quickproxy.go
@@ -28,9 +28,11 @@ type Rule struct {
 	Target string
 
 	// Exclude is a list of URL path prefixes that this rule should not
-	// proxy, even though they fall under URLPrefix. Each entry must start
-	// with "/". A request matching an Exclude prefix is treated as if this
-	// rule didn't match at all: matching continues against any other
+	// proxy, even though they fall under URLPrefix. Each entry is a full
+	// path from root and must itself start with URLPrefix (e.g. rule
+	// URLPrefix "/api" excluding a subpath must use "/api/health", not
+	// "/health"). A request matching an Exclude prefix is treated as if
+	// this rule didn't match at all: matching continues against any other
 	// configured rule, falling back if none match. This is typically used
 	// to carve out paths (e.g. "/health") from a catch-all "/" rule so
 	// they're served by the fallback handler instead of being proxied.
@@ -122,6 +124,9 @@ func NewService(rules []Rule, opts ...Option) (*Service, error) {
 			if !strings.HasPrefix(ex, "/") {
 				return nil, fmt.Errorf("quickproxy: exclude prefix %q for rule %q must start with /", ex, r.URLPrefix)
 			}
+			if !strings.HasPrefix(ex, r.URLPrefix) {
+				return nil, fmt.Errorf("quickproxy: exclude prefix %q for rule %q must itself start with the rule's URLPrefix", ex, r.URLPrefix)
+			}
 		}
 
 		compiled = append(compiled, compiledRule{
diff --git a/pkg/server/quickproxy/quickproxy_test.go b/pkg/server/quickproxy/quickproxy_test.go
index fc329de..7a4ced7 100644
--- a/pkg/server/quickproxy/quickproxy_test.go
+++ b/pkg/server/quickproxy/quickproxy_test.go
@@ -26,10 +26,16 @@ func TestNewService_Validation(t *testing.T) {
 		{"bad exclude prefix", []Rule{
 			{URLPrefix: "/", Target: "http://localhost:1", Exclude: []string{"health"}},
 		}, true},
+		{"exclude outside rule's URLPrefix", []Rule{
+			{URLPrefix: "/api", Target: "http://localhost:1", Exclude: []string{"/health"}},
+		}, true},
 		{"valid", []Rule{{URLPrefix: "/api", Target: "http://localhost:1"}}, false},
 		{"valid with exclude", []Rule{
 			{URLPrefix: "/", Target: "http://localhost:1", Exclude: []string{"/health"}},
 		}, false},
+		{"valid with nested exclude", []Rule{
+			{URLPrefix: "/api", Target: "http://localhost:1", Exclude: []string{"/api/health"}},
+		}, false},
 	}
 
 	for _, tt := range tests {