feat(api): document cache hits in OpenAPI

Co-Authored-By: Virgil <virgil@lethean.io>

openapi.go

@@ -129,7 +129,7 @@ func (sb *SpecBuilder) buildPaths(groups []RouteGroup) map[string]any {
 						"bearerAuth": []any{},
 					},
 				},
-				"responses": operationResponses(rd.Response),
+				"responses": operationResponses(method, rd.Response),
 			}
 
 			// Add request body for methods that accept one.
@@ -170,7 +170,12 @@ func (sb *SpecBuilder) buildPaths(groups []RouteGroup) map[string]any {
 // operationResponses builds the standard response set for a documented API
 // operation. The framework always exposes the common envelope responses, plus
 // middleware-driven 429 and 504 errors.
-func operationResponses(dataSchema map[string]any) map[string]any {
+func operationResponses(method string, dataSchema map[string]any) map[string]any {
+	successHeaders := mergeHeaders(standardResponseHeaders(), rateLimitSuccessHeaders())
+	if method == "get" {
+		successHeaders = mergeHeaders(successHeaders, cacheSuccessHeaders())
+	}
+
 	return map[string]any{
 		"200": map[string]any{
 			"description": "Successful response",
@@ -179,7 +184,7 @@ func operationResponses(dataSchema map[string]any) map[string]any {
 					"schema": envelopeSchema(dataSchema),
 				},
 			},
-			"headers": mergeHeaders(standardResponseHeaders(), rateLimitSuccessHeaders()),
+			"headers": successHeaders,
 		},
 		"400": map[string]any{
 			"description": "Bad request",
@@ -249,7 +254,7 @@ func healthResponses() map[string]any {
 					"schema": envelopeSchema(map[string]any{"type": "string"}),
 				},
 			},
-			"headers": mergeHeaders(standardResponseHeaders(), rateLimitSuccessHeaders()),
+			"headers": mergeHeaders(standardResponseHeaders(), rateLimitSuccessHeaders(), cacheSuccessHeaders()),
 		},
 		"429": map[string]any{
 			"description": "Too many requests",
@@ -402,6 +407,19 @@ func rateLimitSuccessHeaders() map[string]any {
 	}
 }
 
+// cacheSuccessHeaders documents the response header emitted on successful
+// cache hits.
+func cacheSuccessHeaders() map[string]any {
+	return map[string]any{
+		"X-Cache": map[string]any{
+			"description": "Indicates the response was served from the in-memory cache",
+			"schema": map[string]any{
+				"type": "string",
+			},
+		},
+	}
+}
+
 // standardResponseHeaders documents headers emitted by the response envelope
 // middleware on all responses when request IDs are enabled.
 func standardResponseHeaders() map[string]any {

openapi_test.go

@@ -102,6 +102,11 @@ func TestSpecBuilder_Good_EmptyGroups(t *testing.T) {
 	if _, ok := health504Headers["X-RateLimit-Reset"]; !ok {
 		t.Fatal("expected X-RateLimit-Reset header on /health 504 response")
 	}
+	health200 := health["responses"].(map[string]any)["200"].(map[string]any)
+	health200Headers := health200["headers"].(map[string]any)
+	if _, ok := health200Headers["X-Cache"]; !ok {
+		t.Fatal("expected X-Cache header on /health 200 response")
+	}
 
 	// Verify system tag exists.
 	tags := spec["tags"].([]any)
@@ -369,6 +374,9 @@ func TestSpecBuilder_Good_EnvelopeWrapping(t *testing.T) {
 	if _, ok := headers["X-RateLimit-Reset"]; !ok {
 		t.Fatal("expected X-RateLimit-Reset header on 200 response")
 	}
+	if _, ok := headers["X-Cache"]; !ok {
+		t.Fatal("expected X-Cache header on 200 response")
+	}
 	content := resp200["content"].(map[string]any)
 	appJSON := content["application/json"].(map[string]any)
 	schema := appJSON["schema"].(map[string]any)