Use appendedHeaders for response-type-error: one entry per error

Agent-Logs-Url: https://github.com/counterfact/api-simulator/sessions/d4b157a0-4b1d-434f-baa8-f709fcf80a05

Author: Copilot

src/server/dispatcher.ts

@@ -391,10 +391,13 @@ export class Dispatcher {
       if (!validation.valid) {
         return {
           ...normalizedResponse,
-          headers: {
-            ...normalizedResponse.headers,
-            "response-type-error": validation.errors,
-          },
+          appendedHeaders: [
+            ...(normalizedResponse.appendedHeaders ?? []),
+            ...validation.errors.map((error): [string, string] => [
+              "response-type-error",
+              error,
+            ]),
+          ],
         };
       }
     }

src/server/koa-middleware.ts

@@ -148,16 +148,20 @@ export function koaMiddleware(
       for (const [key, value] of Object.entries(response.headers)) {
         if (!HEADERS_TO_DROP.has(key.toLowerCase())) {
           if (Array.isArray(value)) {
-            for (const item of value) {
-              ctx.res.appendHeader(key, item);
-            }
+            ctx.set(key, value);
           } else {
             ctx.set(key, value.toString());
           }
         }
       }
     }
 
+    if (response.appendedHeaders) {
+      for (const [key, value] of response.appendedHeaders) {
+        ctx.res.appendHeader(key, value);
+      }
+    }
+
     ctx.status = response.status ?? HTTP_STATUS_CODE_OK;
 
     return undefined;

src/server/registry.ts

@@ -75,6 +75,7 @@ interface Module {
 }
 
 type CounterfactResponseObject = {
+  appendedHeaders?: [string, string][];
   body?: Uint8Array | string;
   content?: {
     body: unknown;

test/server/dispatcher.test.ts

@@ -1423,10 +1423,11 @@ describe("given a request that contains the differently cased path", () => {
         req: { path: "/widgets" },
       });
 
-      const errors = response.headers?.["response-type-error"];
-      const firstError = Array.isArray(errors) ? errors[0] : errors;
+      const errorValues = response.appendedHeaders
+        ?.filter(([key]) => key === "response-type-error")
+        .map(([, value]) => value);
 
-      expect(firstError).toContain("x-required-header");
+      expect(errorValues?.[0]).toContain("x-required-header");
     });
 
     it("adds response-type-error headers when a response header has the wrong type", async () => {
@@ -1444,10 +1445,11 @@ describe("given a request that contains the differently cased path", () => {
         req: { path: "/widgets" },
       });
 
-      const errors = response.headers?.["response-type-error"];
-      const firstError = Array.isArray(errors) ? errors[0] : errors;
+      const errorValues = response.appendedHeaders
+        ?.filter(([key]) => key === "response-type-error")
+        .map(([, value]) => value);
 
-      expect(firstError).toContain("x-count");
+      expect(errorValues?.[0]).toContain("x-count");
     });
 
     it("does not add error headers when all required response headers are present and valid", async () => {
@@ -1465,7 +1467,11 @@ describe("given a request that contains the differently cased path", () => {
         req: { path: "/widgets" },
       });
 
-      expect(response.headers?.["response-type-error"]).toBeUndefined();
+      expect(
+        response.appendedHeaders?.find(
+          ([key]) => key === "response-type-error",
+        ),
+      ).toBeUndefined();
     });
 
     it("skips response validation when validateResponses is false", async () => {
@@ -1480,7 +1486,11 @@ describe("given a request that contains the differently cased path", () => {
         req: { path: "/widgets" },
       });
 
-      expect(response.headers?.["response-type-error"]).toBeUndefined();
+      expect(
+        response.appendedHeaders?.find(
+          ([key]) => key === "response-type-error",
+        ),
+      ).toBeUndefined();
     });
   });
 });