fix(response): implement BoundedResponseBodyReader so body return patterns evaluate

CHANGELOG.md

@@ -3,6 +3,12 @@
 All notable changes to this project are documented in this file. The format follows
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
+## Unreleased
+
+### Fixed
+
+- Body based `return_pattern` behavior rules now evaluate: `KtorGuardResponse` implements `BoundedResponseBodyReader`, returning a bounded prefix of the materialized body (empty when a streaming body was left unread). Before, the engine could never read the response body in this adapter, so such rules never triggered.
+
 ## [1.1.0] - 2026-10-16
 
 ### Changed

src/main/kotlin/com/guardcore/ktor/KtorGuardResponse.kt

@@ -1,5 +1,6 @@
 package com.guardcore.ktor
 
+import com.guardcore.protocols.BoundedResponseBodyReader
 import com.guardcore.protocols.GuardResponse
 import com.guardcore.protocols.GuardResponseFactory
 import io.ktor.http.ContentType
@@ -18,7 +19,7 @@ import io.ktor.server.response.respondBytes
 public class KtorGuardResponse(
     override val statusCode: Int,
     bodyBytes: ByteArray,
-) : GuardResponse {
+) : GuardResponse, BoundedResponseBodyReader {
   /** Python `create_response` shape: string content. */
   public constructor(
       statusCode: Int,
@@ -36,6 +37,10 @@ public class KtorGuardResponse(
   override val body: ByteArray
     get() = currentBody
 
+  /** Python `read_body_prefix` (adapters.py:131): empty when a streaming body was left unread. */
+  override suspend fun readBodyPrefix(maxBytes: Int): ByteArray =
+      currentBody.copyOf(minOf(maxBytes, currentBody.size))
+
   /** The response-exit path materializes a live view over the outgoing content. */
   internal fun overrideBody(bytes: ByteArray) {
     currentBody = bytes

src/test/kotlin/com/guardcore/ktor/GuardReturnPatternTest.kt

@@ -0,0 +1,38 @@
+package com.guardcore.ktor
+
+import io.ktor.client.request.get
+import io.ktor.client.request.headers
+import io.ktor.http.HttpHeaders
+import io.ktor.http.HttpStatusCode
+import io.ktor.server.application.call
+import io.ktor.server.response.respondText
+import io.ktor.server.routing.get
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+/** Body based return_pattern rules read the response body through BoundedResponseBodyReader. */
+class GuardReturnPatternTest {
+  @Test
+  fun bodyReturnPatternBansTheClientOnceTheThresholdIsExceeded() {
+    val config = baseConfig { behaviorScanResponseBody = true }
+    val decorator = GuardRouteDecorator(config)
+    guardTest(
+        config,
+        decorator = decorator,
+        routes = {
+          get("/leaky") { call.respondText("here is a leaked-token") }
+              .guardReturnMonitor(decorator, pattern = "leaked-token", maxOccurrences = 1)
+        },
+    ) {
+      repeat(2) {
+        val response =
+            client.get("/leaky") { headers { append(HttpHeaders.XForwardedFor, TEST_CLIENT_IP) } }
+        assertEquals(HttpStatusCode.OK, response.status)
+      }
+      // The tracker trips when the count exceeds the threshold, so the second hit bans.
+      val banned =
+          client.get("/leaky") { headers { append(HttpHeaders.XForwardedFor, TEST_CLIENT_IP) } }
+      assertEquals(HttpStatusCode.Forbidden, banned.status)
+    }
+  }
+}