diff --git a/pkl-server/src/main/kotlin/org/pkl/server/Server.kt b/pkl-server/src/main/kotlin/org/pkl/server/Server.kt
index 7bc12f47..70dbb4d5 100644
--- a/pkl-server/src/main/kotlin/org/pkl/server/Server.kt
+++ b/pkl-server/src/main/kotlin/org/pkl/server/Server.kt
@@ -183,49 +183,54 @@ class Server(private val transport: MessageTransport) : AutoCloseable {
   }
 
   private fun createEvaluator(message: CreateEvaluatorRequest, evaluatorId: Long): BinaryEvaluator {
-    val modulePaths = message.modulePaths ?: emptyList()
-    val resolver = ModulePathResolver(modulePaths)
-    val allowedModules = message.allowedModules?.map { Pattern.compile(it) } ?: emptyList()
-    val allowedResources = message.allowedResources?.map { Pattern.compile(it) } ?: emptyList()
-    val rootDir = message.rootDir
-    val env = message.env ?: emptyMap()
-    val properties = message.properties ?: emptyMap()
-    val timeout = message.timeout
-    val cacheDir = message.cacheDir
-    val httpClient =
-      with(HttpClient.builder()) {
-        message.http?.proxy?.let { proxy ->
-          setProxy(proxy.address, proxy.noProxy ?: listOf())
-          proxy.address?.let(IoUtils::setSystemProxy)
-          proxy.noProxy?.let { System.setProperty("http.nonProxyHosts", it.joinToString("|")) }
+    try {
+      val modulePaths = message.modulePaths ?: emptyList()
+      val resolver = ModulePathResolver(modulePaths)
+      val allowedModules = message.allowedModules?.map { Pattern.compile(it) } ?: emptyList()
+      val allowedResources = message.allowedResources?.map { Pattern.compile(it) } ?: emptyList()
+      val rootDir = message.rootDir
+      val env = message.env ?: emptyMap()
+      val properties = message.properties ?: emptyMap()
+      val timeout = message.timeout
+      val cacheDir = message.cacheDir
+      val httpClient =
+        with(HttpClient.builder()) {
+          message.http?.proxy?.let { proxy ->
+            setProxy(proxy.address, proxy.noProxy ?: listOf())
+            proxy.address?.let(IoUtils::setSystemProxy)
+            proxy.noProxy?.let { System.setProperty("http.nonProxyHosts", it.joinToString("|")) }
+          }
+          message.http?.caCertificates?.let(::addCertificates)
+          message.http?.rewrites?.let(::setRewrites)
+          buildLazily()
         }
-        message.http?.caCertificates?.let(::addCertificates)
-        buildLazily()
-      }
-    val dependencies =
-      message.project?.let { proj ->
-        buildDeclaredDependencies(proj.projectFileUri, proj.dependencies, null)
-      }
-    log("Got dependencies: $dependencies")
-    return BinaryEvaluator(
-      StackFrameTransformers.defaultTransformer,
-      SecurityManagers.standard(
-        allowedModules,
-        allowedResources,
-        SecurityManagers.defaultTrustLevels,
-        rootDir,
-      ),
-      httpClient,
-      ClientLogger(evaluatorId, transport),
-      createModuleKeyFactories(message, evaluatorId, resolver),
-      createResourceReaders(message, evaluatorId, resolver),
-      env,
-      properties,
-      timeout,
-      cacheDir,
-      dependencies,
-      message.outputFormat,
-    )
+      val dependencies =
+        message.project?.let { proj ->
+          buildDeclaredDependencies(proj.projectFileUri, proj.dependencies, null)
+        }
+      log("Got dependencies: $dependencies")
+      return BinaryEvaluator(
+        StackFrameTransformers.defaultTransformer,
+        SecurityManagers.standard(
+          allowedModules,
+          allowedResources,
+          SecurityManagers.defaultTrustLevels,
+          rootDir,
+        ),
+        httpClient,
+        ClientLogger(evaluatorId, transport),
+        createModuleKeyFactories(message, evaluatorId, resolver),
+        createResourceReaders(message, evaluatorId, resolver),
+        env,
+        properties,
+        timeout,
+        cacheDir,
+        dependencies,
+        message.outputFormat,
+      )
+    } catch (e: IllegalArgumentException) {
+      throw ProtocolException(e.message ?: "Failed to create an evalutor. $e", e)
+    }
   }
 
   private fun createResourceReaders(
diff --git a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt
index 48d1d7f7..51462d3d 100644
--- a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt
+++ b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt
@@ -99,8 +99,8 @@ class ServerMessagePackDecoder(unpacker: MessageUnpacker) : BaseMessagePackDecod
       getNullable(httpMap, "rewrites")
         ?.asMapValue()
         ?.map()
-        ?.mapKeys { it.key.asStringValue().asString() }
-        ?.mapValues { it.value.asStringValue().asString() }
+        ?.mapKeys { URI(it.key.asStringValue().asString()) }
+        ?.mapValues { URI(it.value.asStringValue().asString()) }
     return Http(caCertificates, proxy, rewrites)
   }
 
diff --git a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackEncoder.kt b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackEncoder.kt
index 25cc63b6..29658130 100644
--- a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackEncoder.kt
+++ b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackEncoder.kt
@@ -48,8 +48,8 @@ class ServerMessagePackEncoder(packer: MessagePacker) : BaseMessagePackEncoder(p
       packString("rewrites")
       packMapHeader(rewrites.size)
       for ((key, value) in rewrites) {
-        packString(key)
-        packString(value)
+        packString(key.toString())
+        packString(value.toString())
       }
     }
   }
diff --git a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessages.kt b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessages.kt
index 0da36bb0..139585c4 100644
--- a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessages.kt
+++ b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessages.kt
@@ -57,7 +57,7 @@ data class Http(
   /** Proxy settings */
   val proxy: Proxy?,
   /** HTTP rewrites */
-  val rewrites: Map<String, String>?,
+  val rewrites: Map<URI, URI>?,
 ) {
   override fun equals(other: Any?): Boolean {
     if (this === other) return true
diff --git a/pkl-server/src/test/kotlin/org/pkl/server/AbstractServerTest.kt b/pkl-server/src/test/kotlin/org/pkl/server/AbstractServerTest.kt
index 21a5c320..1a815ecf 100644
--- a/pkl-server/src/test/kotlin/org/pkl/server/AbstractServerTest.kt
+++ b/pkl-server/src/test/kotlin/org/pkl/server/AbstractServerTest.kt
@@ -60,6 +60,26 @@ abstract class AbstractServerTest {
 
   abstract val client: TestTransport
 
+  private val blankCreateEvaluatorRequest =
+    CreateEvaluatorRequest(
+      requestId = 1,
+      http = null,
+      allowedModules = null,
+      allowedResources = null,
+      clientModuleReaders = null,
+      clientResourceReaders = null,
+      modulePaths = null,
+      env = null,
+      properties = null,
+      timeout = null,
+      rootDir = null,
+      cacheDir = null,
+      outputFormat = null,
+      project = null,
+      externalModuleReaders = null,
+      externalResourceReaders = null,
+    )
+
   @Test
   fun `create and close evaluator`() {
     val evaluatorId = client.sendCreateEvaluatorRequest(123)
@@ -931,6 +951,50 @@ abstract class AbstractServerTest {
       )
   }
 
+  @Test
+  fun `http rewrites`() {
+    val evaluatorId =
+      client.sendCreateEvaluatorRequest(
+        http =
+          Http(
+            caCertificates = null,
+            proxy = null,
+            rewrites = mapOf(URI("https://example.com/") to URI("https://example.example/")),
+          )
+      )
+    client.send(
+      EvaluateRequest(
+        1,
+        evaluatorId,
+        URI("repl:text"),
+        "res = import(\"https://example.com/foo.pkl\")",
+        "output.text",
+      )
+    )
+    val response = client.receive<EvaluateResponse>()
+    assertThat(response.error)
+      .contains(
+        "request was rewritten: https://example.com/foo.pkl -> https://example.example/foo.pkl"
+      )
+  }
+
+  @Test
+  fun `http rewrites -- invalid rule`() {
+    client.send(
+      blankCreateEvaluatorRequest.copy(
+        http =
+          Http(
+            caCertificates = null,
+            proxy = null,
+            rewrites = mapOf(URI("https://example.com") to URI("https://example.example/")),
+          )
+      )
+    )
+    val response = client.receive<CreateEvaluatorResponse>()
+    assertThat(response.error)
+      .contains("Rewrite rule must end with '/', but was 'https://example.com'")
+  }
+
   private val ByteArray.debugYaml
     get() = MessagePackDebugRenderer(this).output.trimIndent()
 
diff --git a/pkl-server/src/test/kotlin/org/pkl/server/ServerMessagePackCodecTest.kt b/pkl-server/src/test/kotlin/org/pkl/server/ServerMessagePackCodecTest.kt
index 20a4daf7..f0de03bd 100644
--- a/pkl-server/src/test/kotlin/org/pkl/server/ServerMessagePackCodecTest.kt
+++ b/pkl-server/src/test/kotlin/org/pkl/server/ServerMessagePackCodecTest.kt
@@ -96,7 +96,7 @@ class ServerMessagePackCodecTest {
           Http(
             proxy = Proxy(URI("http://foo.com:1234"), listOf("bar", "baz")),
             caCertificates = byteArrayOf(1, 2, 3, 4),
-            rewrites = mapOf("https://foo.com" to "https://bar.com"),
+            rewrites = mapOf(URI("https://foo.com/") to URI("https://bar.com/")),
           ),
         externalModuleReaders = mapOf("external" to externalReader, "external2" to externalReader),
         externalResourceReaders = mapOf("external" to externalReader),