diff --git a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/BuildCommand.java b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/BuildCommand.java
index 3fbd352bfeb..e40ae9d50d6 100644
--- a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/BuildCommand.java
+++ b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/BuildCommand.java
@@ -124,7 +124,7 @@ public void execute(Arguments arguments, ClassLoader classLoader) {
     }
 
     private ValidatedResult<Model> buildModel(ClassLoader classLoader, List<String> models, Arguments arguments) {
-        ModelAssembler assembler = Model.assembler(classLoader);
+        ModelAssembler assembler = CommandUtils.createModelAssembler(classLoader);
         CommandUtils.handleModelDiscovery(arguments, assembler, classLoader);
         CommandUtils.handleUnknownTraitsOption(arguments, assembler);
         models.forEach(assembler::addImport);
diff --git a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/CommandUtils.java b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/CommandUtils.java
index 3e83d1b7955..1449edf2046 100644
--- a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/CommandUtils.java
+++ b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/CommandUtils.java
@@ -25,6 +25,7 @@
 import software.amazon.smithy.cli.Arguments;
 import software.amazon.smithy.cli.CliError;
 import software.amazon.smithy.cli.SmithyCli;
+import software.amazon.smithy.model.Model;
 import software.amazon.smithy.model.loader.ModelAssembler;
 
 final class CommandUtils {
@@ -39,6 +40,10 @@ static void handleUnknownTraitsOption(Arguments arguments, ModelAssembler assemb
         }
     }
 
+    static ModelAssembler createModelAssembler(ClassLoader classLoader) {
+        return Model.assembler(classLoader).putProperty(ModelAssembler.DISABLE_JAR_CACHE, true);
+    }
+
     static void handleModelDiscovery(Arguments arguments, ModelAssembler assembler, ClassLoader baseLoader) {
         if (arguments.has(SmithyCli.DISCOVER_CLASSPATH)) {
             discoverModelsWithClasspath(arguments, assembler);
diff --git a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/DiffCommand.java b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/DiffCommand.java
index bbc6cf0e7b0..7acb37d3ae0 100644
--- a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/DiffCommand.java
+++ b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/DiffCommand.java
@@ -58,7 +58,7 @@ public void execute(Arguments arguments, ClassLoader classLoader) {
         List<String> newModels = arguments.repeatedParameter("--new");
         LOGGER.info(String.format("Setting 'new' Smithy models: %s", String.join(" ", newModels)));
 
-        ModelAssembler assembler = Model.assembler(classLoader);
+        ModelAssembler assembler = CommandUtils.createModelAssembler(classLoader);
         Model oldModel = loadModel("old", assembler, oldModels);
         assembler.reset();
         Model newModel = loadModel("new", assembler, newModels);
diff --git a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/ValidateCommand.java b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/ValidateCommand.java
index e314947032b..6bda03135d7 100644
--- a/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/ValidateCommand.java
+++ b/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/ValidateCommand.java
@@ -54,7 +54,7 @@ public void execute(Arguments arguments, ClassLoader classLoader) {
         List<String> models = arguments.positionalArguments();
         LOGGER.info(String.format("Validating Smithy model sources: %s", models));
 
-        ModelAssembler assembler = Model.assembler(classLoader);
+        ModelAssembler assembler = CommandUtils.createModelAssembler(classLoader);
         CommandUtils.handleModelDiscovery(arguments, assembler, classLoader);
         CommandUtils.handleUnknownTraitsOption(arguments, assembler);
 
diff --git a/smithy-model/src/main/java/software/amazon/smithy/model/loader/JarModelLoader.java b/smithy-model/src/main/java/software/amazon/smithy/model/loader/JarModelLoader.java
index 85322dd9e55..35035e77911 100644
--- a/smithy-model/src/main/java/software/amazon/smithy/model/loader/JarModelLoader.java
+++ b/smithy-model/src/main/java/software/amazon/smithy/model/loader/JarModelLoader.java
@@ -18,6 +18,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.URL;
+import java.net.URLConnection;
 import java.util.function.Supplier;
 import java.util.logging.Logger;
 import software.amazon.smithy.utils.IoUtils;
@@ -45,9 +46,17 @@ public boolean load(String filename, Supplier<String> contentSupplier, LoaderVis
         LOGGER.fine(() -> "Loading Smithy model imports from JAR: " + manifestUrl);
 
         for (URL model : ModelDiscovery.findModels(manifestUrl)) {
-            try (InputStream is = model.openStream()) {
-                String contents = IoUtils.toUtf8String(is);
-                delegate.load(model.toExternalForm(), () -> contents, visitor);
+            try {
+                URLConnection connection = model.openConnection();
+
+                if (visitor.hasProperty(ModelAssembler.DISABLE_JAR_CACHE)) {
+                    connection.setUseCaches(false);
+                }
+
+                try (InputStream is = connection.getInputStream()) {
+                    String contents = IoUtils.toUtf8String(is);
+                    delegate.load(model.toExternalForm(), () -> contents, visitor);
+                }
             } catch (IOException e) {
                 throw new ModelImportException(
                         String.format("Error loading Smithy model from URL `%s`: %s", model, e.getMessage()), e);
diff --git a/smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java b/smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java
index ce36a5f197d..665c31fa713 100644
--- a/smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java
+++ b/smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java
@@ -19,6 +19,7 @@
 import java.io.InputStream;
 import java.io.UncheckedIOException;
 import java.net.URL;
+import java.net.URLConnection;
 import java.nio.file.FileVisitOption;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -61,7 +62,22 @@
  * @see Model#assembler()
  */
 public final class ModelAssembler {
+    /**
+     * Allow unknown traits rather than fail.
+     */
     public static final String ALLOW_UNKNOWN_TRAITS = "assembler.allowUnknownTraits";
+
+    /**
+     * Sets {@link URLConnection#setUseCaches} to false.
+     *
+     * <p>When running in a build environment, using caches can cause exceptions
+     * like `java.util.zip.ZipException: ZipFile invalid LOC header (bad signature)`
+     * because a previously loaded JAR might change between builds. The
+     * "assembler.disableJarCache" setting should be set to true when embedding
+     * Smithy into an environment where this can occur.
+     */
+    public static final String DISABLE_JAR_CACHE = "assembler.disableJarCache";
+
     private static final Logger LOGGER = Logger.getLogger(ModelAssembler.class.getName());
     private static final ModelLoader DEFAULT_LOADER = ModelLoader.createDefaultLoader();
 
@@ -126,7 +142,6 @@ public ModelAssembler copy() {
      *     <li>Models registered via {@link #addModel}</li>
      *     <li>Shape registered via {@link #addModel}</li>
      *     <li>Metadata registered via {@link #putMetadata}</li>
-     *     <li>Custom properties set via {@link #putProperty}</li>
      * </ul>
      *
      * <p>The state of {@link #disablePrelude} is reset such that the prelude
@@ -142,7 +157,6 @@ public ModelAssembler reset() {
         validators.clear();
         suppressions.clear();
         documentNodes.clear();
-        properties.clear();
         disablePrelude = false;
         return this;
     }