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 6e17e8df7e7..54d0471f5bb 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
@@ -26,6 +26,7 @@
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
+import java.util.function.Supplier;
 import software.amazon.smithy.build.FileManifest;
 import software.amazon.smithy.build.ProjectionResult;
 import software.amazon.smithy.build.SmithyBuild;
@@ -40,6 +41,7 @@
 import software.amazon.smithy.cli.Style;
 import software.amazon.smithy.cli.dependencies.DependencyResolver;
 import software.amazon.smithy.model.Model;
+import software.amazon.smithy.model.loader.ModelAssembler;
 import software.amazon.smithy.model.validation.Severity;
 
 final class BuildCommand extends ClasspathCommand {
@@ -102,7 +104,14 @@ int runWithClassLoader(SmithyBuildConfig config, Arguments arguments, Env env, L
         // Configure whether the build is quiet or not based on the --quiet option.
         Model model = CommandUtils.buildModel(arguments, models, env, env.stderr(), standardOptions.quiet(), config);
 
-        SmithyBuild smithyBuild = SmithyBuild.create(classLoader)
+        Supplier<ModelAssembler> modelAssemblerSupplier = () -> {
+            ModelAssembler assembler = Model.assembler(classLoader);
+            if (buildOptions.allowUnknownTraits()) {
+                assembler.putProperty(ModelAssembler.ALLOW_UNKNOWN_TRAITS, true);
+            }
+            return assembler;
+        };
+        SmithyBuild smithyBuild = SmithyBuild.create(classLoader, modelAssemblerSupplier)
                 .config(config)
                 .model(model);
 
diff --git a/smithy-cli/src/test/java/software/amazon/smithy/cli/commands/BuildCommandTest.java b/smithy-cli/src/test/java/software/amazon/smithy/cli/commands/BuildCommandTest.java
index 2efcc3e77d8..6946c3ae4bb 100644
--- a/smithy-cli/src/test/java/software/amazon/smithy/cli/commands/BuildCommandTest.java
+++ b/smithy-cli/src/test/java/software/amazon/smithy/cli/commands/BuildCommandTest.java
@@ -52,6 +52,16 @@ public void dumpsOutValidationErrorsAndFails() throws Exception {
         assertThat(result.stderr(), containsString("FAILURE"));
     }
 
+    @Test
+    public void allowsUnknownTraitWithFlag() throws Exception {
+        String model = Paths.get(getClass().getResource("unknown-trait.smithy").toURI()).toString();
+        CliUtils.Result result = CliUtils.runSmithy("build", "--allow-unknown-traits", model);
+
+        assertThat(result.code(), equalTo(0));
+        assertThat(result.stderr(), containsString("Completed projection source"));
+        assertThat(result.stderr(), containsString("Smithy built "));
+    }
+
     @Test
     public void printsSuccessfulProjections() throws Exception {
         String model = Paths.get(getClass().getResource("valid-model.smithy").toURI()).toString();
@@ -74,6 +84,26 @@ public void validationFailuresCausedByProjectionsAreDetected() throws Exception
                    containsString("The following 1 Smithy build projection(s) failed: [exampleProjection]"));
     }
 
+    @Test
+    public void projectionUnknownTraitsAreDisallowed() throws Exception {
+        String config = Paths.get(getClass().getResource("projection-model-import.json").toURI()).toString();
+        CliUtils.Result result = CliUtils.runSmithy("build", "--config", config);
+
+        assertThat(result.code(), not(0));
+        assertThat(result.stderr(), containsString("Unable to resolve trait `some.unknown#trait`"));
+        assertThat(result.stderr(), containsString("Smithy build projection(s) failed: [exampleProjection]\n"));
+    }
+
+    @Test
+    public void projectionUnknownTraitsAreAllowedWithFlag() throws Exception {
+        String config = Paths.get(getClass().getResource("projection-model-import.json").toURI()).toString();
+        CliUtils.Result result = CliUtils.runSmithy("build", "--allow-unknown-traits",  "--config", config);
+
+        assertThat(result.code(), equalTo(0));
+        assertThat(result.stderr(), containsString("Completed projection exampleProjection"));
+        assertThat(result.stderr(), containsString("Smithy built "));
+    }
+
     @Test
     public void exceptionsThrownByProjectionsAreDetected() {
         // TODO: need to make a plugin throw an exception
diff --git a/smithy-cli/src/test/resources/software/amazon/smithy/cli/commands/projection-model-import.json b/smithy-cli/src/test/resources/software/amazon/smithy/cli/commands/projection-model-import.json
new file mode 100644
index 00000000000..dd5ce49f170
--- /dev/null
+++ b/smithy-cli/src/test/resources/software/amazon/smithy/cli/commands/projection-model-import.json
@@ -0,0 +1,8 @@
+{
+  "version": "1.0",
+  "projections": {
+    "exampleProjection": {
+      "imports": ["unknown-trait.smithy"]
+    }
+  }
+}