diff --git a/.chronus/changes/witemple-msft-hsjs-hyperparam-optionality-2025-11-10-11-43-24.md b/.chronus/changes/witemple-msft-hsjs-hyperparam-optionality-2025-11-10-11-43-24.md
new file mode 100644
index 00000000000..0fab7340042
--- /dev/null
+++ b/.chronus/changes/witemple-msft-hsjs-hyperparam-optionality-2025-11-10-11-43-24.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-js"
+---
+
+Fixed a bug that caused optional query/header parameters to be improperly converted to primitive types when not provided in a request.
\ No newline at end of file
diff --git a/packages/http-server-js/src/http/server/index.ts b/packages/http-server-js/src/http/server/index.ts
index 76717b5d215..a54351ccc7b 100644
--- a/packages/http-server-js/src/http/server/index.ts
+++ b/packages/http-server-js/src/http/server/index.ts
@@ -425,7 +425,11 @@ function* emitRawServerOperation(
 
         const encoder = jsScalar.http[httpOperationParam.type];
 
-        paramBaseExpression = encoder.decode(paramNameSafe);
+        const decoded = encoder.decode(paramNameSafe);
+
+        paramBaseExpression = param.optional
+          ? `${paramNameSafe} === undefined ? undefined : (${decoded})`
+          : decoded;
       } else {
         paramBaseExpression = paramNameSafe;
       }