diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9f2381a..def3f97 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 ### Removed
 ### Fixed
 - Restore bound actions/functions on containment navigation paths (regression since v1.2.0)
+- Function parameters annotated with `@mandatory` no longer lead to "Unexpected mandatory after optional parameter" error
 ### Security
 
 ## [1.4.2] - 2026-05-18
diff --git a/lib/compile/index.js b/lib/compile/index.js
index d321158..1bfb40f 100644
--- a/lib/compile/index.js
+++ b/lib/compile/index.js
@@ -39,6 +39,8 @@ function processor(csn, options = {}) {
   , ...options
 }
 
+  _applyMandatoryToFunctionParams(csn);
+
   // must not be part of function* otherwise thrown errors are swallowed
   const csdl = cds.compile.to.edm(csn, edmOptions);
   let openApiDocs = {};
@@ -229,6 +231,24 @@ function _readConfigFile(configFilePath) {
   return result;
 }
 
+/**
+ * The cds-compiler's edmPreprocessor determines function parameter optionality
+ * from the type's nullability (p.notNull), ignoring the CDS `@mandatory` annotation.
+ * A nullable-typed parameter annotated @mandatory` is incorrectly treated as optional,
+ * which then triggers an odata-parameter-order error if a non-nullable parameter follows.
+ *
+ * Fix: honour `@mandatory` by setting notNull on the CSN param before EDM compilation.
+ */
+function _applyMandatoryToFunctionParams(csn) {
+  const functions = Object.values(csn.definitions ?? {})
+    .filter(def => def.kind === 'function');
+  for (const f of functions) {
+    for (const p of Object.values(f.params ?? {})) {
+      p.notNull ??= p['@mandatory'] === true;
+    }
+  }
+}
+
 // we're attaching the events to the main function so they become automatically exposed through the cds facade:
 // cds.compile.to.openapi(...)
 // cds.compile.to.openapi.events.before
diff --git a/test/lib/compile/openapi.test.js b/test/lib/compile/openapi.test.js
index 812ec23..79e5374 100644
--- a/test/lib/compile/openapi.test.js
+++ b/test/lib/compile/openapi.test.js
@@ -684,4 +684,22 @@ service CatalogService {
       cds.removeListener(events.after, handler);
     }
   });
+
+  test('@mandatory on function parameters with nullable types compiles without error', () => {
+    const csn = cds.compile.to.csn(`
+      service ReproService {
+        function getMapping(
+          @mandatory paramA: String,
+          @mandatory paramB: String not null
+        ) returns String;
+      }
+    `);
+    const openAPI = toOpenApi(csn);
+    const params = openAPI.paths['/getMapping'].get.parameters;
+    assert.strictEqual(params.length, 2);
+    assert.strictEqual(params[0].name, 'paramA');
+    assert.strictEqual(params[0].required, true);
+    assert.strictEqual(params[1].name, 'paramB');
+    assert.strictEqual(params[1].required, true);
+  });
 });