Fix body parameter accessor to use methodParameterSegments (#3961)

* Fix body parameter accessor to use methodParameterSegments

When a @bodyRoot property is nested inside a wrapper model (e.g., as a named
property of an anonymous model parameter in ARM templates), the generated
serializer call was using the bare property name (stopParameters) instead of
the correct accessor path (body.stopParameters).

The fix replaces the direct bodyParameter.name lookup with getParamAccessor(),
which resolves the accessor expression from methodParameterSegments — consistent
with how header and query parameters already work.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Update

* update changelog

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Jiao Di (MSFT) <v-jiaodi@microsoft.com>

Author: 3 people

packages/autorest.typescript/CHANGELOG.md

@@ -1,5 +1,6 @@
-## 6.0.70 (2026-05-13)
+## 6.0.70 (2026-05-14)
 
+- [Bugfix] Fix body parameter accessor to use methodParameterSegments. Please refer to [#3961](https://github.com/Azure/autorest.typescript/pull/3961)
 - [Feature] Bump TypeSpec dependencies to latest stable. Please refer to [#3959](https://github.com/Azure/autorest.typescript/pull/3959)
 - [Bugfix] Deduplicate statusCodes in getExpectedStatuses. Please refer to [#3956](https://github.com/Azure/autorest.typescript/pull/3956)
 - [Feature] Align Azure monorepo package.json metadata with new repository schema across codegen packages. Please refer to [#3950](https://github.com/Azure/autorest.typescript/pull/3950)

packages/rlc-common/CHANGELOG.md

@@ -1,5 +1,6 @@
-## 0.53.0 (2026-05-13)
+## 0.53.0 (2026-05-14)
 
+- [Bugfix] Fix body parameter accessor to use methodParameterSegments. Please refer to [#3961](https://github.com/Azure/autorest.typescript/pull/3961)
 - [Feature] Bump TypeSpec dependencies to latest stable. Please refer to [#3959](https://github.com/Azure/autorest.typescript/pull/3959)
 - [Bugfix] Deduplicate statusCodes in getExpectedStatuses. Please refer to [#3956](https://github.com/Azure/autorest.typescript/pull/3956)
 - [Feature] Align Azure monorepo package.json metadata with new repository schema across codegen packages. Please refer to [#3950](https://github.com/Azure/autorest.typescript/pull/3950)

packages/typespec-test/test/batch_modular/generated/typespec-ts/src/api/operations.ts

@@ -1030,9 +1030,7 @@ export function _disableNodeSchedulingSend(
           : {}),
         ...options.requestOptions?.headers,
       },
-      body: !options["body"]
-        ? options["body"]
-        : nodeDisableSchedulingOptionsSerializer(options["body"]),
+      body: !options?.body ? options?.body : nodeDisableSchedulingOptionsSerializer(options?.body),
     });
 }
 
@@ -1099,7 +1097,7 @@ export function _reimageNodeSend(
           : {}),
         ...options.requestOptions?.headers,
       },
-      body: !options["body"] ? options["body"] : nodeReimageOptionsSerializer(options["body"]),
+      body: !options?.body ? options?.body : nodeReimageOptionsSerializer(options?.body),
     });
 }
 
@@ -1165,7 +1163,7 @@ export function _rebootNodeSend(
           : {}),
         ...options.requestOptions?.headers,
       },
-      body: !options["body"] ? options["body"] : nodeRebootOptionsSerializer(options["body"]),
+      body: !options?.body ? options?.body : nodeRebootOptionsSerializer(options?.body),
     });
 }
 
@@ -4001,9 +3999,7 @@ export function _terminateJobSend(
           : {}),
         ...options.requestOptions?.headers,
       },
-      body: !options["body"]
-        ? options["body"]
-        : batchJobTerminateOptionsSerializer(options["body"]),
+      body: !options?.body ? options?.body : batchJobTerminateOptionsSerializer(options?.body),
     });
 }
 

packages/typespec-test/test/loadtesting_modular/generated/typespec-ts/src/loadTestRun/api/operations.ts

@@ -187,7 +187,7 @@ export function _listMetricsSend(
       ...operationOptionsToRequestParameters(options),
       contentType: "application/json",
       headers: { accept: "application/json", ...options.requestOptions?.headers },
-      body: !options["body"] ? options["body"] : metricRequestPayloadSerializer(options["body"]),
+      body: !options?.body ? options?.body : metricRequestPayloadSerializer(options?.body),
     });
 }
 

packages/typespec-ts/CHANGELOG.md

@@ -1,5 +1,6 @@
-## 0.53.0 (2026-05-13)
+## 0.53.0 (2026-05-14)
 
+- [Bugfix] Fix body parameter accessor to use methodParameterSegments. Please refer to [#3961](https://github.com/Azure/autorest.typescript/pull/3961)
 - [Feature] Bump TypeSpec dependencies to latest stable. Please refer to [#3959](https://github.com/Azure/autorest.typescript/pull/3959)
 - [Bugfix] Deduplicate statusCodes in getExpectedStatuses. Please refer to [#3956](https://github.com/Azure/autorest.typescript/pull/3956)
 - [Feature] Align Azure monorepo package.json metadata with new repository schema across codegen packages. Please refer to [#3950](https://github.com/Azure/autorest.typescript/pull/3950)

packages/typespec-ts/src/modular/helpers/operationHelpers.ts

@@ -1702,14 +1702,7 @@ function buildBodyParameter(
     }) as string | undefined;
   }
 
-  const bodyParamName = normalizeName(
-    bodyParameter.name,
-    NameType.Parameter,
-    true
-  );
-  let bodyNameExpression = bodyParameter.optional
-    ? `${optionalParamName}["${bodyParamName}"]`
-    : bodyParamName;
+  let bodyNameExpression = getParamAccessor(bodyParameter, optionalParamName);
 
   // Check if body parameter has a client default value with matching type
   const hasClientDefault =
@@ -1723,15 +1716,17 @@ function buildBodyParameter(
     bodyNameExpression = `(${bodyNameExpression} ?? ${formattedDefault})`;
   }
 
-  // Only apply nullOrUndefinedPrefix if there's no client default value
-  // because the default value already handles null/undefined cases
-  const nullOrUndefinedPrefix = hasClientDefault
-    ? ""
-    : getPropertySerializationPrefix(
-        context,
-        bodyParameter,
-        bodyParameter.optional ? optionalParamName : undefined
-      );
+  // Build null guard using bodyNameExpression so it stays consistent with the
+  // accessor path (especially for nested body parameters like options?.body?.x).
+  // Skip when there's a client default value since it already handles null/undefined.
+  const needsNullGuard =
+    !hasClientDefault &&
+    (bodyParameter.optional ||
+      isTypeNullable(bodyParameter.type) ||
+      bodyNameExpression.includes("?."));
+  const nullOrUndefinedPrefix = needsNullGuard
+    ? `!${bodyNameExpression}? ${bodyNameExpression}:`
+    : "";
 
   // For dual-format operations, check the contentType option at runtime
   if (
@@ -2112,6 +2107,9 @@ function getParamAccessor(
   param: SdkHttpParameter,
   optionalParamName: string = "options"
 ): string {
+  if (param.isGeneratedName) {
+    return param.name;
+  }
   const methodParamExpr = getMethodParamExpr(param, optionalParamName);
   const clientPrefix = "context.";
   if (methodParamExpr) {
@@ -2141,6 +2139,11 @@ function getMethodParamExpr(
   if (segments.length === 0) {
     return undefined;
   }
+  // When there are multiple paths (e.g., a composite body from multiple method
+  // params), we cannot resolve a single accessor — fall back to the caller's logic.
+  if (segments.length > 1) {
+    return undefined;
+  }
   const path = segments[0];
   if (!path || path.length < 1) {
     return undefined;