http-client-java, warning on not spread json-merge-patch payload

.chronus/changes/http-client-java_warning-on-not-spread-json-merge-patch-payload-2026-1-28-11-33-56.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/http-client-java"
+---
+
+Add a warning when emitter does not spread for json-merge-patch payload.

packages/http-client-java/emitter/src/code-model-builder.ts

@@ -2077,6 +2077,11 @@ export class CodeModelBuilder {
 
         if (jsonMergePatch) {
           // skip model flatten, if "application/merge-patch+json"
+          reportDiagnostic(this.program, {
+            code: "spread-json-merge-patch-payload-not-supported",
+            target: sdkMethod.__raw ?? NoTarget,
+          });
+
           if (sdkType.isGeneratedName) {
             schema.language.default.name = pascalCase(op.language.default.name) + "PatchRequest";
           }

packages/http-client-java/emitter/src/lib.ts

@@ -119,6 +119,13 @@ export const $lib = createTypeSpecLibrary({
         default: paramMessage`Constant header '${"headerName"}' is removed from response headers.`,
       },
     },
+    "spread-json-merge-patch-payload-not-supported": {
+      severity: "warning",
+      messages: {
+        default:
+          "Spread JSON merge-patch payload is not supported. The reason is that a property in JSON merge-patch payload class can: set a value; not set so that value does not change; set to null to remove the value. A parameter on method cannot distinguish the latter 2 cases.",
+      },
+    },
   },
   emitter: {
     options: EmitterOptionsSchema,