[tcgc] fix path in override (#4644)

fixes #4257

---------

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

Author: 3 people

.chronus/changes/tcgc-fixPathInOverride-2026-5-16-15-19-10.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Verify all `@path` parameters are present in override

packages/typespec-client-generator-core/src/decorators.ts

@@ -32,6 +32,7 @@ import {
   getServers,
   isBody,
   isBodyRoot,
+  isPathParam,
 } from "@typespec/http";
 import { getVersion, resolveVersions, type Version } from "@typespec/versioning";
 import {
@@ -749,6 +750,24 @@ export const $override = (
       }
     }
 
+    // Warn if original param has @path but override param doesn't,
+    // unless any override param has @clientLocation (indicating an intentional customization
+    // where non-path params are just pass-throughs)
+    if (
+      isPathParam(context.program, originalParam) &&
+      !isPathParam(context.program, overrideParams[index]) &&
+      !overrideParams.some((p) => p.decorators.some((d) => d.decorator.name === "$clientLocation"))
+    ) {
+      reportDiagnostic(context.program, {
+        code: "override-parameters-mismatch",
+        target: context.decoratorTarget,
+        format: {
+          methodName: original.name,
+          checkParameter: overrideParams[index].name,
+        },
+      });
+    }
+
     // Apply the alternate type to the original parameter
     const overrideParam = overrideParams[index];
     overrideParam.decorators

packages/typespec-client-generator-core/src/http.ts

@@ -417,6 +417,7 @@ function getSdkHttpParameters(
       (segment) => segment[segment.length - 1],
     );
   }
+
   return diagnostics.wrap(retval);
 }
 

packages/typespec-client-generator-core/test/decorators/override.test.ts

@@ -547,6 +547,30 @@ it("remove optional query param and add secret name", async () => {
   strictEqual(maxResultsParam.name, "maxresults");
 });
 
+it("reports diagnostic when override parameter is missing @path", async () => {
+  const [_, diagnostics] = await SimpleBaseTester.compileAndDiagnose(
+    createClientCustomizationInput(
+      `
+    @service
+    namespace MyService;
+
+    @route("/items/{itemId}")
+    op getItem(@path itemId: string): void;
+    `,
+      `
+    namespace MyCustomizations;
+
+    op getItemOverride(itemId: string): void;
+
+    @@override(MyService.getItem, MyCustomizations.getItemOverride);
+    `,
+    ),
+  );
+  expectDiagnostics(diagnostics, {
+    code: "@azure-tools/typespec-client-generator-core/override-parameters-mismatch",
+  });
+});
+
 describe("@clientName", () => {
   it("original method", async () => {
     const { program } = await SimpleBaseTester.compile(