diff --git a/.chronus/changes/copilot-fix-xml-test-case-error-2026-2-11-20-7-16.md b/.chronus/changes/copilot-fix-xml-test-case-error-2026-2-11-20-7-16.md
new file mode 100644
index 00000000000..37a44acbc51
--- /dev/null
+++ b/.chronus/changes/copilot-fix-xml-test-case-error-2026-2-11-20-7-16.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-specs"
+---
+
+Accept both `2022-08-26T18:38:00.000Z` and `2022-08-26T18:38:00Z` as valid RFC3339 UTC datetime forms in the `ModelWithDatetime` XML scenario.
\ No newline at end of file
diff --git a/.chronus/changes/copilot-fix-xml-test-case-error-2026-2-11-20-8-54.md b/.chronus/changes/copilot-fix-xml-test-case-error-2026-2-11-20-8-54.md
new file mode 100644
index 00000000000..4446f699c26
--- /dev/null
+++ b/.chronus/changes/copilot-fix-xml-test-case-error-2026-2-11-20-8-54.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/spec-api"
+---
+
+Remove prettier used for ValidationError message, in validateXmlBodyEquals.
\ No newline at end of file
diff --git a/packages/http-specs/specs/payload/xml/mockapi.ts b/packages/http-specs/specs/payload/xml/mockapi.ts
index 1560c5e84ba..de2d3f85834 100644
--- a/packages/http-specs/specs/payload/xml/mockapi.ts
+++ b/packages/http-specs/specs/payload/xml/mockapi.ts
@@ -136,6 +136,16 @@ export const modelWithDatetime = `
 </ModelWithDatetime>
 `;
 
+// Some clients serialize UTC datetimes without trailing zero milliseconds. Both
+// "2022-08-26T18:38:00.000Z" and "2022-08-26T18:38:00Z" are valid RFC3339 representations
+// of the same instant; accept either form.
+const modelWithDatetimeNoMs = `
+<ModelWithDatetime>
+  <rfc3339>2022-08-26T18:38:00Z</rfc3339>
+  <rfc7231>Fri, 26 Aug 2022 14:38:00 GMT</rfc7231>
+</ModelWithDatetime>
+`;
+
 function createServerTests(uri: string, data?: any) {
   return {
     get: passOnSuccess({
@@ -251,12 +261,44 @@ const Payload_Xml_ModelWithEnum = createServerTests("/payload/xml/modelWithEnum"
 Scenarios.Payload_Xml_ModelWithEnumValue_get = Payload_Xml_ModelWithEnum.get;
 Scenarios.Payload_Xml_ModelWithEnumValue_put = Payload_Xml_ModelWithEnum.put;
 
-const Payload_Xml_ModelWithDatetime = createServerTests(
-  "/payload/xml/modelWithDatetime",
-  modelWithDatetime,
-);
-Scenarios.Payload_Xml_ModelWithDatetimeValue_get = Payload_Xml_ModelWithDatetime.get;
-Scenarios.Payload_Xml_ModelWithDatetimeValue_put = Payload_Xml_ModelWithDatetime.put;
+Scenarios.Payload_Xml_ModelWithDatetimeValue_get = passOnSuccess({
+  uri: "/payload/xml/modelWithDatetime",
+  method: "get",
+  request: {},
+  response: {
+    status: 200,
+    body: xml(modelWithDatetime),
+  },
+  kind: "MockApiDefinition",
+});
+
+Scenarios.Payload_Xml_ModelWithDatetimeValue_put = passOnSuccess({
+  uri: "/payload/xml/modelWithDatetime",
+  method: "put",
+  request: {
+    body: xml(modelWithDatetime),
+  },
+  handler: (req: MockRequest) => {
+    req.expect.containsHeader("content-type", "application/xml");
+    // Accept both "2022-08-26T18:38:00.000Z" and "2022-08-26T18:38:00Z" as equivalent UTC datetimes.
+    let firstError: unknown;
+    try {
+      req.expect.xmlBodyEquals(modelWithDatetime);
+    } catch (e) {
+      firstError = e;
+    }
+    if (firstError !== undefined) {
+      req.expect.xmlBodyEquals(modelWithDatetimeNoMs);
+    }
+    return {
+      status: 204,
+    };
+  },
+  response: {
+    status: 204,
+  },
+  kind: "MockApiDefinition",
+});
 
 export const xmlError = `
 <XmlErrorBody>
diff --git a/packages/spec-api/src/request-validations.ts b/packages/spec-api/src/request-validations.ts
index 223b439ce2a..3dc7f7260ab 100644
--- a/packages/spec-api/src/request-validations.ts
+++ b/packages/spec-api/src/request-validations.ts
@@ -1,5 +1,4 @@
 import deepEqual from "deep-equal";
-import * as prettier from "prettier";
 import { parseString } from "xml2js";
 import { CollectionFormat, RequestExt } from "./types.js";
 import { ValidationError } from "./validation-error.js";
@@ -66,11 +65,7 @@ export const validateXmlBodyEquals = (request: RequestExt, expectedBody: string)
   });
 
   if (!deepEqual(actualParsedBody, expectedParsedBody, { strict: true })) {
-    throw new ValidationError(
-      BODY_NOT_EQUAL_ERROR_MESSAGE,
-      prettier.format(expectedBody),
-      prettier.format(request.body),
-    );
+    throw new ValidationError(BODY_NOT_EQUAL_ERROR_MESSAGE, expectedBody, request.rawBody);
   }
 };