diff --git a/.changeset/localecode-preserve-casing.md b/.changeset/localecode-preserve-casing.md
new file mode 100644
index 000000000..fbe64671c
--- /dev/null
+++ b/.changeset/localecode-preserve-casing.md
@@ -0,0 +1,5 @@
+---
+"emdash": patch
+---
+
+Fixes the content and search APIs returning zero results when filtering by a locale with an uppercase region or script subtag (e.g. `?locale=zh-TW`, `pt-BR`, `zh-Hant`). The `localeCode` validator lowercased the value, but config locales, the stored `locale` column, and the public query path all preserve the original casing, so the filter matched nothing. Validation stays case-insensitive; the value is now preserved verbatim. The taxonomy and menu endpoints' shared `?locale=` filter now applies the same BCP-47 validation, so a malformed locale is rejected with a clear error instead of silently matching nothing. Closes #1551.
diff --git a/packages/core/src/api/schemas/common.ts b/packages/core/src/api/schemas/common.ts
index 292655381..4f623e912 100644
--- a/packages/core/src/api/schemas/common.ts
+++ b/packages/core/src/api/schemas/common.ts
@@ -53,16 +53,18 @@ export const httpUrl = z
 	.url()
 	.refine((url) => HTTP_SCHEME_RE.test(url), "URL must use http or https");
 
-/** BCP 47 locale code — language with optional script/region subtags (e.g. en, en-US, pt-BR, es-419, zh-Hant) */
-export const localeCode = z
-	.string()
-	.regex(/^[a-z]{2,3}(-[a-z0-9]{2,8})*$/i, "Invalid locale code")
-	.transform((v) => v.toLowerCase());
+/**
+ * BCP 47 locale code — language with optional script/region subtags (e.g. en, en-US, pt-BR, es-419, zh-Hant).
+ * Validation is case-insensitive, but the value is preserved verbatim: config `locales`/`defaultLocale`, the
+ * stored `locale` column, and the public query path all keep the raw casing, so lowercasing the `?locale=`
+ * filter here made it match zero rows for locales with uppercase subtags (#1551).
+ */
+export const localeCode = z.string().regex(/^[a-z]{2,3}(-[a-z0-9]{2,8})*$/i, "Invalid locale code");
 
 /** Shared `?locale=xx` query shape for endpoints that filter by locale. */
 export const localeFilterQuery = z
 	.object({
-		locale: z.string().min(1).optional(),
+		locale: localeCode.optional(),
 	})
 	.meta({ id: "LocaleFilterQuery" });
 
diff --git a/packages/core/tests/unit/api/schemas.test.ts b/packages/core/tests/unit/api/schemas.test.ts
index 13be8052b..5265c1477 100644
--- a/packages/core/tests/unit/api/schemas.test.ts
+++ b/packages/core/tests/unit/api/schemas.test.ts
@@ -2,10 +2,13 @@ import { describe, it, expect } from "vitest";
 
 import {
 	contentCreateBody,
+	contentListQuery,
 	contentUpdateBody,
 	createFieldBody,
 	updateFieldBody,
 	httpUrl,
+	localeCode,
+	localeFilterQuery,
 	mediaUploadUrlBody,
 	DEFAULT_MAX_UPLOAD_SIZE,
 } from "../../../src/api/schemas/index.js";
@@ -121,6 +124,48 @@ describe("contentUpdateBody schema", () => {
 	});
 });
 
+describe("localeCode validator (#1551)", () => {
+	// Config `locales`/`defaultLocale`, the `locale` column default, and the
+	// public query path all keep the raw BCP-47 casing (e.g. "zh-TW"). The API
+	// schema must do the same — lowercasing the `?locale=` filter (or a create
+	// body's locale) made it miss every row stored under an uppercase subtag.
+	it("preserves region/script subtag casing", () => {
+		expect(localeCode.parse("zh-TW")).toBe("zh-TW");
+		expect(localeCode.parse("pt-BR")).toBe("pt-BR");
+		expect(localeCode.parse("zh-Hant")).toBe("zh-Hant");
+	});
+
+	it("leaves plain language codes unchanged", () => {
+		expect(localeCode.parse("en")).toBe("en");
+	});
+
+	it("still accepts mixed-case input (case-insensitive validation)", () => {
+		expect(localeCode.parse("EN-us")).toBe("EN-us");
+	});
+
+	it("rejects malformed locale codes", () => {
+		expect(() => localeCode.parse("english")).toThrow();
+		expect(() => localeCode.parse("e")).toThrow();
+		expect(() => localeCode.parse("en_US")).toThrow();
+	});
+
+	it("contentListQuery keeps the ?locale= filter casing", () => {
+		const result = contentListQuery.parse({ locale: "zh-TW" });
+		expect(result.locale).toBe("zh-TW");
+	});
+
+	it("contentCreateBody keeps the locale casing", () => {
+		const result = contentCreateBody.parse({ data: { title: "Hi" }, locale: "pt-BR" });
+		expect(result.locale).toBe("pt-BR");
+	});
+
+	it("localeFilterQuery keeps the ?locale= casing and rejects malformed values", () => {
+		expect(localeFilterQuery.parse({ locale: "zh-TW" }).locale).toBe("zh-TW");
+		expect(localeFilterQuery.parse({}).locale).toBeUndefined();
+		expect(() => localeFilterQuery.parse({ locale: "_invalid_" })).toThrow();
+	});
+});
+
 describe("httpUrl validator", () => {
 	it("accepts http URLs", () => {
 		expect(httpUrl.parse("http://example.com")).toBe("http://example.com");