diff --git a/sdk/typescript/README.md b/sdk/typescript/README.md
index 517fc57824..edba4db47c 100644
--- a/sdk/typescript/README.md
+++ b/sdk/typescript/README.md
@@ -2167,6 +2167,12 @@ network binding.
 
 ### API
 
+Mutation requests to `POST /v1/bulk/findings` and `POST /v1/dedupe-groups` require
+`Content-Type: application/json`; charset parameters are accepted. Other media
+types, including a missing content type, return HTTP 400 `invalid_request`
+before embedding or storage. The API remains unauthenticated and requires an
+authenticated TLS proxy before sharing access.
+
 `POST /v1/bulk/findings` accepts `{"findings": [...]}`, using the existing SDK
 `Finding` model, including `findingId`, `occurrenceId`, and `fingerprints`.
 A complete exported `findings.json` document is also accepted; only its
diff --git a/sdk/typescript/src/server/routes.ts b/sdk/typescript/src/server/routes.ts
index 3dfa3d072b..040daf6734 100644
--- a/sdk/typescript/src/server/routes.ts
+++ b/sdk/typescript/src/server/routes.ts
@@ -118,6 +118,17 @@ export async function handleFindingsRequest(
 }
 
 async function readJson(request: IncomingMessage): Promise<unknown> {
+  const mediaType = request.headers["content-type"]
+    ?.split(";", 1)[0]
+    ?.trim()
+    .toLowerCase();
+  if (mediaType !== "application/json") {
+    request.resume();
+    throw new FindingsError(
+      "invalid_request",
+      "Request body must use application/json.",
+    );
+  }
   const chunks: Buffer[] = [];
   for await (const chunk of request) chunks.push(Buffer.from(chunk));
   try {
diff --git a/sdk/typescript/tests-ts/findings-server.test.ts b/sdk/typescript/tests-ts/findings-server.test.ts
index bad754cd7b..be0731349d 100644
--- a/sdk/typescript/tests-ts/findings-server.test.ts
+++ b/sdk/typescript/tests-ts/findings-server.test.ts
@@ -906,6 +906,7 @@ test("imports persist repository associations and keep untagged findings in expl
     (
       await fetch(`${base}/v1/bulk/findings`, {
         method: "POST",
+        headers: { "Content-Type": "application/json" },
         body: JSON.stringify({ findings: [findings[2]] }),
       })
     ).status,
@@ -941,6 +942,74 @@ test("imports persist repository associations and keep untagged findings in expl
   ]);
 });
 
+test.each([
+  undefined,
+  "text/plain;charset=UTF-8",
+  "application/x-www-form-urlencoded",
+  "multipart/form-data; boundary=synthetic-qa",
+])(
+  "rejects non-JSON mutation bodies before side effects: %s",
+  async (mediaType) => {
+    const { store } = await fixture();
+    const embed = mock(embedder.embed);
+    const base = await start(store, { embed });
+    const existing = [finding(1), finding(2)];
+    expect((await insert(base, existing)).status).toBe(201);
+    embed.mockClear();
+    const writeGroups = spyOn(store, "storeDedupeGroups");
+    try {
+      for (const [path, body] of [
+        ["/v1/bulk/findings", { findings: [finding(3)] }],
+        ["/v1/dedupe-groups", { groups: [existing.map((f) => f.findingId)] }],
+      ] as const) {
+        const response = await fetch(base + path, {
+          method: "POST",
+          headers: {
+            Origin: "null",
+            "Sec-Fetch-Site": "cross-site",
+            ...(mediaType === undefined ? {} : { "Content-Type": mediaType }),
+          },
+          body: Buffer.from(JSON.stringify(body)),
+        });
+        expect(response.status).toBe(400);
+        expect(await response.json()).toEqual({
+          error: "invalid_request",
+          message: "Request body must use application/json.",
+        });
+      }
+      expect(embed).not.toHaveBeenCalled();
+      expect(writeGroups).not.toHaveBeenCalled();
+      expect(
+        (await (await fetch(base + "/v1/findings")).json()).findings,
+      ).toEqual(existing);
+      expect(await getGroups(base, existing[0]!.findingId)).toEqual([]);
+    } finally {
+      writeGroups.mockRestore();
+    }
+  },
+);
+
+test.each(["application/json", "Application/JSON; charset=UTF-8"])(
+  "accepts JSON mutation bodies with MIME type %s",
+  async (mediaType) => {
+    const { store } = await fixture();
+    const base = await start(store);
+    const findings = [finding(1), finding(2)];
+    for (const [path, body] of [
+      ["/v1/bulk/findings", { findings }],
+      ["/v1/dedupe-groups", { groups: [findings.map((f) => f.findingId)] }],
+    ] as const) {
+      const response = await fetch(base + path, {
+        method: "POST",
+        headers: { "Content-Type": mediaType },
+        body: JSON.stringify(body),
+      });
+      expect(response.status).toBe(201);
+    }
+    expect(await getGroups(base, findings[0]!.findingId)).toHaveLength(1);
+  },
+);
+
 test("rejects invalid requests before embedding and preserves unknown-route behavior", async () => {
   const { store } = await fixture();
   const embed = mock<() => Promise<never[]>>().mockResolvedValue([]);
@@ -959,6 +1028,7 @@ test("rejects invalid requests before embedding and preserves unknown-route beha
   ]) {
     const response = await fetch(`${base}/v1/bulk/findings`, {
       method: "POST",
+      headers: { "Content-Type": "application/json" },
       body,
     });
     expect(response.status).toBe(400);