diff --git a/src/adapters/channel/api-server/ApiServerChannel.ts b/src/adapters/channel/api-server/ApiServerChannel.ts
index e17634cb1..d95809d79 100644
--- a/src/adapters/channel/api-server/ApiServerChannel.ts
+++ b/src/adapters/channel/api-server/ApiServerChannel.ts
@@ -224,6 +224,16 @@ export class ApiServerChannel implements ChannelAdapter {
 
     const lastMsg = messages[messages.length - 1];
     const userText = normalizeContent(lastMsg?.content);
+    if (userText === null) {
+      sendJson(res, 400, createApiServerErrorBody({
+        event: "api_invalid_content",
+        message: "Unsupported content",
+        code: "invalid_content",
+        status: 400,
+        userHint: "Send content as a string or an array of text parts.",
+      }));
+      return;
+    }
     if (!userText) {
       sendJson(res, 400, createApiServerErrorBody({
         event: "api_empty_message",
@@ -458,7 +468,7 @@ function parseCorsOrigins(value: unknown): string[] {
   return [];
 }
 
-function normalizeContent(content: unknown): string {
+function normalizeContent(content: unknown): string | null {
   if (!content) return "";
   if (typeof content === "string") return content;
   if (Array.isArray(content)) {
@@ -471,6 +481,7 @@ function normalizeContent(content: unknown): string {
       .filter(Boolean)
       .join("\n");
   }
+  if (typeof content === "object") return null;
   return String(content);
 }
 
diff --git a/tests/adapters/channel/api-server-content-object.spec.ts b/tests/adapters/channel/api-server-content-object.spec.ts
new file mode 100644
index 000000000..a2ee83b0d
--- /dev/null
+++ b/tests/adapters/channel/api-server-content-object.spec.ts
@@ -0,0 +1,184 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { Readable } from "node:stream";
+import type { IncomingMessage, ServerResponse } from "node:http";
+
+import { ApiServerChannel } from "../../../src/adapters/channel/api-server/ApiServerChannel.js";
+import { ApiServerSessionMapper } from "../../../src/adapters/channel/api-server/ApiServerSessionMapper.js";
+import type { Gateway, GatewayEvent, GatewaySubmitTurnInput } from "../../../src/gateway/index.js";
+
+type CapturedCall = {
+  sessionKey: string;
+  channelKey: string;
+  message: string;
+};
+
+type CaptureResponse = {
+  statusCode: number;
+  headers: Map<string, string>;
+  ended: boolean;
+  body: string;
+  setHeader(name: string, value: string): void;
+  flushHeaders(): void;
+  write(chunk: string | Buffer): boolean;
+  end(chunk?: string | Buffer): void;
+};
+
+function makeResponse(): CaptureResponse {
+  const chunks: string[] = [];
+  return {
+    statusCode: 0,
+    headers: new Map<string, string>(),
+    ended: false,
+    get body() {
+      return chunks.join("");
+    },
+    setHeader(name, value) {
+      this.headers.set(name.toLowerCase(), String(value));
+    },
+    flushHeaders() {},
+    write(chunk) {
+      chunks.push(String(chunk));
+      return true;
+    },
+    end(chunk) {
+      if (chunk != null) chunks.push(String(chunk));
+      this.ended = true;
+    },
+  };
+}
+
+function makeRequest(content: unknown, stream: boolean, sessionId: string): IncomingMessage {
+  const body = JSON.stringify({
+    model: "fixture-model",
+    messages: [{ role: "user", content }],
+    stream,
+  });
+  const req = Readable.from([Buffer.from(body)]) as unknown as IncomingMessage;
+  (req as any).method = "POST";
+  (req as any).url = "/v1/chat/completions";
+  (req as any).headers = {
+    host: "fixture.invalid",
+    "content-type": "application/json",
+    "x-hermes-session-id": sessionId,
+  };
+  return req;
+}
+
+function makeGateway(calls: CapturedCall[]): Gateway {
+  return {
+    async *submitTurn(input: GatewaySubmitTurnInput): AsyncGenerator<GatewayEvent> {
+      calls.push({
+        sessionKey: input.sessionKey,
+        channelKey: input.channelKey,
+        message: input.message,
+      });
+      yield { type: "assistant_text_delta", text: "fixture-reply" } as GatewayEvent;
+      yield {
+        type: "turn_completed",
+        usage: { inputTokens: 0, outputTokens: 0, totalTokens: 0 },
+        finishReason: "completed",
+      } as unknown as GatewayEvent;
+    },
+  } as unknown as Gateway;
+}
+
+async function runCase(content: unknown, stream: boolean, sessionId: string) {
+  const calls: CapturedCall[] = [];
+  const mapper = new ApiServerSessionMapper({ activeByChatId: {} }, () => "fixture-uuid");
+  const channel = new ApiServerChannel({ mapper, modelName: "fixture-model" });
+  (channel as any).gateway = makeGateway(calls);
+  const res = makeResponse();
+  await (channel as any).handleRequest(
+    makeRequest(content, stream, sessionId),
+    res as unknown as ServerResponse,
+  );
+  return { calls, res, channel, mapper };
+}
+
+test("A. arbitrary plain object is rejected with HTTP 400 and zero Gateway calls", async () => {
+  const { calls, res, channel } = await runCase(
+    { kind: "fixture-object", value: 7 },
+    false,
+    "fixture-501-A",
+  );
+  assert.equal(res.statusCode, 400);
+  assert.equal(calls.length, 0);
+  assert.doesNotMatch(res.body, /\[object Object\]/);
+  const parsed = JSON.parse(res.body);
+  assert.equal(parsed.error?.code, "invalid_content");
+  assert.equal((channel as any).activeChats.size, 0);
+});
+
+test("B. object shaped like a content part is rejected and never becomes [object Object]", async () => {
+  const { calls, res, channel } = await runCase(
+    { type: "text", text: "fixture-object-text" },
+    false,
+    "fixture-501-B",
+  );
+  assert.equal(res.statusCode, 400);
+  assert.equal(calls.length, 0);
+  assert.doesNotMatch(res.body, /\[object Object\]/);
+  assert.doesNotMatch(res.body, /fixture-object-text/);
+  const parsed = JSON.parse(res.body);
+  assert.equal(parsed.error?.code, "invalid_content");
+  assert.equal((channel as any).activeChats.size, 0);
+});
+
+test("C. rejected object with stream=true is rejected before SSE/Gateway admission", async () => {
+  const { calls, res, channel } = await runCase(
+    { kind: "fixture-object", value: 7 },
+    true,
+    "fixture-501-C",
+  );
+  assert.equal(res.statusCode, 400);
+  assert.equal(calls.length, 0);
+  assert.equal(res.headers.get("content-type"), "application/json");
+  assert.doesNotMatch(res.body, /\[object Object\]/);
+  assert.ok(!res.body.includes("data: [DONE]"));
+  assert.equal((channel as any).activeChats.size, 0);
+});
+
+test("D. string content remains accepted with exact original string", async () => {
+  const { calls, res, channel } = await runCase("hello fixture string", false, "fixture-501-D");
+  assert.equal(res.statusCode, 200);
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0]?.message, "hello fixture string");
+  assert.equal((channel as any).activeChats.size, 0);
+});
+
+test("E. intentionally-supported array content preserves existing behavior", async () => {
+  const stringArray = await runCase(["alpha", "beta"], false, "fixture-501-E1");
+  assert.equal(stringArray.res.statusCode, 200);
+  assert.equal(stringArray.calls.length, 1);
+  assert.equal(stringArray.calls[0]?.message, "alpha\nbeta");
+
+  const partArray = await runCase(
+    [
+      { type: "text", text: "part-one" },
+      { type: "input_text", text: "part-two" },
+    ],
+    false,
+    "fixture-501-E2",
+  );
+  assert.equal(partArray.res.statusCode, 200);
+  assert.equal(partArray.calls.length, 1);
+  assert.equal(partArray.calls[0]?.message, "part-one\npart-two");
+});
+
+test("F. object shaped like a content part with stream=true is rejected before SSE/Gateway admission", async () => {
+  const { calls, res, channel } = await runCase(
+    { type: "text", text: "fixture-object-text" },
+    true,
+    "fixture-501-F",
+  );
+  assert.equal(res.statusCode, 400);
+  assert.equal(calls.length, 0);
+  assert.equal(res.headers.get("content-type"), "application/json");
+  assert.doesNotMatch(res.body, /\[object Object\]/);
+  assert.doesNotMatch(res.body, /fixture-object-text/);
+  assert.ok(!res.body.includes("data: [DONE]"));
+  const parsed = JSON.parse(res.body);
+  assert.equal(parsed.error?.code, "invalid_content");
+  assert.equal((channel as any).activeChats.size, 0);
+});