diff --git a/tests/web/web-host.test.ts b/tests/web/web-host.test.ts
index d02d2e36..7ac7b137 100644
--- a/tests/web/web-host.test.ts
+++ b/tests/web/web-host.test.ts
@@ -1,6 +1,7 @@
 import assert from "node:assert/strict";
 import { once } from "node:events";
 import { mkdtemp, rm } from "node:fs/promises";
+import { request as httpRequest } from "node:http";
 import { createConnection } from "node:net";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -783,6 +784,134 @@ async function startTestHost(runtime: WebRuntimeController) {
   return { host, launched, headers };
 }
 
+test("classifies invalid and oversized JSON bodies as client errors", async () => {
+  const cwd = await mkdtemp(join(tmpdir(), "openpi-web-request-body-"));
+  const { host, launched, headers } = await startTestHost(testRuntime(cwd));
+  try {
+    const invalidJson = await fetch(`${launched.origin}/api/workspaces`, {
+      method: "POST",
+      headers: { ...headers, "Content-Type": "application/json" },
+      body: '{"path":',
+    });
+    assert.equal(invalidJson.status, 400);
+    assert.deepEqual(await invalidJson.json(), {
+      code: "INVALID_REQUEST_BODY",
+      error: "request body is invalid JSON",
+    });
+
+    const nonObjectJson = await fetch(`${launched.origin}/api/workspaces`, {
+      method: "POST",
+      headers: { ...headers, "Content-Type": "application/json" },
+      body: "[]",
+    });
+    assert.equal(nonObjectJson.status, 400);
+    assert.deepEqual(await nonObjectJson.json(), {
+      code: "INVALID_REQUEST_BODY",
+      error: "request body must be an object",
+    });
+
+    const oversizedBody = await fetch(`${launched.origin}/api/workspaces`, {
+      method: "POST",
+      headers: { ...headers, "Content-Type": "application/json" },
+      body: JSON.stringify({ path: "x".repeat(16 * 1024) }),
+    });
+    assert.equal(oversizedBody.status, 413);
+    assert.deepEqual(await oversizedBody.json(), {
+      code: "REQUEST_BODY_TOO_LARGE",
+      error: "request body is too large",
+      maxBytes: 16 * 1024,
+    });
+  } finally {
+    await host.stop();
+    await rm(cwd, { recursive: true, force: true });
+  }
+});
+
+test("classifies an oversized body sent in multiple chunks as a client error", async () => {
+  const cwd = await mkdtemp(join(tmpdir(), "openpi-web-request-chunks-"));
+  const { host, launched, headers } = await startTestHost(testRuntime(cwd));
+  const bodyLength = 64 * 1024;
+  let request: ReturnType<typeof httpRequest> | undefined;
+  const responsePromise = new Promise<{
+    statusCode: number | undefined;
+    body: string;
+  }>((resolve, reject) => {
+    request = httpRequest(
+      {
+        hostname: launched.hostname,
+        port: Number(launched.port),
+        path: "/api/workspaces",
+        method: "POST",
+        headers: {
+          ...headers,
+          "Content-Type": "application/json",
+          "Content-Length": bodyLength,
+        },
+      },
+      (response) => {
+        let body = "";
+        response.setEncoding("utf8");
+        response.on("data", (chunk: string) => {
+          body += chunk;
+        });
+        response.on("end", () =>
+          resolve({ statusCode: response.statusCode, body }),
+        );
+      },
+    );
+    request.once("error", reject);
+    request.write(Buffer.alloc(20 * 1024, "a"));
+  });
+
+  try {
+    const timeout = new Promise<never>((_, reject) => {
+      const timer = setTimeout(
+        () => reject(new Error("timed out waiting for early 413 response")),
+        2_000,
+      );
+      timer.unref();
+    });
+    const response = await Promise.race([responsePromise, timeout]);
+    assert.equal(response.statusCode, 413);
+    assert.deepEqual(JSON.parse(response.body), {
+      code: "REQUEST_BODY_TOO_LARGE",
+      error: "request body is too large",
+      maxBytes: 16 * 1024,
+    });
+  } finally {
+    request?.destroy();
+    await host.stop();
+    await rm(cwd, { recursive: true, force: true });
+  }
+});
+
+test("keeps unexpected Web Host failures classified as server errors", async () => {
+  const cwd = await mkdtemp(join(tmpdir(), "openpi-web-server-error-"));
+  const { host, launched, headers } = await startTestHost(testRuntime(cwd));
+  const adapter = (
+    host as unknown as {
+      adapter: { importWorkspace(path: string): Promise<string> };
+    }
+  ).adapter;
+  adapter.importWorkspace = async () => {
+    throw new Error("unexpected adapter failure");
+  };
+  try {
+    const response = await fetch(`${launched.origin}/api/workspaces`, {
+      method: "POST",
+      headers: { ...headers, "Content-Type": "application/json" },
+      body: JSON.stringify({ path: cwd }),
+    });
+    assert.equal(response.status, 500);
+    assert.deepEqual(await response.json(), {
+      error: "unexpected adapter failure",
+    });
+  } finally {
+    await host.stop();
+    await rm(cwd, { recursive: true, force: true });
+  }
+});
+
 test("adapter initialization fails before the Host starts listening", async () => {
   const cwd = await mkdtemp(join(tmpdir(), "openpi-web-startup-failure-"));
   const runtime = testRuntime(cwd);
diff --git a/web/host/web-host.ts b/web/host/web-host.ts
index 779d502a..c61c9612 100644
--- a/web/host/web-host.ts
+++ b/web/host/web-host.ts
@@ -40,6 +40,29 @@ const SERVER_CLOSE_DRAIN_MS = 500;
 const DEFAULT_SHUTDOWN_TIMEOUT_MS = 5_000;
 const execFileAsync = promisify(execFile);
 
+type WebRequestErrorCode =
+  | "INVALID_REQUEST_BODY"
+  | "REQUEST_BODY_TOO_LARGE";
+
+class WebRequestError extends Error {
+  readonly code: WebRequestErrorCode;
+  readonly statusCode: 400 | 413;
+  readonly maxBytes?: number;
+
+  constructor(
+    message: string,
+    code: WebRequestErrorCode,
+    statusCode: 400 | 413,
+    maxBytes?: number,
+  ) {
+    super(message);
+    this.name = "WebRequestError";
+    this.code = code;
+    this.statusCode = statusCode;
+    this.maxBytes = maxBytes;
+  }
+}
+
 export interface WebHostOptions {
   runtime: WebRuntimeController;
   onEvent?: (type: string, detail?: Record<string, unknown>) => void;
@@ -269,6 +292,15 @@ export class WebHost {
       await this.handle(request, response);
     } catch (error) {
       if (response.destroyed || response.writableEnded) return;
+      if (error instanceof WebRequestError) {
+        return this.json(response, error.statusCode, {
+          code: error.code,
+          error: error.message,
+          ...(error.maxBytes === undefined
+            ? {}
+            : { maxBytes: error.maxBytes }),
+        });
+      }
       this.json(response, 500, {
         error: error instanceof Error ? error.message : "request failed",
       });
@@ -649,23 +681,34 @@ export class WebHost {
     for await (const chunk of request) {
       const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
       bytes += buffer.length;
-      if (bytes > MAX_COMMAND_BYTES)
-        throw new Error("request body is too large");
+      if (bytes > MAX_COMMAND_BYTES) {
+        throw new WebRequestError(
+          "request body is too large",
+          "REQUEST_BODY_TOO_LARGE",
+          413,
+          MAX_COMMAND_BYTES,
+        );
+      }
       chunks.push(buffer);
     }
+    let value: unknown;
     try {
-      const value: unknown = JSON.parse(Buffer.concat(chunks).toString("utf8"));
-      if (!value || typeof value !== "object" || Array.isArray(value)) {
-        throw new Error("request body must be an object");
-      }
-      return value as Record<string, unknown>;
-    } catch (error) {
-      throw new Error(
-        error instanceof SyntaxError
-          ? "request body is invalid JSON"
-          : String(error),
+      value = JSON.parse(Buffer.concat(chunks).toString("utf8"));
+    } catch {
+      throw new WebRequestError(
+        "request body is invalid JSON",
+        "INVALID_REQUEST_BODY",
+        400,
+      );
+    }
+    if (!value || typeof value !== "object" || Array.isArray(value)) {
+      throw new WebRequestError(
+        "request body must be an object",
+        "INVALID_REQUEST_BODY",
+        400,
       );
     }
+    return value as Record<string, unknown>;
   }
 
   private authorized(request: IncomingMessage) {