diff --git a/src/lib/upload-validator.ts b/src/lib/upload-validator.ts
index d516d129..86fa0aa9 100644
--- a/src/lib/upload-validator.ts
+++ b/src/lib/upload-validator.ts
@@ -17,7 +17,8 @@ export interface UploadValidationOptions {
  *  - File extension is in `allowedExtensions` (case-insensitive)
  *  - No path segment is a hidden dir/file (starts with `.`); blocks ~/.ssh,
  *    ~/.aws, .env, etc. even after symlink resolution
- *  - If `allowedBaseDir` is set, the canonical path must live inside it
+ *  - `allowedBaseDir` is mandatory: uploads are refused unless it is configured
+ *    (via MCP_UPLOAD_BASE_DIR), and the canonical path must live inside it
  */
 export function validateUploadPath(
   filePath: string,
@@ -71,23 +72,28 @@ export function validateUploadPath(
     );
   }
 
-  if (options.allowedBaseDir) {
-    let baseCanonical: string;
-    try {
-      baseCanonical = fs.realpathSync(path.resolve(options.allowedBaseDir));
-    } catch {
-      throw new Error(
-        `Upload rejected: configured MCP_UPLOAD_BASE_DIR does not exist (${options.allowedBaseDir}).`,
-      );
-    }
-    const baseWithSep = baseCanonical.endsWith(path.sep)
-      ? baseCanonical
-      : baseCanonical + path.sep;
-    if (canonical !== baseCanonical && !canonical.startsWith(baseWithSep)) {
-      throw new Error(
-        `Upload rejected: file must be located inside ${baseCanonical}.`,
-      );
-    }
+  if (!options.allowedBaseDir) {
+    throw new Error(
+      "Upload rejected: MCP_UPLOAD_BASE_DIR is not set. Set it to the directory " +
+        "containing the files to upload, then restart the MCP server.",
+    );
+  }
+
+  let baseCanonical: string;
+  try {
+    baseCanonical = fs.realpathSync(path.resolve(options.allowedBaseDir));
+  } catch {
+    throw new Error(
+      `Upload rejected: configured MCP_UPLOAD_BASE_DIR does not exist (${options.allowedBaseDir}).`,
+    );
+  }
+  const baseWithSep = baseCanonical.endsWith(path.sep)
+    ? baseCanonical
+    : baseCanonical + path.sep;
+  if (canonical !== baseCanonical && !canonical.startsWith(baseWithSep)) {
+    throw new Error(
+      `Upload rejected: file must be located inside ${baseCanonical}.`,
+    );
   }
 
   return canonical;
diff --git a/tests/tools/upload-validator.test.ts b/tests/tools/upload-validator.test.ts
index 51eed475..f423f194 100644
--- a/tests/tools/upload-validator.test.ts
+++ b/tests/tools/upload-validator.test.ts
@@ -32,10 +32,32 @@ describe("validateUploadPath", () => {
     const resolved = validateUploadPath(file, {
       allowedExtensions: APP_BINARY_EXTENSIONS,
       maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      allowedBaseDir: workDir,
     });
     expect(resolved).toBe(fs.realpathSync(file));
   });
 
+  it("refuses when no base directory is configured (MCP_UPLOAD_BASE_DIR unset)", () => {
+    const file = write("app.apk");
+    expect(() =>
+      validateUploadPath(file, {
+        allowedExtensions: APP_BINARY_EXTENSIONS,
+        maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      }),
+    ).toThrow(/MCP_UPLOAD_BASE_DIR is not set/);
+  });
+
+  it("refuses when the configured base directory does not exist", () => {
+    const file = write("app.apk");
+    expect(() =>
+      validateUploadPath(file, {
+        allowedExtensions: APP_BINARY_EXTENSIONS,
+        maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+        allowedBaseDir: path.join(os.tmpdir(), "no-such-base-dir-xyz"),
+      }),
+    ).toThrow(/does not exist/);
+  });
+
   it("rejects an empty path", () => {
     expect(() =>
       validateUploadPath("   ", {
@@ -185,6 +207,7 @@ describe("validateUploadPath", () => {
     const resolved = validateUploadPath(file, {
       allowedExtensions: APP_BINARY_EXTENSIONS,
       maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      allowedBaseDir: workDir,
     });
     expect(resolved).toBe(fs.realpathSync(file));
   });