diff --git a/README.md b/README.md
index f56f62e..dd52f71 100644
--- a/README.md
+++ b/README.md
@@ -215,6 +215,8 @@ NEXT_PUBLIC_API_BASE_URL=http://localhost:8000
 | `MAX_HTML_LENGTH` | integer | `200000` | 单次 HTML 最大字节数 |
 | `API_KEY` | string | 空 | 非空时启用 `X-API-Key` 鉴权 |
 | `ALLOW_UNSAFE_HTML` | boolean | `true` | 是否关闭 HTML 安全拦截；当前默认开启，便于执行 JS |
+| `ENABLE_REQUEST_DEBUG_LOG` | boolean | `true` | 是否输出 422 请求调试日志 |
+| `REQUEST_LOG_MAX_CHARS` | integer | `10000` | 单次请求日志最多记录多少字符 |
 
 ## 给腾讯云智能体的最小参数说明
 
diff --git a/backend/.env b/backend/.env
index 3214aae..637be23 100644
--- a/backend/.env
+++ b/backend/.env
@@ -8,3 +8,5 @@ MAX_RETENTION_DAYS=30
 MAX_HTML_LENGTH=200000
 API_KEY=""
 ALLOW_UNSAFE_HTML=true
+ENABLE_REQUEST_DEBUG_LOG=true
+REQUEST_LOG_MAX_CHARS=10000
diff --git a/backend/.env.example b/backend/.env.example
index 3214aae..637be23 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -8,3 +8,5 @@ MAX_RETENTION_DAYS=30
 MAX_HTML_LENGTH=200000
 API_KEY=""
 ALLOW_UNSAFE_HTML=true
+ENABLE_REQUEST_DEBUG_LOG=true
+REQUEST_LOG_MAX_CHARS=10000
diff --git a/backend/app/config.py b/backend/app/config.py
index 3a63211..0ebc304 100644
--- a/backend/app/config.py
+++ b/backend/app/config.py
@@ -76,6 +76,8 @@ class Settings:
     max_html_length = max(1024, _get_int_env("MAX_HTML_LENGTH", 200_000))
     api_key = os.getenv("API_KEY", "").strip()
     allow_unsafe_html = _get_bool_env("ALLOW_UNSAFE_HTML", False)
+    enable_request_debug_log = _get_bool_env("ENABLE_REQUEST_DEBUG_LOG", True)
+    request_log_max_chars = max(256, _get_int_env("REQUEST_LOG_MAX_CHARS", 10_000))
 
 
 settings = Settings()
diff --git a/backend/app/main.py b/backend/app/main.py
index 7204d9c..3e6cb19 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -1,7 +1,9 @@
 import logging
 
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
 
 from app.config import settings
 from app.database import Base, SessionLocal, engine, ensure_database_schema
@@ -39,6 +41,46 @@ app.add_middleware(
 app.include_router(html.router, prefix=settings.api_prefix)
 
 
+def _truncate_for_log(value: str) -> str:
+    limit = settings.request_log_max_chars
+    if len(value) <= limit:
+        return value
+
+    return f"{value[:limit]}... [truncated {len(value) - limit} chars]"
+
+
+@app.exception_handler(RequestValidationError)
+async def validation_exception_handler(
+    request: Request,
+    exc: RequestValidationError,
+) -> JSONResponse:
+    if settings.enable_request_debug_log:
+        try:
+            raw_body = (await request.body()).decode("utf-8", errors="replace")
+        except Exception as body_error:
+            raw_body = f"<failed to read body: {body_error}>"
+
+        interesting_headers = {
+            key: value
+            for key, value in request.headers.items()
+            if key.lower() in {"content-type", "content-length", "x-api-key", "user-agent"}
+        }
+        logger.error(
+            "422 validation error: method=%s path=%s query=%s headers=%s errors=%s body=%s",
+            request.method,
+            request.url.path,
+            dict(request.query_params),
+            interesting_headers,
+            exc.errors(),
+            _truncate_for_log(raw_body),
+        )
+
+    return JSONResponse(
+        status_code=422,
+        content={"detail": exc.errors()},
+    )
+
+
 @app.on_event("startup")
 def cleanup_on_startup() -> None:
     db = SessionLocal()