fix(proxy): set Content-Length: 0 on empty-body replacement

build_raw_request only auto-adds Content-Length for a non-empty body, so replacing the body with an empty string produced a request with no Content-Length. Set it to 0 explicitly in apply_modifications (scoped to the body-replacement path, so body-less requests like GET are unaffected). Adds a test.

strix/tools/proxy/caido_api.py

@@ -269,6 +269,10 @@ def apply_modifications(
         if "Content-Length" not in explicit_cl:
             for key in [k for k in headers if k.title() == "Content-Length"]:
                 del headers[key]
+            # build_raw_request only auto-adds Content-Length for a non-empty
+            # body, so an empty replacement body needs it set explicitly.
+            if body == "":
+                headers["Content-Length"] = "0"
     if "cookies" in modifications:
         cookies: dict[str, str] = {}
         if headers.get("Cookie"):

tests/test_proxy_caido_api.py

@@ -57,3 +57,19 @@ def test_no_body_modification_keeps_content_length() -> None:
     }
     result = apply_modifications(components, {"headers": {"X-Test": "1"}}, "http://x.test/")
     assert result["headers"].get("Content-Length") == "3"
+
+
+def test_empty_body_replacement_sets_content_length_zero() -> None:
+    components = {
+        "method": "POST",
+        "headers": {"Host": "x.test", "Content-Length": "3"},
+        "body": "old",
+    }
+    result = apply_modifications(components, {"body": ""}, "http://x.test/")
+    _conn, raw = build_raw_request(
+        method=result["method"],
+        url=result["url"],
+        headers=result["headers"],
+        body=result["body"],
+    )
+    assert _content_length(raw) == "0"