diff --git a/strix/tools/proxy/caido_api.py b/strix/tools/proxy/caido_api.py
index 926d5c74..5ce03478 100644
--- a/strix/tools/proxy/caido_api.py
+++ b/strix/tools/proxy/caido_api.py
@@ -261,6 +261,14 @@ def apply_modifications(
         headers.update(modifications["headers"])
     if "body" in modifications:
         body = modifications["body"]
+        # Replacing the body invalidates any carried-over Content-Length.
+        # Drop it (case-insensitively) so build_raw_request recomputes the
+        # correct length for the new body — unless the caller deliberately
+        # set Content-Length in this call's header modifications.
+        explicit_cl = {k.title() for k in modifications.get("headers", {})}
+        if "Content-Length" not in explicit_cl:
+            for key in [k for k in headers if k.title() == "Content-Length"]:
+                del headers[key]
     if "cookies" in modifications:
         cookies: dict[str, str] = {}
         if headers.get("Cookie"):
diff --git a/tests/test_proxy_caido_api.py b/tests/test_proxy_caido_api.py
new file mode 100644
index 00000000..3299db18
--- /dev/null
+++ b/tests/test_proxy_caido_api.py
@@ -0,0 +1,59 @@
+"""Tests for raw-request building in the Caido proxy helper."""
+
+from __future__ import annotations
+
+from strix.tools.proxy.caido_api import apply_modifications, build_raw_request
+
+
+def _content_length(raw: bytes) -> str | None:
+    for line in raw.decode("utf-8").split("\r\n"):
+        if line.lower().startswith("content-length:"):
+            return line.split(":", 1)[1].strip()
+    return None
+
+
+def test_body_replacement_recomputes_content_length() -> None:
+    components = {
+        "method": "POST",
+        "headers": {"Host": "x.test", "Content-Length": "3"},
+        "body": "old",
+    }
+    new_body = "this is the new and much longer body!"
+    result = apply_modifications(components, {"body": new_body}, "http://x.test/submit")
+    _conn, raw = build_raw_request(
+        method=result["method"],
+        url=result["url"],
+        headers=result["headers"],
+        body=result["body"],
+    )
+    assert _content_length(raw) == str(len(new_body.encode("utf-8")))
+
+
+def test_explicit_content_length_override_is_preserved() -> None:
+    components = {
+        "method": "POST",
+        "headers": {"Host": "x.test", "Content-Length": "3"},
+        "body": "old",
+    }
+    result = apply_modifications(
+        components,
+        {"body": "new body", "headers": {"Content-Length": "999"}},
+        "http://x.test/",
+    )
+    _conn, raw = build_raw_request(
+        method=result["method"],
+        url=result["url"],
+        headers=result["headers"],
+        body=result["body"],
+    )
+    assert _content_length(raw) == "999"
+
+
+def test_no_body_modification_keeps_content_length() -> None:
+    components = {
+        "method": "POST",
+        "headers": {"Host": "x.test", "Content-Length": "3"},
+        "body": "old",
+    }
+    result = apply_modifications(components, {"headers": {"X-Test": "1"}}, "http://x.test/")
+    assert result["headers"].get("Content-Length") == "3"