diff --git a/codex-rs/utils/path-uri/src/lib.rs b/codex-rs/utils/path-uri/src/lib.rs
index 28ac14e54f..cf0160cb97 100644
--- a/codex-rs/utils/path-uri/src/lib.rs
+++ b/codex-rs/utils/path-uri/src/lib.rs
@@ -432,7 +432,8 @@ impl PathUri {
                 .path_segments()
                 .and_then(|mut segments| segments.find(|segment| !segment.is_empty()))
                 .is_some_and(|segment| {
-                    matches!(segment.as_bytes(), [drive, b':'] if drive.eq_ignore_ascii_case(&path_bytes[0]))
+                    is_windows_drive_uri_segment(segment)
+                        && segment.as_bytes()[0].eq_ignore_ascii_case(&path_bytes[0])
                 });
             if !same_drive {
                 return Err(PathUriParseError::InvalidFileUriPath {
@@ -497,9 +498,13 @@ impl PathUri {
     /// The URI's inferred path convention must match the current host. Conversion should succeed
     /// when the URI was created from an [`AbsolutePathBuf`] on the current host, including fallback
     /// URIs created by [`Self::from_abs_path`]. Foreign conventions are rejected rather than being
-    /// projected onto a syntactically valid but unrelated host path.
+    /// projected onto a syntactically valid but unrelated host path. Encoded Windows path
+    /// separators are rejected before native conversion can reinterpret URI segment boundaries.
     pub fn to_abs_path(&self) -> io::Result<AbsolutePathBuf> {
-        if self.infer_path_convention() != Some(PathConvention::native()) {
+        if self.infer_path_convention() != Some(PathConvention::native())
+            || (PathConvention::native() == PathConvention::Windows
+                && containment_path_segments(&self.0, PathConvention::Windows).is_none())
+        {
             return Err(io::Error::new(
                 io::ErrorKind::InvalidInput,
                 PathUriParseError::InvalidFileUriPath {
@@ -700,7 +705,10 @@ fn decode_bad_path_uri(url: &Url) -> Option<Vec<u8>> {
 }
 
 fn is_windows_drive_uri_segment(segment: &str) -> bool {
-    matches!(segment.as_bytes(), [drive, b':'] if drive.is_ascii_alphabetic())
+    matches!(
+        segment.as_bytes(),
+        [drive, b':'] | [drive, b'%', b'3', b'A' | b'a'] if drive.is_ascii_alphabetic()
+    )
 }
 
 fn containment_path_segments(url: &Url, convention: PathConvention) -> Option<Vec<&str>> {
diff --git a/codex-rs/utils/path-uri/src/tests.rs b/codex-rs/utils/path-uri/src/tests.rs
index c5863fc1aa..ecbd87d1e5 100644
--- a/codex-rs/utils/path-uri/src/tests.rs
+++ b/codex-rs/utils/path-uri/src/tests.rs
@@ -79,6 +79,29 @@ fn non_native_uri_io_conversion_is_invalid_input() {
     }
 }
 
+#[test]
+fn windows_uri_native_conversion_rejects_encoded_separators() {
+    for uri in [
+        "file:///C%3A/plugins/demo/..%5Coutside.json",
+        "file:///C%3a/plugins/demo/..%5coutside.json",
+        "file:///C:/plugins/demo/..%2Foutside.json",
+        "file://server/share/plugins/demo/..%5Coutside.json",
+    ] {
+        let uri = PathUri::parse(uri).expect("valid Windows file URI");
+
+        assert_eq!(uri.infer_path_convention(), Some(PathConvention::Windows));
+        assert!(containment_path_segments(&uri.0, PathConvention::Windows).is_none());
+
+        #[cfg(windows)]
+        assert_eq!(
+            uri.to_abs_path()
+                .expect_err("encoded Windows separators must not reach native conversion")
+                .kind(),
+            io::ErrorKind::InvalidInput
+        );
+    }
+}
+
 #[test]
 fn file_uri_parses_a_windows_path_on_any_host() {
     let uri = PathUri::parse("file:///C:/Users/Alice%20Smith/src/main.rs")
@@ -142,6 +165,14 @@ fn infers_path_conventions_from_uri_shape() {
         ("file:///home/alice/src", Some(PathConvention::Posix)),
         ("file:///C:/Users/Alice/src", Some(PathConvention::Windows)),
         ("file:///d:", Some(PathConvention::Windows)),
+        (
+            "file:///c%3A/Users/Alice/src",
+            Some(PathConvention::Windows),
+        ),
+        (
+            "file:///D%3a/Users/Alice/src",
+            Some(PathConvention::Windows),
+        ),
         ("file://server/share/src", Some(PathConvention::Windows)),
         // Opaque fallback for POSIX bytes `/tmp/null-\0-\xff-byte`.
         (
@@ -208,6 +239,10 @@ fn inferred_native_path_string_uses_the_inferred_convention() {
             "file:///C:/Users/Alice%20Smith/main.rs",
             r"C:\Users\Alice Smith\main.rs",
         ),
+        (
+            "file:///c%3A/Users/Alice/src/main.rs",
+            r"C:\Users\Alice\src\main.rs",
+        ),
         ("file://server/share/main.rs", r"\\server\share\main.rs"),
         ("file://server/", "file://server/"),
         ("file:///%00/bad/path/YQ", "file:///%00/bad/path/YQ"),
@@ -994,6 +1029,8 @@ fn join_resolves_windows_same_drive_relative_path() {
     for (base, path, expected) in [
         ("file:///C:/base", r"C:tmp", "file:///C:/base/tmp"),
         ("file:///C:/base", r"c:tmp", "file:///C:/base/tmp"),
+        ("file:///C%3A/base", r"C:tmp", "file:///C%3A/base/tmp"),
+        ("file:///C%3a/base", r"c:tmp", "file:///C%3a/base/tmp"),
         ("file:///C:/base/dir", r"C:..\tmp", "file:///C:/base/tmp"),
         ("file:///C:/base", "C:", "file:///C:/base"),
     ] {