diff --git a/codex-rs/utils/path-uri/src/api_path_string.rs b/codex-rs/utils/path-uri/src/api_path_string.rs
index 84222c6eb2..6876f56e28 100644
--- a/codex-rs/utils/path-uri/src/api_path_string.rs
+++ b/codex-rs/utils/path-uri/src/api_path_string.rs
@@ -56,6 +56,12 @@ impl fmt::Display for PathConvention {
 pub struct ApiPathString(String);
 
 impl ApiPathString {
+    /// Wraps an API path string without interpreting it using the current
+    /// host's path rules.
+    pub fn new(path: impl Into<String>) -> Self {
+        Self(path.into())
+    }
+
     /// Converts an absolute path to a [`PathUri`] before rendering it using the
     /// requested native path convention.
     pub fn from_abs_path(
@@ -156,6 +162,198 @@ fn parse_windows_path(path: &str) -> Option<PathUri> {
     None
 }
 
+pub(crate) fn resolve_native_path(
+    base: &PathUri,
+    path: &str,
+    convention: PathConvention,
+) -> Result<PathUri, ApiPathStringError> {
+    if path.is_empty() {
+        return Ok(base.clone());
+    }
+
+    match convention {
+        PathConvention::Posix => {
+            if path.starts_with('/') {
+                return parse_resolved_posix_path(path);
+            }
+            reject_opaque_base(base)?;
+            let base = render_posix_path(base)?;
+            parse_resolved_posix_path(&format!("{}/{path}", base.trim_end_matches('/')))
+        }
+        PathConvention::Windows => resolve_windows_path(base, path),
+    }
+}
+
+fn resolve_windows_path(base: &PathUri, path: &str) -> Result<PathUri, ApiPathStringError> {
+    if is_absolute_windows_path(path) {
+        return parse_resolved_windows_path(path);
+    }
+
+    reject_opaque_base(base)?;
+    let base = render_windows_path(base)?;
+    let path = if path.starts_with(['\\', '/']) {
+        let root_end = windows_root_end(&base);
+        format!(
+            "{}\\{}",
+            &base[..root_end],
+            path.trim_start_matches(['\\', '/'])
+        )
+    } else {
+        format!("{}\\{path}", base.trim_end_matches(['\\', '/']))
+    };
+    parse_resolved_windows_path(&path)
+}
+
+fn parse_resolved_posix_path(path: &str) -> Result<PathUri, ApiPathStringError> {
+    let Some(path) = path.strip_prefix('/') else {
+        return Err(invalid_native_path(path, PathConvention::Posix));
+    };
+    if path.contains('\0') {
+        return Err(invalid_native_path(path, PathConvention::Posix));
+    }
+    build_resolved_path_uri(
+        /*host*/ None,
+        path.split('/'),
+        /*protected_segments*/ 0,
+        path,
+        PathConvention::Posix,
+    )
+}
+
+fn parse_resolved_windows_path(path: &str) -> Result<PathUri, ApiPathStringError> {
+    if path.contains('\0') {
+        return Err(invalid_native_path(path, PathConvention::Windows));
+    }
+
+    if let Some(rest) = path.strip_prefix(r"\\").or_else(|| path.strip_prefix("//")) {
+        let mut segments = rest.split(is_windows_separator_char);
+        let Some(host) = segments.next().filter(|host| !host.is_empty()) else {
+            return Err(invalid_native_path(path, PathConvention::Windows));
+        };
+        let Some(share) = segments.next().filter(|share| !share.is_empty()) else {
+            return Err(invalid_native_path(path, PathConvention::Windows));
+        };
+        return build_resolved_path_uri(
+            Some(host),
+            std::iter::once(share).chain(segments),
+            /*protected_segments*/ 1,
+            path,
+            PathConvention::Windows,
+        );
+    }
+
+    let bytes = path.as_bytes();
+    if !matches!(
+        bytes,
+        [drive, b':', separator, ..]
+            if drive.is_ascii_alphabetic() && is_windows_separator_byte(*separator)
+    ) {
+        return Err(invalid_native_path(path, PathConvention::Windows));
+    }
+    build_resolved_path_uri(
+        /*host*/ None,
+        std::iter::once(&path[..2]).chain(path[3..].split(is_windows_separator_char)),
+        /*protected_segments*/ 1,
+        path,
+        PathConvention::Windows,
+    )
+}
+
+fn build_resolved_path_uri<'a>(
+    host: Option<&str>,
+    segments: impl IntoIterator<Item = &'a str>,
+    protected_segments: usize,
+    native_path: &str,
+    convention: PathConvention,
+) -> Result<PathUri, ApiPathStringError> {
+    let segments = segments.into_iter().collect::<Vec<_>>();
+    let preserve_trailing_separator = segments
+        .last()
+        .is_some_and(|segment| matches!(*segment, "" | "." | ".."));
+    let mut normalized_segments = Vec::with_capacity(segments.len());
+    for segment in segments {
+        match segment {
+            "" | "." => {}
+            ".." => {
+                if normalized_segments.len() > protected_segments {
+                    normalized_segments.pop();
+                }
+            }
+            _ => {
+                let is_drive = convention == PathConvention::Windows
+                    && host.is_none()
+                    && normalized_segments.is_empty()
+                    && is_windows_drive(segment);
+                if convention == PathConvention::Windows
+                    && !is_drive
+                    && !is_valid_windows_component(segment)
+                {
+                    return Err(invalid_native_path(native_path, convention));
+                }
+                normalized_segments.push(segment);
+            }
+        }
+    }
+    if normalized_segments.len() < protected_segments {
+        return Err(invalid_native_path(native_path, convention));
+    }
+    if preserve_trailing_separator {
+        normalized_segments.push("");
+    }
+
+    path_uri_from_segments(host, normalized_segments.into_iter())
+        .ok_or_else(|| invalid_native_path(native_path, convention))
+}
+
+fn is_absolute_windows_path(path: &str) -> bool {
+    path.strip_prefix(r"\\")
+        .or_else(|| path.strip_prefix("//"))
+        .is_some()
+        || matches!(
+            path.as_bytes(),
+            [drive, b':', separator, ..]
+                if drive.is_ascii_alphabetic() && is_windows_separator_byte(*separator)
+        )
+}
+
+fn windows_root_end(path: &str) -> usize {
+    if path.starts_with(r"\\") {
+        path.match_indices('\\')
+            .nth(3)
+            .map_or(path.len(), |(index, _)| index)
+    } else {
+        2
+    }
+}
+
+fn is_windows_drive(component: &str) -> bool {
+    let bytes = component.as_bytes();
+    bytes.len() == 2 && bytes[0].is_ascii_alphabetic() && bytes[1] == b':'
+}
+
+fn is_valid_windows_component(component: &str) -> bool {
+    !component
+        .chars()
+        .any(|character| character <= '\u{1f}' || r#"<>:"/\|?*"#.contains(character))
+        && !component.ends_with([' ', '.'])
+}
+
+fn reject_opaque_base(path: &PathUri) -> Result<(), ApiPathStringError> {
+    if path.opaque_fallback_bytes().is_some() {
+        return Err(ApiPathStringError::OpaqueFallback {
+            path: path.to_string(),
+        });
+    }
+    Ok(())
+}
+
+fn invalid_native_path(path: &str, convention: PathConvention) -> ApiPathStringError {
+    ApiPathStringError::InvalidNativePath {
+        path: path.to_string(),
+        convention,
+    }
+}
+
 fn path_uri_from_segments<'a>(
     host: Option<&str>,
     segments: impl Iterator<Item = &'a str>,
diff --git a/codex-rs/utils/path-uri/src/api_path_string_tests.rs b/codex-rs/utils/path-uri/src/api_path_string_tests.rs
index c123568889..73f4056665 100644
--- a/codex-rs/utils/path-uri/src/api_path_string_tests.rs
+++ b/codex-rs/utils/path-uri/src/api_path_string_tests.rs
@@ -418,3 +418,78 @@ fn serializes_and_deserializes_as_a_string() {
         rendered
     );
 }
+
+#[test]
+fn resolves_posix_paths_without_host_path_rules() {
+    let base = PathUri::parse("file:///workspace/src").expect("base URI");
+
+    for (path, expected) in [
+        ("", "file:///workspace/src"),
+        ("../README.md", "file:///workspace/README.md"),
+        ("../../../README.md", "file:///README.md"),
+        ("/tmp/output", "file:///tmp/output"),
+        (
+            r"generated\output",
+            "file:///workspace/src/generated%5Coutput",
+        ),
+    ] {
+        assert_eq!(
+            base.resolve_native(path, PathConvention::Posix),
+            Ok(PathUri::parse(expected).expect("resolved URI")),
+            "resolving {path}"
+        );
+    }
+}
+
+#[test]
+fn resolves_windows_drive_and_unc_paths_without_host_path_rules() {
+    let drive = PathUri::parse("file:///C:/workspace/src").expect("drive URI");
+    let share = PathUri::parse("file://server/share/workspace/src").expect("share URI");
+
+    for (base, path, expected) in [
+        (&drive, "", "file:///C:/workspace/src"),
+        (&drive, r"..\README.md", "file:///C:/workspace/README.md"),
+        (&drive, r"\logs\output.txt", "file:///C:/logs/output.txt"),
+        (&drive, r"D:\other\file.txt", "file:///D:/other/file.txt"),
+        (
+            &drive,
+            r"\\other\share\file.txt",
+            "file://other/share/file.txt",
+        ),
+        (
+            &share,
+            r"..\..\..\README.md",
+            "file://server/share/README.md",
+        ),
+        (
+            &share,
+            r"\logs\output.txt",
+            "file://server/share/logs/output.txt",
+        ),
+    ] {
+        assert_eq!(
+            base.resolve_native(path, PathConvention::Windows),
+            Ok(PathUri::parse(expected).expect("resolved URI")),
+            "resolving {path}"
+        );
+    }
+}
+
+#[test]
+fn relative_resolution_rejects_incompatible_and_opaque_bases() {
+    let posix = PathUri::parse("file:///workspace").expect("POSIX URI");
+    let opaque = PathUri::parse("file:///%00/bad/path/YQ").expect("opaque fallback URI");
+
+    assert!(matches!(
+        posix.resolve_native("child", PathConvention::Windows),
+        Err(ApiPathStringError::IncompatibleConvention { .. })
+    ));
+    assert!(matches!(
+        opaque.resolve_native("child", PathConvention::Posix),
+        Err(ApiPathStringError::OpaqueFallback { .. })
+    ));
+    assert_eq!(
+        opaque.resolve_native("/tmp", PathConvention::Posix),
+        Ok(PathUri::parse("file:///tmp").expect("absolute URI"))
+    );
+}
diff --git a/codex-rs/utils/path-uri/src/lib.rs b/codex-rs/utils/path-uri/src/lib.rs
index 468d577c1a..75030542fc 100644
--- a/codex-rs/utils/path-uri/src/lib.rs
+++ b/codex-rs/utils/path-uri/src/lib.rs
@@ -206,6 +206,20 @@ impl PathUri {
         Self::try_from(url)
     }
 
+    /// Resolves a native absolute or relative path against this URI using the
+    /// supplied path convention.
+    ///
+    /// This is host-independent: Windows paths can be resolved on POSIX and
+    /// vice versa. Relative navigation is lexical and cannot escape the drive,
+    /// UNC share, or POSIX root represented by this URI.
+    pub fn resolve_native(
+        &self,
+        path: &str,
+        convention: PathConvention,
+    ) -> Result<Self, ApiPathStringError> {
+        api_path_string::resolve_native_path(self, path, convention)
+    }
+
     /// Converts this file URI to a path using the current host's path rules.
     ///
     /// Conversion should succeed when the URI was created from an