Always emit packages table for pylock.toml

crates/uv-resolver/src/lock/export/pylock_toml.rs

@@ -1021,7 +1021,10 @@ impl<'lock> PylockToml {
             };
             doc.insert("attestation-identities", value(attestation_identities));
         }
-        if !self.packages.is_empty() {
+        if self.packages.is_empty() {
+            // `packages` is a required key in PEP 751, even when empty.
+            doc.insert("packages", value(Array::new()));
+        } else {
             let mut packages = ArrayOfTables::new();
             for dist in &self.packages {
                 packages.push(dist.to_toml()?);

crates/uv/tests/project/export.rs

@@ -4074,6 +4074,41 @@ fn pep_751_dependency() -> Result<()> {
     Ok(())
 }
 
+/// `packages` is a required key in PEP 751, even when there are no packages to export.
+#[test]
+fn pep_751_no_packages() -> Result<()> {
+    let context = uv_test::test_context!("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        "#,
+    )?;
+
+    context.lock().assert().success();
+
+    uv_snapshot!(context.filters(), context.export().arg("--format").arg("pylock.toml").arg("--no-emit-project"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv export --cache-dir [CACHE_DIR] --format pylock.toml --no-emit-project
+    lock-version = "1.0"
+    created-by = "uv"
+    requires-python = ">=3.12"
+    packages = []
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    "#);
+
+    Ok(())
+}
+
 #[test]
 fn pep_751_export_no_header() -> Result<()> {
     let context = uv_test::test_context!("3.12");