diff --git a/moon_config/README.mbt.md b/moon_config/README.mbt.md
index f59c3b4b..f62bb1e4 100644
--- a/moon_config/README.mbt.md
+++ b/moon_config/README.mbt.md
@@ -11,6 +11,12 @@ post-processing the raw AST, so their diagnostics include duplicate and
 unexpected top-level keys. Use `validate_moon_mod`, `validate_moon_pkg`, or
 `validate_moon_work` only when validating an already post-processed `Ast`.
 
+`moon.mod` accepts an optional boolean `private` field. An absent field or
+`private = false` denotes public access; `private = true` denotes private access
+in a registry that supports it. Both a top-level assignment and
+`options(private: true)` are preserved in the JSON output. Non-boolean values
+and duplicate fields produce diagnostics.
+
 ## Examples
 
 Parse a `moon.mod` file and convert the returned `Ast` to JSON:
diff --git a/moon_config/post_process.mbt b/moon_config/post_process.mbt
index 03beadaa..99aea4fe 100644
--- a/moon_config/post_process.mbt
+++ b/moon_config/post_process.mbt
@@ -99,6 +99,10 @@ fn post_process_moon_mod(ast : Ast, diagnostics : Array[Report]) -> Ast {
           | "description"
           | "source"
           | "rule" => results.push(field)
+          "private" => {
+            validate_private(value, diagnostics)
+            results.push(field)
+          }
           "preferred_target" => results.push(("preferred-target", value))
           "warnings" =>
             match value {
@@ -146,6 +150,9 @@ fn post_process_moon_mod(ast : Ast, diagnostics : Array[Report]) -> Ast {
                     seen, diagnostics, "moon.mod", allowed_duplicate, option_key,
                     option_value,
                   )
+                  if option_key == "private" {
+                    validate_private(option_value, diagnostics)
+                  }
                   results.push(option_field)
                 }
               _ => {
diff --git a/moon_config/private_wbtest.mbt b/moon_config/private_wbtest.mbt
new file mode 100644
index 00000000..fb789da6
--- /dev/null
+++ b/moon_config/private_wbtest.mbt
@@ -0,0 +1,44 @@
+///|
+test "moon.mod private is optional and preserves boolean values" {
+  let (absent, absent_reports) = parse_moon_mod("name = \"user/mod\"")
+  assert_true(absent_reports.is_empty())
+  json_inspect(ToJson::to_json(absent), content={ "name": "user/mod" })
+  let (public_mod, public_reports) = parse_moon_mod("private = false")
+  assert_true(public_reports.is_empty())
+  json_inspect(ToJson::to_json(public_mod), content={ "private": false })
+  let (private_mod, private_reports) = parse_moon_mod("private = true")
+  assert_true(private_reports.is_empty())
+  assert_true(validate_moon_mod(private_mod).is_empty())
+  json_inspect(ToJson::to_json(private_mod), content={ "private": true })
+}
+
+///|
+test "moon.mod options preserves private" {
+  let (ast, reports) = parse_moon_mod("options(private: true)")
+  assert_true(reports.is_empty())
+  json_inspect(ToJson::to_json(ast), content={ "private": true })
+}
+
+///|
+test "moon.mod rejects non-boolean private" {
+  let (_, reports) = parse_moon_mod("private = \"true\"")
+  assert_eq(reports.map(report => report.msg), [
+    "Invalid moon.mod config: `private` must be a boolean.",
+  ])
+  let (_, reports) = parse_moon_mod("options(private: 1)")
+  assert_eq(reports.map(report => report.msg), [
+    "Invalid moon.mod config: `private` must be a boolean.",
+  ])
+  let (ast, _) = parse_moon_mod("private = []")
+  assert_eq(validate_moon_mod(ast).map(report => report.msg), [
+    "Invalid moon.mod config: `private` must be a boolean.",
+  ])
+}
+
+///|
+test "moon.mod rejects duplicate private across options" {
+  let (_, reports) = parse_moon_mod("private = true\noptions(private: false)")
+  assert_eq(reports.map(report => report.msg), [
+    "Duplicate key `private` found in moon.mod.",
+  ])
+}
diff --git a/moon_config/validate.mbt b/moon_config/validate.mbt
index e8967b3a..6818a0a1 100644
--- a/moon_config/validate.mbt
+++ b/moon_config/validate.mbt
@@ -51,6 +51,16 @@ fn validate_pkgtype_kind(value : Ast, diagnostics : Array[Report]) -> Unit {
   }
 }
 
+///|
+fn validate_private(value : Ast, diagnostics : Array[Report]) -> Unit {
+  if !(value is Bool(_, ..)) {
+    diagnostics.push({
+      loc: value.loc(),
+      msg: "Invalid moon.mod config: `private` must be a boolean.",
+    })
+  }
+}
+
 ///|
 fn _validate_moon_pkg(ast : Ast, diagnostics : Array[Report]) -> Unit {
   match ast {
@@ -88,6 +98,7 @@ fn _validate_moon_mod(ast : Ast, diagnostics : Array[Report]) -> Unit {
           seen, diagnostics, "moon.mod", allowed_duplicate, key, value,
         )
         match key {
+          "private" => validate_private(value, diagnostics)
           "warnings"
           | "supported_targets"
           | "preferred_target"