diff --git a/src/cargo/util/context/mod.rs b/src/cargo/util/context/mod.rs
index 0503e8ff35c..8954c02cedd 100644
--- a/src/cargo/util/context/mod.rs
+++ b/src/cargo/util/context/mod.rs
@@ -1298,7 +1298,7 @@ impl GlobalContext {
     ) -> CargoResult<()> {
         let includes = self.include_paths(cv, false)?;
         for include in includes {
-            let Some(abs_path) = include.resolve_path(self) else {
+            let Some(abs_path) = include.resolve_path(self)? else {
                 continue;
             };
 
@@ -1413,7 +1413,7 @@ impl GlobalContext {
         // Accumulate all values here.
         let mut root = CV::Table(HashMap::new(), value.definition().clone());
         for include in includes {
-            let Some(abs_path) = include.resolve_path(self) else {
+            let Some(abs_path) = include.resolve_path(self)? else {
                 continue;
             };
 
@@ -2305,9 +2305,9 @@ impl ConfigInclude {
     /// For CLI based include (e.g., `--config 'include = "foo.toml"'`),
     /// it is relative to the current working directory.
     ///
-    /// Returns `None` if this is an optional include and the file doesn't exist.
-    /// Otherwise returns `Some(PathBuf)` with the absolute path.
-    fn resolve_path(&self, gctx: &GlobalContext) -> Option<PathBuf> {
+    /// Returns `Ok(None)` if this is an optional include and the file doesn't exist.
+    /// Otherwise returns `Ok(Some(PathBuf))` with the absolute path.
+    fn resolve_path(&self, gctx: &GlobalContext) -> CargoResult<Option<PathBuf>> {
         let abs_path = match &self.def {
             Definition::Path(p) | Definition::Cli(Some(p)) => p.parent().unwrap(),
             Definition::Environment(_) | Definition::Cli(None) | Definition::BuiltIn => gctx.cwd(),
@@ -2321,10 +2321,22 @@ impl ConfigInclude {
                 self.def,
                 abs_path.display(),
             );
-            None
-        } else {
-            Some(abs_path)
+            return Ok(None);
         }
+
+        let reserved_paths = ["config.toml.d", "user.config.toml", "user.config.toml.d"];
+        for reserved in &reserved_paths {
+            let suffix = Path::new(".cargo").join(reserved);
+            if abs_path.ends_with(&suffix) {
+                bail!(
+                    "`{}` is a reserved configuration path, but was included in `{}`",
+                    suffix.display(),
+                    self.def,
+                )
+            }
+        }
+
+        Ok(Some(abs_path))
     }
 }
 
diff --git a/tests/testsuite/config_include.rs b/tests/testsuite/config_include.rs
index 06e71f2901d..2f08fe1be35 100644
--- a/tests/testsuite/config_include.rs
+++ b/tests/testsuite/config_include.rs
@@ -684,3 +684,96 @@ Caused by:
 "#]],
     );
 }
+
+#[cargo_test]
+fn user_config_toml_reserved() {
+    write_config_at(
+        ".cargo/config.toml",
+        "
+        [[include]]
+        path = 'user.config.toml'
+        ",
+    );
+
+    write_config_at(
+        ".cargo/user.config.toml",
+        "
+        key1 = 1
+        ",
+    );
+
+    let gctx = GlobalContextBuilder::new()
+        .unstable_flag("config-include")
+        .build_err();
+    assert_error(
+        gctx.unwrap_err(),
+        str![[r#"
+could not load Cargo configuration
+
+Caused by:
+  `.cargo/user.config.toml` is a reserved configuration path, but was included in `[ROOT]/.cargo/config.toml`
+"#]],
+    );
+}
+
+#[cargo_test]
+fn config_toml_fragment_dir_reserved() {
+    write_config_at(
+        ".cargo/config.toml",
+        "
+        [[include]]
+        path = 'config.toml.d'
+        ",
+    );
+
+    write_config_at(
+        ".cargo/config.toml.d/00-myconfig.toml",
+        "
+        key1 = 1
+        ",
+    );
+
+    let gctx = GlobalContextBuilder::new()
+        .unstable_flag("config-include")
+        .build_err();
+    assert_error(
+        gctx.unwrap_err(),
+        str![[r#"
+could not load Cargo configuration
+
+Caused by:
+  expected a config include path ending with `.toml`, but found `config.toml.d` from `[ROOT]/.cargo/config.toml`
+"#]],
+    );
+}
+
+#[cargo_test]
+fn user_config_toml_fragment_dir_reserved() {
+    write_config_at(
+        ".cargo/config.toml",
+        "
+        [[include]]
+        path = 'user.config.toml.d'
+        ",
+    );
+
+    write_config_at(
+        ".cargo/user.config.toml.d/00-myconfig.toml",
+        "
+        key1 = 1
+        ",
+    );
+
+    let gctx = GlobalContextBuilder::new()
+        .unstable_flag("config-include")
+        .build_err();
+    assert_error(
+        gctx.unwrap_err(),
+        str![[r#"
+could not load Cargo configuration
+
+Caused by:
+  expected a config include path ending with `.toml`, but found `user.config.toml.d` from `[ROOT]/.cargo/config.toml`
+"#]],
+    );
+}