diff --git a/src/cortex-cli/src/run_cmd/execution.rs b/src/cortex-cli/src/run_cmd/execution.rs
index 18bb58d5b..81384636b 100644
--- a/src/cortex-cli/src/run_cmd/execution.rs
+++ b/src/cortex-cli/src/run_cmd/execution.rs
@@ -17,6 +17,21 @@ use super::output::{copy_to_clipboard, send_notification};
 use super::session::{SessionMode, resolve_session_id};
 use super::system::check_file_descriptor_limits;
 
+fn validate_attach_url(server_url: &str) -> Result<()> {
+    let parsed = reqwest::Url::parse(server_url).with_context(|| {
+        format!(
+            "--attach expects an HTTP(S) server URL, got '{server_url}'. Use --file/-f to attach local files."
+        )
+    })?;
+
+    match parsed.scheme() {
+        "http" | "https" => Ok(()),
+        _ => bail!(
+            "--attach expects an HTTP(S) server URL, got '{server_url}'. Use --file/-f to attach local files."
+        ),
+    }
+}
+
 impl RunCli {
     /// Run the command.
     pub async fn run(self) -> Result<()> {
@@ -136,6 +151,7 @@ impl RunCli {
 
         // Execute based on whether we're attaching to a server or running locally
         if let Some(ref server_url) = self.attach {
+            validate_attach_url(server_url)?;
             self.run_attached(server_url, &message, &attachments, session_mode)
                 .await
         } else {
diff --git a/src/cortex-cli/tests/run_attach.rs b/src/cortex-cli/tests/run_attach.rs
new file mode 100644
index 000000000..9230e4087
--- /dev/null
+++ b/src/cortex-cli/tests/run_attach.rs
@@ -0,0 +1,52 @@
+use std::process::Command;
+
+#[test]
+fn run_attach_rejects_filesystem_paths() {
+    let output = Command::new(env!("CARGO_BIN_EXE_Cortex"))
+        .args([
+            "run",
+            "--dry-run",
+            "--attach",
+            "/nonexistent/file.txt",
+            "test",
+        ])
+        .output()
+        .expect("cortex binary should run");
+
+    assert!(
+        !output.status.success(),
+        "expected non-zero exit; stdout: {}; stderr: {}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("--attach expects an HTTP(S) server URL"),
+        "stderr did not explain the invalid --attach value: {stderr}"
+    );
+}
+
+#[test]
+fn run_attach_allows_http_server_urls() {
+    let output = Command::new(env!("CARGO_BIN_EXE_Cortex"))
+        .args([
+            "run",
+            "--dry-run",
+            "--attach",
+            "http://localhost:3000",
+            "test",
+        ])
+        .output()
+        .expect("cortex binary should run");
+
+    assert!(
+        output.status.success(),
+        "expected zero exit; stdout: {}; stderr: {}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(stderr.contains("Server attachment not yet fully implemented"));
+}