feat: emit stable json errors

crates/geth-cli/src/lib.rs

@@ -1227,6 +1227,18 @@ pub struct EmptyArgs {}
 
 pub async fn run() -> Result<()> {
     let cli = Cli::parse();
+    let json = cli.json || cli.jsonl;
+    match run_inner(cli).await {
+        Ok(()) => Ok(()),
+        Err(error) if json => {
+            print_json_error(&error)?;
+            std::process::exit(1);
+        }
+        Err(error) => Err(error),
+    }
+}
+
+async fn run_inner(cli: Cli) -> Result<()> {
     if let Command::Completions { shell } = &cli.command {
         print_completions(*shell);
         return Ok(());
@@ -1328,6 +1340,47 @@ pub async fn run() -> Result<()> {
     Ok(())
 }
 
+fn print_json_error(error: &anyhow::Error) -> Result<()> {
+    let message = error.to_string();
+    let detail = format!("{error:#}");
+    let hint = json_error_hint(&detail);
+    println!(
+        "{}",
+        serde_json::to_string_pretty(&serde_json::json!({
+            "type": "error",
+            "code": json_error_code(&detail),
+            "message": message,
+            "detail": detail,
+            "hint": hint,
+        }))?
+    );
+    Ok(())
+}
+
+fn json_error_code(detail: &str) -> &'static str {
+    let lower = detail.to_ascii_lowercase();
+    if lower.contains("connect to daemon") || lower.contains("connection refused") {
+        "daemon_unavailable"
+    } else if lower.contains("unauthorized") || lower.contains("missing grant") {
+        "unauthorized"
+    } else if lower.contains("peer candidate not found") {
+        "peer_not_found"
+    } else if lower.contains("resource not found") {
+        "resource_not_found"
+    } else if lower.contains("invalid") {
+        "invalid_input"
+    } else {
+        "command_failed"
+    }
+}
+
+fn json_error_hint(detail: &str) -> Option<String> {
+    detail
+        .lines()
+        .find_map(|line| line.strip_prefix("next: "))
+        .map(ToOwned::to_owned)
+}
+
 fn print_guide(topic: Option<GuideTopic>, json: bool) -> Result<()> {
     let (name, body) = match topic {
         None => ("index", GUIDE_INDEX),
@@ -3931,3 +3984,30 @@ fn shell_quote_command(command: &[String]) -> String {
         .collect::<Vec<_>>()
         .join(" ")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn json_error_classification_is_stable_for_common_failures() {
+        assert_eq!(
+            json_error_code("connect to daemon at /tmp/geth.sock: No such file or directory"),
+            "daemon_unavailable"
+        );
+        assert_eq!(
+            json_error_code("unauthorized: missing grant"),
+            "unauthorized"
+        );
+        assert_eq!(
+            json_error_code("peer candidate not found: node:missing"),
+            "peer_not_found"
+        );
+        assert_eq!(
+            json_error_hint(
+                "peer candidate not found: node:missing\nnext: import a peer card with `geth peer import <path>`",
+            ),
+            Some("import a peer card with `geth peer import <path>`".to_owned())
+        );
+    }
+}