nautechsystems/nautilus_trader · error

Invalid data_type JSON: {e}

Error message

Invalid data_type JSON: {e}

What it means

`DataType::from_persistence_json` parses a persisted `DataType` from a JSON string. If the string is not syntactically valid JSON, `serde_json::from_str` fails and the serde error is wrapped as `Invalid data_type JSON: {e}`.

Source

Thrown at crates/model/src/data/mod.rs:805

        if let Some(ref id) = self.identifier {
            map.insert(
                "identifier".to_string(),
                serde_json::Value::String(id.clone()),
            );
        }
        serde_json::to_string(&serde_json::Value::Object(map))
    }

    /// Deserializes from JSON produced by `to_persistence_json`.
    /// Accepts legacy JSON with `topic` (ignored); topic is rebuilt from `type_name` + metadata.
    ///
    /// # Errors
    ///
    /// Returns an error if the string is not valid JSON or missing required fields.
    pub fn from_persistence_json(s: &str) -> Result<Self, anyhow::Error> {
        let value: serde_json::Value =
            serde_json::from_str(s).map_err(|e| anyhow::anyhow!("Invalid data_type JSON: {e}"))?;
        let obj = value
            .as_object()
            .ok_or_else(|| anyhow::anyhow!("data_type must be a JSON object"))?;
        let type_name = obj
            .get("type_name")
            .and_then(|v| v.as_str())
            .ok_or_else(|| anyhow::anyhow!("data_type must have type_name"))?;
        let metadata = obj.get("metadata").and_then(|m| {
            if m.is_null() {
                None
            } else {
                let p: Params = serde_json::from_value(m.clone()).ok()?;
                if p.is_empty() { None } else { Some(p) }
            }
        });
        let identifier = obj
            .get("identifier")
            .and_then(|v| v.as_str())

View on GitHub (pinned to 18893faf8b)

Solutions

1. Print the failing string (from the serde error offset) and validate it with a JSON linter to find the syntax fault.
2. Check the source file for truncation/corruption; restore from backup or re-persist the DataType.
3. Ensure you pass only the JSON document, not surrounding log lines or a Python/Rust debug repr.
4. Strip BOM/whitespace and confirm file encoding (UTF-8) before parsing.

Example fix

// before
let dt = DataType::from_persistence_json(&raw_line)?;

// after
let trimmed = raw_line.trim().trim_start_matches('\u{feff}');
let dt = DataType::from_persistence_json(trimmed)
    .with_context(|| format!("bad DataType JSON: {trimmed}"))?;

fn is_plausible_json(s: &str) -> bool {
    serde_json::from_str::<serde_json::Value>(s.trim().trim_start_matches('\u{feff}')).is_ok()
}

match DataType::from_persistence_json(s) {
    Ok(dt) => dt,
    Err(e) => { log::warn!("skipping bad DataType record: {e}"); continue; }
};

When it happens

Trigger: Calling `DataType::from_persistence_json` with a string that is truncated, empty, concatenated with other log text, or otherwise not valid JSON (e.g. a single-quoted or comment-containing string).

Common situations: Reading a corrupted or partially written catalog/journal file; copy-pasting a DataType repr instead of its JSON; a persistence file written by an older version with a different format; loading a file with BOM or trailing garbage bytes.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

Related errors

• from_json not implemented for {}
• Failed to decode position replay state: {e}
• data_type must be a JSON object
• CustomData JSON missing 'payload' field
• Invalid NodeState value

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/2c0e76adb908e276. Report an issue: GitHub.