nikivdev/code · error

failed to decode json line with simd-json: {err}

Error message

failed to decode json line with simd-json: {err}

What it means

parse_json_line decodes a single JSON line into T. On linux x86_64/aarch64 with the linux-host-simd-json feature, it uses simd-json for speed; when simd_json::serde::from_slice fails, this error wraps the simd-json error. Note simd-json can be stricter than serde_json (e.g. requires mutable buffer, different number handling).

Source

Thrown at src/json_parse.rs:14

use anyhow::{Result, anyhow};
use serde::de::DeserializeOwned;

#[inline]
pub fn parse_json_line<T: DeserializeOwned>(line: &str) -> Result<T> {
    #[cfg(all(
        feature = "linux-host-simd-json",
        target_os = "linux",
        any(target_arch = "x86_64", target_arch = "aarch64")
    ))]
    {
        let mut buf = line.as_bytes().to_vec();
        return simd_json::serde::from_slice(&mut buf)
            .map_err(|err| anyhow!("failed to decode json line with simd-json: {err}"));
    }

    #[cfg(not(all(
        feature = "linux-host-simd-json",
        target_os = "linux",
        any(target_arch = "x86_64", target_arch = "aarch64")
    )))]
    {
        serde_json::from_str(line).map_err(|err| anyhow!("failed to decode json line: {err}"))
    }
}

#[inline]
pub fn parse_json_bytes_in_place<T: DeserializeOwned>(bytes: &mut [u8]) -> Result<T> {
    #[cfg(all(
        feature = "linux-host-simd-json",
        target_os = "linux",
        any(target_arch = "x86_64", target_arch = "aarch64")

View on GitHub (pinned to a747e741ae)

Solutions

1. Log/print the offending line and the wrapped simd-json error to see the exact offset/reason
2. Verify the line is complete, non-empty JSON before parsing
3. Compare struct T with the actual JSON shape (field names/types)
4. If simd-json strictness is the issue, test the same input without the linux-host-simd-json feature

Example fix

// before
let v: Event = parse_json_line(line)?; // panics-equivalent on blank line
// after
if line.trim().is_empty() { return Ok(None); }
let v: Event = parse_json_line(line)?;

fn is_plausible_json(line: &str) -> bool {
    let t = line.trim();
    !t.is_empty() && (t.starts_with('{') || t.starts_with('['))
}

fn looks_like_json_object(s: &str) -> bool {
    s.trim_start().starts_with('{') && s.trim_end().ends_with('}')
}

match parse_json_line::<Event>(line) {
    Ok(v) => handle(v),
    Err(e) if e.to_string().contains("simd-json") => {
        eprintln!("skipping malformed line: {e:#}"); // or buffer for retry
    }
    Err(e) => return Err(e),
}

When it happens

Trigger: A line of log/output is fed to parse_json_line under the simd-json build and is not valid JSON, is truncated, or has a type mismatch with T.

Common situations: Partially written or truncated log lines; the stream emits non-JSON lines (plain text, empty line); schema drift between producer and consumer struct.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• failed to decode json bytes with simd-json: {err}
• failed to decode json line: {err}
• failed to decode json bytes: {err}
• 'codanna mcp get_index_info' failed: {}

AI-assisted analysis of nikivdev/code@a747e741ae (2026-09-01). Data as JSON: /api/errors/d41bfe23bce9bde7. Report an issue: GitHub.