What causes "Error parsing JSON from URL {} {}" in SeleniumHQ/selenium?

Manually fetch the URL and confirm the body is valid JSON matching the expected schema.

SeleniumHQ/selenium · error · anyhow::Error

Error parsing JSON from URL {} {}

Error message

Error parsing JSON from URL {} {}

What it means

Raised by parse_json_from_url() in rust/src/downloads.rs when serde_json::from_str fails on the body fetched from a URL. This wraps the serde error and the URL so the caller knows which endpoint returned malformed JSON. It is used to parse version-discovery endpoints (CfT, NuGet, msedgedriver) and any deserialization failure is fatal for that discovery path.

Source

Thrown at rust/src/downloads.rs:108

pub async fn read_redirect_from_link(
    http_client: &Client,
    url: String,
    log: &Logger,
) -> Result<String, Error> {
    parse_version(
        http_client.get(&url).send().await?.url().path().to_string(),
        log,
    )
}

pub fn parse_json_from_url<T>(http_client: &Client, url: &str) -> Result<T, Error>
where
    T: Serialize + for<'a> Deserialize<'a>,
{
    let content = read_content_from_link(http_client, url)?;
    match serde_json::from_str(&content) {
        Ok(json) => Ok(json),
        Err(err) => Err(anyhow!(format!(
            "Error parsing JSON from URL {} {}",
            url, err
        ))),
    }
}

View on GitHub (pinned to aa36b38e69)

Solutions

Manually fetch the URL and confirm the body is valid JSON matching the expected schema.
Retry to rule out a transient partial response.
Check for proxy/TLS interception that rewrites the body.
If the upstream schema changed, update the deserializing struct T.

Defensive patterns

Strategy: try-catch

Validate before calling

// Rust: validate JSON shape before deserializing into the typed struct
let content = read_content_from_link(http_client, url)?;
let v: serde_json::Value = serde_json::from_str(&content)
    .map_err(|e| anyhow!("Endpoint {} returned non-JSON: {}", url, e))?;

Try / catch

match parse_json_from_url::<T>(http_client, url) {
    Ok(data) => Ok(data),
    Err(e) if e.to_string().contains("Error parsing JSON") => {
        log::warn!("Version endpoint {} returned malformed JSON; falling back", url);
        fallback_discovery()
    }
    Err(e) => Err(e),
}

Prevention

Log the raw body when JSON parsing fails for diagnosis.
Pin the deserializing structs to the documented API schema.
Detect HTML/error pages before attempting JSON parse.

When it happens

Trigger: read_content_from_link returns successfully but the text is not valid JSON for the expected type T. Causes: endpoint returns HTML error page, a truncated response, a schema mismatch where the JSON doesn't fit T, or a 200 with an empty body.

Common situations: A version endpoint is temporarily replaced by a maintenance HTML page; corporate proxy injects a block page; the upstream API schema changed; TLS interception corrupts the body.

Related errors

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/18f83200522a47a7. Report an issue: GitHub.