What causes "server info value must serialize" in vllm-project/vllm?

If running a patched build: audit recently added Config fields for types serde_json cannot serialize (non-string map keys without custom serde) and add #[serde(with = ...)] or convert keys to String.

vllm-project/vllm · critical

server info value must serialize

Error message

server info value must serialize

What it means

A panic (expect) in ServerInfoSnapshot::from_config: serde_json::to_value(config) must succeed when serializing the frontend Config for the /server info endpoints. Config derives Serialize, so this failing means a Config field holds data serde_json cannot represent — an internal invariant break, not user error in the normal path.

Source

Thrown at rust/src/server/src/server_info.rs:32

pub(crate) enum ServerInfoConfigFormat {
    Text,
    Json,
}

/// Snapshot returned by `/server_info`.
#[derive(Debug, Clone)]
pub(crate) struct ServerInfoSnapshot {
    vllm_config_text: String,
    vllm_config_json: Value,
    vllm_env: BTreeMap<String, String>,
    system_env: BTreeMap<String, String>,
}

impl ServerInfoSnapshot {
    /// Capture the runtime configuration fields available to the Rust frontend.
    pub(crate) fn from_config(config: &Config) -> Self {
        let vllm_config_json =
            serde_json::to_value(config).expect("server info value must serialize");

        Self {
            vllm_config_text: render_config_text(&vllm_config_json),
            vllm_config_json,
            vllm_env: collect_vllm_env(),
            system_env: collect_system_env(),
        }
    }

    pub(crate) fn response(&self, config_format: ServerInfoConfigFormat) -> Value {
        let vllm_config = match config_format {
            ServerInfoConfigFormat::Text => Value::String(self.vllm_config_text.clone()),
            ServerInfoConfigFormat::Json => self.vllm_config_json.clone(),
        };

        json!({
            "vllm_config": vllm_config,
            "vllm_env": self.vllm_env.clone(),

View on GitHub (pinned to c794754062)

Solutions

If running a patched build: audit recently added Config fields for types serde_json cannot serialize (non-string map keys without custom serde) and add #[serde(with = ...)] or convert keys to String.
Check that the Serialize derive covers enum variants you added (no untagged variants with conflicting shapes).
On stock builds, report the panic with the Config dump — it indicates a bug in the server.

Example fix

// before
pub struct Config { pub extra: HashMap<u32, String>, .. }  // serde_json: key must be a string

// after
pub struct Config {
    #[serde(with = "serde_json::map_tuple_keys")]
    pub extra: HashMap<u32, String>,
    ..
}

Defensive patterns

Strategy: try-catch

Validate before calling

// if you build custom Config types, prove serializability in a test:
#[test]
fn config_serializes() {
    let cfg = test_config();
    assert!(serde_json::to_value(&cfg).is_ok());
}

Try / catch

let snapshot = std::panic::catch_unwind(|| {
    ServerInfoSnapshot::from_config(&config)
});
// a panic here is an internal invariant break — capture it in a crash handler that logs the Config's Debug form.

Prevention

For any new Config field, add a serde_json round-trip test in CI.
Avoid non-string map keys in serializable config without explicit serde adapters.
Report stock-build occurrences upstream with the panic backtrace.

When it happens

Trigger: A Config field (possibly added by a new feature or non-default transport-mode variant) containing a value that cannot map to JSON — e.g. a map with non-string keys serialized without a helper, or untagged enum data that hits an unserializable state. Fires during snapshot construction at startup or on config reload.

Common situations: Building a custom/patched frontend that adds a Config field with a map key type serde_json rejects; version skew between config producer and serializer; essentially never seen on stock builds.

Related errors

AI-assisted analysis of vllm-project/vllm@c794754062 (2026-08-14). Data as JSON: /api/errors/d06ad1a9e43d24b5. Report an issue: GitHub.