vllm-project/vllm · error · Error

{kind} parser `{name}` is not registered{}

Error message

{kind} parser `{name}` is not registered{}

What it means

Thrown when a chat frontend requests a named structured-output or tool-call parser by name, but the parser registry has no creator registered under that name. The message is augmented by `available_parser_hint`, which appends the list of registered parser names (e.g. ` (available: guided_json, regex)`). Registry lookups happen in `rust/src/chat/src/parser/tool/mod.rs:156`, `parser/unified.rs:72`, `parser/reasoning/mod.rs:139`, and in `lib.rs:73/84` when the frontend config selects a parser.

Source

Thrown at rust/src/chat/src/error.rs:37

    ChatTemplate(String),
    #[error("multimodal input is not supported by this chat renderer")]
    UnsupportedMultimodalRenderer,
    #[error("unsupported multimodal content: {0}")]
    UnsupportedMultimodalContent(&'static str),
    #[error("`{modality}` input is not supported by this model")]
    UnsupportedModality { modality: String },
    #[error("At most {limit} {modality}(s) may be provided in one prompt.")]
    MmLimitExceeded { modality: String, limit: usize },
    #[error("multimodal preprocessing error: {0}")]
    Multimodal(#[message] String),
    #[error("{kind} parsing is not available for model `{model_id}`")]
    ParserUnavailableForModel {
        kind: &'static str,
        model_id: String,
    },
    #[error("{kind} parsing is disabled by frontend configuration")]
    ParserDisabled { kind: &'static str },
    #[error(
        "{kind} parser `{name}` is not registered{}",
        available_parser_hint(.available_names)
    )]
    ParserUnavailableByName {
        kind: &'static str,
        name: String,
        available_names: Vec<String>,
    },
    #[error("failed to initialize {kind} parser `{name}`")]
    ParserInitialization {
        kind: &'static str,
        name: String,
        #[source]
        error: BoxedError,
    },
    #[error(
        "gpt_oss uses native Harmony output parsing; generic {kind} parser override `{selection}` is not supported"
    )]

View on GitHub (pinned to c794754062)

Solutions

1. Read the `(available: ...)` suffix in the message and switch the config to one of the listed names.
2. Check the parser kind: tool parsers, unified (structured) parsers, and reasoning parsers each have separate registries; a name valid for one kind is invalid for another.
3. If the parser should exist, rebuild the Rust workspace with the cargo feature that gates it (see rust/src/chat/src/parser/*/mod.rs registration lists).
4. Search the codebase for the exact name string to confirm it is registered at startup.

Example fix

// before
structured_outputs.parser = "json_mode"; // not registered

// after
structured_outputs.parser = "guided_json"; // name taken from the available list in the error

let registered: &[&str] = registry.available_names(kind);
let name = config.parser_name.as_str();
if !registered.contains(&name) {
    return Err(format!("parser `{name}` not registered; available: {registered:?}"));
}

fn is_registered_parser(registry: &ParserRegistry, kind: ParserKind, name: &str) -> bool {
    registry.available_names(kind).iter().any(|n| n == name)
}

match result {
    Err(vllm_chat::Error::ParserUnavailableByName { name, available_names, .. }) => {
        eprintln!("parser `{name}` unknown; pick from {available_names:?}");
    }
    other => other?,
}

When it happens

Trigger: Passing `structured_outputs.parser = "<name>"` or `tool_call_parser = "<name>"` in the Rust frontend config where <name> is misspelled or was compiled out (parser crates are behind feature flags). Calling `ParserRegistry::creator(name)` for a kind (tool / structured / reasoning) whose registry only contains a subset of names.

Common situations: Typos like `hermes` vs `llama3_json`; migrating from Python vLLM where a parser exists but the Rust build does not enable that feature; assuming hermetic-style names, then running against a build compiled without them.

Related errors

• gpt_oss uses native Harmony output parsing; generic {kind} p
• failed to initialize {kind} parser `{name}`
• failed to read chat template file
• chat template looks like a file path but does not exist
• cannot combine `--headless` with `--data-parallel-size-local

AI-assisted analysis of vllm-project/vllm@c794754062 (2026-08-14). Data as JSON: /api/errors/c7f0efcddc777560. Report an issue: GitHub.