BoundaryML/baml · error

unknown family member `{name}`

Error message

unknown family member `{name}`

What it means

In the same macro parser (from_input), family member references are resolved by name against the declared members. An ident that matches no declared member produces a compile-time syn::Error spanned on that ident.

Source

Thrown at baml_language/crates/baml_type_macros/src/parse.rs:305

}

impl Family {
    pub(crate) fn from_input(input: FamilyInput) -> syn::Result<Self> {
        let FamilyInput {
            axes,
            members,
            satellites,
            master,
        } = input;

        let axis_index = |name: &Ident| -> syn::Result<usize> {
            axes.iter()
                .position(|a| a == name)
                .ok_or_else(|| syn::Error::new(name.span(), format!("unknown axis `{name}`")))
        };
        let member_index = |name: &Ident| -> syn::Result<usize> {
            members.iter().position(|m| &m.name == name).ok_or_else(|| {
                syn::Error::new(name.span(), format!("unknown family member `{name}`"))
            })
        };

        let master_ident = master.ident.clone();

        let resolved_members = members
            .iter()
            .enumerate()
            .map(|(i, m)| {
                let includes = m
                    .includes
                    .iter()
                    .map(&axis_index)
                    .collect::<syn::Result<Vec<_>>>()?;
                let child = match &m.child {
                    ChildRef::SelfRef => Child::Member(i),
                    ChildRef::Named(name) => {
                        let idx = member_index(name)?;

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Correct the member name in the macro invocation to match a declared member
2. Inspect the family definition's members list for valid names
3. If a member was renamed, update every macro call site referencing the old name

Example fix

// before
member: ChatMesage,

// after
member: ChatMessage,

fn known_member(name: &str, members: &[Member]) -> Option<usize> {
    members.iter().position(|m| m.name == name)
}

// Compile-time error; fix the source. To surface a friendlier message in build scripts:
match known_member("ChatMessage", &members) {
    Some(_) => {}
    None => panic!("member `ChatMessage` not declared; available: {:?}", members.iter().map(|m| &m.name).collect::<Vec<_>>()),
}

When it happens

Trigger: Expanding the macro with input referencing a family member name not present in the members declaration — misspelling, or referencing a member deleted/renamed in the family definition.

Common situations: Typos in member names within macro invocations; adding a member to the invocation but not the family (or removing it from the family but not call sites); copy-paste from a different type family.

Understand the failure class

Background: "invalid id" errors: invalid identifier format — why libraries reject IDs before lookup, and how to fix them — this error's family across 37 libraries.

Related errors

• unknown axis `{name}`
• interned member `{name}` cannot be another member's child
• the master member cannot be interned: the master is the plai
• variant `{}` has more than one `#[axis(..)]`
• variant `{}` must declare exactly one `#[axis(..)]`

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/fe93305ff09d60cb. Report an issue: GitHub.