Invalid client property. Should have been a vertex property

Error message

Invalid client property. Should have been a vertex property but got: {}

What it means

resolve_properties resolves a BAML client's properties against the provider and expects the result to be a Vertex-specific resolved property. If the resolved property is of any other kind (i.e. the client was declared for a different provider or with malformed options), it bails with this message naming the actual resolved property type.

Source

Thrown at engine/baml-runtime/src/internal/llm_client/primitive/vertex/vertex_client.rs:58

pub struct VertexClient {
    pub name: String,
    pub client: reqwest::Client,
    pub retry_policy: Option<String>,
    pub context: RenderContext_Client,
    pub features: ModelFeatures,
    properties: ResolvedVertex,
}

fn resolve_properties(
    provider: &ClientProvider,
    properties: &UnresolvedClientProperty<()>,
    ctx: &RuntimeContext,
) -> Result<ResolvedVertex, anyhow::Error> {
    let properties = properties.resolve(provider, &ctx.eval_ctx(false))?;

    let ResolvedClientProperty::Vertex(mut props) = properties else {
        anyhow::bail!(
            "Invalid client property. Should have been a vertex property but got: {}",
            properties.name()
        );
    };

    if props.anthropic_version.is_none() && props.model.starts_with("claude") {
        props.anthropic_version =
            Some(internal_llm_client::anthropic::DEFAULT_ANTHROPIC_VERSION.to_string());
    }

    if let Some(anthropic_version) = &props.anthropic_version {
        props
            .properties
            .entry("anthropic_version".into())
            .or_insert_with(|| json!(anthropic_version));
        props
            .properties
            .entry("max_tokens".into())

View on GitHub (pinned to bd85ce9dee)

Solutions

Check that the client's options block contains Vertex properties (model, project_id, location, etc.) and not another provider's fields
If using dynamic_runtime, ensure the client properties JSON specifies provider 'vertex' and vertex-shaped options
Rename or remove the misconfigured client and re-declare it with the correct provider/options pair

Example fix

// before (options copied from another provider)
client<MyVertex> MyClient {
  provider vertex
  options { api_key ... }
}
// after
client<MyVertex> MyClient {
  provider vertex
  options {
    model gemini-1.5-pro
    project_id my-gcp-project
    location us-central1
  }
}

Defensive patterns

Strategy: validation

Validate before calling

function isVertexClient(client) {
  return client?.provider === 'vertex' &&
    typeof client.options?.model === 'string' &&
    (client.options?.project_id === undefined || typeof client.options.project_id === 'string');
}

Type guard

function isVertexProps(p) { return !!p && p.provider === 'vertex' && typeof p.options === 'object'; }

Try / catch

try {
  const resolved = baml_client.withOptions({}).MyClient;
} catch (err) {
  if (String(err).includes('Should have been a vertex property')) {
    throw new Error('Client options are not vertex-shaped; check provider/options in baml_clients.baml');
  }
  throw err;
}

Prevention

Match the options block to the declared provider; don't copy options across providers
Validate dynamic client properties against the vertex schema before handing them to dynamic_runtime
Lint BAML client definitions so provider/option pairs are checked

When it happens

Trigger: Declaring a BAML client with provider 'vertex' but whose resolved options are not a Vertex property block — e.g. copying options from an OpenAI/Anthropic client, or using dynamic_runtime context whose properties resolve to a different provider type.

Common situations: Copy-pasting a client definition and changing the provider string without changing options, or passing dynamically loaded client properties (dynamic_new) whose shape doesn't match the vertex provider.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/e994b67f583f2f4e. Report an issue: GitHub.