Hmbown/CodeWhale · error

.size must be an integer

Error message

{context}.size must be an integer

What it means

parse_resource_entry validates fields returned by an MCP server when building a resource descriptor for the catalog. MCP requires resource 'size' to be an integer byte count; if the server returned a non-integer JSON value (string, float, bool, null) this error is thrown to reject the malformed entry rather than propagate bad metadata.

Solutions

1. Fix the MCP server (or fixture) to emit 'size' as a JSON integer number of bytes
2. Coerce/normalize the value in an intermediary proxy before it reaches this client
3. Remove the 'size' field if the real byte count is unknown — the field is optional

Example fix

// before (server response)
{"name":"doc.txt","size":"1024"}
// after
{"name":"doc.txt","size":1024}

fn has_valid_size(v: &Value) -> bool {
    v.get("size").map_or(true, |s| s.is_i64() || s.is_u64())
}

fn size_is_integer(v: &serde_json::Value) -> bool {
    v.as_i64().is_some() || v.as_u64().is_some()
}

match client.list_resources_with_metadata().await {
    Ok(entries) => use(entries),
    Err(e) if e.to_string().contains(".size must be an integer") => {
        log::warn("server sent non-integer resource size; skipping");
    }
    Err(e) => return Err(e),
}

When it happens

Trigger: list_resources_with_metadata parses a resources/list response whose resource entry contains a 'size' field that is neither a JSON integer (i64/u64) — e.g. "size": "1024" or "size": 10.5.

Common situations: Third-party MCP servers serializing size as a string or float; hand-written mock servers or fixtures with stringified numbers; schema drift after an MCP server upgrade.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• .annotations is not a valid MCP annotations object
• invalid JSON payload at key
• bad_target
• Burn rate is optional. When set, it must be a positive $/hr.
• Cargo metadata dependencies for

AI-assisted analysis of Hmbown/CodeWhale@73e0f67d83 (2026-09-22). Data as JSON: /api/errors/1583ccfbfd5ddf20. Report an issue: GitHub.

Appendix: source

Thrown at crates/mcp/src/stdio_client.rs:199

        .get("uri")
        .and_then(Value::as_str)
        .with_context(|| format!("{context}.uri must be a string"))?
        .to_string();
    let name = fields
        .get("name")
        .and_then(Value::as_str)
        .with_context(|| format!("{context}.name must be a string"))?
        .to_string();
    let description = optional_string_field(fields, "description", &context)?;
    let mime_type = optional_string_field(fields, "mimeType", &context)?;

    let mut metadata = json!({"name": name});
    if let Some(mime_type) = mime_type {
        metadata["mimeType"] = Value::String(mime_type);
    }
    if let Some(size) = fields.get("size") {
        if size.as_i64().is_none() && size.as_u64().is_none() {
            bail!("{context}.size must be an integer");
        }
        metadata["size"] = size.clone();
    }
    if let Some(annotations) = fields.get("annotations") {
        if !valid_annotations(annotations) {
            bail!("{context}.annotations is not a valid MCP annotations object");
        }
        metadata["annotations"] = annotations.clone();
    }

    Ok((
        McpResourceDescriptor {
            server_name: server_name.to_string(),
            uri,
            description,
        },
        metadata,
    ))

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