What causes "field `{field_name}` cannot be empty" in openai/codex?

Give the field a non-empty value

openai/codex · error · ConstraintError

field `{field_name}` cannot be empty

Error message

field `{field_name}` cannot be empty

What it means

ConstraintError::EmptyField (codex-rs/config/src/constraint.rs:22, built via ConstraintError::empty_field) is returned by a Constrained validator when a field that must be non-empty receives an empty string. field_name in the message identifies exactly which field failed.

Source

Thrown at codex-rs/config/src/constraint.rs:22

use crate::config_requirements::RequirementSource;
use thiserror::Error;

#[derive(Debug, Error, PartialEq, Eq)]
pub enum ConstraintError {
    #[error(
        "invalid value for `{field_name}`: `{candidate}` is not in the allowed set {allowed} (set by {requirement_source})"
    )]
    InvalidValue {
        field_name: &'static str,
        candidate: String,
        allowed: String,
        requirement_source: RequirementSource,
    },

    #[error("To use model `{model}`, you need to use auto review.")]
    AutoReviewRequired { model: String },

    #[error("field `{field_name}` cannot be empty")]
    EmptyField { field_name: String },

    #[error("invalid rules in requirements (set by {requirement_source}): {reason}")]
    ExecPolicyParse {
        requirement_source: RequirementSource,
        reason: String,
    },

    #[error(
        "invalid requirement for MCP server `{server_name}` (set by {requirement_source}): {reason}"
    )]
    McpServerRequirementParse {
        server_name: String,
        requirement_source: RequirementSource,
        reason: String,
    },
}

View on GitHub (pinned to 339751715c)

Solutions

Give the field a non-empty value
Remove the key entirely instead of setting it to an empty string
If the value comes from an env var, default it or skip writing the key when the variable is empty

Example fix

# before
name = ""

# after
name = "my-server"

Defensive patterns

Strategy: validation

Validate before calling

fn non_empty(field: &str) -> Result<&str, String> {
    let t = field.trim();
    if t.is_empty() {
        Err("field cannot be empty".into())
    } else {
        Ok(t)
    }
}

Type guard

fn is_non_empty(s: &str) -> bool {
    !s.trim().is_empty()
}

Try / catch

match constrained.set(value) {
    Err(ConstraintError::EmptyField { field_name }) => {
        eprintln!("{field_name} must not be empty");
    }
    r => r?,
}

Prevention

Trim and reject empty inputs at the form or API boundary before they reach config
Omit keys rather than writing them as empty strings in generated configs
Unit-test config writers against empty env vars

When it happens

Trigger: Setting a constrained non-empty string field (a name or identifier in config or requirements) to an empty string via config.toml, an override, or an API set call; the validator does not trim, so empty and whitespace-only values fail.

Common situations: Template configs generated with empty placeholders (name = ""); env-var-driven overrides that expand to an empty string; scripts writing config keys unconditionally even when the source value is missing.

Related errors

AI-assisted analysis of openai/codex@339751715c (2026-08-25). Data as JSON: /api/errors/9b0bc7ceb72c1ea9. Report an issue: GitHub.