Error Reporting

Q: anyhow in libraries?

Avoid it. Library consumers need typed errors. Use thiserror and let binaries wrap with anyhow.

Q: How do I add error codes?

Use miette::Diagnostic with a code(...) attribute for searchable error IDs.

Q: JSON error output?

Define an ErrorResponse { code, message, details } struct and print serde JSON on failure when --json is set.

Q: Suppress warnings but show errors?

Warnings go to stderr with WARN: prefix. Errors use non-zero exit and distinct formatting.

Q: Localized errors?

Use fluent or rust-i18n for translated messages. Keep error codes stable across locales.

Q: Testing error messages?

Assert on err.to_string() or format!("{err:#}") in integration tests.

Q: Log errors and print them?

Log full detail with tracing::error!. Print a short user-facing summary to stderr.

Q: Multiple errors at once?

Collect errors in a Vec and print all before exiting. Common in linters and validators.

Q: Caused by chains?

anyhow supports .context() and #[source]. Each layer adds operator-relevant detail.

Q: miette without nightly?

miette works on stable Rust. Enable fancy feature for graphical reports when stderr is a TTY.

Present actionable errors to CLI users with anyhow and miette.

Recipe

use anyhow::Context;
 
fn read_config(path: &str) -> anyhow::Result<String> {
    std::fs::read_to_string(path)
        .with_context(|| format!("failed to read config at {path}"))
}
 
fn main() -> anyhow::Result<()> {
    let cfg = read_config("app.toml")?;
    println!("loaded {cfg}");
    Ok(())
}

When to reach for this: Application binaries where users need context-rich messages, not library-style typed errors.

Working Example

use miette::{miette, IntoDiagnostic, Result};
 
#[derive(Debug, thiserror::Error, Diagnostic)]
#[error("invalid port: {0}")]
#[diagnostic(code(app::invalid_port))]
struct InvalidPort(u16);
 
fn parse_port(s: &str) -> Result<u16> {
    let port: u16 = s.parse().into_diagnostic()?;
    if port == 0 {
        return Err(miette!(InvalidPort(port)));
    }
    Ok(port)
}
 
fn main() -> Result<()> {
    let port = parse_port("0")?;
    println!("listening on {port}");
    Ok(())
}

What this demonstrates:

anyhow::Context adds location context to std errors
miette provides diagnostic codes and formatted reports
thiserror defines small domain error types
main returns Result for automatic non-zero exit

Deep Dive

anyhow vs thiserror vs miette

Crate	Role
`anyhow`	Ergonomic `Result` in binaries
`thiserror`	Typed errors in libraries
`miette`	Beautiful reports with source spans

CLI Error Guidelines

One problem per error message
Say what failed, why, and how to fix it
Print to stderr; never mix errors into stdout JSON
Chain context as you cross subsystem boundaries

Exit Codes

use std::process::ExitCode;
 
fn main() -> ExitCode {
    if let Err(e) = run() {
        eprintln!("{e:#}");
        return ExitCode::FAILURE;
    }
    ExitCode::SUCCESS
}

Use {:#} for the full anyhow chain.

Gotchas

Printing Display only - Users miss the error chain. Fix: Use {:#} or miette::Report.
Panics in CLI - Ugly stack traces for users. Fix: Return Result and reserve panics for bugs.
Library uses anyhow - Callers cannot match error types. Fix: Libraries use thiserror; binaries wrap with anyhow.
Verbose RUST_BACKTRACE by default - Overwhelms users. Fix: Show backtrace only with --verbose or env hint.
Leaking internal paths - /home/ci/... confuses users. Fix: Strip or relativize paths in user messages.

Alternatives

Alternative	Use When	Don't Use When
`thiserror` only	Libraries with matchable errors	Top-level binary needing context chains
Plain `eprintln!`	Tiny scripts	Multi-step pipelines needing context
`color-eyre`	Dev tools during development	Minimal dependency production CLIs

FAQs

anyhow in libraries?

Avoid it. Library consumers need typed errors. Use thiserror and let binaries wrap with anyhow.

How do I add error codes?

Use miette::Diagnostic with a code(...) attribute for searchable error IDs.

JSON error output?

Define an ErrorResponse { code, message, details } struct and print serde JSON on failure when --json is set.

Suppress warnings but show errors?

Warnings go to stderr with WARN: prefix. Errors use non-zero exit and distinct formatting.

Localized errors?

Use fluent or rust-i18n for translated messages. Keep error codes stable across locales.

Testing error messages?

Assert on err.to_string() or format!("{err:#}") in integration tests.

Log errors and print them?

Log full detail with tracing::error!. Print a short user-facing summary to stderr.

Multiple errors at once?

Collect errors in a Vec and print all before exiting. Common in linters and validators.

Caused by chains?

anyhow supports .context() and #[source]. Each layer adds operator-relevant detail.

miette without nightly?

miette works on stable Rust. Enable fancy feature for graphical reports when stderr is a TTY.

CLI Basics - exit codes
clap - validation errors
Configuration & Env - config load failures
CLI Best Practices - actionable messages

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Recipe

use anyhow::Context;
 
fn read_config(path: &str) -> anyhow::Result<String> {
    std::fs::read_to_string(path)
        .with_context(|| format!("failed to read config at {path}"))
}
 
fn main() -> anyhow::Result<()> {
    let cfg = read_config("app.toml")?;
    println!("loaded {cfg}");
    Ok(())
}

When to reach for this: Application binaries where users need context-rich messages, not library-style typed errors.

Working Example

use miette::{miette, IntoDiagnostic, Result};
 
#[derive(Debug, thiserror::Error, Diagnostic)]
#[error("invalid port: {0}")]
#[diagnostic(code(app::invalid_port))]
struct InvalidPort(u16);
 
fn parse_port(s: &str) -> Result<u16> {
    let port: u16 = s.parse().into_diagnostic()?;
    if port == 0 {
        return Err(miette!(InvalidPort(port)));
    }
    Ok(port)
}
 
fn main() -> Result<()> {
    let port = parse_port("0")?;
    println!("listening on {port}");
    Ok(())
}

What this demonstrates:

anyhow::Context adds location context to std errors
miette provides diagnostic codes and formatted reports
thiserror defines small domain error types
main returns Result for automatic non-zero exit

Deep Dive

anyhow vs thiserror vs miette

Crate	Role
`anyhow`	Ergonomic `Result` in binaries
`thiserror`	Typed errors in libraries
`miette`	Beautiful reports with source spans

CLI Error Guidelines

One problem per error message
Say what failed, why, and how to fix it
Print to stderr; never mix errors into stdout JSON
Chain context as you cross subsystem boundaries

Exit Codes

use std::process::ExitCode;
 
fn main() -> ExitCode {
    if let Err(e) = run() {
        eprintln!("{e:#}");
        return ExitCode::FAILURE;
    }
    ExitCode::SUCCESS
}

Use {:#} for the full anyhow chain.

Gotchas

Printing Display only - Users miss the error chain. Fix: Use {:#} or miette::Report.
Panics in CLI - Ugly stack traces for users. Fix: Return Result and reserve panics for bugs.
Library uses anyhow - Callers cannot match error types. Fix: Libraries use thiserror; binaries wrap with anyhow.
Verbose RUST_BACKTRACE by default - Overwhelms users. Fix: Show backtrace only with --verbose or env hint.
Leaking internal paths - /home/ci/... confuses users. Fix: Strip or relativize paths in user messages.

Alternatives

Alternative	Use When	Don't Use When
`thiserror` only	Libraries with matchable errors	Top-level binary needing context chains
Plain `eprintln!`	Tiny scripts	Multi-step pipelines needing context
`color-eyre`	Dev tools during development	Minimal dependency production CLIs

FAQs

anyhow in libraries?

Avoid it. Library consumers need typed errors. Use thiserror and let binaries wrap with anyhow.

How do I add error codes?

Use miette::Diagnostic with a code(...) attribute for searchable error IDs.

JSON error output?

Define an ErrorResponse { code, message, details } struct and print serde JSON on failure when --json is set.

Suppress warnings but show errors?

Warnings go to stderr with WARN: prefix. Errors use non-zero exit and distinct formatting.

Localized errors?

Use fluent or rust-i18n for translated messages. Keep error codes stable across locales.

Testing error messages?

Assert on err.to_string() or format!("{err:#}") in integration tests.

Log errors and print them?

Log full detail with tracing::error!. Print a short user-facing summary to stderr.

Multiple errors at once?

Collect errors in a Vec and print all before exiting. Common in linters and validators.

Caused by chains?

anyhow supports .context() and #[source]. Each layer adds operator-relevant detail.

miette without nightly?

miette works on stable Rust. Enable fancy feature for graphical reports when stderr is a TTY.

CLI Basics - exit codes
clap - validation errors
Configuration & Env - config load failures
CLI Best Practices - actionable messages

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Error Reporting

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Error Reporting