Guides Rust error handling strategy. Use when choosing between Result/Option/panic, using anyhow vs thiserror, propagating errors with ?, or designing custom error types.
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Layer 1: Language Mechanics
Is this failure expected or a bug?
Before choosing error handling strategy:
Before handling an error:
What kind of failure is this?
Result<T, E>Option<T>panic!panic!Who handles this?
?match/if-letWhat context is needed?
anyhow::ContextIs failure expected?
├─ Yes → Is absence the only "failure"?
│ ├─ Yes → Option<T>
│ └─ No → Result<T, E>
│ ├─ Library → thiserror
│ └─ Application → anyhow
└─ No → Is it a bug?
├─ Yes → panic!, assert!
└─ No → Consider if really unrecoverable
Use ? → Need context?
├─ Yes → .context("message")
└─ No → Plain ?
| Pattern | When | Example |
|---|---|---|
Result<T, E> | Recoverable error | fn read() -> Result<String, io::Error> |
Option<T> | Absence is normal | fn find() -> Option<&Item> |
? | Propagate error | let data = file.read()?; |
unwrap() | Dev/test only | config.get("key").unwrap() |
expect() | Invariant holds | env.get("HOME").expect("HOME set") |
panic! | Unrecoverable | panic!("critical failure") |
| Context | Error Crate | Why |
|---|---|---|
| Library | thiserror | Typed errors for consumers |
| Application | anyhow | Ergonomic error handling |
| Mixed | Both | thiserror at boundaries, anyhow internally |
| Mistake | Fix |
|---|---|
.unwrap() in production | Use ? or match |
Type mismatch on ? | Use anyhow or impl From<E> |
| Lost error context | Add .context("what was happening") |
Box<dyn Error> everywhere | Use thiserror for typed errors |
| Anti-Pattern | Why Bad | Better |
|---|---|---|
.unwrap() everywhere | Panics in production | .expect("reason") or ? |
| Ignore errors silently | Bugs hidden | Handle or propagate |
panic! for expected errors | Bad UX, no recovery | Result |
Box<dyn Error> everywhere | Lost type info | thiserror |
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Babysit a GitHub pull request during a user-approved monitoring window by polling review comments, CI checks/workflow runs, and mergeability state. Diagnose failures, retry likely flaky failures up to 3 times, auto-fix/push branch-related issues when appropriate, and report fresh review feedback. Use when the user asks Codex to monitor a PR, watch CI, handle review comments, or keep an eye on failures and feedback on an open PR.
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