## TL;DR

The var was never defined, so dbt has nothing to substitute. Define it in dbt_project.yml under vars, pass it with --vars on the command line, or give the var() call a default like var('my_var', 'fallback'). Pick one; the default argument is the quickest unbreak.

## Error

```text
Compilation Error: var is undefined in dbt model
```

## Steps

1. Find the var call in your model: search for var('name') matching the undefined name in the error.
   Expected: You locate the exact line and the var name dbt complains about.

2. Check dbt_project.yml for a vars: block defining it, and check whether your run passes --vars '{name: value}'. One of these is missing.
   Expected: You confirm the var is defined nowhere, or only defined for a different target.

3. Fix it: add it under vars: in dbt_project.yml, or change the call to var('name', 'sane_default') so it degrades gracefully.
   Expected: The var resolves; you can verify with dbt compile and inspect the rendered SQL in target/compiled.

4. Run dbt compile --select the model, then a full dbt compile to catch other models using the same var.
   Expected: Whole project compiles with no undefined-var errors.

## When to use

- Compilation Error names a var as undefined
- a model works for one target but fails for another (vars differ per target)
- a new model copied a var() call from elsewhere without the definition

## When not to use

- the error is about env_var, not var (that is an environment variable, different fix)
- the var is defined but has the wrong value (that is a config bug, not undefined)
- the failure happens at warehouse runtime, after successful compile

## Tool compatibility

- dbt-core 1.0 and later; --vars CLI flag syntax unchanged
- dbt Cloud: set vars in the job's deployment environment or job settings
- dbt_project.yml vars block works across all adapters

## Variant phrasings

### var is undefined only in CI
CI runs dbt without the --vars flag your local alias includes. Put the var in dbt_project.yml instead of relying on local shell aliases.

### var undefined inside a macro
Same fix, but check the macro is not called before vars load. Macro-level var() calls resolve fine as long as the var is defined project-wide.

## Why it happens

var() is dbt's project-level variable lookup. If the name is not in dbt_project.yml vars and not passed via --vars, there is nothing to return and dbt raises at compile time rather than rendering an empty string. Copy-paste between projects is the usual way the definition gets lost.

## Edge cases

- vars: defined under a target-specific block only apply to that target; a model compiled for prod will not see dev-only vars.
- Watch quoting: --vars '{my_var: abc}' needs the braces quoted or the shell eats them.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_8OfGIJcGExCf1pn7B1_A7g