What causes "mapping values are not allowed here" in YAML?

This usually means a colon appears where YAML did not expect a key/value boundary, often due to indentation drift, mixed tabs/spaces, or unquoted values containing colons.

Should I use tabs in YAML files?

No. YAML indentation should use spaces only. Tabs are a common source of parser failures in CI and deployment pipelines.

How can I validate YAML before committing?

Run a YAML validator after edits and before push. Validate the smallest failing block first, then expand to the full document once the block parses correctly.

YAML Parser Errors: Fast Fix Guide for Indentation and Syntax Failures (2026)

Published February 28, 2026 · 7 min read

Validate before redeploy: run suspicious snippets through the YAML Validator to pinpoint indentation and key/value parse failures before CI blocks your pipeline.

Most YAML parse failures are small structural mistakes that become expensive only because they are hard to spot in long config files. This guide uses a deterministic triage order so you can isolate the failing block and fix it in minutes.

Recognize the common parser errors
Run a fast triage workflow
Fix patterns with before/after examples
Harden your CI check
Pre-commit checklist

1. Recognize the common parser errors

These messages usually point to formatting, not logic:

mapping values are not allowed here
did not find expected key
found character '\t' that cannot start any token
block sequence entries are not allowed in this context

Warning: Fix one error at a time and re-validate. A single indentation mismatch can cascade into many false follow-up errors.

2. Run a fast triage workflow

Copy the exact failing line range from CI output.
Replace all tabs with spaces in that block.
Align sibling keys to the same indentation depth.
Quote values containing :, #, or leading special characters.
Validate the small block first, then the full document.

# locate tabs quickly
rg -n "\t" your-config.yaml

# visualize indentation width
sed -n 'start,endp' your-config.yaml | sed 's/ /·/g'

3. Fix patterns with before/after examples

Error: mapping values are not allowed here

# bad
env:
  API_URL: https://api.example.com:v1

# good
env:
  API_URL: "https://api.example.com:v1"

Error: did not find expected key

# bad
services:
  web:
    image: myapp:latest
     ports:
      - "8080:8080"

# good
services:
  web:
    image: myapp:latest
    ports:
      - "8080:8080"

Error: tab character in indentation

# bad (tab before image)
services:
	web:
    image: myapp:latest

# good
services:
  web:
    image: myapp:latest

Tip: Keep YAML indentation at two spaces per level and enforce it with editor settings plus a lint rule in CI.

4. Harden your CI check

Catch formatting errors before deploy by validating YAML on pull requests:

# example: validate all yaml files in repo
find . -type f \( -name "*.yml" -o -name "*.yaml" \) -print0 \
  | xargs -0 -n1 yamllint

If you need a quick browser-based spot check during debugging, validate suspect blocks first, then re-run your repo-level lint command.

5. Pre-commit checklist

No tabs in YAML files.
Consistent indentation for sibling keys.
Strings with colon/hash safely quoted.
List items aligned with parent key depth.
At least one YAML validation step passes locally before push.

FAQ

Why does my YAML parse locally but fail in CI?

Different linters/parsers can be stricter in CI. Standardize on one linter version and run the same command locally.

Can comments break YAML parsing?

Yes, if a value includes # without quoting, the parser may treat the rest as a comment and break structure.

What is the safest first step when the error line looks wrong?

Start a few lines above the reported line. YAML parser errors often surface after the true root cause.