Config Merge Tool

About this tool

Real applications usually read more than one configuration file: a checked-in default, an environment-specific override, a local file, and finally deployment variables. This tool merges those layers in the same order. Paste JSON, YAML, TOML or .env text into the four layer boxes; blank layers are skipped, and later layers override earlier layers.

The result is a normalized config document, not a textual patch. Comments, blank lines and YAML anchors are not preserved because they are not part of the merged value tree. The upside is that layers can use different formats: a JSON base, YAML staging override and .env deployment layer can all merge into one JSON, YAML, TOML, .env or report output.

Worked example — JSON base, YAML override and env values

Layer 1:

{
  "db": { "host": "localhost", "port": 5432 },
  "debug": false
}

Layer 2:

db:
  host: staging.internal

Layer 3:

DEBUG=true
DB__PORT=6543

With the defaults, the merged JSON is:

{
  "db": {
    "host": "staging.internal",
    "port": "6543"
  },
  "debug": "true"
}

The YAML layer changed only db.host. The .env layer used DB__PORT to address db.port and DEBUG to override debug using the default case-matching mode. Env values are strings, so the port and debug values are strings in the result.

Merge options

• Object merge: deep merge nested objects, or shallow-replace the whole top-level value.
• Array merge: replace lists (default), append lists, or append only values that are not already present.
• Null deletes: with the default on, a later null removes an inherited key; turn it off to keep null as a value.
• Key case matching: default match lets env-style DB__HOST override db.host; preserve treats case differences as separate keys.
• Variable substitution: ${VAR}, ${VAR:-default} and ${VAR-default} expand after the merge. Values come from the Vars box first, then from the merged config itself (${db.host} or ${DB__HOST}). $$ is a literal dollar sign, and unresolvable references are left as written.

Limits and edge cases

• Total pasted input is capped at 256 KiB across all layers and vars.
• Nesting is capped at 64 levels.
• Each layer must parse to a top-level mapping/object. Bare lists and scalars are rejected.
• TOML output cannot represent null, so nulls are dropped before TOML serialization.
• .env output flattens nested paths with __ and uppercases them. It rejects lists of objects and key collisions such as db.host plus db__host becoming the same env name.
• Auto-detection is intentionally simple: valid JSON wins first, [section] means TOML, KEY=value without spaces means .env, TOML-style key = value is TOML when it parses, and the remaining mapping syntax is YAML. Force the input format when your file is ambiguous.

FAQ