How is this different from the Formatter or Merge?

Both of those parse the document into a data structure and write a fresh one, which is correct TOML and a terrible diff: comments disappear, key order normalises, and quoting style changes. This tool never re-emits the file. It finds the line that defines your key and rewrites the characters after the equals sign, so a version bump is a one-line diff instead of a whole-file rewrite.

What happens to my comments?

They stay exactly where they are, including a comment on the same line as the key you are changing — and including its alignment, so version = "1.0" # pinned keeps its three spaces. A # inside a string is correctly treated as part of the string, not as the start of a comment.

Can it change a value that spans several lines?

No, and it refuses rather than guessing. A multi-line array or a triple-quoted string cannot be patched by rewriting one line without risking a broken file, so the tool says so and changes nothing. Use Merge for those.

What if the key does not exist yet?

It is added. If the parent table already has a header the key goes in underneath it; otherwise a new table header is appended at the end of the file. Choose only if it already exists to make a missing key an error instead — which is what you want in a script.

How do I reach an item in an array of tables?

Use its zero-based index as a path segment, the same as the Query tool: servers.1.ip is the ip of the second [[servers]] block.

Is my config uploaded?

Never. The file is parsed and patched in your browser. You can disconnect from the network first and it still works.

TOML Set Value

updated 26 August 2026

Change one value and change nothing else. Give it a dotted path — package.version, tool.poetry.name, servers.1.ip — and a new value, and it rewrites just the characters after that key's equals sign. Comments stay, key order stays, spacing stays, quoting style stays. The diff is one line, which is the difference between a reviewable commit and an unreviewable one.

— S., [email protected]

Path New value Type If missing

one line changes, nothing else

TOML input

Result

Why a text edit rather than a rewrite

The obvious way to change a TOML value is to parse the file into a data structure, set the field, and serialise it back out. Every other write-capable tool on this site works that way, and for converting between formats it is exactly right.

For editing a file you intend to keep, it is wrong. Parsing throws away everything that is not data: the comment explaining why a dependency is pinned, the blank line separating two sections, the decision to write 'literal' rather than "basic", the order the author chose. Serialising puts back a canonical form. The value you wanted to change is one line; the diff is the whole file, and nobody can review it.

So this tool never parses-and-re-emits. It reads the file once to confirm it is valid TOML — patching text you cannot parse is how you produce a broken Cargo.toml — then scans the lines tracking which table it is inside, finds the line that defines your key, and replaces the characters between the equals sign and the end of the value. Everything else is copied through byte for byte, down to the line endings: a CRLF file stays CRLF.

What it will not do

A value spread over several lines — a multi-line array, a triple-quoted string — cannot be patched by rewriting a single line, and half-rewriting one is how you get a file that no longer parses. When the target turns out to span lines the tool refuses and changes nothing, rather than doing something clever. Merge handles those, at the cost of reformatting.

It also will not invent structure it cannot place. A missing key is added under its table if that table exists, or in a new table appended at the end if it does not — but a key whose parent is an array of tables that has no matching index is an error, because there is no right answer to which block you meant.

Types, and when Auto guesses wrong

On Auto, a value that TOML would read as a number, a boolean, a date or a date-time is written bare, and anything else is quoted as a basic string. That is right almost always and wrong in one memorable case: a version like 2.0 is a valid float, so version = 2.0 is what you get, and Cargo will reject it. Set the type to String when the value only looks like a number.

Raw writes exactly what you typed with no quoting or checking, which is how you set an inline table or an array on one line: { version = "1.0", optional = true } or ["derive", "rc"]. It is also how you write a syntax error, so the status line's validation is worth reading afterwards.