The question that actually matters before pasting anything into an online diff checker: does this tool process the comparison on their servers, or entirely in your browser? If you're diffing API keys, a client contract, or proprietary code, that's not a minor detail — several popular diff tools upload your text to compare it, some even generate shareable public links by default. I compared 5 tools specifically on that axis, plus what each one actually does once the diff is computed.
1. ToolTiny — fully local, with a merge workspace and JSON-aware diffing
ToolTiny's Text Compare tool runs the entire diff — built on the jsdiff library — inside your browser tab, so nothing you paste or upload ever reaches a server. Word-by-Word mode handles contracts and prose; Line-by-Line handles config files, logs, and source code. You can paste directly or upload .txt, .log, .json, .py, .csv, and similar files without copy-pasting first.
Past the basic highlight-the-difference stage, it now covers what you'd expect from a full comparison workspace: a JSON-normalize mode that parses both inputs and sorts keys recursively before comparing, so two JSON files that only differ in key order or indentation correctly show as identical rather than flagging false differences; ignore-whitespace and ignore-case toggles; a live stats bar showing characters/lines added, removed, and modified alongside an overall similarity percentage; a minimap sidebar for jumping straight to a change in a long document; a merge panel where each changed segment gets its own "Keep Original" / "Keep New" choice, building a combined result you can copy out; and one-click export of the whole comparison as a self-contained HTML file. All of it still runs client-side — the feature list grew, the privacy model didn't change.
2. Diffchecker — the most polished workspace, with a privacy trade-off to know about
Diffchecker offers switchable views, a rule system to ignore timestamps, build numbers, or session IDs before comparing, and merge functionality with a genuinely refined interface. Free to try, unlimited on the paid tier. Worth knowing explicitly: Diffchecker's core convenience feature — shareable diff links — means comparisons can be processed and stored server-side rather than staying local to your browser. For non-sensitive comparisons where sharing a link matters more than privacy, that's a reasonable trade. For anything confidential, it's the one thing to check before pasting.
3. Mergely — built for developers who want to edit while diffing
The most code-editor-like interface here: syntax highlighting, line numbers, and inline editing directly within the diff view — you can fix either side of the comparison without leaving the tool and re-diff immediately. If your workflow is "compare, then fix what's wrong," this saves a copy-out-edit-copy-back cycle that even merge-panel tools like ToolTiny still involve.
4. EveryTool — the other fully-local option with a deep feature set
Also 100% browser-based using the Myers diff algorithm (the same one Git uses): JSON mode, whitespace/casing filters, a minimap, live change counts with similarity percentage, and merge-by-accepting-individual-changes. It's a close match to ToolTiny feature-for-feature at this point — the meaningful difference is that ToolTiny also takes direct file uploads for common code and data formats, where EveryTool's flow is paste-based.
5. OnlineTextCompare — simplest two-way merge tool, fully local
A lighter-weight alternative to Mergely's full-editor approach: local processing, no signup, no usage limits, with arrow-based merging between the two versions. Good middle ground if you want basic merge capability without the deeper feature set of the tools above.
Quick comparison
| Tool | Fully local | Merge function | JSON-aware diff | Notable feature |
|---|---|---|---|---|
| ToolTiny | ✅ | ✅ | ✅ (key-order normalized) | Direct file upload + HTML export |
| Diffchecker | Partial (shareable links processed server-side) | ✅ | Not specified | Ignore-rules for timestamps/build numbers |
| Mergely | ✅ | ✅ | Not specified | Inline editing within the diff view |
| EveryTool | ✅ | ✅ | ✅ | Minimap, similarity %, HTML export |
| OnlineTextCompare | ✅ | ✅ | Not specified | Simple arrow-based merge, unlimited use |
Which one for which situation
- Comparing something confidential — contracts, API keys, proprietary code? ToolTiny, Mergely, EveryTool, or OnlineTextCompare — all fully local, nothing leaves your device.
- Need to share a diff with a non-technical stakeholder via link? Diffchecker, understanding that means server-side processing.
- Want to fix issues while comparing, not after? Mergely's inline editing is still the one dedicated tool for that specific workflow.
- Comparing JSON files that might just be reordered or reformatted? ToolTiny or EveryTool — both normalize before diffing so formatting noise doesn't show up as a false change.
- Want the result as a standalone file to attach to a ticket or email? ToolTiny or EveryTool's HTML export.
The honest note
"Runs in your browser" is doing real work as a phrase, but it's worth verifying rather than assuming for any tool not on this list. A diff tool with a "share this comparison" button is, by necessity, sending your text somewhere to generate that link — the convenience and the privacy guarantee are in direct tension, and a tool can't honestly offer both for the same feature. Before pasting anything you wouldn't want on a stranger's server, check specifically whether the tool's sharing or export features require a round-trip through their backend, not just whether the initial diff computation happens locally.
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