Shadow vs Harmonic Security — open-source Harmonic alternative for AI data protection
Harmonic Security is a browser extension, deployed through device management, that uses small language models to spot sensitive data and risky intent in employee AI use, then warns, blocks or redirects users to approved tools. Shadow is an open-source privacy layer that translates sensitive values in AI API requests into realistic, reversible fakes and restores them in the response. Harmonic shapes how people use AI web apps; Shadow protects the content of API, agent and file traffic while the tools keep working.
At a glance
Section titled “At a glance”| Shadow | Harmonic Security | |
|---|---|---|
| Open source / licence | Yes, Apache-2.0 | No, commercial |
| Deployment | Endpoint HTTPS proxy, gateway (/gw/<provider>/), Rust library | Browser extension deployed via MDM, with a cloud console |
| Protection method | Reversible realistic fakes from a persona; typed tokens for reports; optional redaction | Warn, block, redirect; redaction per vendor |
| Restores responses | Yes, automatically | No |
| Streaming | Yes, SSE and NDJSON with split-fake reassembly | Not applicable (acts before submission) |
| Tool calls | Yes, arguments restored locally; intent-aware for provider-hosted tools | Not applicable |
| Files | DOCX, XLSX, PPTX masked in place; PDF to text; CSV, JSON, HTML | Upload monitoring and control |
| Self-hosted | Yes, single binary, no account | No |
| Price model | Free | Quote |
What Harmonic does well
Section titled “What Harmonic does well”- Context-aware detection. Small language models judge what is being shared rather than relying only on patterns, which can catch things like unreleased financials or source code that regex misses. Harmonic claims it is “96% more accurate than regex” (vendor claim).
- Low friction for users. Warnings and redirects explain the risk and point people to sanctioned tools, which tends to reduce workarounds compared with hard blocks.
- Shadow-AI visibility. Security teams see which AI apps are used across the organisation, including new ones.
- Fast deployment. An extension pushed via MDM covers managed browsers without network changes.
- Upload awareness. File uploads to AI apps are covered as well as typed prompts.
- Low latency claims. The vendor states under 200 ms per check.
Where Shadow is different
Section titled “Where Shadow is different”What happens after sensitive data is detected?
Section titled “What happens after sensitive data is detected?”Harmonic mostly decides whether to warn, block or redirect. Shadow decides how to transform: every sensitive value it detects becomes a realistic fake from a persistent persona, the request proceeds, and the real values are restored in the answer. Users don’t have to choose between privacy and getting the work done.
Traffic a browser extension cannot see
Section titled “Traffic a browser extension cannot see”IDE assistants, CLIs, agents, CI jobs and back-end services call model APIs directly. Shadow covers them when they call a supported AI API host through proxy mode, gateway mode or the Rust library, and restores streams and tool-call arguments.
Intent that changes the transformation
Section titled “Intent that changes the transformation”Harmonic’s intent detection decides whether to flag. Shadow’s intent-aware policy decides how to transform: typed tokens for bulk reports, kept recipients and provider-hosted action targets, fakes elsewhere.
Files masked in place
Section titled “Files masked in place”Shadow masks DOCX, XLSX and PPTX uploads in place and turns PDFs into masked text. See file masking.
Open source and local
Section titled “Open source and local”No account, no telemetry, auditable code.
When should you choose Harmonic?
Section titled “When should you choose Harmonic?”- Your priority is employee behaviour in AI web apps across managed browsers.
- You want contextual detection beyond regex with coaching and redirects.
- You need visibility into which AI tools staff use.
- You want a fast, extension-based rollout.
When should you choose Shadow?
Section titled “When should you choose Shadow?”- You need to protect model API traffic from tools, agents and services.
- Users must keep working with sensitive data and get answers with real values.
- You need streaming, tool-call and Office-file support.
- You want an open-source, self-hosted component.
Using Harmonic and Shadow together
Section titled “Using Harmonic and Shadow together”- Use Harmonic in managed browsers for web-chat AI use and coaching.
- Install Shadow on developer machines, CI runners and services that call model APIs.
- Use Harmonic’s console for organisation-wide visibility, and Shadow’s per-request logs (counts and kinds, never values) for API traffic.
- Optionally strengthen Shadow’s detection with the open-weight NER sidecar or Presidio. See detection.
How do they compare on accuracy?
Section titled “How do they compare on accuracy?”Harmonic’s accuracy figures are vendor claims without a public dataset we could reproduce. Shadow publishes its methodology and code on the benchmarks page.
Sources
Section titled “Sources”Last reviewed: 2026-10-31. Harmonic Security is a trademark of its owner; this comparison is based on public documentation as of the review date.
Frequently asked questions
Is Shadow a free alternative to Harmonic Security?
For API and agent traffic, partly. Shadow is free and open source and masks prompts to model APIs with reversible fakes. Harmonic is a commercial browser-extension product that watches employee use of AI apps with small language models and warns, blocks or redirects. Shadow does not provide browser-level visibility or user coaching.
How does Harmonic Security detect sensitive data?
Harmonic says it uses small language models in a browser extension to judge the intent and sensitivity of what users share. It claims under 200 ms latency and 96% better accuracy than regex; these are vendor claims and we have not seen independent results.
Does Harmonic restore redacted values in AI responses?
Not that public documentation describes. Harmonic focuses on warning, blocking, redirecting to approved tools and, per the vendor, redaction. Shadow restores its fakes in responses, streams and tool calls.
How much does Harmonic Security cost?
Harmonic pricing is by quote. Shadow is free under Apache-2.0.
Can Harmonic and Shadow be used together?
Yes. Harmonic can cover employee use of AI web apps on managed browsers while Shadow masks traffic from developer tools, agents and services that a browser extension does not see.