Shadow vs Zscaler — open-source Zscaler GenAI DLP alternative for AI APIs
Zscaler’s GenAI protection, part of the Zscaler Zero Trust Exchange, lets security teams allow, block, isolate and log employee use of AI apps, with DLP inspection of what is sent. 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. Zscaler governs who uses which AI apps across a fleet; Shadow keeps the content of API, agent and file traffic private while the tools keep working.
At a glance
Section titled “At a glance”| Shadow | Zscaler GenAI protection | |
|---|---|---|
| Open source / licence | Yes, Apache-2.0 | No, commercial |
| Deployment | Endpoint HTTPS proxy, gateway (/gw/<provider>/), Rust library | SASE inline (Zero Trust Exchange, Client Connector) |
| Protection method | Reversible realistic fakes from a persona; typed tokens for reports; optional redaction | Allow, block, isolate, log; DLP inspection |
| Restores responses | Yes, automatically | Not documented |
| Streaming | Yes, SSE and NDJSON with split-fake reassembly | Not applicable to block or isolate decisions; not documented for transformation |
| Tool calls | Yes, arguments restored locally; intent-aware for provider-hosted tools | Not documented |
| Files | DOCX, XLSX, PPTX masked in place; PDF to text; CSV, JSON, HTML | Upload inspection and blocking |
| Self-hosted | Yes, single binary, no account | No, cloud service |
| Price model | Free | Quote |
What Zscaler does well
Section titled “What Zscaler does well”- Scale and coverage. Zscaler inspects traffic from managed devices wherever they are, so AI use on laptops at home, in offices and on the road falls under one policy.
- Shadow-AI discovery. Security teams can see which AI apps are used and by whom, and decide which to sanction.
- Browser isolation. Instead of a hard block, risky AI apps can be opened in an isolated browser where, depending on policy, pasting or uploading is restricted. That keeps the tool usable for low-risk work.
- Unified DLP. The same dictionaries and engines used for web and SaaS DLP apply to AI prompts.
- Logging and SOC integration. Prompts and decisions can be logged centrally and streamed to SIEM tools, which supports investigations and audit evidence.
- Part of a wider zero-trust platform. Organisations already using Zscaler for internet and private-app access add AI controls without new agents.
Where Shadow is different
Section titled “Where Shadow is different”Does Zscaler keep AI tools working when data is sensitive?
Section titled “Does Zscaler keep AI tools working when data is sensitive?”Zscaler’s documented actions for AI apps are allow, block, isolate and log. Blocking protects data but stops the work, and people find workarounds. Shadow lets the request through with every sensitive value it detects translated: jane@acme.com becomes hui.zhao@zhilan.cn, a card number stays Luhn-valid, and a codename becomes another plausible codename from a persistent persona. The model answers, and Shadow restores the real values before you see them.
Streaming and tool calls
Section titled “Streaming and tool calls”Shadow restores fakes in SSE and NDJSON streams, even when a fake is split across events, and in tool-call arguments so agents act on real targets. Public Zscaler documentation does not describe value transformation and restoration for these flows.
Intent-aware handling
Section titled “Intent-aware handling”Shadow treats an agent action, a report over customer rows, a code question and a chat differently. See intent-aware policy.
Files are masked, not only blocked
Section titled “Files are masked, not only blocked”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, no plaintext sent to a security cloud. Shadow runs on the endpoint, in a container or inside a Rust service.
When should you choose Zscaler?
Section titled “When should you choose Zscaler?”- You need company-wide visibility and control over AI app use on managed devices.
- You already run Zscaler for internet access and want AI policy in the same console.
- Browser isolation or block-and-coach fits your risk appetite.
- You need central logs and reporting for your SOC and auditors.
When should you choose Shadow?
Section titled “When should you choose Shadow?”- The traffic you care about is API calls from developer tools, agents, CI and services.
- People must keep using AI with sensitive data, with answers restored to real values.
- You need streaming and tool-call support.
- You want an open-source control you can audit and run without a vendor cloud.
Using Zscaler and Shadow together
Section titled “Using Zscaler and Shadow together”- Keep Zscaler as the fleet control for browser and SaaS AI use.
- Install Shadow on machines and services that call model APIs (proxy mode for whole machines, gateway mode for SDKs, or the Docker sidecar).
- Avoid double TLS interception: exclude the AI API hosts Shadow handles from SSL inspection on those machines, or send Shadow’s outbound traffic through Zscaler with
upstream_proxyso Zscaler inspects already-masked requests. - Keep allow-listing decisions in Zscaler; let Shadow handle the content of allowed requests.
How do they compare on accuracy?
Section titled “How do they compare on accuracy?”Zscaler does not publish PII detection results on public datasets, and an independent comparison would need a tenant. Shadow publishes its methodology and code on the benchmarks page.
Sources
Section titled “Sources”- Zscaler: Make generative AI tools like ChatGPT safe and secure
- Zscaler product site
- Shadow: how it works
- Shadow: security model
Last reviewed: 2026-10-31. Zscaler 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 Zscaler for AI?
Only for one slice of the problem. Shadow is a free, open-source layer that masks sensitive values in AI API traffic on the machines where it runs. Zscaler is a commercial zero-trust platform that governs AI app use across a whole fleet, alongside web, SaaS and private-app access. Shadow does not replace that.
Can Zscaler redact sensitive data in ChatGPT prompts?
Zscaler's public material on generative AI describes allowing, blocking, isolating (browser isolation) and logging AI apps, plus DLP inspection of prompts. Reversible pseudonymisation of prompt values is not documented.
Does Zscaler restore values in the AI response?
Not that public documentation describes. Shadow replaces values with realistic fakes and restores the real values in responses, SSE streams and tool calls locally.
How much does Zscaler cost?
Zscaler sells by quote and bundle; list prices are not published. Shadow is free under Apache-2.0.
Can Zscaler and Shadow be used together?
Yes. Zscaler can govern browser and SaaS AI use on managed devices while Shadow masks API, agent and developer traffic. Exclude the AI API hosts Shadow intercepts from Zscaler SSL inspection on those machines, or chain Shadow through Zscaler with upstream_proxy.