Observed Signal · May 6, 2026 · Technical Release · Source: DEV Community · Impact: 2/5 · Sentiment: Neutral
Missing CSP 'wasm-unsafe-eval' Broke WebLLM on Vercel
A developer post describes a production failure where a WebLLM-based WebAssembly model loaded successfully in local Vite development but failed silently when deployed to Vercel. Root cause: the site's Content-Security-Policy omitted the 'wasm-unsafe-eval' keyword from script-src, which prevents WebAssembly compilation APIs (e.g., WebAssembly.instantiateStreaming) and caused a CompileError inside a worker. The error path produced no visible logs because diagnostic logs were gated behind import.meta.env.DEV and stripped from production builds. The fix added 'wasm-unsafe-eval' to script-src in vercel.json, removed DEV gating from error-path logs so failures surface in production, and hooked the securitypolicyviolation event into Umami analytics for rapid future diagnostics. The post highlights dev/prod CSP asymmetry (Vite vs Vercel) and recommends treating error logs as production-critical.
Practical operational fix for WASM-based in-browser LLM deployments and production observability; relevant to web developers and teams deploying WebAssembly/LLM workloads but not industry-shifting.
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Key Takeaways & Evidence Grounding
- WebLLM called WebAssembly.instantiateStreaming() to load a WASM model in a worker; in production this threw a CompileError due to CSP.
- The deployed Content-Security-Policy on Vercel lacked 'wasm-unsafe-eval' in script-src, preventing WASM compilation.
- Fixes applied: add 'wasm-unsafe-eval' to script-src in vercel.json; surface error-path logs in production; subscribe to securitypolicyviolation events and route them to Umami analytics.
- Vite's dev server does not apply vercel.json headers, causing a dev/prod asymmetry where the issue reproduced only on Vercel.
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