denoland/deno · error · anyhow::Error

unsupported media type

Error message

unsupported media type {} for {}

What it means

When eszip stores a module in its JavaScript source slot, it handles a fixed set of media types (JS/JSX served as-is, TS/TSX transpiled with the given emit options). If the module's media type falls into the catch-all arm, the build rejects it with 'unsupported media type <type> for <specifier>' — the module kind cannot be represented as embedded JS source.

Solutions

1. Import real JS/TS modules; load assets with Deno.readFile/inline bytes instead of `import`
2. Give the file a .js/.ts (or .mjs/.mts) extension so it maps to a supported media type
3. For local dev servers, send correct Content-Type (text/javascript or application/typescript) for module responses
4. If you need Wasm, use WebAssembly APIs / a loader path that stores it as a Wasm module rather than a JS-source slot

Example fix

// before
import shaderSrc from "./shader.wasm";  // unsupported media type Wasm

// after
const shaderBytes = await Deno.readFile(new URL("./shader.wasm", import.meta.url));

// Reject non-code imports before building
const CODE_EXT = /\.(m?jsx?|m?tsx?|json|jsonc)$/;
export function assertImportable(specifier: string) {
  if (!CODE_EXT.test(new URL(specifier, import.meta.url).pathname)) {
    throw new Error(`Refusing to import non-code asset as module: ${specifier}`);
  }
}

function isEmbeddableMediaType(path: string): boolean {
  return /\.(m?jsx?|m?tsx?|json|jsonc)$/i.test(path);
}

const { success, stderr } = await new Deno.Command("deno", { args: ["compile", "src/mod.ts"] }).output();
const err = new TextDecoder().decode(stderr);
const m = err.match(/unsupported media type (\S+) for (\S+)/);
if (!success && m) switchToAssetLoader(m[2]); // readFile instead of import

When it happens

Trigger: A static import resolving to a file with an extension Deno maps to a non-code type (e.g. .wasm, .html, .sql, .md or an unknown/extensionless type under certain media-type providers) being visited as a regular module; a server serving JS content with a Content-Type that deno_graph classifies as something else.

Common situations: Importing asset files as if they were JS modules (works in bundlers with asset loaders, not in Deno's eszip path); extensionless files or misconfigured media types on dev servers; older Deno versions classifying some extensions differently than the current runtime.

Related errors

• module not found
• failed to load
• Could not resolve package req
• Unknown media type value
• An executable name was not provided. One could not be…

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/49c27814bdaf688c. Report an issue: GitHub.

Appendix: source

Thrown at libs/eszip/v2.rs:1438

                }
                _ => Cow::Borrowed(emit_options),
              };
              let emit = parsed_source
                .transpile(
                  transpile_options,
                  &TranspileModuleOptions {
                    module_kind: module_kind_provider.module_kind(module),
                  },
                  &emit_options,
                )?
                .into_source();
              source = emit.text.into_bytes().into();
              source_map = Arc::from(
                emit.source_map.map(|s| s.into_bytes()).unwrap_or_default(),
              );
            }
            _ => {
              return Err(anyhow::anyhow!(
                "unsupported media type {} for {}",
                module.media_type,
                visited.specifier()
              ));
            }
          };

          let eszip_module = EszipV2Module::Module {
            kind: ModuleKind::JavaScript,
            source: EszipV2SourceSlot::Ready(source),
            source_map: EszipV2SourceSlot::Ready(source_map),
          };
          modules.insert(specifier_key.into_owned(), eszip_module);

          Ok(Some(Box::new(module.dependencies.values().filter_map(
            |dependency| {
              Some(ToVisit::Module {
                specifier: dependency.get_code()?,

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