What causes "Unexpected media type {media_type} for {specifier}" in denoland/deno?

Load the asset with a read API instead of an ESM import

Unexpected media type {media_type} for {specifier}

Error message

Unexpected media type {media_type} for {specifier}

What it means

The graph-based module loader in libs/resolver hands sources to the runtime only for media types it can emit as JavaScript. When a module resolves to Css, Html, Jsonc, Json5, Markdown, Sql, Wasm, or SourceMap on this path, it panics because no JS emit exists for that type.

Source

Thrown at libs/resolver/loader/module_loader.rs:627

            MediaType::TypeScript
            | MediaType::Mts
            | MediaType::Jsx
            | MediaType::Tsx => {
              return Ok(Some(CodeOrDeferredEmit::DeferredEmit {
                specifier,
                media_type: *media_type,
                source: &source.text,
              }));
            }
            MediaType::Css
            | MediaType::Html
            | MediaType::Jsonc
            | MediaType::Json5
            | MediaType::Markdown
            | MediaType::Sql
            | MediaType::Wasm
            | MediaType::SourceMap => {
              panic!("Unexpected media type {media_type} for {specifier}")
            }
          };

          // at this point, we no longer need the parsed source in memory, so free it
          self.parsed_source_cache.free(specifier);

          Ok(Some(CodeOrDeferredEmit::Source(LoadedModule {
            source: LoadedModuleSource::ArcStr(code),
            specifier: Cow::Borrowed(specifier),
            media_type: *media_type,
          })))
        }
      },
      Some(deno_graph::Module::Wasm(WasmModule {
        source, specifier, ..
      })) => Ok(Some(CodeOrDeferredEmit::Source(LoadedModule {
        source: LoadedModuleSource::ArcBytes(source.clone()),
        specifier: Cow::Borrowed(specifier),

View on GitHub (pinned to 9ad36f7a2c)

Solutions

Load the asset with a read API instead of an ESM import
Import a supported module type (JS/TS/JSON as allowed by your runtime configuration)
Configure the transpile or asset pipeline for that media type before this loader path runs
Check the resolved MediaType up front and reject non-JS/TS types with a real error

Example fix

// before
import styles from "./styles.css"; // panics: Unexpected media type Css

// after
const styles = await Deno.readTextFile(new URL("./styles.css", import.meta.url));

Defensive patterns

Strategy: validation

Validate before calling

use deno_ast::MediaType;

fn unusable_as_module_source(mt: MediaType) -> bool {
  matches!(
    mt,
    MediaType::Css | MediaType::Html | MediaType::Jsonc | MediaType::Json5
      | MediaType::Markdown | MediaType::Sql | MediaType::Wasm | MediaType::SourceMap
  )
}

// reject before the loader panics
if unusable_as_module_source(media_type) {
  return Err(format!("{specifier} cannot be loaded as a module source"));
}

Prevention

Keep ESM imports limited to JS/TS-family files; load assets with read APIs
In embedders, resolve the MediaType and reject unsupported types with a real error before handing off to the loader
Lint the codebase for imports of .css/.md/.sql/.html extensions in CI

When it happens

Trigger: Importing a .css, .html, .jsonc, .json5, .md, .sql, or .wasm file through a loader configuration that expects JS source output, without the asset/transpile pipeline the Deno CLI normally runs.

Common situations: Porting web code that imports stylesheets or markdown; importing JSON5/JSONC configs as modules; embedders reusing the resolver loader without configuring transpilation.

Related errors

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/4bb195c2dfee505f. Report an issue: GitHub.