What causes "could not read manifest file" in leptos-rs/leptos?

Rebuild with cargo leptos build (clean the target/site directory first) to regenerate a valid manifest.

could not read manifest file

Error message

could not read manifest file

What it means

When HydrationScripts finds __wasm_split_manifest.json in the package directory, it parses it with serde_json and expects success. The panic means the manifest file exists but is not valid JSON matching the expected WasmSplitManifest shape (or serde failed reading its contents).

Source

Thrown at leptos/src/hydration/mod.rs:113

                }
            }
            (split, manifest)
        } else {
            (
                "__wasm_split.______________________.js".to_string(),
                "__wasm_split_manifest.json".to_string(),
            )
        };

        let site_dir = &options.site_root;
        let pkg_dir = &options.site_pkg_dir;
        let path = PathBuf::from(site_dir.to_string());
        let path = path.join(pkg_dir.to_string()).join(wasm_split_manifest);
        let file = std::fs::read_to_string(path).ok()?;

        let manifest = WasmSplitManifest(ArcStoredValue::new((
            format!("{root}/{pkg_dir}"),
            serde_json::from_str(&file).expect("could not read manifest file"),
            wasm_split_js,
        )));

        Some(manifest)
    }) {
        provide_context(splits.clone());
    }

    let mut js_file_name = options.output_name.to_string();
    let mut wasm_file_name = options.output_name.to_string();
    if options.hash_files {
        let hash_path = std::env::current_exe()
            .map(|path| {
                path.parent().map(|p| p.to_path_buf()).unwrap_or_default()
            })
            .unwrap_or_default()
            .join(options.hash_file.as_ref());
        if hash_path.exists() {

View on GitHub (pinned to 32d20f6c9d)

Solutions

Rebuild with cargo leptos build (clean the target/site directory first) to regenerate a valid manifest.
Verify __wasm_split_manifest.json parses as the expected JSON (object with the wasm-split entries) with jq.
Align leptos and wasm-split tool versions across all crates in the workspace.
Purge stale CI caches of target/site so an old manifest isn't reused.

Example fix

// before
rm target/site/pkg/__wasm_split_manifest.json # stale schema
// after
cargo clean -p app && cargo leptos build # regenerates valid manifest

Defensive patterns

Strategy: validation

Validate before calling

let raw = std::fs::read_to_string("target/site/pkg/__wasm_split_manifest.json")?;
let v: serde_json::Value = serde_json::from_str(&raw)?; // fails fast if corrupt

Type guard

fn valid_manifest(s: &str) -> bool {
    serde_json::from_str::<serde_json::Value>(s).map(|v| v.is_object()).unwrap_or(false)
}

Prevention

Clean target/site before release builds to avoid stale manifests
Pin leptos and wasm-split tool versions across the workspace
Validate the manifest JSON in CI after each build
Avoid caching target/site/pkg across different tool versions

When it happens

Trigger: A corrupted, truncated, or hand-edited __wasm_split_manifest.json in the target/site pkg dir; a manifest produced by an incompatible wasm-split version that changed the JSON schema.

Common situations: Partial/stale build artifacts from an interrupted cargo leptos build; mixing leptos/wasm-split versions across a workspace; CI caching a pkg dir from a different build; manually editing output hashes in the manifest.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of leptos-rs/leptos@32d20f6c9d (2026-09-01). Data as JSON: /api/errors/651f6ac6f1de314b. Report an issue: GitHub.