sveltejs/kit · error · Error

Data returned from action inside ${route_id} is not serializ

Error message

Data returned from action inside ${route_id} is not serializable: ${error.message} (data.${error.path})

What it means

Devalue failed to serialize a property of the object returned from a form action (or one nested under `fail` data). Because the serialization error carries a `path`, SvelteKit augments the message with the exact offending property (e.g. `data.items.0.date`) so you can fix the returned shape.

Source

Thrown at packages/kit/src/runtime/server/page/actions.js:320

	try {
		return fn(data);
	} catch (e) {
		// If we're here, the data could not be serialized with devalue
		const error = /** @type {any} */ (e);

		// if someone tries to use `json()` in their action
		if (data instanceof Response) {
			throw new Error(
				`Data returned from action inside ${route_id} is not serializable. Form actions need to return plain objects or fail(). E.g. return { success: true } or return fail(400, { message: "invalid" });`,
				{ cause: e }
			);
		}

		// if devalue could not serialize a property on the object, etc.
		if ('path' in error) {
			let message = `Data returned from action inside ${route_id} is not serializable: ${error.message}`;
			if (error.path !== '') message += ` (data.${error.path})`;
			throw new Error(message, { cause: e });
		}

		throw error;
	}
}

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Map the offending property (see `data.<path>` in the message) to a plain value: `String(bigintValue)`, `.toISOString()` for dates, spread ORM rows into POJOs
2. Strip functions, class instances, and `undefined` from the returned object
3. Verify the return type of every branch of your action (including `fail` data)

Example fix

// before
export const actions = {
  default: async () => {
    const user = await db.user.findFirst();
    return { user }; // contains BigInt id
  }
};
// after
export const actions = {
  default: async () => {
    const user = await db.user.findFirst();
    return { user: { ...user, id: String(user.id), createdAt: user.createdAt.toISOString() } };
  }
};

// sanity check before returning
const data = { items };
JSON.stringify(data); // throws/spot-checks most non-serializable values

When it happens

Trigger: Returning values devalue cannot encode from an action: `undefined` in arrays, class instances, functions, `BigInt`, `Symbol`, cyclic references, `Date` inside certain positions, or a promise stored on the returned object.

Common situations: Returning ORM entities/Mongoose docs or Prisma objects with methods; returning rows containing `BigInt` IDs; accidental inclusion of functions (e.g. attaching helpers to the result); non-JSON dates or Maps/Sets in older devalue configurations.

Related errors

• Data returned from action inside ${route_id} is not serializ
• called uneval before init_transport
• called stringify before init_transport
• called parse before init_transport
• Cannot `return redirect(...)` — use `redirect(...)` instead

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/bb118ebfa3befcbd. Report an issue: GitHub.