Jackson: Cannot deserialize value of type from Array or Object value

What the error means

The JSON is valid, but its shape does not match the Java type Jackson was asked to build. from Object value means Jackson found a JSON object where the type expects something else, such as a list. from Array value means it found an array where it expected an object or a single value. The message names the target type and the JSON token. Versions before 2.12 said out of START_OBJECT token or out of START_ARRAY token for the same mismatch.

Exact messages by runtime

Messages depend on the input and runtime version. Node and Python rows were reproduced locally. Other rows cite official documentation or source. Rows marked as fragments or templates preserve the source wording and are not complete exception transcripts. Browsers and libraries may append location details.

Causes you can reproduce

record Item(long id) {}

String json = "{\"items\":[{\"id\":1}]}";
List<Item> items = new ObjectMapper().readValue(json, new TypeReference<List<Item>>() {});
// MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<...Item>` from Object value

record Order(List<String> tags) {}

new ObjectMapper().readValue("{\"tags\":\"new\"}", Order.class);
// MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<java.lang.String>` from String value

record User(String address) {}

new ObjectMapper().readValue("{\"address\":{\"city\":\"Pune\"}}", User.class);
// Cannot deserialize value of type `java.lang.String` from Object value (token `JsonToken.START_OBJECT`)

How to fix it

1. Read the target type and the JSON token. The (through reference chain: ...) suffix names the class and property where it failed.
2. Look at the JSON at that property. An object starts with {, an array with [.
3. Change the Java type to match: a wrapper record for {"items": [...]}, a List or array for [...], and a nested record for {...}.
4. If the API really sends either one value or a list, enable ACCEPT_SINGLE_VALUE_AS_ARRAY and document that contract.

record Item(long id) {}
record ItemsResponse(List<Item> items) {}

ItemsResponse response = new ObjectMapper()
    .readValue("{\"items\":[{\"id\":1}]}", ItemsResponse.class);
List<Item> items = response.items();

List<Item> items = new ObjectMapper()
    .readValue("[{\"id\":1}]", new TypeReference<List<Item>>() {});

ObjectMapper mapper = JsonMapper.builder()
    .enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
    .build();
Order order = mapper.readValue("{\"tags\":\"new\"}", Order.class); // tags = [new]

Compare the JSON shape with your types

1. Copy the exact JSON your code received, for example from a log or a failing test.
2. Paste it into the Tool Kumar JSON viewer. The document stays in your browser.
3. Open the tree at the property named in the error and check whether the value there is an object, an array or a single value. The JSON path finder copies the path to any value.
4. Generate types from the same document with a converter listed under related tools, then compare them with your own classes or structs.

Frequently asked questions

Related errors and tools

• Jackson: Unrecognized field, not marked as ignorable
• Jackson: Unrecognized token / Unexpected character ('<' (code 60))
• Go JSON: cannot unmarshal string, object or array into Go value
• All JSON error guides
• JSON syntax reference
• Generate Java records from the JSON you received
• Find the path to the value that failed

Standards and diagnostic sources

RFC 8259 and ECMA-404 define JSON syntax. Diagnostic wording belongs to the parser. The table links each cited message to its source.

• Jackson source: abstract List is created as ArrayList