Clojure library for fast http api format negotiation, encoding and decoding.

License: EPL-2.0

Language: Clojure

Keywords: clojure, content-negotiation, edn, http, interceptor, json, middleware, ring, transit

Muuntaja Continuous Integration status

Clojure library for fast http format negotiation, encoding and decoding. Standalone library, but ships with adapters for ring (async) middleware & Pedestal-style interceptors. Explicit & extendable, supporting out-of-the-box JSON, EDN and Transit (both JSON & Msgpack). Ships with optional adapters for MessagePack and YAML.

Based on ring-middleware-format, but a complete rewrite (and up to 30x faster).


  • both standalone & http (ring & pedestal)
  • explicit configuration over mutable state (e.g. multimethods)
  • lazy streams when possible
  • fast with good defaults
  • extendable & pluggable: new formats, behavior
  • typed exceptions - caught elsewhere
  • support runtime docs (like swagger) & inspection (negotiation results)
  • support runtime configuration (negotiation overrides)

Check the Wiki & api-docs for more details.

Latest version

Clojars Project

Muuntaja requires Java 1.8+



(require '[muuntaja.middleware :as middleware])

(defn echo [request]
  {:status 200
   :body (:body-params request)})

; with defaults
(def app (middleware/wrap-format echo))

(app {:headers
      {"content-type" "application/edn"
       "accept" "application/json"}
      :body "{:kikka 42}"})
; {:status 200,
;  :body #object[]
;  :muuntaja/format "application/json",
;  :headers {"Content-Type" "application/json; charset=utf-8"}}

There is a Ring guide in a wiki. See also differences to ring-middleware-format & ring-json if you are migrating from those.


See muuntaja.interceptor.


Create a Muuntaja and use it to encode & decode JSON:

(require '[muuntaja.core :as m])

;; with defaults
(def m (muuntaja/create))

(->> {:kikka 42}
     (m/encode m "application/json")
; => "{\"kikka\":42}"

(->> {:kikka 42}
     (m/encode m "application/json")
     (m/decode m "application/json"))
; => {:kikka 42}

With custom EDN decoder opts:

(def m
      [:formats "application/edn" :decoder-opts]
      {:readers {'INC inc}})))

(->> "{:value #INC 41}"
     (m/decode m "application/edn"))
; => {:value 42}

A function to encode Transit-json:

(def encode-transit-json
  (m/encoder m "application/transit+json"))

(slurp (encode-transit-json {:kikka 42}))
; => "[\"^ \",\"~:kikka\",42]"


Muuntaja ships with streaming encoders for both JSON & Transit. With these, the encoded data can be lazily written to provided OutputStream, avoiding intermediate byte-streams. These encoders return a muuntaja.protocols.StreamableResponse type, which satisifies the following protocols & interfaces:

  • ring.protocols.StreamableResponseBody, Ring 1.6.0 will stream these for you
  • clojure.lang.IFn, invoke the result with an OutputStream to write the results into the stream
  •, so you can slurp the response
(require '[muuntaja.format.json :as json-format])

(def m
    (-> m/default-options

(->> {:kikka 42}
     (m/encode m "application/json"))
; <<StreamableResponse>>

(->> {:kikka 42}
     (m/encode m "application/json")
; "{\"kikka\":42}"

(->> {:kikka 42}
     (m/encode m "application/json")
     (m/decode m "application/json"))
; {:kikka 42}

HTTP format negotiation

HTTP format negotiation is done via request headers for both request (content-type, including the charset) and response (accept and accept-charset). With the default options, a full match on the content-type is required, e.g. application/json. Adding a :matches regexp for formats enables more loose matching. See Configuration wiki-page for more info.

Results of the negotiation are published into request & response under namespaced keys for introspection. These keys can also be set manually, overriding the content negotiation process.


When something bad happens, an typed exception is thrown. You should handle it elsewhere. Thrown exceptions have an ex-data with the following :type value (plus extra info to generate descriptive erros to clients):

  • :muuntaja/decode, input can't be decoded with the negotiated format & charset.
  • :muuntaja/request-charset-negotiation, request charset is illegal.
  • :muuntaja/response-charset-negotiation, could not negotiate a charset for the response.
  • :muuntaja/response-format-negotiation, could not negotiate a format for the response.

Server Spec


  • :muuntaja/format, format name that was used to decode the request body, e.g. application/json. If the key is already present in the request map, muuntaja middleware/interceptor will skip the decoding process.
  • :muuntaja/request, client-negotiated request format and charset as FormatAndCharset record. Will be used in the response pipeline.
  • :muuntaja/response, client-negotiated response format and charset as FormatAndCharset record. Will be used in the response pipeline.
  • :body-params decoded body is here.


  • :muuntaja/encode?, if set to true, the response body will be encoded regardles of the type (primitives!)
  • :muuntaja/format, format name that was used to encode the response body, e.g. application/json. If the key is already present in the response map, muuntaja middleware/interceptor will skip the encoding process.
  • :muuntaja/content-type, handlers can use this to override the negotiated content-type for response encoding, e.g. setting it to application/edn will cause the response to be formatted in JSON.


Default options

{:http {:extract-content-type extract-content-type-ring
        :extract-accept-charset extract-accept-charset-ring
        :extract-accept extract-accept-ring
        :decode-request-body? (constantly true)
        :encode-response-body? encode-collections-with-override}

 :allow-empty-input? true

 :default-charset "utf-8"
 :charsets available-charsets

 :default-format "application/json"
 :formats {"application/json" json-format/json-format
           "application/edn" edn-format/edn-format
           "application/transit+json" transit-format/transit-json-format
           "application/transit+msgpack" transit-format/transit-msgpack-format}}


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By Unknown. The drawing is signed "E. Ducretet", indicating that the apparatus was made by Eugene Ducretet, a prominent Paris scientific instrument manufacturer and radio researcher. The drawing was undoubtedly originally from the Ducretet instrument catalog. [Public domain], via Wikimedia Commons.

Original Code (ring-middleware-format)

Copyright © 2011, 2012, 2013, 2014 Nils Grunwald
Copyright © 2015, 2016 Juho Teperi

This library

Copyright © 2016-2017 Metosin Oy

Distributed under the Eclipse Public License, the same as Clojure.

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Tommi Reiman Nils Grünwald Juho Teperi Miikka Koskinen Christopher Mark Gore kalekale Dmitri Sotnikov r0man Ian Eure Howard M. Lewis Ship Craig P Jolicoeur Luke Amdor Sergio Alvarez-Napagao Chris McDevitt Justin Balthrop Michael Blume Anthony Grimes Wei Hsu Ducky Paul Bauer

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Clojure library for fast http format negotiation, encoding and decoding.
Latest release 0.5.0 - Updated - 149 stars

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