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The buf curl command lets you invoke RPCs on a Connect, gRPC, or gRPC-Web server. This improves usability over the standard cURL utility since buf curl handles aspects of the relevant protocol for you, without you having to define the right headers and craft message envelopes. It also allows you to provide request data and view response data in JSON format, even while the on-the-wire format is binary.

$ buf curl \    --data '{"sentence": "I feel happy."}' \
{"sentence":"Do you often feel happy?"}

You can view a listing of all supported options and how to use them by running buf help curl.

RPC Target#

The only positional argument is the URL of the RPC method to invoke. The name of the method to invoke comes from the last two path components of the URL, which should be the fully-qualified service name and method name, respectively.

$ buf curl \

The URL can use either http or https as the scheme. If http is used then the HTTP 1.1 protocol will be used unless the --http2-prior-knowledge flag is set. If https is used then HTTP/2 will be preferred during protocol negotiation and HTTP 1.1 used only if the server does not support HTTP/2.

The default RPC protocol used will be Connect. To use a different protocol, gRPC or gRPC-Web, use the --protocol flag.

Using gRPC without TLS
$ buf curl --protocol grpc --http2-prior-knowledge \    http://localhost:20202/

The gRPC protocol cannot be used with HTTP 1.1, but the other two can as long as the method being invoked is not a bidirectional streaming method.

Request Data#

The input request is specified via the -d or --data flag. If absent, an empty request is sent unless the method uses a request stream, in which case an empty stream is sent.

There are two ways to provide the request body:

  1. Immediate: The value of the --data flag is the request body.
  2. File: The value of the --data flag starts with an at-sign (@). The rest of the flag value is interpreted as a filename from which to read the request body. If that filename is just a dash (-) then the request body is read from stdin.

The request body is a JSON document that contains the JSON formatted request message. If the RPC method being invoked is a client-streaming method, the request body may consist of multiple JSON values, appended to one another. Multiple JSON documents should usually be separated by whitespace, though this is not strictly required unless the request message type has a custom JSON representation that is not a JSON object.

If the --data flag is specified multiple times, only the value of the last occurrence is used.

Request Metadata#

Request metadata (i.e. headers) are defined using -H or --header flags. The flag value is in name: value format.

There are two ways to provide the request metadata:

  1. Immediate: The flag value is in name: value format and defines a single header to add to the request.
  2. File: The value of the --header flag starts with an at-sign (@). The rest of the flag value is interpreted as a filename from which to read request headers, each header on a separate line. If that filename is just a dash (-) then the request headers are read from stdin.

If headers and the request body are both to be read from the same file (or both read from stdin), the file must include headers first, then a blank line, and then the request body:

Example file with both metadata and data
Custom-Header-1: foo-bar-bazAuthorization: token jas8374hgnkvje9wpkerebncjqol4
{   "sentence": "Hi, doc. I feel hungry."}

The --header flag may be specified multiple times. The headers sent with the request are the union of all values provided.

RPC Schema#

Transcoding from the binary Protobuf format to JSON requires access to the schema for the messages in question. By default, buf curl will expect the server to expose the server reflection service.

If the server does not support reflection, you can instead indicate the schema to use via a --schema option. This option accepts the same kind of inputs as buf build and buf generate, letting you point to Protobuf sources on disk, in a Git repo, or in a BSR module.

$ buf curl \   --schema \   --data '{"name": "Bob Loblaw"}' \

Server Reflection#

By default, buf curl will try the latest and most appropriate server reflection protocol and then fallback to other protocols if necessary. As of this writing, there are two protocols supported:

  1. grpc-v1: This corresponds to version "v1" of the gRPC server reflection service (source). This is the preferred version and is attempted first.
  2. grpc-v1alpha: This corresponds to version "v1alpha" of the same reflection service (source). Many gRPC servers only support this older version.

If you know the server only supports v1alpha, you can use --reflect-protocol=grpc-v1alpha to have buf curl use that instead of first trying v1.

You can separately configure headers used for reflection requests using --reflect-header flags. If you want buf curl to send all of the same headers as for the main RPC invocation, you can use --reflect-header=*.

If server reflection is used, the assumed URL for the reflection service is the same as the given RPC target URL, but with the last two path elements removed and replaced with the service and method name for the server reflection protocol.


Server reflection does not currently work with HTTP 1.1 since the supported reflection protocols rely on bidirectional streaming.


By default, buf curl will print the response message(s) to stdout in JSON format. If an RPC error occurs, the error information is printed to stderr, also in JSON format, and the exit code will be the gRPC error code shifted three bits to the left. A non-zero exit code that is less than eight indicates incorrect usage or other unexpected error.

You can enable verbose output via the -v or --verbose flag. This will generate a lot of output to stderr which may be helpful for troubleshooting, including details related to TLS and traces of all RPC activity, including server reflection calls.


Issue a unary RPC to a plain-text (i.e. "h2c") gRPC server, where the schema for the service is in a Buf module in the current directory, using an empty request message:

$ buf curl --schema . --protocol grpc --http2-prior-knowledge \   http://localhost:20202/

Issue an RPC to a Connect server, where the schema comes from the Buf Schema Registry, using a request that is defined as a command-line argument:

$ buf curl \   --schema \   --data '{"name": "Bob Loblaw"}' \
{"sentence":"Hi Bob Loblaw. I'm Eliza."}{"sentence":"Before we begin, Bob Loblaw, let me tell you something about myself."}{"sentence":"I was created by Joseph Weizenbaum."}{"sentence":"How are you feeling today?"}

Issue a unary RPC to a server that supports reflection, with verbose output:

$ buf curl -v \   --data '{"sentence": "I am not feeling well."}' \
buf: * Using server reflection to resolve "buf.connect.demo.eliza.v1.ElizaService"buf: * Dialing (tcp) * Connected to * TLS connection using TLSv1.3 / TLS_AES_128_GCM_SHA256buf: * ALPN, server accepted protocol h2buf: * Server certificate:buf: * subject: CN=demo.connect.buildbuf: * start date: 2022-12-25 19:53:17 +0000 UTCbuf: * end date: 2023-03-25 20:44:49 +0000 UTCbuf: * subjectAltNames: []buf: * issuer: CN=GTS CA 1D4,O=Google Trust Services LLC,C=USbuf: * Server certificate chain verifiedbuf: * Server certificate is valid for demo.connect.buildbuf: > (#1) POST /grpc.reflection.v1.ServerReflection/ServerReflectionInfobuf: > (#1) Accept-Encoding: identitybuf: > (#1) Connect-Accept-Encoding: gzipbuf: > (#1) Connect-Protocol-Version: 1buf: > (#1) Connect-Timeout-Ms: 119999buf: > (#1) Content-Type: application/connect+protobuf: > (#1) User-Agent: connect-go/1.4.1 (go1.19.3)buf: > (#1)buf: } (#1) [5 bytes data]buf: } (#1) [40 bytes data]buf: < (#1) HTTP/2.0 200 OKbuf: < (#1) Alt-Svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000buf: < (#1) Connect-Accept-Encoding: gzipbuf: < (#1) Connect-Content-Encoding: gzipbuf: < (#1) Content-Type: application/connect+protobuf: < (#1) Date: Wed, 11 Jan 2023 18:42:27 GMTbuf: < (#1) Server: Google Frontendbuf: < (#1) Traceparent: 00-42685e452dcc166e0071a9d14b7adda4-97faf80b4e2ea581-01buf: < (#1) Vary: Originbuf: < (#1) Via: 1.1 googlebuf: < (#1) X-Cloud-Trace-Context: 42685e452dcc166e0071a9d14b7adda4/10951338171344790913;o=1buf: < (#1)buf: { (#1) [5 bytes data]buf: { (#1) [975 bytes data]buf: * Server reflection has resolved file "buf/connect/demo/eliza/v1/eliza.proto"buf: * Invoking RPC buf.connect.demo.eliza.v1.ElizaService.Saybuf: > (#2) POST /buf.connect.demo.eliza.v1.ElizaService/Saybuf: > (#2) Accept-Encoding: gzipbuf: > (#2) Connect-Protocol-Version: 1buf: > (#2) Connect-Timeout-Ms: 119823buf: > (#2) Content-Type: application/protobuf: > (#2) User-Agent: buf/1.12.0 connect-go/1.4.1 (go1.19.3)buf: > (#2)buf: } (#2) [24 bytes data]buf: * (#2) Finished uploadbuf: < (#2) HTTP/2.0 200 OKbuf: < (#2) Accept-Encoding: gzipbuf: < (#2) Alt-Svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000buf: < (#2) Content-Length: 42buf: < (#2) Content-Type: application/protobuf: < (#2) Date: Wed, 11 Jan 2023 18:42:27 GMTbuf: < (#2) Server: Google Frontendbuf: < (#2) Traceparent: 00-0c204b31fb17714d8203dd77d177b699-6b7f2a2cbcae59cf-00buf: < (#2) Vary: Originbuf: < (#2) Via: 1.1 googlebuf: < (#2) X-Cloud-Trace-Context: 0c204b31fb17714d8203dd77d177b699/7745956255733012943buf: < (#2)buf: { (#2) [42 bytes data]buf: * (#2) Call complete{"sentence":"How long have you been not feeling well?"}buf: { (#1) [5 bytes data]buf: { (#1) [2 bytes data]buf: * (#1) Call complete

Issue a client-streaming RPC to a gRPC-web server that supports reflection, where custom headers and request data are both in a heredoc:

$ buf curl --data @- --header @- --protocol grpcweb \ \   <<EOMCustom-Header-1: foo-bar-bazAuthorization: token jas8374hgnkvje9wpkerebncjqol4
{"sentence": "Hi, doc. I feel hungry."}{"sentence": "What is the answer to life, the universe, and everything?"}{"sentence": "If you were a fish, what kind of fish would you be?."}EOM
{"sentence":"When you feel hungry, what do you do?"}{"sentence":"How would an answer to that help you?"}{"sentence":"Why do you say that about me?"}