How to control Expect: 100-continue handling in cURL

Large uploads can stall or waste bandwidth when an API, proxy, or object store rejects the request after curl has already started sending the body. Expect: 100-continue gives an HTTP/1.1 server a chance to approve or reject the request headers before the upload body moves.

curl adds Expect: 100-continue automatically for PUT uploads and for POST bodies known or suspected to be larger than one megabyte. An empty Expect: header removes the handshake, an explicit Expect: 100-continue header forces it on smaller bodies, and --expect100-timeout changes how long curl waits for the provisional response.

The setting matters only for HTTP/1.1. Keep the header when early rejection can avoid sending a large body, remove it when a broken gateway turns the provisional response into a needless pause, and shorten the timeout when the server eventually answers but curl's default one-second wait is longer than the upload path can tolerate.

1. Run a verbose HTTP/1.1 upload and confirm that curl adds Expect: 100-continue.

   $ curl --silent --show-error --verbose --http1.1 \
     --upload-file ./release-bundle-2026.04.22.tar.gz \
     --output /dev/null \
     "https://uploads-api.example.net/v1/releases/release-bundle-2026.04.22.tar.gz"
   ##### snipped #####
   > PUT /v1/releases/release-bundle-2026.04.22.tar.gz HTTP/1.1
   > Host: uploads-api.example.net
   > User-Agent: curl/8.x
   > Accept: */*
   > Content-Length: 1048704
   > Expect: 100-continue
   >
   < HTTP/1.1 100 Continue
   * upload completely sent off: 1048704 bytes
   < HTTP/1.1 201 Created

   In verbose output, > lines are request headers sent by curl, < lines are server responses, and the blank > line ends the request-header block.

   Related: How to upload files with cURL
   Related: How to view HTTP request headers with cURL

2. Remove the handshake with an empty Expect: header when the upstream accepts immediate body transmission more consistently.

   $ curl --silent --show-error --verbose --http1.1 \
     --header "Expect:" \
     --upload-file ./release-bundle-2026.04.22.tar.gz \
     --output /dev/null \
     "https://uploads-api.example.net/v1/releases/release-bundle-2026.04.22.tar.gz"
   ##### snipped #####
   > PUT /v1/releases/release-bundle-2026.04.22.tar.gz HTTP/1.1
   > Host: uploads-api.example.net
   > User-Agent: curl/8.x
   > Accept: */*
   > Content-Length: 1048704
   >
   * upload completely sent off: 1048704 bytes
   < HTTP/1.1 201 Created

   Removing Expect: means an authentication failure, quota failure, or object-policy rejection can still receive the full request body before the failure is visible.

3. Force the provisional check on a small request body when the server should approve the headers before reading the payload.

   $ curl --silent --show-error --verbose --http1.1 \
     --header "Expect: 100-continue" \
     --header "Content-Type: application/json" \
     --data-binary @./release-bundle-2026.04.22.json \
     --output /dev/null \
     "https://uploads-api.example.net/v1/releases/release-bundle-2026.04.22/metadata"
   ##### snipped #####
   > POST /v1/releases/release-bundle-2026.04.22/metadata HTTP/1.1
   > Host: uploads-api.example.net
   > User-Agent: curl/8.x
   > Accept: */*
   > Expect: 100-continue
   > Content-Type: application/json
   > Content-Length: 72
   >
   < HTTP/1.1 100 Continue
   * upload completely sent off: 72 bytes
   < HTTP/1.1 201 Created

   Setting the header explicitly applies the same approval step to small request bodies that curl would otherwise send immediately.

   Related: Send data in HTTP requests with cURL

4. Shorten the wait with --expect100-timeout when delayed provisional responses should not hold the body for the full default pause.

   $ curl --silent --show-error --verbose --http1.1 \
     --expect100-timeout 0.2 \
     --upload-file ./release-bundle-2026.04.22.tar.gz \
     --output /dev/null \
     "https://uploads-api.example.net/v1/releases/release-bundle-2026.04.22.tar.gz"
   ##### snipped #####
   > PUT /v1/releases/release-bundle-2026.04.22.tar.gz HTTP/1.1
   > Host: uploads-api.example.net
   > User-Agent: curl/8.x
   > Accept: */*
   > Content-Length: 1048704
   > Expect: 100-continue
   >
   * Done waiting for 100-continue
   * upload completely sent off: 1048704 bytes
   < HTTP/1.1 100 Continue
   < HTTP/1.1 201 Created

   --expect100-timeout uses seconds, accepts decimal values with a dot, and falls back to sending the body when the timer expires. The default wait is one second.