> For the complete documentation index, see [llms.txt](https://andrewmcodes.gitbook.io/vcr/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://andrewmcodes.gitbook.io/vcr/v5.0.0/cassettes/update_content_length_header.md).

# Update content\_length header

When the `:update_content_length_header` option is set to a truthy value, VCR will ensure that the `Content-Length` header will have the correct value. This is useful in several situations:

* When you manually edit the cassette file and change the resonse body

  length.  You can use this option so you don't have to manually calculate

  and update the body length.
* When you use ERB, the response body length may vary.  This will ensure

  it is always correct.
* Syck, the default YAML engine for ruby 1.8 (and 1.9, unless you compile it to use Psych), has a bug where it sometimes will remove some whitespace strings when you serialize them. This may cause the `Content-Length` header to have the wrong value.

  This is especially important when you use a client that checks the `Content-Length` header. Mechanize, for example, will raise an `EOFError` when the header value does not match the actual body length.

## Background ()

*Given* a previously recorded cassette file "cassettes/example.yml" with:

```
--- 
http_interactions: 
- request: 
    method: get
    uri: http://example.com/
    body: 
      encoding: UTF-8
      string: ""
    headers: {}
  response: 
    status: 
      code: 200
      message: OK
    headers: 
      Content-Type: 
      - text/html;charset=utf-8
      Content-Length: 
      - "11"
    body: 
      encoding: UTF-8
      string: Hello <modified>
    http_version: "1.1"
  recorded_at: Tue, 01 Nov 2011 04:58:44 GMT
recorded_with: VCR 2.0.0
```

*And* a file named "common\_stuff.rb" with:

```
require 'vcr'

VCR.configure do |c|
  c.cassette_library_dir = 'cassettes'
  c.hook_into :webmock
end

def make_request_and_print_results
  response = Net::HTTP.get_response('example.com', '/')
  puts "Body length: #{response.body.length}"
  puts "Header value: #{response['Content-Length']}"
end
```

## Default :update\_content\_length\_header setting

*Given* a file named "default.rb" with:

```
require 'common_stuff'

VCR.use_cassette('example') do
  make_request_and_print_results
end
```

*When* I run `ruby default.rb`

*Then* the output should contain:

```
Body length: 16
Header value: 11
```

## :update\_content\_length\_header => false

*Given* a file named "false.rb" with:

```
require 'common_stuff'

VCR.use_cassette('example', :update_content_length_header => false) do
  make_request_and_print_results
end
```

*When* I run `ruby false.rb`

*Then* the output should contain:

```
Body length: 16
Header value: 11
```

## :update\_content\_length\_header => true

*Given* a file named "true.rb" with:

```
require 'common_stuff'

VCR.use_cassette('example', :update_content_length_header => true) do
  make_request_and_print_results
end
```

*When* I run `ruby true.rb`

*Then* the output should contain:

```
Body length: 16
Header value: 16
```