Modify headers

modifyHeaders changes request headers before they are sent or response headers after they are received.

Header modification fields

Field	Required	Behavior
Name	Yes	Trimmed header name. Empty names are ignored.
Operation	Yes	set, append, or remove
Value	For set and append	Trimmed value. An empty value makes the item inactive.

Disabled items remain in the profile but do not generate DNR modifications.

Operations

set

Creates the header or replaces its value. Lower-priority modifications may be restricted after a set operation.

append

Adds another value. Chrome chooses the appropriate separator where possible.

remove

Removes the header. It does not use a value. Lower-priority rules cannot modify that header afterward.

Request and response timing

Request-header modifications run before Chrome sends headers to the server. Response-header modifications run after response headers arrive. A response modification cannot undo data already sent in the request.

Chrome DevTools may not display DNR response-header changes in the Network panel because of Chromium issue 40196848. This is a display limitation, not evidence that the modification failed. See Verify a modified response header.

Append restrictions

Chrome only permits append for these request headers, using the exact lowercase spelling shown:

accept, accept-encoding, accept-language, access-control-request-headers, cache-control, connection, content-language, cookie, forwarded, if-match, if-none-match, keep-alive, range, te, trailer, transfer-encoding, upgrade, user-agent, via, want-digest, x-forwarded-for.

This allowlist is browser-defined and case-sensitive. See Chromium's kDNRRequestHeaderAppendAllowList for the current list and the delimiter used by each header. Response-header append is not limited by this request-header list.

Conflicts

Matching modifyHeaders profiles are evaluated from higher to lower priority. An earlier operation limits later operations on the same header:

• after append, later rules may only append;
• after set, only lower-priority rules from the same extension may append;
• after remove, no later rule may modify the header.

Matching allow or allowAllRequests rules can suppress lower-priority header modifications. See Priority and conflicts.

Source

See Chrome's Declarative Net Request header modification.