Use cURL’s -A or --user-agent option to send a custom HTTP User-Agent value. For example, curl -A "MyApp/1.0" https://example.com identifies the request as MyApp/1.0. The option changes request metadata; it does not turn cURL into a web browser.
The cURL options you need
The dedicated option is -A, with --user-agent as its long form. Both set the User-Agent string sent to an HTTP server.
| Purpose | Command form | What it does |
|---|---|---|
| Set a value | -A "MyApp/1.0" |
Sends MyApp/1.0 as the User-Agent. |
| Set a value with the long option | --user-agent "My App/1.0" |
The readable equivalent of -A; quote values containing spaces. |
| Set the header generically | -H "User-Agent: MyApp/1.0" |
Sets the same HTTP header through cURL’s general header option. |
| Remove the header | --user-agent "" |
Disables cURL’s User-Agent header for the request. |
| Send a blank value | --user-agent " " |
Sends a User-Agent header whose value is one space; this is different from removing it. |
If an option appears more than once, the last User-Agent value supplied is the one cURL uses.
Set a User-Agent in a cURL command
Use a descriptive application identifier
A maintainable User-Agent normally identifies your application and, when useful, its version:
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curl -A "MyApp/1.0" https://example.comThe long form is equivalent:
curl --user-agent "My App/1.0" https://example.comShell quoting matters. Put the value in single or double quotes when it contains spaces, parentheses, semicolons, or other shell punctuation. Without an explicit value, the cURL command-line tool sends a default in the form curl/VERSION; the current man-page example shows curl/8.23.0. A custom value replaces that default.
Set the header with -H
You can express the same request with general header syntax:
curl -H "User-Agent: MyApp/1.0" https://example.comUse -A when you are specifically controlling the User-Agent: it communicates intent and avoids manually writing the header name. Use -H when you are already assembling several headers in one place.
Remove or blank the User-Agent
To stop cURL from sending a User-Agent header, pass an empty string:
curl --user-agent "" https://example.comThat is not the same as passing one space:
curl --user-agent " " https://example.comThe first command removes the option’s header. The second deliberately sends a header with a blank-looking value. Choose the first behavior when a server or test requires no User-Agent field; choose the second only when you specifically need a present-but-empty value.
Control precedence when scripts add options
When a command contains multiple -A or --user-agent options, cURL uses the last one. This makes a later command-line setting an effective override for a value added by a shared script fragment:
curl -A "OldClient/1.0" --user-agent "NewClient/2.0" https://example.comThe request uses NewClient/2.0. Keep one authoritative setting in production commands when possible so that the identity is easy to audit.
What a User-Agent can—and cannot—do
Use browser-style values only for a clear reason
cURL’s tutorial demonstrates a browser-style value such as Mozilla/3.0 (Win95; I). You can supply a similar string:
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curl -A 'Mozilla/3.0 (Win95; I)' https://example.comThis changes the identifying string that the server receives. It does not add a browser’s JavaScript engine, cookie handling, layout engine, TLS behavior, or other capabilities. A site may still serve different content, require JavaScript, or reject the request for reasons unrelated to the text of the User-Agent.
For software that is making an automated request, a descriptive identifier such as InventorySync/2.4 is easier to maintain than pretending to be a particular browser. Follow the destination site’s terms, robots policy, rate limits, and access rules; changing an identifying header is not permission to bypass controls.
Proxy requests and the User-Agent header
-A controls the User-Agent sent to the HTTP server. If you need to control headers addressed to an HTTP proxy instead, cURL provides --proxy-header. Keep the two destinations separate in your design: an origin-server header and a proxy header are not interchangeable.
Set the User-Agent from code
C with libcurl
In libcurl, set the option on an easy handle with CURLOPT_USERAGENT:
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int main(void) {
CURL *curl = curl_easy_init();
if (!curl) return 1;
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
curl_easy_setopt(curl, CURLOPT_USERAGENT, "MyApp/1.0");
CURLcode result = curl_easy_perform(curl);
curl_easy_cleanup(curl);
return result == CURLE_OK ? 0 : 1;
}The API reference defines the string as the User-Agent header for HTTP requests. A later setting overrides an earlier one; passing NULL disables the option. CURLOPT_USERAGENT has been available since curl 7.1. The command-line -A option and this libcurl setting are separate interfaces, so use the one that matches your integration.
Python with Requests
Python’s HTTP libraries generally expose headers as a mapping. This complete example sends the same metadata:
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import requests
url = "https://example.com"
headers = {"User-Agent": "MyApp/1.0"}
response = requests.get(url, headers=headers, timeout=30)
response.raise_for_status()
print(response.status_code)To remove the header, omit it from the mapping rather than supplying an empty-looking value. Whether a library adds its own default headers is library-specific, so inspect the actual outgoing request when exact wire behavior matters.
Node.js with the built-in Fetch API
In a current Node.js release with fetch, put the header in the request options:
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headers: { 'User-Agent': 'MyApp/1.0' }
});
if (!response.ok) {
throw new Error(`HTTP ${response.status}`);
}
console.log(await response.text());As with Python, removing your explicit header does not necessarily mean that the runtime sends no User-Agent at all; a client library may provide a default. Test the behavior of the exact runtime and version you deploy.
Choosing between the three cURL approaches
| Approach | Best fit | Important detail |
|---|---|---|
-A / --user-agent |
Normal cURL commands and scripts | Purpose-built, concise, and supports explicit removal with an empty value. |
-H "User-Agent: ..." |
Commands that centralize all HTTP headers | Uses general header syntax; you manage the header name yourself. |
CURLOPT_USERAGENT |
Applications embedding libcurl | Set it on each relevant easy handle; later settings replace earlier ones. |
The cURL project records --user-agent/-A as introduced in curl 4.5.1. That history does not change the syntax used by current cURL versions.
Troubleshooting common User-Agent problems
The value is cut off or the shell reports a syntax error
Cause: the value contains spaces or shell metacharacters and was not quoted.
Fix: wrap the complete value in single or double quotes, for example --user-agent "My App/1.0". If the value itself contains a quote, choose the other quote style or escape the character according to your shell.
The request still identifies as the old client
Cause: another User-Agent option appears later in the command or in a wrapper script.
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Fix: search the assembled command for every -A, --user-agent, and -H "User-Agent: ...". The last dedicated User-Agent option wins; remove the unintended override.
The server sees a blank value when you wanted no header
Cause: the command used one space rather than an empty string.
Fix: use --user-agent "" for removal. Reserve --user-agent " " for tests that require a present header with a one-space value.
A site still rejects the request after you copied a browser string
Cause: User-Agent is only request metadata. It does not supply browser capabilities, and a destination may apply other access checks.
Fix: use an honest application identifier, check the site’s access requirements, and determine whether the workflow actually needs a browser automation tool rather than cURL. Do not assume that changing the string bypasses a bot check or other restriction.
The origin works but the proxy behaves differently
Cause: the header needed by the proxy was configured as an origin-server header.
Fix: configure proxy-directed headers with --proxy-header, while keeping the origin User-Agent under -A or -H.
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Performance, reliability, and cost considerations
Setting a User-Agent is a local option that changes the request header; it does not add a separate network round trip or a paid service. The main reliability concern is accuracy: a misleading browser claim can produce content or access behavior that your client cannot actually handle. A versioned application identifier makes logs and support investigations easier, while an empty or blank value should be used only when your protocol test calls for it.
For repeatable jobs, keep the value in one configuration variable and apply it consistently across command-line, Python, Node.js, or libcurl implementations. Document whether the requirement is “no header,” “blank header,” or a particular identifier—those are different wire-level requests.
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Bottom line
Use -A or --user-agent for a clear, versioned identifier, quote values that contain spaces, and remember that the last setting wins. Use an empty value to remove the header, a one-space value to send a blank header, --proxy-header for proxy-directed metadata, and CURLOPT_USERAGENT when embedding libcurl. A User-Agent changes identification only; it does not create browser functionality.
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