- Software and digital services
- HTTP service
431
HTTP 431 Request Header Fields Too Large: meaning and next diagnostic branch
Risk level
Low
Usually safe to check without specialist tools.
Urgency
Medium
Quick answer
HTTP 431 Request Header Fields Too Large means: The server refuses the request because its header fields are too large. It identifies a protocol result, not the failed component. Preserve one request/response pair and discriminate the raw method, target, Content-Length, header sizes and the first hop that rejects the same bytes.
Safety first
Stop and check this first
- Redact credentials before sharing traces. Do not raise global body/header limits or disable the WAF from one request.
Safe checks you can perform
- Freeze one complete failing exchange
- Change one variable and repeat once
- Stop before retries hide the first failure
Leave these tasks to a qualified professional
- Do not attempt this professional task: Trace-led handoff for HTTP 431
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Trace-led handoff for HTTP 431
What they doCompare exact byte counts and parsed request fields at client, WAF/proxy and origin; identify the first parser or configured limit that diverges.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Freeze one complete failing exchange
Record HTTP 431, timestamp, method, scheme/host/path without secrets, response headers and body, request ID, client, deployment version and every proxy/CDN/gateway hop. Redact credentials and personal data.
Redact credentials before sharing traces. Do not raise global body/header limits or disable the WAF from one request.
Evidence: IANA HTTP Status Code Registry · RFC 6585 - Additional HTTP Status Codes
Change one variable and repeat once
Capture the request without secrets and replay once in a safe test environment. Change only the named method, length, URI or header field and compare edge versus origin.
Redact credentials before sharing traces. Do not raise global body/header limits or disable the WAF from one request.
Evidence: IANA HTTP Status Code Registry · RFC 6585 - Additional HTTP Status Codes
Stop before retries hide the first failure
If the edge rejects bytes the origin accepts, fix the edge limit/rule. If both reject identically, fix the client contract or origin route; a generic server upgrade is unproved.
Redact credentials before sharing traces. Do not raise global body/header limits or disable the WAF from one request.
Evidence: IANA HTTP Status Code Registry · RFC 6585 - Additional HTTP Status Codes
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Software and digital services
- Brand
- HTTP
- Product type
- HTTP service
Known code variants
- 431 Anfragekopfzeilen zu groß
- 431 Campi di intestazione troppo grandi
- 431 Campos de cabecera demasiado grandes
- 431 Champs d’en-tête trop volumineux
- 431 Request Header Fields Too Large
- événement Fronius 431
- evento Fronius 431
- Fronius Ereignis 431
- Fronius event 431
- Fronius inverter 431
- Fronius Symo / Eco 431
- Fronius Wechselrichter 431
- HTTP 431
- HTTP status 431
- inversor Fronius 431
- inverter Fronius 431
- onduleur Fronius 431
What it means
Registered meaning: The server refuses the request because its header fields are too large. The next decision boundary is the raw method, target, Content-Length, header sizes and the first hop that rejects the same bytes. Before changing infrastructure or buying support, require: Compare exact byte counts and parsed request fields at client, WAF/proxy and origin; identify the first parser or configured limit that diverges.
Warnings and stop conditions
- Lowsource verified
Redact credentials before sharing traces. Do not raise global body/header limits or disable the WAF from one request.
Evidence: IANA HTTP Status Code Registry · RFC 6585 - Additional HTTP Status Codes
Symptoms and causes
Probable causes · in order
- What the registered HTTP status establishesConfirmed
IANA registers 431 as Request Header Fields Too Large. The controlling specification establishes: The server refuses the request because its header fields are too large.
Evidence: IANA HTTP Status Code Registry · RFC 6585 - Additional HTTP Status Codes
- The evidence that changes the next branchPossible
Capture the raw method, target, Content-Length, header sizes and the first hop that rejects the same bytes. If the edge rejects bytes the origin accepts, fix the edge limit/rule. If both reject identically, fix the client contract or origin route; a generic server upgrade is unproved.
Evidence: IANA HTTP Status Code Registry · RFC 6585 - Additional HTTP Status Codes
Virtual technician
Does 431 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
IANA HTTP Status Code Registry
Internet Assigned Numbers Authority · 2025 · standard
- 2
RFC 6585 - Additional HTTP Status Codes
IETF / RFC Editor · 2012 · standard
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