Fastly VCL constraints and limitations
VCL services are subject to the following restrictions or limits:
| Item | Limit | Implications of exceeding the limit |
|---|---|---|
| URL size | 8KB | VCL processing is skipped and a "Too long request string" error is emitted. |
Cookie header size | 32KB | The cookie header will be unset and Fastly will set req.http.Fastly-Cookie-Overflow = "1", then run your VCL as normal. |
| Request header size | 69KB | Depending on the circumstances, exceeding the limit can result in Fastly closing the client connection abruptly, the client receiving a 502 Gateway Error response with "I/O error" in the body, or receiving a 503 Service Unavailable response with the text "Header overflow" in the body. |
| Response header size | 69KB | A 503 error is triggered with obj.response value of "backend read error". This error can be intercepted in vcl_error. See Fastly generated errors to learn about all synthetic errors generated by Fastly. |
| Request header count | 255 | VCL processing is skipped or aborted if in progress, and a response with "Header overflow" in the body is emitted. A number of headers are added to the request by Fastly, so the practical limit is lower, but is not a predictable constant. Assuming a practical limit of 200 is safe. |
| Response header count | 96 | VCL processing is skipped or aborted if in progress, and a response with "Header overflow" in the body is emitted. A number of headers are added to the response by Fastly, so the practical limit is lower, but is not a predictable constant. Assuming a practical limit of 85 is safe. |
req.body size | 8KB | Larger requests will have an empty req.body, so request body payload is available in req.body only for payloads smaller than 8KB. |
| Surrogate key size | 1KB | Requests to the purge API that cite longer keys will fail, so in practical terms it is useless to tag content with keys exceeding the length limit. |
| Surrogate key header size | 16KB | Only keys that are entirely within the first 16KB of the surrogate key header value will be applied to the cache object. |
| VCL file size | 1MB | Attempts to upload VCL via the API will fail if the VCL payload is larger. |
| VCL total size | 3MB | Attempts to upload VCL via the API will fail if the VCL payload would cause your total service VCL to be larger than this. |
restart limit | 3 restarts | The 4th invocation of the restart statement will trigger a 503 error. This error can be intercepted in vcl_error. |
| Dictionary item count | 1000 | Attempts to create dictionary items will fail if they exceed the limit. Contact Fastly support to discuss raising this limit. |
| Dictionary item key length | 256 characters | Attempts to create dictionary items will fail. |
| Dictionary item value length | 8000 characters | Attempts to create dictionary items will fail. |
WARNING: Personal data should not be incorporated into VCL. Our Compliance and Law FAQ describes in detail how Fastly handles personal data privacy.
