Chainlink VRF request stuck 'pending, low balance' — subscription must cover callbackGasLimit times the gas-lane max price

Symptom: a VRF v2.5 subscription on Sepolia is funded with 15 LINK, requestRandomWords is emitted, upkeep runs, and the Chainlink UI shows the request pending with a low-balance warning implying a cost above 100 LINK. Nothing reverts; the request simply never fulfils.

Cause: VRF does not bill actual usage up front — it requires the subscription to be able to pay the worst-case fulfilment cost. That bound is roughly (callbackGasLimit + verification gas) multiplied by the *maximum* gas price of the chosen key hash / gas lane, plus the premium, converted to LINK via the LINK/ETH feed. On Sepolia the default lane caps at 500 gwei, so a callbackGasLimit of 1,000,000 reserves an enormous amount even though the real fulfilment would use a fraction of it. The pending state is the coordinator waiting for a balance that can cover the bound.

Fix: size callbackGasLimit to what fulfillRandomWords actually consumes plus margin — measure it in a local/forked test and set something like 70,000-200,000 rather than a round million:

callbackGasLimit: 70000,   // was 1000000
numWords: 1,

Also keep numWords minimal (each word adds gas), pick a lower-max-price gas lane if the chain offers one, top the subscription up, and confirm the consumer contract is actually added to the subscription — a request from a non-consumer fails differently but is often diagnosed as a funding issue.

General lesson: with VRF, callbackGasLimit is a cost lever, not just a safety ceiling, and heavy logic in fulfillRandomWords (storage writes, transfers, loops) is what forces it up. Figures here are VRF v2.5 on Sepolia as of 2025; lane parameters change per network.

Source: adapted from a Stack Exchange answer by Hamza (question by Hamza), https://ethereum.stackexchange.com/questions/170233/why-is-chainlink-vrf-request-costing-me-over-120-link-on-sepolia-testnet — CC BY-SA 4.0