Essentially the same as this question, but the solutions there don't seem applicable here. Both isolated and in compounds, I can only find 匙 as either shi⁰ or chi² “spoon”.
Can this character in this context have a specific tone, and if so in what resource can it be found? Or is looking at tone alone misleading; is/was it possible to read 钥匙 as yao⁴chi²? (Where?)
匙 http://www.cantonese.sheik.co.uk/dictionary/characters/371/
[粵] ci4 | [國] chi2 : spoon
[粵] si4 | [國] shi5 : key
When '匙' means 'spoon', it is read as /chi2/ in Mandarin
When '匙' means 'key', it is read as /shi5/ in Mandarin
It doesn't matter it is by itself or in a compound word
There are other examples of single character that have more than one meaning and pronounced differently for each meaning. 樂 is one of them. When 樂 means 'joy' it is read as /le4/; When 樂 means 'music' it is read as /yue4/
My digital copy of《汉语大词典》gives a first tone:
匙
shī
钥匙
I'd like to check this against a paper copy though, just to be sure.
Grand Ricci gives:
shí (or chí)
1. Cuiller
2. N. f.
This seems to be the more likely of the two as it tonally fits the chí pronunciation.
In Mandarin Chinese, 钥匙 is yao4shi0. The 匙 here can only have the neutral tone.
《现代汉语词典》 has an entry for 匙(shi0), with a very brief explanation: 见1466页【钥匙】（see 钥匙 on page 1466）. The word 钥匙 only has one pronunciation listed: yao5shi0. (I don't have digital version of this dictionary.)
Also you can see 匙 and 钥匙 on zdic.net, which also only lists only "shi5" for the 匙 in 钥匙.
匙 seems to be "shi5" only in 钥匙，and 钥匙 is always yao4shi5.
