If I share my bank credentials with TrueLayer, will I be breaching my bank's terms and conditions?

No, you will not.

If you use our Credential Sharing method to share your account information with a provider, you will need to provide your bank credentials to TrueLayer so that the application can access your information temporarily.

TrueLayer is an authorised AIS and PIS provider, licensed by the UK Financial Conduct Authority (FCA), therefore we have permission to use your credentials when you authorise us. This does not violate any terms and conditions in the agreement between you and your bank.

According to the FCA:

Your banking terms and conditions should not prevent you from sharing your credentials with regulated AIS or PIS providers. Your bank cannot hold you responsible for unauthorised transactions just because you have shared your credentials with regulated AIS and PIS providers.

See the FCA website for more details.