Full details, release notes, download instructions and links can be found on GitHub:
https://github.com/espressif/esp-idf/re ... tag/v3.1.5
Important: An app using ESP-IDF v3.1.5 will fail to boot if DIO flash mode & PSRAM are both used together with a bootloader from an earlier version. If using this configuration in the bootloader or app, do not update to v3.1.5. This bug will be fixed in ESP-IDF v3.1.6.
At time of writing, the latest stable release is V3.2.2.