`KeyError: 'deprecated'` because of bad cache

Crash Report

I ran mypy in my venv and the cache somehow got into a bad state, leading to crashes until the cache was removed. I don't know what triggered this. If it matters, the mypy that caused the bad cache was installed with faster-cache.

Traceback

Traceback (most recent call last):
  File "[...]/.venv/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
             ~~~~~~~~~~~~~^^
  File "[...]/.venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
    ~~~~^^
  File "mypy/main.py", line 109, in main
  File "mypy/main.py", line 193, in run_build
  File "mypy/build.py", line 194, in build
  File "mypy/build.py", line 269, in _build
  File "mypy/build.py", line 2940, in dispatch
  File "mypy/build.py", line 3331, in process_graph
  File "mypy/build.py", line 3409, in process_fresh_modules
  File "mypy/build.py", line 2096, in load_tree
  File "mypy/nodes.py", line 402, in deserialize
  File "mypy/nodes.py", line 4058, in deserialize
  File "mypy/nodes.py", line 3999, in deserialize
  File "mypy/nodes.py", line 263, in deserialize
  File "mypy/nodes.py", line 3420, in deserialize
  File "mypy/nodes.py", line 4058, in deserialize
  File "mypy/nodes.py", line 3999, in deserialize
  File "mypy/nodes.py", line 263, in deserialize
  File "mypy/nodes.py", line 952, in deserialize
  File "mypy/nodes.py", line 879, in deserialize
KeyError: 'deprecated'

To Reproduce

repro.zip

unzip repro.zip
cd repro/
uv venv
source .venv/bin/activate.fish
uv pip install -r requirements.txt
mypy foo.py

Your Environment

• Mypy version used: 1.14.1 (compiled: yes)
• Mypy configuration options from mypy.ini (and other config files): strict = true
• Python version used: 3.13.1
• Operating system and version: macOS Sequoia 15.2

Start with the deserialization path in mypy/nodes.py, especially the deserialize methods near the reported KeyError, and reproduce the failure using the commands and repro.zip provided. Trace how the bad cache is read after running mypy with faster-cache. Done means the reproduced cache state no longer crashes with KeyError: 'deprecated'.