libmamba reports `PackagesNotFoundError` for `python=3.12` from `defaults` on win-64, while `conda search` sees it and `--solver classic` succeeds

### Checklist

- [x] I added a descriptive title
- [x] I searched open reports and couldn't find a duplicate

### What happened?

libmamba reports `PackagesNotFoundError` for `python=3.12` from `defaults` on win-64, while `conda search` sees it and `--solver classic` succeeds

### Conda Info

```shell
active environment : None
            shell level : 0
       user config file : C:\Users\顾籽黎\.condarc
 populated config files : C:\D\miniconda3\.condarc
                          C:\D\miniconda3\condarc.d\anaconda-auth.yml
          conda version : 26.1.1
    conda-build version : not installed
         python version : 3.13.11.final.0
                 solver : libmamba (default)
       virtual packages : __archspec=1=skylake
                          __conda=26.1.1=0
                          __cuda=12.8=0
                          __win=10.0.26200=0
       base environment : C:\D\miniconda3  (read only)
      conda av data dir : C:\D\miniconda3\etc\conda
  conda av metadata url : None
           channel URLs : https://repo.anaconda.com/pkgs/main/win-64
                          https://repo.anaconda.com/pkgs/main/noarch
                          https://repo.anaconda.com/pkgs/r/win-64
                          https://repo.anaconda.com/pkgs/r/noarch
                          https://repo.anaconda.com/pkgs/msys2/win-64
                          https://repo.anaconda.com/pkgs/msys2/noarch
          package cache : C:\D\miniconda3\pkgs
                          C:\Users\顾籽黎\.conda\pkgs
                          C:\Users\顾籽黎\AppData\Local\conda\conda\pkgs
       envs directories : C:\Users\顾籽黎\.conda\envs
                          C:\D\miniconda3\envs
                          C:\Users\顾籽黎\AppData\Local\conda\conda\envs
    temporary directory : C:\Users\顾籽黎\AppData\Local\Temp
               platform : win-64
             user-agent : conda/26.1.1 requests/2.32.5 CPython/3.13.11 Windows/11 Windows/10.0.26200 solver/libmamba conda-libmamba-solver/25.11.0 libmambapy/2.3.2 aau/0.7.5 c/. s/. e/.
          administrator : False
             netrc file : None
           offline mode : False
```

### Conda Config

```shell
==> C:\D\miniconda3\.condarc <==
channels:
  - defaults

==> C:\D\miniconda3\condarc.d\anaconda-auth.yml <==
channel_settings:
  - frozendict.frozendict({'channel': 'https://repo.anaconda.cloud/*', 'auth': 'anaconda-auth'})
```

### Conda list

```shell
# packages in environment at C:\D\miniconda3:
#
# Name                        Version          Build               Channel
anaconda-anon-usage           0.7.5            pyhb46e38b_100      defaults
anaconda-auth                 0.12.3           py313haa95532_0     defaults
anaconda-cli-base             0.7.0            py313haa95532_0     defaults
anaconda_powershell_prompt    1.1.0            haa95532_1          defaults
anaconda_prompt               1.1.0            haa95532_1          defaults
annotated-types               0.6.0            py313haa95532_1     defaults
archspec                      0.2.5            pyhd3eb1b0_0        defaults
boltons                       25.0.0           py313haa95532_0     defaults
brotlicffi                    1.2.0.0          py313h885b0b7_0     defaults
bzip2                         1.0.8            h2bbff1b_6          defaults
ca-certificates               2025.12.2        haa95532_0          defaults
certifi                       2026.01.04       py313haa95532_0     defaults
cffi                          2.0.0            py313h02ab6af_1     defaults
charset-normalizer            3.4.4            py313haa95532_0     defaults
click                         8.2.1            py313haa95532_1     defaults
colorama                      0.4.6            py313haa95532_0     defaults
conda                         26.1.1           py313haa95532_0     defaults
conda-anaconda-telemetry      0.3.0            pyhd3eb1b0_1        defaults
conda-anaconda-tos            0.2.2            py313haa95532_1     defaults
conda-content-trust           0.2.0            py313haa95532_1     defaults
conda-libmamba-solver         25.11.0          pyhdf14ebd_0        defaults
conda-package-handling        2.4.0            py313haa95532_1     defaults
conda-package-streaming       0.12.0           py313haa95532_1     defaults
cpp-expected                  1.1.0            h214f63a_0          defaults
cryptography                  46.0.3           py313habbc9f9_0     defaults
distro                        1.9.0            py313haa95532_0     defaults
fmt                           11.2.0           h58b7f6e_0          defaults
frozendict                    2.4.6            py313h02ab6af_0     defaults
idna                          3.11             py313haa95532_0     defaults
jaraco.classes                3.4.0            py313haa95532_0     defaults
jaraco.context                6.0.0            py313haa95532_0     defaults
jaraco.functools              4.1.0            py313haa95532_0     defaults
jsonpatch                     1.33             py313haa95532_1     defaults
jsonpointer                   3.0.0            py313haa95532_0     defaults
keyring                       25.7.0           py313haa95532_0     defaults
libarchive                    3.8.2            h6c023e8_0          defaults
libcurl                       8.16.0           h97e0424_0          defaults
libexpat                      2.7.3            h885b0b7_4          defaults
libffi                        3.4.4            hd77b12b_1          defaults
libiconv                      1.16             h2bbff1b_3          defaults
libmamba                      2.3.2            hc213065_1          defaults
libmambapy                    2.3.2            py313h364efb6_1     defaults
libmpdec                      4.0.0            h827c3e9_0          defaults
libsolv                       0.7.30           h23a355e_2          defaults
libssh2                       1.11.1           h2addb87_0          defaults
libxml2                       2.13.9           h6201b9f_0          defaults
libzlib                       1.3.1            h02ab6af_0          defaults
lz4-c                         1.9.4            h2bbff1b_1          defaults
markdown-it-py                4.0.0            py313haa95532_0     defaults
mdurl                         0.1.2            py313haa95532_0     defaults
menuinst                      2.4.2            py313h885b0b7_1     defaults
more-itertools                10.8.0           py313haa95532_0     defaults
msgpack-python                1.1.1            py313h5da7b33_0     defaults
nlohmann_json                 3.11.2           h6c2663c_0          defaults
openssl                       3.5.5            hbb43b14_0          defaults
packaging                     25.0             py313haa95532_1     defaults
pcre2                         10.46            h5740b90_0          defaults
pip                           25.3             pyhc872135_0        defaults
pkce                          1.0.3            py313haa95532_0     defaults
platformdirs                  4.5.0            py313haa95532_0     defaults
pluggy                        1.5.0            py313haa95532_0     defaults
pybind11-abi                  5                hd3eb1b0_0          defaults
pycosat                       0.6.6            py313h827c3e9_2     defaults
pycparser                     2.23             py313haa95532_0     defaults
pydantic                      2.12.4           py313haa95532_0     defaults
pydantic-core                 2.41.5           py313h114bc41_1     defaults
pydantic-settings             2.12.0           py313haa95532_0     defaults
pygments                      2.19.2           py313haa95532_0     defaults
pyjwt                         2.10.1           py313haa95532_1     defaults
pysocks                       1.7.1            py313haa95532_1     defaults
python                        3.13.11          h260b955_100_cp313  defaults
python-dotenv                 1.1.0            py313haa95532_0     defaults
python_abi                    3.13             3_cp313             defaults
pywin32-ctypes                0.2.2            py313haa95532_0     defaults
readchar                      4.2.1            py313haa95532_0     defaults
reproc                        14.2.4           hd77b12b_2          defaults
reproc-cpp                    14.2.4           hd77b12b_2          defaults
requests                      2.32.5           py313haa95532_1     defaults
rich                          14.2.0           py313haa95532_0     defaults
ruamel.yaml                   0.18.16          py313hb9a58be_0     defaults
ruamel.yaml.clib              0.2.14           py313hb9a58be_0     defaults
semver                        3.0.4            py313haa95532_0     defaults
setuptools                    80.9.0           py313haa95532_0     defaults
shellingham                   1.5.4            py313haa95532_0     defaults
simdjson                      3.10.1           h214f63a_0          defaults
sqlite                        3.51.0           hda9a48d_0          defaults
tk                            8.6.15           hf199647_0          defaults
tomli                         2.2.1            py313haa95532_0     defaults
tqdm                          4.67.1           py313h4442805_1     defaults
truststore                    0.10.1           py313haa95532_1     defaults
typer                         0.20.0           py313haa95532_0     defaults
typer-slim                    0.20.0           py313haa95532_0     defaults
typer-slim-standard           0.20.0           py313haa95532_0     defaults
typing-extensions             4.15.0           py313haa95532_0     defaults
typing-inspection             0.4.2            py313haa95532_0     defaults
typing_extensions             4.15.0           py313haa95532_0     defaults
tzdata                        2025b            h04d1e81_0          defaults
ucrt                          10.0.22621.0     haa95532_0          defaults
urllib3                       2.6.1            py313haa95532_0     defaults
vc                            14.3             h2df5915_10         defaults
vc14_runtime                  14.44.35208      h4927774_10         defaults
vs2015_runtime                14.44.35208      ha6b5a95_10         defaults
wheel                         0.45.1           py313haa95532_0     defaults
win_inet_pton                 1.1.0            py313haa95532_1     defaults
xz                            5.6.4            h4754444_1          defaults
yaml-cpp                      0.8.0            hd77b12b_1          defaults
zlib                          1.3.1            h02ab6af_0          defaults
zstandard                     0.24.0           py313he335c29_0     defaults
zstd                          1.5.7            h56299aa_0          defaults
```

### Additional Context

## Body

### Checklist

- [x] I searched existing issues and did not find an exact match
- [x] I can reproduce this consistently
- [x] This happens with the official `defaults` channel
- [x] `conda search` can see the package
- [x] `--solver classic` succeeds with the same spec

---

### Summary

On Windows 11 (`win-64`), `conda create -n test python=3.12 -c defaults -y` fails with:

```text
PackagesNotFoundError: The following packages are not available from current channels:

  - python=3.12
```

However:

1. `conda search -c defaults python=3.12` clearly shows multiple available builds from `pkgs/main`
2. `conda search -c defaults python=3.12 --info` shows valid package metadata and URLs
3. `conda create -n test python=3.12 -c defaults --solver classic -y` succeeds
4. My `base` environment itself is installed successfully and is functional (currently running Python from base without issue)

Because of this, it looks like a `conda-libmamba-solver` / `libmamba` solving bug or incorrect error classification, rather than an actual missing package in `defaults`.

---

### Environment

```text
OS: Windows 11
Platform: win-64
conda: 26.1.1
conda-libmamba-solver: 25.11.0
libmambapy: 2.3.2
solver: libmamba (default)
channel: defaults
```

Relevant `conda info` values:

```text
platform : win-64
solver : libmamba
base environment : C:\D\miniconda3
channel URLs :
  https://repo.anaconda.com/pkgs/main/win-64
  https://repo.anaconda.com/pkgs/main/noarch
  https://repo.anaconda.com/pkgs/r/win-64
  https://repo.anaconda.com/pkgs/r/noarch
  https://repo.anaconda.com/pkgs/msys2/win-64
  https://repo.anaconda.com/pkgs/msys2/noarch
```

---

### Minimal reproduction

#### 1. Confirm that the package exists in `defaults`

```bash
conda search -c defaults python=3.12
```

This returns visible results such as:

```text
python  3.12.0   h1d929f7_0  pkgs/main
...
python  3.12.12  h716150d_1  pkgs/main
```

#### 2. Try creating an environment with the default solver (`libmamba`)

```bash
conda create -n test python=3.12 -c defaults -y
```

This fails with:

```text
PackagesNotFoundError: The following packages are not available from current channels:

  - python=3.12
```

#### 3. Try the same spec with the classic solver

```bash
conda create -n test python=3.12 -c defaults --solver classic -y
```

This succeeds.

---

### Additional observations

I also confirmed the package metadata is available:

```bash
conda search -c defaults python=3.12 --info
```

This shows valid package metadata for multiple Windows builds, including package URLs under:

```text
https://repo.anaconda.com/pkgs/main/win-64/
```

For example, `python-3.12.12-h716150d_1.conda` is visible.

I also tried:

```bash
conda create -n test python=3.12.12 -c defaults -y
```

and it still failed under `libmamba`.

I also tried forcing full repodata:

```bash
conda create -n test python=3.12 -c defaults --repodata-fn repodata.json -y
```

and it still failed under `libmamba`.

I also tried restricting to `pkgs/main` only:

```bash
conda create -n test python=3.12 -c https://repo.anaconda.com/pkgs/main --override-channels -y
```

and it still failed under `libmamba`.

So this does not appear to be caused by:
- missing package metadata
- missing package on `defaults`
- old conda version
- extra channels interfering

---

### What I tried during troubleshooting

1. Verified `conda search -c defaults python=3.12` returns available builds
2. Verified `conda search -c defaults python=3.12 --info` returns valid package metadata
3. Cleared index cache:

```bash
conda clean -i -y
```

4. Retried create with default solver → still failed
5. Retried with exact version `python=3.12.12` → still failed
6. Retried with `--repodata-fn repodata.json` → still failed
7. Retried using only `https://repo.anaconda.com/pkgs/main` + `--override-channels` → still failed
8. Updated conda from `25.11.1` to `26.1.1`
9. Retried after update → still failed
10. Retried with `--solver classic` → succeeded

---

### Expected behavior

`conda create -n test python=3.12 -c defaults -y` should succeed, since:

- the package is visible in `conda search`
- the package metadata is valid
- the same spec succeeds with `--solver classic`

---

### Actual behavior

With the default `libmamba` solver, conda reports:

```text
PackagesNotFoundError: The following packages are not available from current channels:

  - python=3.12
```

even though the package clearly exists in `defaults`.

---

### Suspected issue

This looks like one of the following:

1. a `libmamba` solve bug on Windows for this spec/channel combination
2. incorrect handling of package metadata / constraints for `python=3.12` on `win-64`
3. a misclassified solver failure being surfaced as `PackagesNotFoundError`, even though the package actually exists

Because `classic` succeeds and `search` sees the package, this seems unlikely to be a real package availability problem.

---

### Relevant verbose behavior

With verbose logs, `libmamba` appears to read repodata successfully and receives the requested spec:

```text
The solver will handle these requests:
{
  "Install": [
    "python=3.12"
  ]
}
```

But it still ends with `PackagesNotFoundError`.

This is why I suspect the package-not-found message may be masking a deeper solver issue.

---

### Workaround

Using the classic solver works:

```bash
conda create -n test python=3.12 -c defaults --solver classic -y
```

---

### Notes

My `base` environment itself is installed and working correctly, and Python in `base` is usable.
So this does not appear to be a general inability to install/use Python from the same conda installation.

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libmamba reports `PackagesNotFoundError` for `python=3.12` from `defaults` on win-64, while `conda search` sees it and `--solver classic` succeeds #887

Checklist

What happened?

Conda Info

Conda Config

Conda list

Additional Context

Body

Checklist

Summary

Environment

Minimal reproduction

1. Confirm that the package exists in `defaults`

2. Try creating an environment with the default solver (`libmamba`)

3. Try the same spec with the classic solver

Additional observations

What I tried during troubleshooting

Expected behavior

Actual behavior

Suspected issue

Relevant verbose behavior

Workaround

Notes

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

libmamba reports PackagesNotFoundError for python=3.12 from defaults on win-64, while conda search sees it and --solver classic succeeds #887

Description

Checklist

What happened?

Conda Info

Conda Config

Conda list

Additional Context

Body

Checklist

Summary

Environment

Minimal reproduction

1. Confirm that the package exists in defaults

2. Try creating an environment with the default solver (libmamba)

3. Try the same spec with the classic solver

Additional observations

What I tried during troubleshooting

Expected behavior

Actual behavior

Suspected issue

Relevant verbose behavior

Workaround

Notes

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

libmamba reports `PackagesNotFoundError` for `python=3.12` from `defaults` on win-64, while `conda search` sees it and `--solver classic` succeeds #887

1. Confirm that the package exists in `defaults`

2. Try creating an environment with the default solver (`libmamba`)