Libraries installed under `directories.user` are not discovered

### Describe the problem

Arduino CLI 1.5.1 does not seem to discover libraries installed in the configured user libraries folder.

The library is correctly installed and detected by `arduino-cli lib list`, but during compilation the library resolver does not find it.

This behavior happens on both Windows and Ubuntu 24.04.

## Environment

* OS: Ubuntu 24.04 (also reproduced on Windows)
* Arduino CLI version:

```
arduino-cli  Version: 1.5.1
Commit: 01f3d4f2b
Date: 2026-06-05T10:22:17Z
```

* Installation method:

```bash
curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
```

* Board:

```
arduino:avr:uno
```

* Arduino AVR core:

```
arduino:avr 1.8.8
```

## Arduino CLI configuration

`arduino-cli config dump`:

```yaml
board_manager:
  additional_urls: []

directories:
  data: /home/tiago/.arduino15
  downloads: /home/tiago/.arduino15/staging
  user: /home/tiago/Arduino
```

## Installed libraries

`arduino-cli lib list`:

```
Name  Installed  Available  Location Description
Servo 1.3.0      -          user     -
```

The library exists at:

```
/home/tiago/Arduino/libraries/Servo
```

with the expected structure:

```
Servo/
├── library.properties
└── src/
    └── Servo.h
```

## Sketch

`MyFirstSketch.ino`:

```cpp
#include <Servo.h>

Servo myServo;

void setup() {
  myServo.attach(9);
}

void loop() {
}
```

## Compile command

```bash
arduino-cli compile
```

or:

```bash
arduino-cli compile --fqbn arduino:avr:uno
```

## Error output

Compilation fails with:

```
fatal error: Servo.h: No such file or directory
#include <Servo.h>
         ^~~~~~~~~

compilation terminated.
```

Verbose compilation shows:

```
ResolveLibrary(Servo.h)
  -> candidates: []
```

No candidates are found for the library.

## Additional information

The same issue was reproduced on Windows.

## Workaround

Moving the library folder from the user library location:

```
~/Arduino/libraries/Servo
```

to the AVR platform libraries folder:

```
~/.arduino15/packages/arduino/hardware/avr/1.8.8/libraries
```

makes compilation succeed.

Command used:

```bash
cp -rv ~/Arduino/libraries/Servo \
~/.arduino15/packages/arduino/hardware/avr/1.8.8/libraries
```

After moving the library, `arduino-cli compile` finds `Servo.h` and the sketch builds successfully.

## Expected behavior

Libraries installed with:

```bash
arduino-cli lib install Servo
```

should be automatically discovered from the configured user libraries directory:

```
directories:
  user: /home/tiago/Arduino
```

without requiring the library to be copied into the platform-specific `libraries` folder.

## Additional logs

`arduino-cli compile -v` output:

```
FQBN: arduino:avr:uno
Using board 'uno' from platform in folder: /home/tiago/.arduino15/packages/arduino/hardware/avr/1.8.8
Using core 'arduino' from platform in folder: /home/tiago/.arduino15/packages/arduino/hardware/avr/1.8.8

Detecting libraries used...
/home/tiago/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/tiago/.arduino15/packages/arduino/hardware/avr/1.8.8/cores/arduino -I/home/tiago/.arduino15/packages/arduino/hardware/avr/1.8.8/variants/standard /home/tiago/.cache/arduino/sketches/0B532B3C68DF5C9F947628F4FA330A9D/sketch/MyFirstSketch.ino.cpp.merged -o /dev/null -MMD -MF /home/tiago/.cache/arduino/sketches/0B532B3C68DF5C9F947628F4FA330A9D/sketch/MyFirstSketch.ino.cpp.merged.libsdetect.d
Alternatives for Servo.h: []
ResolveLibrary(Servo.h)
  -> candidates: []

Used platform Version Path
arduino:avr      1.8.8      /home/tiago/.arduino15/packages/arduino/hardware/avr/1.8.8
Error during build: exit status 1
```


### To reproduce

1. Install Arduino CLI 1.5.1:

```bash
curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
```

2. Initialize Arduino CLI:

```bash
arduino-cli config init
```

3. Configure the Arduino user directory in `arduino-cli.yaml`:

```yaml
directories:
  user: /home/$user/Arduino
```

4. Install the Servo library:

```bash
arduino-cli lib install Servo
```

5. Verify that the library is installed in the user libraries folder:

```bash
arduino-cli lib list
```

Expected output:

```text
Name  Installed  Location
Servo 1.3.0      user
```

The library is located at:

```text
/home/$user/Arduino/libraries/Servo
```

6. Create a new Arduino sketch:

```bash
arduino-cli sketch new MyFirstSketch
cd MyFirstSketch
```

7. Add the following code to `MyFirstSketch.ino`:

```cpp
#include <Servo.h>

Servo myServo;

void setup() {
  myServo.attach(9);
}

void loop() {
}
```

8. Install the Arduino AVR platform:

```bash
arduino-cli core install arduino:avr
```

9. Compile the sketch:

```bash
arduino-cli compile --fqbn arduino:avr:uno
```

10. Compilation fails with:

```text
fatal error: Servo.h: No such file or directory
#include <Servo.h>
         ^~~~~~~~~

compilation terminated.
```

Verbose compilation output shows:

```text
ResolveLibrary(Servo.h)
  -> candidates: []
```

indicating that the library resolver does not find the installed user library.


### Expected behavior

Libraries installed using:

```bash
arduino-cli lib install <library-name>
```

should be automatically discovered by Arduino CLI during compilation from the configured user libraries directory.

Given the following configuration:

```yaml
directories:
  user: /home/tiago/Arduino
```

and an installed library located at:

```text
/home/tiago/Arduino/libraries/Servo
```

running:

```bash
arduino-cli compile --fqbn arduino:avr:uno
```

should resolve the dependency and compile the sketch successfully.

The verbose output should show the library being detected, for example:

```text
Alternatives for Servo.h: [Servo@1.3.0]
ResolveLibrary(Servo.h)
  -> Servo@1.3.0
```

The user should not need to manually copy libraries into platform-specific directories such as:

```text
~/.arduino15/packages/arduino/hardware/avr/1.8.8/libraries
```

because libraries installed in the configured user library folder should be available to all compatible boards and platforms.


### Arduino CLI version

arduino-cli  Version: 1.5.1 Commit: 01f3d4f2b Date: 2026-06-05T10:22:17Z

### Operating system

Linux

### Operating system version

24.04

### Additional context

_No response_

### Issue checklist

- [x] I searched for previous reports in [the issue tracker](https://github.com/arduino/arduino-cli/issues?q=)
- [x] I verified the problem still occurs when using the [nightly build](https://arduino.github.io/arduino-cli/dev/installation/#nightly-builds)
- [x] My report contains all necessary details

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Uh oh!

Libraries installed under `directories.user` are not discovered #3253

Describe the problem

Environment

Arduino CLI configuration

Installed libraries

Sketch

Compile command

Error output

Additional information

Workaround

Expected behavior

Additional logs

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Expected behavior

Arduino CLI version

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Uh oh!

Libraries installed under directories.user are not discovered #3253

Description

Describe the problem

Environment

Arduino CLI configuration

Installed libraries

Sketch

Compile command

Error output

Additional information

Workaround

Expected behavior

Additional logs

To reproduce

Expected behavior

Arduino CLI version

Operating system

Operating system version

Additional context

Issue checklist

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

Libraries installed under `directories.user` are not discovered #3253