kcserver starting then abrutly exiting

[jake-wittman]

Opening a new discussion based on my comment on another discussion. Requests from @juliasilge

I have kernalSupervisor show terminal set to true and the kernal supervisor logs on trace.
The server process is exiting without leaving any output behind, which is part of what makes this interesting. To capture more, set these two in your settings and then restart to run through the problem again:

Here's the Kernel Supervisor output:

2026-06-04 09:50:38.731 [info] Positron Kernel Supervisor activated
2026-06-04 09:50:38.731 [info] 16:50:37 [Positron] Generated connection file path: C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-848931d5-4732.json
2026-06-04 09:50:38.731 [info] 16:50:37 [Positron] Using named pipe transport
2026-06-04 09:50:38.731 [info] 16:50:37 [Positron] Starting Kallichore server c:\Positron\resources\app\extensions\positron-supervisor\resources\kallichore\kcserver.exe with connection file C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-848931d5-4732.json and 30000ms startup timeout
2026-06-04 09:50:38.733 [info] 16:50:38 [Positron] Kallichore terminal started in 638ms with PID 29716
16:50:39 [Positron] Kallichore server PID 29716 is not running
16:50:39 [Positron] The supervisor process exited unexpectedly during startup
16:50:39 [Positron] Failed to start Kallichore server: Error: The supervisor process exited unexpectedly during startup
16:50:40 [Positron] Generated connection file path: C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-aa933af4-4732.json
16:50:40 [Positron] Using named pipe transport
16:50:40 [Positron] Starting Kallichore server c:\Positron\resources\app\extensions\positron-supervisor\resources\kallichore\kcserver.exe with connection file C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-aa933af4-4732.json and 30000ms startup timeout
16:50:40 [Positron] Kallichore terminal started in 199ms with PID 28992
16:50:41 [Positron] Kallichore server PID 28992 is not running
16:50:41 [Positron] The supervisor process exited unexpectedly during startup

I'm not seeing any Kallichore terminal though. I'm assuming that'd show up as an option in the terminal window? Or is it maybe these pop-ups that I get?

The terminal process "c:\Positron\resources\app\extensions\positron-supervisor\resources\supervisor-wrapper.bat 'C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-848931d5-4732.out.log', 'c:\Positron\resources\app\extensions\positron-supervisor\resources\kallichore\kcserver.exe', '--log-level', 'trace', '--log-file', 'C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-848931d5-4732.log', '--connection-file', 'C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-848931d5-4732.json', '--resource-sample-interval', '1000', '--transport', 'named-pipe', '--idle-shutdown-hours', '1'" terminated with exit code: 1.

Is your H: drive (where Positron is installed) a local disk, a mapped network drive, or removable storage? The H: drive is just another local disk I have that I was trying the install on. I moved it to my C drive and see the same thing.

What security / protection / antivirus is running on this machine? I have Windows defender running b ut I've whitelisted kcserver and the whole Positron folder

Here's the rest of my startup diagnostics# Positron Runtime Startup Diagnostics

System Information

• Positron: 2026.06.0 build 211 (Code OSS 1.111.0)
• Commit: d5f94f0
• OS: win32(10.0.19045)
• CPUs: AMD Ryzen 9 3900X 12-Core Processor (24 x 3800)
• Memory(System): 63.93 GB (42.54GB free)
• Initial Startup: true

Workspace-Affiliated Runtimes

• No affiliated runtimes for this workspace

Active Sessions

• No active runtime sessions

Time to First Runtime Ready

• No runtime has reached ready state yet

Startup Phase Progression

• Current Phase: Starting

Phase	Timestamp (ms)	Duration (ms)
initializing	1780591831714	-
awaitingTrust	1780591831715	1
starting	1780591834963	3248

Positron Runtime Performance Marks

Name	Timestamp	Delta	Total
code/positron/runtimeStartupBegin	1780591831714	0	0
code/positron/runtimeStartupPhase/initializing	1780591831714	0	0
code/positron/runtimeStartupPhase/awaitingTrust	1780591831715	1	1
code/positron/runtimeStartupPhase/starting	1780591834963	3248	3249
code/positron/runtimeStartup/extensionPreActivate/ms-python.python	1780591835044	81	3330
code/positron/runtimeStartup/extensionPreActivate/positron.positron-r	1780591835044	0	3330

Individual Session Startup Times

• No session start marks recorded

Interpreter Settings

Startup Behavior

• interpreters.startupBehavior: auto
• interpreters.restartOnCrash: true

Python

• python.defaultInterpreterPath: python
• python.interpreters.include: []
• python.interpreters.exclude: []
• python.interpreters.override: []
• python.environmentProviders.enable: {"Venv":true,"Conda":true,"uv":true}
• python.locator: native

R

• positron.r.interpreters.default: (not set)
• positron.r.customBinaries: []
• positron.r.customRootFolders: []
• positron.r.interpreters.exclude: []
• positron.r.interpreters.override: []
• positron.r.interpreters.condaDiscovery: false
• positron.r.interpreters.pixiDiscovery: false

Session Launch Parameters

• No active sessions

Discovered Runtimes

• No runtimes have been discovered

Discovery Cache

This-session counters

• Foreground cache-hit registrations: 0
• Background revalidations attempted / succeeded / failed: 0 / 0 / 0
• Evictions this session: 0
• Full-discovery passes this session: 0
• Full-discovery passes triggered by root changes: 0

Per-bucket state

Extension	Language	Entries	Oldest firstSeen	Newest lastValidated	lastFullDiscovery
positron.positron-reticulate	python	0	-	-	10m ago
positron.positron-r	r	1	10m ago	10m ago	10m ago
ms-python.python	python	1	10m ago	10m ago	10m ago

Discovery Root Signatures

positron.positron-r / r

• Roots: 6 (with opaque blob)
  | Path | Exists | mtime |
  | -------------------------------------------- | ------ | ------------------------ |
  | C:\Program Files\R | yes | 2026-04-17T23:32:14.822Z |
  | C:\Users\Mandelbrat\AppData\Local\Programs\R | no | - |
  | /usr/bin/R | no | - |
  | /usr/local/bin/R | no | - |
  | /opt/local/bin/R | no | - |
  | /opt/homebrew/bin/R | no | - |

ms-python.python / python

• Roots: 18 (with opaque blob)
  | Path | Exists | mtime |
  | ------------------------------------------------------- | ------ | ------------------------ |
  | C:\Users\Mandelbrat.pyenv\pyenv-win\versions | no | - |
  | /usr/bin | no | - |
  | /usr/local/bin | no | - |
  | /opt/homebrew/bin | no | - |
  | /opt/local/bin | no | - |
  | /opt/python | no | - |
  | C:\Users\Mandelbrat.pyenv\versions | no | - |
  | C:\Users\Mandelbrat\anaconda3\envs | no | - |
  | C:\Users\Mandelbrat\miniconda3\envs | no | - |
  | C:\Users\Mandelbrat\anaconda\envs | no | - |
  | C:\Users\Mandelbrat\miniconda\envs | no | - |
  | C:\Users\Mandelbrat.conda\envs | no | - |
  | C:\Users\Mandelbrat\AppData\Roaming\uv\python | no | - |
  | C:\Users\Mandelbrat\AppData\Local\hatch\env\virtual | no | - |
  | C:\Users\Mandelbrat\AppData\Local\Programs\Python | yes | 2025-06-26T14:37:40.144Z |
  | C:\Users\Mandelbrat\AppData\Local\Microsoft\WindowsApps | yes | 2026-06-01T22:42:11.265Z |
  | C:\Program Files\Python | no | - |
  | H:\Positron | yes | 2026-06-01T23:18:28.196Z |

Extension Host Status

• Registered extensions: 133

Extension Host	State
Local Process	Responsive

Output Channels

• No output channel content available

[juliasilge]

Thanks for the thorough report and diagnostics, Jake! This is really useful.

Here is what I have pieced together from your logs. kcserver is exiting within about a second of launching, and it is not leaving anything behind in either log file. 😱 That tells us it is failing very early, before it can open its own log. The reason you are seeing empty logs is a gap on our side. The Windows launch wrapper only captures the server's standard output, not its error output, so the actual error message is getting discarded. We are going to fix the wrapper so these errors surface, but in the meantime you can capture the error directly.

Open a Command Prompt (cmd.exe) and run the server yourself in a visible window. This is essentially the same command Positron runs, minus the wrapper that is swallowing the output:

c:\Positron\resources\app\extensions\positron-supervisor\resources\kallichore\kcserver.exe --log-level trace --log-file %TEMP%\kcserver-test.log --connection-file %TEMP%\kcserver-test.json --transport named-pipe

Whatever it prints (a panic message, a missing DLL, an access-denied error?) is the real cause. Could you paste that output here? If %TEMP%\kcserver-test.log ends up with any contents, that would help too.

A couple of things to try while you are at it:

1. As a possible workaround, switch the supervisor transport from named pipe to TCP. Open Settings, search for "Kernel Supervisor Transport", set it to TCP, then restart Positron. If that gets you running, it points to a named-pipe-specific problem on this machine.
2. Does running the command above produce a visible error, or does it also exit silently?

Thanks for bearing with us while we get to the bottom of this.

[juliasilge]

This looks related to #13753 and of course possibly our old friend #5905.

[juliasilge]

I opened #14063 for us to fix the Windows logging.

[jake-wittman]

For reasons unknown to me I actually can't open cmd.exe - when I ttry to a window open and immediately closes (my computer has been like this for 2 years at least and I have never been able to figure out why it happens or what caused it to behave this way one day). I can however open Powershell, so assuming that's functionally the same here is what I see:

PS C:\Users\Mandelbrat> c:\Positron\resources\app\extensions\positron-supervisor\resources\kallichore\kcserver.exe --log-level trace --log-file %TEMP%\kcserver-test.log --connection-file %TEMP%\kcserver-test.json --transport named-pipe

thread 'main' (14488) panicked at crates\kcserver\src\main.rs:282:44:
called `Result::unwrap()` on an `Err` value: Os { code: 3, kind: NotFound, message: "The system cannot find the path specified." }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I cannot find a kcserver-test.log on my computer anywhere, including in %TEMP%

When I change to TCP I still get the error but it only seems to start up and stop once instead of twice:

2026-06-05 21:22:44.278 [info] Positron Kernel Supervisor activated
2026-06-05 21:22:44.278 [info] 04:22:43 [Positron] Generated connection file path: C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-1acd5f47-32276.json
2026-06-05 21:22:44.278 [info] 04:22:43 [Positron] Using TCP transport
2026-06-05 21:22:44.278 [info] 04:22:43 [Positron] Starting Kallichore server c:\Positron\resources\app\extensions\positron-supervisor\resources\kallichore\kcserver.exe with connection file C:\Users\MANDEL~1\AppData\Local\Temp\kallichore-1acd5f47-32276.json and 30000ms startup timeout
2026-06-05 21:22:44.278 [info] 04:22:44 [Positron] Kallichore terminal started in 497ms with PID 32360
04:22:45 [Positron] Kallichore server PID 32360 is not running
04:22:45 [Positron] The supervisor process exited unexpectedly during startup
04:22:45 [Positron] Failed to start Kallichore server: Error: The supervisor process exited unexpectedly during startup

[jmcphers]

The "file not found" makes me wonder if you're running into a version of this: #13388. We use temp files to pass connection info around, and if that handoff doesn't happen you'll get startup failures.