[Bug]: Custom `probes` may break worker registration with `type: sglang` router

[un-def]

Steps to reproduce

type: service
name: smg

env:
  - MODEL_ID=Qwen/Qwen2.5-0.5B-Instruct

groups:
  - replicas: 1
    python: "3.12"
    commands:
      - pip install smg
      - |
        smg launch \
          --host 0.0.0.0 \
          --port 8000 \
          --enable-igw \
          --policy cache_aware \
          --model-path $MODEL_ID
    router:
      type: sglang
    resources:
      gpu: 0

  - replicas: 1
    image: lmsysorg/sglang:v0.5.10.post1
    commands:
      - |
        python3 -m sglang.launch_server \
          --model-path $MODEL_ID \
          --host 0.0.0.0 \
          --port 8000 \
          --context-length 4096
    resources:
      gpu:
        memory: 16GB..
        # SGLang's CuTe DSL kernels don't compile for Turing (sm_75, e.g. T4)
        compute_capability: 8.0

port: 8000

model:
  type: chat
  name: Qwen/Qwen2.5-0.5B-Instruct
  format: openai
  prefix: /v1

probes:
  - type: http
    url: /health_generate
    interval: 5s

/health_generate is a correct probe for both Shepherd Model Gateway and SGLang Server – “Check the health [...] by generating one token” – roughly the same as dstack's _openai_model_probe_spec; but adding this probe reintroduces the same kind of deadlock that was fixed in #4270. Replacing the custom probe with probes: [] solves the issue.

Actual behaviour

The worker is ready, the router is never ready (see the PROBES column):

 NAME                               BACKEND         RESOURCES                                    INSTANCE TYPE  PRICE    STATUS   PROBES  SUBMITTED   ERROR
 smg deployment=0                                   -                                                           -        running          7 mins ago
    group=0 replica=0 deployment=0  gcp (us-west1)  cpu=x86:2 mem=2GB disk=20GB                  e2-highcpu-2   $0.0495  running  ×       7 mins ago
    group=1 replica=1 deployment=0  gcp (us-west1)  cpu=x86:4 mem=16GB disk=100GB gpu=L4:24GB:1  g2-standard-4  $0.7068  running  ✓       7 mins ago

Server logs:

# Both are not ready
dstack._internal.server.background.scheduled_tasks.probes:122 probe(fa83a6)smg-0-0-0: probe status code: 503
dstack._internal.server.background.scheduled_tasks.probes:125 probe(ea13f1)smg-0-1-0: probe failed: ReadError('')

# Both are not ready
dstack._internal.server.background.scheduled_tasks.probes:122 probe(fa83a6)smg-0-0-0: probe status code: 503
dstack._internal.server.background.scheduled_tasks.probes:122 probe(ea13f1)smg-0-1-0: probe status code: 503

# The worker is ready; the router returns 503 infinitely as there is no registered worker
dstack._internal.server.background.scheduled_tasks.probes:122 probe(fa83a6)smg-0-0-0: probe status code: 503
dstack._internal.server.background.scheduled_tasks.probes:122 probe(ea13f1)smg-0-1-0: probe status code: 200

/health_generate response from the router:

GET /health_generate HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:8000
User-Agent: HTTPie/3.2.4



HTTP/1.1 503 Service Unavailable
access-control-allow-origin: *
access-control-expose-headers: *
content-length: 110
content-type: application/json
date: Thu, 24 Sep 2026 09:19:06 GMT
x-request-id: req-N5bKymgO02OuEHl01IoT6pE5
x-smg-error-code: no_workers

{
    "error": {
        "code": "no_workers",
        "message": "No workers are available",
        "param": null,
        "type": "Service Unavailable"
    }
}

/health_generate response from the worker:

GET /health_generate HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:8000
User-Agent: HTTPie/3.2.4



HTTP/1.1 200 OK
content-length: 0
date: Thu, 24 Sep 2026 09:19:59 GMT
server: uvicorn

Expected behaviour

No response

dstack version

39b28c2

Server logs

DEBUG    dstack._internal.server.services.runs.router_worker_sync:568 run(f7ba4f)smg: no ready router job in group 0, skipping worker sync

Additional information

No response