SGLang Bisect CI Regression

Investigate a consistently failing CI test to find the root cause - whether it's a code regression from a specific PR, a hardware/runner-specific issue, or an environment change. Optionally reproduce the failure on a remote GPU server.

Slash Command

/sglang-bisect-ci-regression <test_name_or_ci_url> [ssh_target] [docker_container]

When to Use This Skill

• A CI test is failing consistently on main (scheduled runs)
• You need to find which PR introduced a regression
• You suspect a runner-specific or GPU-specific issue
• You want to reproduce a CI failure on a remote server

Arguments

• First argument (required): Test file name (e.g. test_lora_tp.py) or a GitHub Actions job URL
• Second argument (optional): SSH target for remote reproduction (e.g. user@host)
• Third argument (optional): Docker container name on the SSH target (e.g. sglang_dev)

If SSH target and docker container are not provided, the skill will only perform the CI log analysis and bisection, without remote reproduction. Ask the user for these if reproduction is needed and they weren't provided.

Background: Scheduled CI Runs

SGLang uses the pr-test.yml workflow with scheduled runs (cron-triggered) to periodically test the main branch. These runs are the primary data source for detecting regressions:

• Workflow: pr-test.yml with event: schedule
• Branch: main
• Dashboard: https://github.com/sgl-project/sglang/actions/workflows/pr-test.yml?query=event%3Aschedule
• Frequency: Runs multiple times daily, each pinned to the HEAD of main at trigger time
• Purpose: Catches regressions that slip through PR-level CI (e.g., interaction bugs between merged PRs, hardware-specific issues)

Always use these scheduled runs (not PR-triggered runs) when bisecting regressions on main. The --event schedule filter in gh run list ensures you only see these periodic main-branch runs.

Workflow

Phase 1: Extract the Failure Signature

1. Get the failing test details from CI logs. If given a URL, fetch logs directly. If given a test name, find recent scheduled runs of pr-test.yml on main that failed:

# List recent scheduled runs targeting main (the primary source of truth for regressions)
# These are cron-triggered runs visible at:
# https://github.com/sgl-project/sglang/actions/workflows/pr-test.yml?query=event%3Aschedule
gh run list --repo sgl-project/sglang --workflow="pr-test.yml" --event schedule --branch main --limit 20 --json databaseId,conclusion,createdAt,headSha

# Find the job containing the test
gh run view {RUN_ID} --repo sgl-project/sglang --json jobs --jq '.jobs[] | select(.conclusion == "failure") | {name, conclusion, databaseId}'

# Get the failure details
gh run view {RUN_ID} --repo sgl-project/sglang --job {JOB_ID} --log 2>&1 | grep -E -B 5 -A 30 "AssertionError|FAIL|Error|{TEST_NAME}"

2. Record the failure signature:
   • Exact error message and assertion
   • Affected test method name
   • Model/config involved
   • Numeric values (e.g., tolerance diffs, scores)
   • Whether the failure is deterministic (same values across runs)

Phase 2: Temporal Bisection

3. Find the boundary between passing and failing runs. Walk through the scheduled run history (from the pr-test.yml schedule runs on main) to identify:
   • Last known PASSING run (sha + date)
   • First known FAILING run (sha + date)

# For each scheduled run, check the specific partition/job status
gh run view {RUN_ID} --repo sgl-project/sglang --json jobs --jq '.jobs[] | select(.name == "{JOB_NAME}") | {conclusion, databaseId}'

# Verify a specific test passed or failed in a run
gh run view {RUN_ID} --repo sgl-project/sglang --job {JOB_ID} --log 2>&1 | grep -E "{TEST_NAME}|PASSED|FAILED|logprobs mismatch" | head -10

4. List commits between the boundary:

git log --oneline {LAST_PASS_SHA}..{FIRST_FAIL_SHA}

5. Filter for relevant commits that touch files related to the failing test (model layers, kernels, test utilities, etc.):

git log --oneline {LAST_PASS_SHA}..{FIRST_FAIL_SHA} -- {relevant_paths}

Phase 3: Runner/Hardware Analysis

6. Check if the failure is runner-specific. Extract the runner identity from each failing and passing run:

# Get runner name and machine
gh run view {RUN_ID} --repo sgl-project/sglang --job {JOB_ID} --log 2>&1 | grep -E "Runner name|Machine name" | head -5

# Get GPU/driver info
gh run view {RUN_ID} --repo sgl-project/sglang --job {JOB_ID} --log 2>&1 | grep -i -E "NVIDIA-SMI|Driver Version|CUDA Version" | head -5

# Get package versions
gh run view {RUN_ID} --repo sgl-project/sglang --job {JOB_ID} --log 2>&1 | grep -E "sgl.kernel.*==|flashinfer.*==" | head -5

7. Correlate runners with pass/fail outcomes. Build a table:

Run ID	Date	Runner	GPU Type	Driver	Result

If all failures map to a specific runner type/GPU and all passes map to another, the issue is hardware-specific, not a code regression.

Phase 4: Code Analysis

8. If a code regression is suspected (failures not runner-specific), examine the candidate commits:

   • Read the changed files
   • Understand how the changes could affect the failing test
   • Look for prefill-vs-decode differences, TP-specific paths, kernel changes

9. If a hardware issue is suspected, analyze:

   • Kernel compatibility (CUDA compute capability)
   • Driver version differences
   • All-reduce / NCCL behavior differences
   • CUDA graph capture differences across GPU architectures

Phase 5: Remote Reproduction (Optional)

Only if SSH target and docker container were provided.

10. Verify the remote environment:

ssh {SSH_TARGET} "docker exec {CONTAINER} nvidia-smi --query-gpu=name,driver_version --format=csv"
ssh {SSH_TARGET} "docker exec {CONTAINER} pip show sgl-kernel sglang flashinfer-python 2>&1 | grep -E 'Name:|Version:'"

11. Ensure latest code is installed. If the container is stale, update:

# Try fetching latest main
ssh {SSH_TARGET} "docker exec {CONTAINER} bash -c 'cd /path/to/sglang && git fetch origin main && git checkout origin/main'"
# Or download and install from tarball if git auth fails
ssh {SSH_TARGET} "docker exec {CONTAINER} bash -c 'cd /tmp && curl -L https://github.com/sgl-project/sglang/archive/refs/heads/main.tar.gz | tar xz && cd sglang-main && pip install -e \"python[all]\"'"
# Reinstall (after git fetch)
ssh {SSH_TARGET} "docker exec {CONTAINER} bash -c 'cd /path/to/sglang && pip install -e \"python[all]\"'"
# Install test dependencies if needed
ssh {SSH_TARGET} "docker exec {CONTAINER} pip install peft rouge-score"

12. Create a minimal reproduction script that:

    • Uses if __name__ == '__main__' with mp.set_start_method("spawn")
    • Runs the specific failing test configuration
    • Prints key metrics (diffs, scores, outputs)
    • Exits with code 1 on failure

13. Copy and run the reproduction script:

scp /tmp/repro_script.py {SSH_TARGET}:/tmp/
ssh {SSH_TARGET} "docker cp /tmp/repro_script.py {CONTAINER}:/tmp/"
ssh {SSH_TARGET} "docker exec -e CUDA_VISIBLE_DEVICES=0,1 {CONTAINER} python3 /tmp/repro_script.py"

14. Run control experiments to isolate the variable:
    • If suspecting TP issue: run with TP=1 as control
    • If suspecting GPU issue: compare same code on different GPU
    • If suspecting a specific commit: test before/after that commit

Phase 6: Report

15. Produce a structured report:

## CI Regression Bisection Report

### Failure Signature
- **Test**: {test_file}::{test_method}
- **Error**: {exact error message}
- **Key metrics**: {numeric values}
- **Deterministic**: Yes/No

### Root Cause Classification
One of:
- **Code Regression**: PR #{number} introduced the bug
- **Hardware-Specific**: Fails on {GPU_TYPE}, passes on others
- **Environment Change**: New runner/driver/package version
- **Pre-existing Flakiness**: Intermittent, not a new regression

### Evidence
| Condition | Result |
|-----------|--------|
| {condition1} | PASS/FAIL |
| {condition2} | PASS/FAIL |

### Timeline
- {date}: Last known pass ({sha}, {runner})
- {date}: First known fail ({sha}, {runner})
- {date}: Confirmed reproduction on {server}

### Recommended Fix
- **Short-term**: {workaround}
- **Long-term**: {proper fix}

Key Patterns to Recognize

Pattern	Diagnosis
Same SHA passes on runner A, fails on runner B	Hardware/runner-specific
All runners fail after commit X	Code regression from commit X
Intermittent - same runner sometimes passes/fails	Flaky test or race condition
Prefill OK but decode fails	TP/all-reduce issue in decode path
Works with TP=1, fails with TP>1	Tensor parallelism bug
Exact same numeric diff every time	Deterministic bug, not flakiness

Important Notes

• Always check runner identity before concluding it's a code regression. Many "consistent" failures are actually runner-specific.
• Test partition assignments change over time as tests are added/removed. A test may move between partitions, landing on different runner types.
• H200 runners use /root/actions-runner/ path and machine names like gpu-h200-worker-*. Non-H200 runners use /public_sglang_ci/runner-* paths.
• When running remote reproduction, use run_in_background for long-running tests and check output with TaskOutput.
• Container environments may be stale - always verify package versions match CI before drawing conclusions.