Drive Positron

Drive Positron through CDP

Use this skill to inspect and interact with a running development build of Positron. Treat the resulting profile and application state as disposable.

This workflow complements automated tests; it does not replace them. Add an appropriate test when the verified behavior needs regression coverage. See .claude/skills/author-e2e-tests.

Do not use this workflow to hand a persistent Positron instance to a person:

• the profile is deleted during cleanup;
• native file dialogs are replaced with quick input;
• no watch process recompiles subsequent source edits.

Use the launch-positron command for that case.

Know what this changes in your checkout

The disposable profile is isolated. The build state is not.

Before it starts the application, scripts/launch.sh runs build/lib/preLaunch.ts against your real checkout. Pre-launch writes to directories that your normal development build also uses:

• .build/builtInExtensions/<name>: pre-launch deletes and re-downloads this directory for every built-in extension whose version on disk does not match product.json. A rebase that bumps a built-in extension version is enough to trigger it.
• .build/electron: pre-launch deletes and re-downloads the whole directory when the installed Electron version does not match the expected one.
• out/: pre-launch runs npm run compile when this directory is absent. That competes with the build daemons, which own compilation.

An interrupted or failed pre-launch can leave a built-in extension deleted or partially written. Your normal development build then fails to start until you repair it. To repair:

npm run download-builtin-extensions
npm run electron

Do not interrupt the script while it reports that it is running pre-launch.

Platform support

These scripts run on macOS, Linux, and Windows. On Windows, run them from Git Bash; they are bash scripts and will not work from PowerShell or cmd.

Tools they expect on PATH:

Windows launches the app out-of-process on purpose

On Windows launch.sh does not spawn the app directly. It writes launch-app.cmd and launch-app.ps1 into the run directory and has WMI (Win32_Process.Create) start the app, which reparents it to WmiPrvSE while keeping it in the interactive session. Do not "simplify" this back to a direct background spawn.

The reason is Ark, the R kernel. It statically links a ZeroMQ built with the wepoll poller, which opens \Device\Afd directly, and that call fails for any process inside an agent session's process tree. A directly spawned app therefore starts, but every R session dies immediately with exit code 1073741845 and not a socket (...epoll.cpp:73). Python is unaffected, because its ZeroMQ uses the select poller -- so the symptom looks like an R-specific bug and is not.

macOS and Linux keep the plain background spawn: their ZeroMQ uses kqueue and kernel epoll, neither of which opens a device handle.

Launch Positron

Run:

.claude/skills/drive-positron/scripts/launch.sh -- \
	--use-mock-keychain \
	--disable-workspace-trust \
	--skip-welcome \
	--folder-uri file:///private/tmp/myworkspace

Wait for the script to print one JSON object. Record at least:

• pid
• cdpPort
• runDir
• logFile

The launcher:

• copies the source profile from $POSITRON_DEV_USER_DATA_DIR or ~/.positron-dev, using rsync when present and tar otherwise;
• writes only to the disposable copy;
• creates an isolated shared-data directory;
• uses a short run directory under /tmp by default;
• assigns unique ports for CDP and the debug endpoints;
• converts the profile paths for the native binary on Windows;
• waits for CDP and verifies that the app remains alive before returning.

Required launch arguments

The launcher supplies --shared-data-dir automatically, so the disposable instance does not use the normal ~/.positron-shared store.

Protect the source profile

The launcher reads the source profile with a one-way rsync into the run directory. It does not use --delete, and it applies files.simpleDialog.enable only to the disposable copy.

It excludes lock files, sockets, singleton state, caches, logs, and workspace storage so the copied profile can run alongside a normal development instance.

To avoid reading the normal development profile at all, create a minimal seed:

mkdir -p /tmp/positron-seed/User
echo '{"positron.notebook.enabled": true}' \
	> /tmp/positron-seed/User/settings.json

Then launch with:

--source-user-data-dir /tmp/positron-seed

Attach Playwright

Use a literal session name and reuse it for every command:

npx @playwright/cli -s=positron \
	attach --cdp=http://127.0.0.1:"$CDP_PORT"

npx @playwright/cli -s=positron snapshot

Do not derive the session name from $$. Separate shell invocations receive different process IDs and would silently create different sessions.

Common operations:

npx @playwright/cli -s=positron click e153
npx @playwright/cli -s=positron click e980 right
npx @playwright/cli -s=positron type "some text"
npx @playwright/cli -s=positron press Enter
npx @playwright/cli -s=positron resize 1600 1100
npx @playwright/cli -s=positron eval '(() => document.title)()'
npx @playwright/cli -s=positron \
	screenshot --filename="$PWD/shots/01.png"

Use element references from the latest snapshot. Do not substitute screen coordinates, and use the positional right argument for a right-click.

Filter a large snapshot when looking for a known control:

R=$(npx @playwright/cli -s=positron snapshot 2>&1 \
	| grep -oE 'button "Run Cell" \[ref=e[0-9]+' \
	| grep -oE 'e[0-9]+$' \
	| head -1)

Take a screenshot early when the UI does not match expectations. A screenshot often reveals blocking dialogs, an unopened workspace, missing kernels, or focus in the wrong editor faster than DOM inspection.

Enter text in Monaco

Do not use type or fill for notebook cell editors or chat inputs backed by Monaco. Use:

.claude/skills/drive-positron/scripts/monaco-paste.sh \
	--session positron "text to insert"

Use individual press operations when testing actual keyboard handling.

Account for Positron behavior

• Restore focus to the notebook before invoking a notebook action. Opening an output in a plot tab moves focus away from the notebook.

• Ensure the selected Python environment contains the packages the scenario needs. A fresh profile may discover a bare interpreter without packages such as matplotlib or pandas.

• To prioritize Positron's development environment, expose it as the workspace environment:

  ln -s <repo>/extensions/positron-python/.venv \
    /private/tmp/myworkspace/.venv
  

• Allow interpreter discovery and marketplace extension installation to finish before concluding that a kernel is unavailable.

• Set positron.notebook.enabled to true in the workspace or seed profile when testing the Positron notebook editor.

• Expect selectors to change. Prefer the maintained page objects under test/e2e/pages/ when locating Positron controls; otherwise take a fresh snapshot.

Use upstream debugging guidance

scripts/launch.sh is a maintained fork of .agents/skills/launch/scripts/launch.sh. It does not inherit upstream fixes.

When the upstream script changes:

1. Compare it with this fork.
2. Port applicable fixes without removing the Positron-specific profile, path, isolation, and liveness behavior.
3. Revalidate the launch workflow.

Read .agents/skills/launch/SKILL.md when you need:

• the debug-port mapping;
• dap-cli breakpoint instructions;
• parallel-instance guidance;
• additional Monaco input details.

Clean up

Always stop the disposable instance; Positron can retain several gigabytes of memory.

npx @playwright/cli -s=positron close
.claude/skills/drive-positron/scripts/stop.sh \
	--cdp-port "$CDP_PORT" --run-dir "$RUN_DIR"

stop.sh signals the process that owns the CDP port, waits for the port to stop answering, forces the stop if it does not, and then removes the run directory. It exits non-zero if the instance is still reachable, so a silent failure to clean up is not possible.

Do not signal the Electron helper processes yourself. Positron reads a terminated renderer as a window crash: it respawns the helpers, shows a "window terminated unexpectedly" dialog, and then ignores the signal sent to the main process, leaving the instance running.

Pass --run-dir only when it is the exact runDir the launcher reported. The script refuses any path that does not contain a generated positron-dev-launch component, and stops the instance without deleting anything when --run-dir is omitted.

Do not use kill "$PID" on its own. On Windows the reported pid belongs to the MSYS shell that exec'd the native Electron binary, so killing it can leave the application running. stop.sh locates the real process through the CDP port on every platform.

Remove .playwright-cli only if it is the session directory created in the intended workspace.

To confirm independently that no process remains, check that the CDP port no longer answers:

curl -sf -o /dev/null "http://127.0.0.1:$CDP_PORT/json/version" \
	&& echo "still running" || echo "stopped"

This works on every platform. pgrep -f "remote-debugging-port=$CDP_PORT" is equivalent on macOS and Linux, but pgrep is absent from Git Bash on Windows.

Troubleshoot failures

• Attach reports connect ECONNREFUSED: The application exited after opening CDP. Inspect the path reported as logFile.

• The log reports listen EINVAL or an IPC path longer than 103 characters: The run-directory base is too long. Unset $POSITRON_LAUNCH_TMP or point it at a shorter directory. This affects macOS and Linux only; Windows uses named pipes and has no such limit.

• rsync: command not found (Windows): You are on an older copy of launch.sh. The current script falls back to tar when rsync is absent.

• Snapshot references disappear: Look for a modal dialog with a screenshot, then take a new snapshot.

• A built-in extension does not load: Compile extensions with:

  npm run gulp compile-extensions
  

  Do not rely on watch-extensions when another extension is already preventing the watch task from starting.