diff --git a/electron/main/process-runner.test.mjs b/electron/main/process-runner.test.mjs
new file mode 100644
index 00000000..6ce5656a
--- /dev/null
+++ b/electron/main/process-runner.test.mjs
@@ -0,0 +1,114 @@
+/**
+ * Process-extension runners are cached per extension id and reused across
+ * workflow runs. The cache must not hand back a runner that was built for
+ * different arguments: the workspace folder is baked into each runner, so after
+ * the user moves the workspace in Settings (which updates the backend at
+ * runtime, without a restart) a stale runner keeps writing node output into the
+ * old folder, where the viewer can no longer find it.
+ */
+import test from 'node:test'
+import assert from 'node:assert/strict'
+import { buildSync } from 'esbuild'
+import { createRequire } from 'node:module'
+import { mkdtempSync, mkdirSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join, resolve } from 'node:path'
+
+function loadModule() {
+  const outfile = join(mkdtempSync(join(tmpdir(), 'modly-runner-test-')), 'process-runner.cjs')
+  const require = createRequire(import.meta.url)
+  const result = buildSync({
+    entryPoints: [resolve('electron/main/process-runner.ts')],
+    bundle: true,
+    platform: 'node',
+    format: 'cjs',
+    write: false,
+  })
+  writeFileSync(outfile, result.outputFiles[0].text, 'utf8')
+  return require(outfile)
+}
+
+// A JS process extension that reports the workspace it was given, plus how many
+// runs this worker has served (module state survives only while it is reused).
+function makeJsExtension(root) {
+  const extDir = join(root, 'js-ext')
+  mkdirSync(extDir, { recursive: true })
+  writeFileSync(join(extDir, 'processor.js'), [
+    'let runs = 0',
+    'module.exports = async (input, params, context) => {',
+    '  runs += 1',
+    '  return { filePath: context.workspaceDir, text: String(runs) }',
+    '}',
+    '',
+  ].join('\n'))
+  return extDir
+}
+
+// A "Python" process extension driven through the same stdin/stdout protocol.
+// Node stands in for the interpreter so the test needs no Python install.
+function makeStdioExtension(root) {
+  const extDir = join(root, 'py-ext')
+  mkdirSync(extDir, { recursive: true })
+  writeFileSync(join(extDir, 'processor.cjs'), [
+    "let raw = ''",
+    "process.stdin.on('data', (chunk) => { raw += chunk })",
+    "process.stdin.on('end', () => {",
+    '  const data = JSON.parse(raw)',
+    "  process.stdout.write(JSON.stringify({ type: 'done', result: { filePath: data.workspaceDir } }) + '\\n')",
+    '})',
+    '',
+  ].join('\n'))
+  return extDir
+}
+
+test('a JS process runner follows the workspace after it moves', async () => {
+  const { getProcessRunner, terminateAllProcessRunners } = loadModule()
+  const root = mkdtempSync(join(tmpdir(), 'modly-runner-js-'))
+  const extDir = makeJsExtension(root)
+  const oldWorkspace = join(root, 'workspace-old')
+  const newWorkspace = join(root, 'workspace-new')
+  try {
+    const before = await getProcessRunner('js-ext', extDir, 'processor.js', oldWorkspace, root).run({}, {})
+    assert.equal(before.filePath, oldWorkspace)
+
+    const after = await getProcessRunner('js-ext', extDir, 'processor.js', newWorkspace, root).run({}, {})
+    assert.equal(after.filePath, newWorkspace)
+  } finally {
+    terminateAllProcessRunners()
+  }
+})
+
+test('a Python process runner follows the workspace after it moves', async () => {
+  const { getPythonProcessRunner, terminateAllProcessRunners } = loadModule()
+  const root = mkdtempSync(join(tmpdir(), 'modly-runner-py-'))
+  const extDir = makeStdioExtension(root)
+  const oldWorkspace = join(root, 'workspace-old')
+  const newWorkspace = join(root, 'workspace-new')
+  try {
+    const before = await getPythonProcessRunner('py-ext', process.execPath, extDir, 'processor.cjs', oldWorkspace, root).run({}, {})
+    assert.equal(before.filePath, oldWorkspace)
+
+    const after = await getPythonProcessRunner('py-ext', process.execPath, extDir, 'processor.cjs', newWorkspace, root).run({}, {})
+    assert.equal(after.filePath, newWorkspace)
+  } finally {
+    terminateAllProcessRunners()
+  }
+})
+
+test('unchanged arguments keep reusing the same warm runner', async () => {
+  const { getProcessRunner, terminateAllProcessRunners } = loadModule()
+  const root = mkdtempSync(join(tmpdir(), 'modly-runner-reuse-'))
+  const extDir = makeJsExtension(root)
+  const workspace = join(root, 'workspace')
+  try {
+    const first = getProcessRunner('js-ext', extDir, 'processor.js', workspace, root)
+    assert.equal((await first.run({}, {})).text, '1')
+
+    const second = getProcessRunner('js-ext', extDir, 'processor.js', workspace, root)
+    assert.equal(second, first)
+    // Same worker thread: its module state carried over instead of reloading.
+    assert.equal((await second.run({}, {})).text, '2')
+  } finally {
+    terminateAllProcessRunners()
+  }
+})
diff --git a/electron/main/process-runner.ts b/electron/main/process-runner.ts
index 758f62d2..f29e5dae 100644
--- a/electron/main/process-runner.ts
+++ b/electron/main/process-runner.ts
@@ -270,6 +270,19 @@ export function getExtPythonExe(extDir: string): string | null {
 // ─── Registry (one runner per extension id, reused across calls) ──────────────
 
 const registry = new Map<string, IProcessRunner>()
+// The arguments each cached runner was built with. A runner bakes them in at
+// construction, so a call with different ones — e.g. after the workspace is
+// moved in Settings, which updates paths without a restart — must not get the
+// old runner back, or node output keeps landing in the previous folder.
+const registryArgs = new Map<string, string>()
+
+function canReuseRunner(extensionId: string, args: string[]): boolean {
+  const key = JSON.stringify(args)
+  if (registry.has(extensionId) && registryArgs.get(extensionId) === key) return true
+  terminateProcessRunner(extensionId)
+  registryArgs.set(extensionId, key)
+  return false
+}
 
 export function getProcessRunner(
   extensionId:  string,
@@ -278,7 +291,7 @@ export function getProcessRunner(
   workspaceDir: string,
   tempDir:      string,
 ): ProcessRunner {
-  if (!registry.has(extensionId)) {
+  if (!canReuseRunner(extensionId, [extDir, entry, workspaceDir, tempDir])) {
     registry.set(extensionId, new ProcessRunner(extDir, entry, workspaceDir, tempDir))
   }
   return registry.get(extensionId)! as ProcessRunner
@@ -292,7 +305,7 @@ export function getPythonProcessRunner(
   workspaceDir: string,
   tempDir:      string,
 ): PythonProcessRunner {
-  if (!registry.has(extensionId)) {
+  if (!canReuseRunner(extensionId, [pythonExe, extDir, entry, workspaceDir, tempDir])) {
     registry.set(extensionId, new PythonProcessRunner(pythonExe, extDir, entry, workspaceDir, tempDir))
   }
   return registry.get(extensionId)! as PythonProcessRunner