diff --git a/pkg/workflow/compiler_custom_job_memory_test.go b/pkg/workflow/compiler_custom_job_memory_test.go
index 1023115dbf5..de2cdcf9388 100644
--- a/pkg/workflow/compiler_custom_job_memory_test.go
+++ b/pkg/workflow/compiler_custom_job_memory_test.go
@@ -81,6 +81,103 @@ Reads cache memory and dispatches tasks.
 	assert.NotContains(t, yamlStr, "update_cache_memory:", "update_cache_memory job should not be created without threat detection")
 }
 
+// TestCustomJobRestoreMemoryUsesDefaultRunsOn verifies that custom restore-memory
+// jobs inherit the workflow runner when runs-on is omitted.
+func TestCustomJobRestoreMemoryUsesDefaultRunsOn(t *testing.T) {
+	tmpDir := testutil.TempDir(t, "custom-job-restore-default-runs-on")
+
+	frontmatter := `---
+name: Orchestrator
+on: workflow_dispatch
+permissions:
+  contents: read
+engine: copilot
+strict: false
+tools:
+  cache-memory: true
+jobs:
+  setup:
+    restore-memory: true
+    steps:
+      - name: Verify setup
+        run: echo "ok"
+---
+
+# Orchestrator Workflow
+`
+
+	testFile := filepath.Join(tmpDir, "test.md")
+	require.NoError(t, os.WriteFile(testFile, []byte(frontmatter), 0644))
+
+	compiler := NewCompiler()
+	require.NoError(t, compiler.CompileWorkflow(testFile))
+
+	lockFile := filepath.Join(tmpDir, "test.lock.yml")
+	content, err := os.ReadFile(lockFile)
+	require.NoError(t, err)
+
+	section := extractJobSection(string(content), "setup")
+	require.NotEmpty(t, section, "Expected setup job section in lock file")
+	assert.Contains(t, section, "runs-on: ubuntu-latest", "custom job should inherit default workflow runner when runs-on is omitted")
+	assert.Contains(t, section, "Restore cache-memory", "restore-memory steps should still be present")
+}
+
+// TestCustomJobRestoreMemoryInheritsArrayRunsOn verifies that a custom job with
+// restore-memory: true and no explicit runs-on properly inherits a workflow-level
+// array/object runs-on, with continuation lines correctly indented.
+func TestCustomJobRestoreMemoryInheritsArrayRunsOn(t *testing.T) {
+	tmpDir := testutil.TempDir(t, "custom-job-restore-array-runs-on")
+
+	frontmatter := `---
+name: Orchestrator
+on: workflow_dispatch
+permissions:
+  contents: read
+engine: copilot
+strict: false
+runs-on:
+  group: ubuntu-runners
+  labels: [self-hosted]
+tools:
+  cache-memory: true
+jobs:
+  setup:
+    restore-memory: true
+    steps:
+      - name: Verify setup
+        run: echo "ok"
+---
+
+# Orchestrator Workflow
+`
+
+	testFile := filepath.Join(tmpDir, "test.md")
+	require.NoError(t, os.WriteFile(testFile, []byte(frontmatter), 0644))
+
+	compiler := NewCompiler()
+	require.NoError(t, compiler.CompileWorkflow(testFile))
+
+	lockFile := filepath.Join(tmpDir, "test.lock.yml")
+	content, err := os.ReadFile(lockFile)
+	require.NoError(t, err)
+
+	section := extractJobSection(string(content), "setup")
+	require.NotEmpty(t, section, "Expected setup job section in lock file")
+	assert.Contains(t, section, "runs-on:", "custom job should inherit workflow-level runs-on")
+	assert.Contains(t, section, "group: ubuntu-runners", "custom job should include group from inherited runs-on")
+	// Verify continuation lines are properly indented (4 spaces) inside the job section.
+	for line := range strings.SplitSeq(section, "\n") {
+		if strings.TrimSpace(line) == "" || strings.HasPrefix(strings.TrimSpace(line), "setup:") {
+			continue
+		}
+		if strings.TrimSpace(line) != "" {
+			assert.True(t, strings.HasPrefix(line, "    "),
+				"all non-empty lines inside job section should be indented with at least 4 spaces, got: %q", line)
+		}
+	}
+	assert.Contains(t, section, "Restore cache-memory", "restore-memory steps should still be present")
+}
+
 // TestCustomJobRestoreMemoryRepoMemory verifies that a custom job with
 // restore-memory: true gets repo-memory clone steps injected when repo-memory is configured.
 func TestCustomJobRestoreMemoryRepoMemory(t *testing.T) {
diff --git a/pkg/workflow/compiler_custom_jobs.go b/pkg/workflow/compiler_custom_jobs.go
index 5ba478129f5..ec80f26e5c5 100644
--- a/pkg/workflow/compiler_custom_jobs.go
+++ b/pkg/workflow/compiler_custom_jobs.go
@@ -477,6 +477,13 @@ func configureCustomReusableWorkflow(job *Job, jobName string, usesStr string, c
 }
 
 func (c *Compiler) configureCustomJobSteps(job *Job, jobName string, configMap map[string]any, data *WorkflowData) error {
+	if job.RunsOn == "" {
+		job.RunsOn = c.indentYAMLLines(data.RunsOn, "    ")
+		if job.RunsOn == "" {
+			job.RunsOn = "runs-on: ubuntu-latest"
+		}
+	}
+
 	// Add basic steps if specified (only for non-reusable workflow jobs).
 	// `setup-steps` and `pre-steps` stay distinct so setup-steps can remain the
 	// first injected steps in the job, followed by compiler scaffolding,