diff --git a/modules/actions/jobparser/model.go b/modules/actions/jobparser/model.go
index 5950feb5c8..1df8d89350 100644
--- a/modules/actions/jobparser/model.go
+++ b/modules/actions/jobparser/model.go
@@ -75,7 +75,22 @@ func (w *SingleWorkflow) SetJob(id string, job *Job) error {
 }
 
 func (w *SingleWorkflow) Marshal() ([]byte, error) {
-	return yaml.Marshal(w)
+	// Encode with the same indentation SetJob uses (2). yaml.Marshal's default
+	// indentation (4) makes the encoder emit multi-line block scalars (e.g. a
+	// `run:` step that begins with blank lines) with a wrong explicit indentation
+	// indicator (`run: |4`) that then fails to re-parse, which silently strands
+	// the job during concurrency evaluation. Keeping both encoders at indent 2
+	// makes the serialized single workflow round-trip.
+	var buf bytes.Buffer
+	enc := yaml.NewEncoder(&buf)
+	enc.SetIndent(2)
+	if err := enc.Encode(w); err != nil {
+		return nil, err
+	}
+	if err := enc.Close(); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
 }
 
 type Job struct {
diff --git a/modules/actions/jobparser/roundtrip_test.go b/modules/actions/jobparser/roundtrip_test.go
new file mode 100644
index 0000000000..ae4fa4dee2
--- /dev/null
+++ b/modules/actions/jobparser/roundtrip_test.go
@@ -0,0 +1,64 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package jobparser
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// A step whose `run:` block starts with blank lines must still survive the
+// Parse -> SingleWorkflow.Marshal -> Parse round-trip. Previously Marshal used a
+// different indentation than SetJob, which made the encoder emit the block scalar
+// with a wrong explicit indentation indicator (`run: |4`) that no longer parsed;
+// the job then stayed silently blocked during concurrency evaluation.
+func TestSingleWorkflowRoundTripRunBlockLeadingBlankLines(t *testing.T) {
+	const wf = `name: demo
+on:
+  workflow_call:
+    inputs:
+      app_name:
+        type: string
+        required: true
+jobs:
+  build:
+    name: build
+    env:
+      IMAGE_TAG: ${{ inputs.app_name }}
+    runs-on: ubuntu-latest
+    steps:
+      - if: ${{ inputs.app_name != '' }}
+        name: packages
+        run: |
+
+
+          echo start
+          echo done
+`
+	sws, err := Parse([]byte(wf))
+	require.NoError(t, err)
+	require.Len(t, sws, 1)
+
+	// pin the original run block as the baseline
+	_, origJob := sws[0].Job()
+	require.Len(t, origJob.Steps, 1)
+	const wantRun = "\n\necho start\necho done\n"
+	require.Equal(t, wantRun, origJob.Steps[0].Run)
+
+	payload, err := sws[0].Marshal()
+	require.NoError(t, err)
+
+	// the serialized single workflow must be parseable again -- this is what the
+	// server does in EvaluateJobConcurrencyFillModel -> ParseJob. Before the fix
+	// Marshal emitted `run: |4`, which failed here and left the job blocked.
+	roundTripped, err := Parse(payload)
+	require.NoError(t, err, "serialized single workflow must round-trip; got payload:\n%s", payload)
+	require.Len(t, roundTripped, 1)
+
+	// the round-trip must preserve the run block byte-for-byte
+	_, gotJob := roundTripped[0].Job()
+	require.Len(t, gotJob.Steps, 1)
+	require.Equal(t, wantRun, gotJob.Steps[0].Run, "round-trip must preserve run content; got payload:\n%s", payload)
+}