diff --git a/scripts/bash/check-prerequisites.sh b/scripts/bash/check-prerequisites.sh
index 7d6dba1353..1eadd991b2 100644
--- a/scripts/bash/check-prerequisites.sh
+++ b/scripts/bash/check-prerequisites.sh
@@ -148,6 +148,12 @@ if [[ ! -f "$IMPL_PLAN" ]]; then
     exit 1
 fi
 
+if [[ ! -s "$IMPL_PLAN" ]]; then
+    echo "ERROR: plan.md is empty in $FEATURE_DIR" >&2
+    echo "Populate it, or remove the empty file and rerun $(format_speckit_command plan "$REPO_ROOT") with a valid plan template." >&2
+    exit 1
+fi
+
 # Check for spec.md if required
 if $REQUIRE_SPEC && [[ ! -f "$FEATURE_SPEC" ]]; then
     echo "ERROR: spec.md not found in $FEATURE_DIR" >&2
diff --git a/scripts/bash/setup-plan.sh b/scripts/bash/setup-plan.sh
index aa394183cf..9c8d163409 100644
--- a/scripts/bash/setup-plan.sh
+++ b/scripts/bash/setup-plan.sh
@@ -43,7 +43,15 @@ if [[ -f "$IMPL_PLAN" ]]; then
         echo "Plan already exists at $IMPL_PLAN, skipping template copy"
     fi
 else
-    if resolve_template_content "plan-template" "$REPO_ROOT" > "$IMPL_PLAN"; then
+    template_file=$(mktemp)
+    trap 'rm -f "$template_file"' EXIT
+    if resolve_template_content "plan-template" "$REPO_ROOT" > "$template_file"; then
+        if [[ ! -s "$template_file" ]]; then
+            echo "ERROR: Plan template is empty; no new plan.md was created." >&2
+            echo "Restore a valid plan template and rerun plan setup." >&2
+            exit 1
+        fi
+        cat "$template_file" > "$IMPL_PLAN"
         if $JSON_MODE; then
             echo "Copied plan template to $IMPL_PLAN" >&2
         else
@@ -51,16 +59,12 @@ else
         fi
     else
         resolve_status=$?
-        rm -f "$IMPL_PLAN"
         if [ "$resolve_status" -ne 1 ]; then
             exit "$resolve_status"
         fi
-        if $JSON_MODE; then
-            echo "Warning: Plan template not found" >&2
-        else
-            echo "Warning: Plan template not found"
-        fi
-        touch "$IMPL_PLAN"
+        echo "ERROR: Plan template not found; no new plan.md was created." >&2
+        echo "Restore a valid plan template and rerun plan setup." >&2
+        exit 1
     fi
 fi
 
diff --git a/scripts/powershell/check-prerequisites.ps1 b/scripts/powershell/check-prerequisites.ps1
index 0e9434d063..04814e6283 100644
--- a/scripts/powershell/check-prerequisites.ps1
+++ b/scripts/powershell/check-prerequisites.ps1
@@ -101,13 +101,22 @@ if (-not (Test-Path $paths.FEATURE_DIR -PathType Container)) {
     exit 1
 }
 
-if (-not (Test-Path $paths.IMPL_PLAN -PathType Leaf)) {
+if (-not (Test-Path -LiteralPath $paths.IMPL_PLAN -PathType Leaf)) {
     [Console]::Error.WriteLine("ERROR: plan.md not found in $($paths.FEATURE_DIR)")
     $planCommand = Format-SpecKitCommand -CommandName 'plan' -RepoRoot $paths.REPO_ROOT
     [Console]::Error.WriteLine("Run $planCommand first to create the implementation plan.")
     exit 1
 }
 
+if ((Get-Item -LiteralPath $paths.IMPL_PLAN).Length -eq 0) {
+    [Console]::Error.WriteLine("ERROR: plan.md is empty in $($paths.FEATURE_DIR)")
+    $planCommand = Format-SpecKitCommand -CommandName 'plan' -RepoRoot $paths.REPO_ROOT
+    [Console]::Error.WriteLine(
+        "Populate it, or remove the empty file and rerun $planCommand with a valid plan template."
+    )
+    exit 1
+}
+
 # Check for spec.md if required
 if ($RequireSpec -and -not (Test-Path $paths.FEATURE_SPEC -PathType Leaf)) {
     [Console]::Error.WriteLine("ERROR: spec.md not found in $($paths.FEATURE_DIR)")
diff --git a/scripts/powershell/setup-plan.ps1 b/scripts/powershell/setup-plan.ps1
index 0d888584a7..88b342c8bb 100644
--- a/scripts/powershell/setup-plan.ps1
+++ b/scripts/powershell/setup-plan.ps1
@@ -48,6 +48,13 @@ if (Test-Path -LiteralPath $paths.IMPL_PLAN -PathType Leaf) {
 } else {
     $content = Resolve-TemplateContent -TemplateName 'plan-template' -RepoRoot $paths.REPO_ROOT
     if ($null -ne $content) {
+        if ($content.Length -eq 0) {
+            [Console]::Error.WriteLine(
+                "ERROR: Plan template is empty; no new plan.md was created."
+            )
+            [Console]::Error.WriteLine("Restore a valid plan template and rerun plan setup.")
+            exit 1
+        }
         $utf8NoBom = New-Object System.Text.UTF8Encoding($false)
         [System.IO.File]::WriteAllText($paths.IMPL_PLAN, $content, $utf8NoBom)
         # Emit the copy status like the bash twin (setup-plan.sh); route to stderr
@@ -58,16 +65,11 @@ if (Test-Path -LiteralPath $paths.IMPL_PLAN -PathType Leaf) {
             Write-Output "Copied plan template to $($paths.IMPL_PLAN)"
         }
     } else {
-        # Match the bash twin's wording and stream routing (stderr in -Json so
-        # stdout stays pure JSON, stdout otherwise), consistent with the sibling
-        # "Copied plan template" message above.
-        if ($Json) {
-            [Console]::Error.WriteLine("Warning: Plan template not found")
-        } else {
-            Write-Output "Warning: Plan template not found"
-        }
-        # Create a basic plan file if template doesn't exist
-        New-Item -ItemType File -Path $paths.IMPL_PLAN -Force | Out-Null
+        [Console]::Error.WriteLine(
+            "ERROR: Plan template not found; no new plan.md was created."
+        )
+        [Console]::Error.WriteLine("Restore a valid plan template and rerun plan setup.")
+        exit 1
     }
 }
 
diff --git a/scripts/python/check_prerequisites.py b/scripts/python/check_prerequisites.py
index e025b4d672..c7834fe456 100644
--- a/scripts/python/check_prerequisites.py
+++ b/scripts/python/check_prerequisites.py
@@ -236,6 +236,16 @@ def main(argv: list[str] | None = None) -> int:
         )
         return 1
 
+    if paths.impl_plan.stat().st_size == 0:
+        print(f"ERROR: plan.md is empty in {paths.feature_dir}", file=sys.stderr)
+        print(
+            "Populate it, or remove the empty file and rerun "
+            f"{format_speckit_command('plan', paths.repo_root)} "
+            "with a valid plan template.",
+            file=sys.stderr,
+        )
+        return 1
+
     if args.require_spec and not paths.feature_spec.is_file():
         print(f"ERROR: spec.md not found in {paths.feature_dir}", file=sys.stderr)
         print(
diff --git a/scripts/python/setup_plan.py b/scripts/python/setup_plan.py
index 721eed8f39..3eaa9f4a3f 100644
--- a/scripts/python/setup_plan.py
+++ b/scripts/python/setup_plan.py
@@ -70,11 +70,28 @@ def main(argv: list[str] | None = None) -> int:
             print(f"Error: {exc}", file=sys.stderr)
             return 1
         if template_content is not None:
+            if template_content == "":
+                print(
+                    "ERROR: Plan template is empty; no new plan.md was created.",
+                    file=sys.stderr,
+                )
+                print(
+                    "Restore a valid plan template and rerun plan setup.",
+                    file=sys.stderr,
+                )
+                return 1
             paths.impl_plan.write_bytes(template_content.encode("utf-8"))
             print(f"Copied plan template to {paths.impl_plan}", file=status_stream)
         else:
-            print("Warning: Plan template not found", file=status_stream)
-            paths.impl_plan.touch()
+            print(
+                "ERROR: Plan template not found; no new plan.md was created.",
+                file=sys.stderr,
+            )
+            print(
+                "Restore a valid plan template and rerun plan setup.",
+                file=sys.stderr,
+            )
+            return 1
 
     if json_mode:
         sys.stdout.write(
diff --git a/tests/test_check_prerequisites_python_parity.py b/tests/test_check_prerequisites_python_parity.py
index 69372d3f17..3a53887ff8 100644
--- a/tests/test_check_prerequisites_python_parity.py
+++ b/tests/test_check_prerequisites_python_parity.py
@@ -562,6 +562,70 @@ def test_python_negative_errors_are_stderr_only(
     assert py.stdout.strip() == ""
 
 
+@pytest.mark.parametrize(
+    "port",
+    [
+        pytest.param("bash", marks=requires_bash),
+        pytest.param(
+            "powershell",
+            marks=pytest.mark.skipif(
+                not (HAS_PWSH or _WINDOWS_POWERSHELL), reason="no PowerShell available"
+            ),
+        ),
+        pytest.param("python"),
+    ],
+)
+@pytest.mark.parametrize("json_mode", [True, False], ids=["json", "text"])
+@pytest.mark.parametrize("plan_state", ["missing", "empty", "nonempty", "whitespace"])
+def test_plan_prerequisite_diagnostics(
+    tmp_path: Path, port: str, json_mode: bool, plan_state: str
+) -> None:
+    repo = tmp_path / "proj"
+    repo.mkdir()
+    _git_init(repo)
+    (repo / ".specify").mkdir()
+    _install_scripts(repo)
+    _write_feature_json(repo)
+    feature = repo / "specs" / "001-my-feature"
+    feature.mkdir(parents=True)
+    plan = feature / "plan.md"
+    bodies = {"empty": b"", "nonempty": b"# Plan\n", "whitespace": b" \t\n"}
+    if plan_state != "missing":
+        plan.write_bytes(bodies[plan_state])
+        before = plan.stat().st_mtime_ns
+    if port == "powershell":
+        command = _ps_cmd(repo, *(("-Json",) if json_mode else ()))
+    else:
+        command_fn = _bash_cmd if port == "bash" else _py_cmd
+        command = command_fn(repo, *(("--json",) if json_mode else ()))
+
+    result = _run(command, repo)
+
+    if plan_state in {"missing", "empty"}:
+        assert result.returncode == 1
+        assert result.stdout == ""
+        lines = result.stderr.replace("\\", "/").splitlines()
+        diagnostic = "not found" if plan_state == "missing" else "is empty"
+        assert lines[0].startswith(f"ERROR: plan.md {diagnostic} in ")
+        assert lines[0].endswith("/specs/001-my-feature")
+        guidance = (
+            "Run /speckit.plan first to create the implementation plan."
+            if plan_state == "missing"
+            else "Populate it, or remove the empty file and rerun /speckit.plan "
+            "with a valid plan template."
+        )
+        assert lines[1:] == [guidance]
+    else:
+        assert result.returncode == 0, result.stderr
+        if json_mode:
+            assert "FEATURE_DIR" in _json_stdout(result)
+    if plan_state == "missing":
+        assert not plan.exists()
+    else:
+        assert plan.read_bytes() == bodies[plan_state]
+        assert plan.stat().st_mtime_ns == before
+
+
 def test_python_branch_falls_back_to_feature_dir_basename(prereq_repo: Path) -> None:
     (prereq_repo / "specs" / "001-my-feature").mkdir(parents=True)
     _write_feature_json(prereq_repo)
diff --git a/tests/test_setup_plan_no_overwrite.py b/tests/test_setup_plan_no_overwrite.py
index c92f038f25..2f4734ca0d 100644
--- a/tests/test_setup_plan_no_overwrite.py
+++ b/tests/test_setup_plan_no_overwrite.py
@@ -281,10 +281,7 @@ def test_ps_setup_plan_copied_message_on_stderr_in_json_mode(plan_repo: Path) ->
 
 @pytest.mark.skipif(not (HAS_PWSH or _WINDOWS_POWERSHELL), reason="no PowerShell available")
 def test_ps_setup_plan_template_not_found_warning_matches_bash(plan_repo: Path) -> None:
-    """When no plan template resolves, -Json mode must emit 'Warning: Plan template
-    not found' on stderr (matching the bash twin's wording and stream routing) while
-    keeping stdout pure JSON. Before the fix the PowerShell script used Write-Warning,
-    producing a different 'WARNING:' prefix on the warning stream instead."""
+    """When no plan template resolves, setup must fail without creating plan.md."""
     # Remove the template the fixture installs so resolution finds nothing.
     (plan_repo / ".specify" / "templates" / "plan-template.md").unlink()
     script = plan_repo / ".specify" / "scripts" / "powershell" / "setup-plan.ps1"
@@ -297,7 +294,10 @@ def test_ps_setup_plan_template_not_found_warning_matches_bash(plan_repo: Path)
         check=False,
         env=_clean_env(),
     )
-    assert result.returncode == 0, result.stderr
-    data = json.loads(result.stdout)
-    assert "IMPL_PLAN" in data
-    assert "Warning: Plan template not found" in result.stderr
+    assert result.returncode == 1
+    assert result.stdout == ""
+    assert result.stderr == (
+        "ERROR: Plan template not found; no new plan.md was created.\n"
+        "Restore a valid plan template and rerun plan setup.\n"
+    )
+    assert not (plan_repo / "specs" / "001-my-feature" / "plan.md").exists()
diff --git a/tests/test_setup_plan_python_parity.py b/tests/test_setup_plan_python_parity.py
index 09356a142f..0ac08cf626 100644
--- a/tests/test_setup_plan_python_parity.py
+++ b/tests/test_setup_plan_python_parity.py
@@ -26,6 +26,21 @@
 
 SCRIPT = "setup-plan"
 TEMPLATE_BODY = "# Plan Template\n\nBody.\n"
+PORTS = [
+    pytest.param("bash", marks=requires_bash),
+    pytest.param(
+        "powershell",
+        marks=pytest.mark.skipif(not HAS_POWERSHELL, reason="no PowerShell available"),
+    ),
+    pytest.param("python"),
+]
+
+
+def _command(repo: Path, port: str, json_mode: bool) -> list[str]:
+    if port == "powershell":
+        return ps_cmd(repo, SCRIPT, *(("-Json",) if json_mode else ()))
+    command = bash_cmd if port == "bash" else py_cmd
+    return command(repo, SCRIPT, *(("--json",) if json_mode else ()))
 
 
 def _setup_repo(tmp_path: Path, name: str = "proj", template: bool = True) -> Path:
@@ -45,6 +60,91 @@ def repo(tmp_path: Path) -> Path:
     return _setup_repo(tmp_path)
 
 
+@pytest.mark.parametrize("port", PORTS)
+@pytest.mark.parametrize("json_mode", [True, False], ids=["json", "text"])
+@pytest.mark.parametrize("template_state", ["missing", "empty", "empty_override"])
+def test_plan_template_failure_and_retry(
+    tmp_path: Path, port: str, json_mode: bool, template_state: str
+) -> None:
+    repo = _setup_repo(tmp_path, template=False)
+    template = repo / ".specify" / "templates" / "plan-template.md"
+    if template_state != "missing":
+        template.parent.mkdir(parents=True)
+        template.write_bytes(b"")
+        if template_state == "empty_override":
+            template.write_bytes(TEMPLATE_BODY.encode("utf-8"))
+            template = template.parent / "overrides" / template.name
+            template.parent.mkdir()
+            template.write_bytes(b"")
+    plan = repo / "specs" / "001-my-feature" / "plan.md"
+
+    result = run(_command(repo, port, json_mode), repo)
+
+    diagnostic = "not found" if template_state == "missing" else "is empty"
+    assert result.returncode == 1
+    assert result.stdout == ""
+    assert result.stderr == (
+        f"ERROR: Plan template {diagnostic}; no new plan.md was created.\n"
+        "Restore a valid plan template and rerun plan setup.\n"
+    )
+    assert not plan.exists()
+
+    template.parent.mkdir(parents=True, exist_ok=True)
+    template.write_bytes(TEMPLATE_BODY.encode("utf-8"))
+    retry = run(_command(repo, port, json_mode), repo)
+
+    assert retry.returncode == 0, retry.stderr
+    assert plan.read_bytes() == TEMPLATE_BODY.encode("utf-8")
+    if json_mode:
+        assert sorted(json_stdout(retry)) == [
+            "BRANCH", "FEATURE_DIR", "FEATURE_SPEC", "IMPL_PLAN"
+        ]
+    else:
+        assert "IMPL_PLAN: " in retry.stdout
+
+
+@pytest.mark.parametrize("port", PORTS)
+@pytest.mark.parametrize("json_mode", [True, False], ids=["json", "text"])
+@pytest.mark.parametrize("template_state", ["missing", "empty", "valid"])
+@pytest.mark.parametrize("body", [b"", b"# Existing plan\n"], ids=["empty", "nonempty"])
+def test_existing_plan_is_preserved(
+    tmp_path: Path, port: str, json_mode: bool, template_state: str, body: bytes
+) -> None:
+    repo = _setup_repo(tmp_path, template=False)
+    if template_state != "missing":
+        template = repo / ".specify" / "templates" / "plan-template.md"
+        template.parent.mkdir(parents=True)
+        template.write_bytes(b"" if template_state == "empty" else TEMPLATE_BODY.encode())
+    plan = repo / "specs" / "001-my-feature" / "plan.md"
+    plan.write_bytes(body)
+    before = plan.stat().st_mtime_ns
+
+    result = run(_command(repo, port, json_mode), repo)
+
+    assert result.returncode == 0, result.stderr
+    assert plan.read_bytes() == body
+    assert plan.stat().st_mtime_ns == before
+    status = result.stderr if json_mode else result.stdout
+    assert "already exists" in status
+    assert "skipping template copy" in status
+    if json_mode:
+        assert "IMPL_PLAN" in json_stdout(result)
+
+
+@pytest.mark.parametrize("port", PORTS)
+@pytest.mark.parametrize("json_mode", [True, False], ids=["json", "text"])
+def test_whitespace_plan_template_is_not_empty(
+    repo: Path, port: str, json_mode: bool
+) -> None:
+    body = b" \t\n\n"
+    (repo / ".specify" / "templates" / "plan-template.md").write_bytes(body)
+
+    result = run(_command(repo, port, json_mode), repo)
+
+    assert result.returncode == 0, result.stderr
+    assert (repo / "specs" / "001-my-feature" / "plan.md").read_bytes() == body
+
+
 @requires_bash
 def test_python_fresh_copy_matches_bash(tmp_path: Path) -> None:
     repo_a = _setup_repo(tmp_path, "proj-a")
@@ -162,13 +262,27 @@ def test_python_missing_template_matches_bash(tmp_path: Path) -> None:
     bash = run(bash_cmd(repo_a, SCRIPT, "--json"), repo_a)
     py = run(py_cmd(repo_b, SCRIPT, "--json"), repo_b)
 
-    assert py.returncode == bash.returncode == 0
+    assert py.returncode == bash.returncode == 1
+    assert py.stdout == bash.stdout == ""
     assert normalize_repo_paths(py.stderr, repo_b) == normalize_repo_paths(
         bash.stderr, repo_a
     )
     for repo in (repo_a, repo_b):
         plan = repo / "specs" / "001-my-feature" / "plan.md"
-        assert plan.read_text(encoding="utf-8") == ""
+        assert not plan.exists()
+
+    for repo in (repo_a, repo_b):
+        templates = repo / ".specify" / "templates"
+        templates.mkdir(parents=True)
+        (templates / "plan-template.md").write_text(TEMPLATE_BODY, encoding="utf-8")
+
+    bash_retry = run(bash_cmd(repo_a, SCRIPT, "--json"), repo_a)
+    py_retry = run(py_cmd(repo_b, SCRIPT, "--json"), repo_b)
+    assert py_retry.returncode == bash_retry.returncode == 0
+    for repo in (repo_a, repo_b):
+        assert (
+            repo / "specs" / "001-my-feature" / "plan.md"
+        ).read_text(encoding="utf-8") == TEMPLATE_BODY
 
 
 @requires_bash