Fix smriti current crash on list-valued blocked_by

External-machine validation found GET /api/v5/current 500s on a real
project: a structured task can carry `blocked_by` as a list of dependency
labels, but CurrentTask.blocked_by is typed Optional[str], so constructing
the model raised a Pydantic ValidationError.

Add a field validator that normalizes `blocked_by` — string, list, or null —
to a single display string. The API contract is unchanged (blocked_by stays
a string), so the CLI and the Project Current State UI panel, which both
read this endpoint, need no change. Adds a regression test.

backend/app/api/routes/current.py

@@ -17,7 +17,7 @@ from datetime import datetime
 from typing import Optional
 
 from fastapi import APIRouter, Depends
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
@@ -108,6 +108,25 @@ class CurrentTask(BaseModel):
     blocked_by: Optional[str] = None
     status: str = "open"
 
+    @field_validator("blocked_by", mode="before")
+    @classmethod
+    def _coerce_blocked_by(cls, value: object) -> Optional[str]:
+        """Normalize blocked_by from whatever real task payloads carry.
+
+        A structured task's `blocked_by` shows up as a string, a list of
+        dependency labels (a task blocked by several others), or null.
+        Coerce any of them to a single display string so the current-state
+        surface never 500s on a list-valued blocked_by.
+        """
+        if value is None:
+            return None
+        if isinstance(value, str):
+            return value.strip() or None
+        if isinstance(value, (list, tuple)):
+            labels = [str(item).strip() for item in value if str(item).strip()]
+            return ", ".join(labels) or None
+        return str(value).strip() or None
+
 
 class RecentActivityEntry(BaseModel):
     """One recent checkpoint, newest-first. Spans all branches so the

backend/tests/integration/test_current_state.py

@@ -201,6 +201,45 @@ def test_current_open_tasks_accept_legacy_intent_type(client):
     assert cur["open_tasks_by_intent"]["implement"][0]["id"] == "middleware"
 
 
+def test_current_handles_list_valued_blocked_by(client):
+    """Real projects produce tasks whose `blocked_by` is a list of
+    dependency labels. The current-state surface must normalize that to a
+    display string, not 500 on it (regression: HTTP 500 from a Pydantic
+    ValidationError when blocked_by was a list)."""
+    repo_id = _create_repo(client, "List Blocked-By")
+    session_id = _create_session(client, repo_id)
+    _commit(
+        client,
+        repo_id,
+        session_id,
+        message="Tasks with varied blocked_by shapes",
+        tasks=[
+            {
+                "text": "Wire the limiter into the gateway",
+                "intent_hint": "implement",
+                "id": "wire-gateway",
+                "status": "open",
+                "blocked_by": ["middleware", "load-test"],
+            },
+            {
+                "text": "Single-dependency task",
+                "intent_hint": "implement",
+                "id": "single-dep",
+                "status": "open",
+                "blocked_by": "wire-gateway",
+            },
+        ],
+    )
+
+    # Pre-fix, a list-valued blocked_by raised a Pydantic ValidationError
+    # and this request 500ed; _get_current asserts a 200.
+    cur = _get_current(client, repo_id)
+
+    by_id = {t["id"]: t for t in cur["open_tasks_by_intent"]["implement"]}
+    assert by_id["wire-gateway"]["blocked_by"] == "middleware, load-test"
+    assert by_id["single-dep"]["blocked_by"] == "wire-gateway"
+
+
 def test_current_recent_milestones(client):
     """Milestone notes surface in recent_milestones; plain notes do not."""
     repo_id = _create_repo(client, "Milestones")