fix: coerce JSON-string tags to list in MemoryItem and MCP tools (#682)

MCP tool bridges sometimes serialize JSON arrays as strings during
transport, e.g. '["a", "b"]' arrives as the literal string '["a", "b"]'
instead of a native JSON array. This causes Pydantic to reject the
input with a validation error.

Add defensive coercion at two layers:

1. HTTP API (http.py): Pydantic field_validator on MemoryItem.tags
   with mode="before" that parses JSON strings back into lists.
2. MCP tools (mcp_tools.py): Same coercion in build_content_dict
   before tags reach the Pydantic model.

A plain non-JSON string is wrapped in a single-element list.
Correctly-formatted input is passed through unchanged.

Co-authored-by: Philipp <philipp@Philipps-MacBook-Pro.local>

hindsight-api-slim/hindsight_api/api/http.py

@@ -424,6 +424,27 @@ class MemoryItem(BaseModel):
         default=None,
         description="Optional tags for visibility scoping. Memories with tags can be filtered during recall.",
     )
+
+    @field_validator("tags", mode="before")
+    @classmethod
+    def coerce_tags(cls, v):
+        """Coerce JSON-string tags to list.
+
+        MCP tool bridges sometimes serialize JSON arrays as strings during
+        transport, e.g. '["a", "b"]' instead of ["a", "b"]. This validator
+        parses such strings back into lists so the retain call succeeds.
+        A plain non-JSON string is wrapped in a single-element list.
+        """
+        if isinstance(v, str):
+            try:
+                parsed = json.loads(v)
+                if isinstance(parsed, list):
+                    return parsed
+            except (json.JSONDecodeError, TypeError):
+                pass
+            return [v]
+        return v
+
     observation_scopes: Literal["per_tag", "combined", "all_combinations"] | list[list[str]] | None = Field(
         default=None,
         title="ObservationScopes",

hindsight-api-slim/hindsight_api/mcp_tools.py

@@ -117,6 +117,19 @@ def build_content_dict(
     Returns:
         Tuple of (content_dict, error_message). error_message is None if successful.
     """
+    # Coerce tags from JSON string to list if needed.
+    # MCP tool bridges sometimes serialize JSON arrays as strings during
+    # transport, e.g. '["a", "b"]' instead of ["a", "b"].
+    if isinstance(tags, str):
+        try:
+            parsed = json.loads(tags)
+            if isinstance(parsed, list):
+                tags = parsed
+        except (json.JSONDecodeError, TypeError):
+            pass
+        if isinstance(tags, str):
+            tags = [tags]
+
     content_dict: dict[str, Any] = {"content": content, "context": context}
 
     if timestamp: