fix: coerce string params to list/dict in MCP tools (#657)

Signed-off-by: phernandez <paul@basicmachines.co>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: phernandez

src/basic_memory/mcp/tools/canvas.py

@@ -4,12 +4,14 @@
 """
 
 import json
-from typing import Dict, List, Any, Optional
+from typing import Annotated, Dict, List, Any, Optional
 
 from loguru import logger
 from fastmcp import Context
+from pydantic import BeforeValidator
 
 from basic_memory.mcp.project_context import get_project_client
+from basic_memory.utils import coerce_list
 from basic_memory.mcp.server import mcp
 from basic_memory.mcp.tools.utils import call_put, call_post, resolve_entity_id
 
@@ -19,8 +21,8 @@
     annotations={"destructiveHint": False, "idempotentHint": True, "openWorldHint": False},
 )
 async def canvas(
-    nodes: List[Dict[str, Any]],
-    edges: List[Dict[str, Any]],
+    nodes: Annotated[List[Dict[str, Any]], BeforeValidator(coerce_list)],
+    edges: Annotated[List[Dict[str, Any]], BeforeValidator(coerce_list)],
     title: str,
     directory: str,
     project: Optional[str] = None,

src/basic_memory/mcp/tools/search.py

@@ -6,8 +6,10 @@
 
 from loguru import logger
 from fastmcp import Context
+from pydantic import BeforeValidator
 
 from basic_memory.config import ConfigManager
+from basic_memory.utils import coerce_dict, coerce_list
 from basic_memory.mcp.container import get_container
 from basic_memory.mcp.project_context import (
     detect_project_from_url_prefix,
@@ -307,18 +309,26 @@ async def search_notes(
     output_format: Literal["text", "json"] = "text",
     note_types: Annotated[
         List[str] | None,
+        BeforeValidator(coerce_list),
         "Filter by the 'type' field in note frontmatter (e.g. 'note', 'chapter', 'person'). "
         "Case-insensitive.",
     ] = None,
     entity_types: Annotated[
         List[str] | None,
+        BeforeValidator(coerce_list),
         "Filter by knowledge graph item type: 'entity' (whole notes), 'observation', or "
         "'relation'. Defaults to 'entity'. Do NOT pass schema/frontmatter types like "
         "'Chapter' here — use note_types instead.",
     ] = None,
     after_date: Optional[str] = None,
-    metadata_filters: Optional[Dict[str, Any]] = None,
-    tags: Optional[List[str]] = None,
+    metadata_filters: Annotated[
+        Dict[str, Any] | None,
+        BeforeValidator(coerce_dict),
+    ] = None,
+    tags: Annotated[
+        List[str] | None,
+        BeforeValidator(coerce_list),
+    ] = None,
     status: Optional[str] = None,
     min_similarity: Optional[float] = None,
     context: Context | None = None,

src/basic_memory/mcp/tools/write_note.py

@@ -1,16 +1,17 @@
 """Write note tool for Basic Memory MCP server."""
 
 import textwrap
-from typing import List, Union, Optional, Literal
+from typing import Annotated, List, Union, Optional, Literal
 
 from loguru import logger
+from pydantic import BeforeValidator
 
 from basic_memory.config import ConfigManager
 from basic_memory.mcp.project_context import get_project_client, add_project_metadata
 from basic_memory.mcp.server import mcp
 from fastmcp import Context
 from basic_memory.schemas.base import Entity
-from basic_memory.utils import parse_tags, validate_project_path
+from basic_memory.utils import coerce_dict, parse_tags, validate_project_path
 
 # Define TagType as a Union that can accept either a string or a list of strings or None
 TagType = Union[List[str], str, None]
@@ -28,7 +29,7 @@ async def write_note(
     workspace: Optional[str] = None,
     tags: list[str] | str | None = None,
     note_type: str = "note",
-    metadata: dict | None = None,
+    metadata: Annotated[dict | None, BeforeValidator(coerce_dict)] = None,
     overwrite: bool | None = None,
     output_format: Literal["text", "json"] = "text",
     context: Context | None = None,

src/basic_memory/utils.py

@@ -1,13 +1,14 @@
 """Utility functions for basic-memory."""
 
+import json
 import os
 
 import logging
 import re
 import sys
 from datetime import datetime, timezone
 from pathlib import Path
-from typing import Protocol, Union, runtime_checkable, List, Optional
+from typing import Any, Protocol, Union, runtime_checkable, List, Optional
 
 from loguru import logger
 from unidecode import unidecode
@@ -385,6 +386,36 @@ def parse_tags(tags: Union[List[str], str, None]) -> List[str]:
         return []
 
 
+def coerce_list(v: Any) -> Any:
+    """Coerce string input to list for MCP clients that serialize lists as strings."""
+    if v is None:
+        return v
+    if isinstance(v, str):
+        try:
+            parsed = json.loads(v)
+            if isinstance(parsed, list):
+                return parsed
+        except (json.JSONDecodeError, TypeError):
+            pass
+        # Single string value — wrap in a list
+        return [v]
+    return v
+
+
+def coerce_dict(v: Any) -> Any:
+    """Coerce string input to dict for MCP clients that serialize dicts as strings."""
+    if v is None:
+        return v
+    if isinstance(v, str):
+        try:
+            parsed = json.loads(v)
+            if isinstance(parsed, dict):
+                return parsed
+        except (json.JSONDecodeError, TypeError):
+            pass
+    return v
+
+
 def normalize_newlines(multiline: str) -> str:
     """Replace any \r\n, \r, or \n with the native newline.
 

test-int/mcp/test_string_params_integration.py

@@ -0,0 +1,167 @@
+"""Integration tests for MCP tools accepting string-serialized list/dict params.
+
+Goes through the full FastMCP Client → validate_call → tool function path,
+which is where Pydantic rejects strings for list/dict params.
+"""
+
+import pytest
+from fastmcp import Client
+
+
+@pytest.mark.asyncio
+async def test_search_notes_entity_types_as_string(mcp_server, app, test_project):
+    """search_notes should accept entity_types as a JSON string via MCP protocol."""
+    async with Client(mcp_server) as client:
+        await client.call_tool(
+            "write_note",
+            {
+                "project": test_project.name,
+                "title": "Entity Type Coerce Test",
+                "directory": "test",
+                "content": "# Test\nContent for entity type coercion",
+            },
+        )
+
+        # MCP client sends entity_types as a string
+        result = await client.call_tool(
+            "search_notes",
+            {
+                "project": test_project.name,
+                "query": "coercion",
+                "entity_types": '["entity"]',
+            },
+        )
+        text = result.content[0].text
+        assert "Search Failed" not in text
+
+
+@pytest.mark.asyncio
+async def test_search_notes_note_types_as_string(mcp_server, app, test_project):
+    """search_notes should accept note_types as a JSON string via MCP protocol."""
+    async with Client(mcp_server) as client:
+        await client.call_tool(
+            "write_note",
+            {
+                "project": test_project.name,
+                "title": "Note Type Coerce Test",
+                "directory": "test",
+                "content": "# Test\nContent for note type coercion",
+            },
+        )
+
+        result = await client.call_tool(
+            "search_notes",
+            {
+                "project": test_project.name,
+                "query": "coercion",
+                "note_types": '["note"]',
+            },
+        )
+        text = result.content[0].text
+        assert "Search Failed" not in text
+
+
+@pytest.mark.asyncio
+async def test_search_notes_tags_as_string(mcp_server, app, test_project):
+    """search_notes should accept tags as a JSON string via MCP protocol."""
+    async with Client(mcp_server) as client:
+        await client.call_tool(
+            "write_note",
+            {
+                "project": test_project.name,
+                "title": "Tags Coerce Test",
+                "directory": "test",
+                "content": "# Test\nTagged content for coercion",
+                "tags": "alpha",
+            },
+        )
+
+        result = await client.call_tool(
+            "search_notes",
+            {
+                "project": test_project.name,
+                "query": "tagged",
+                "tags": '["alpha"]',
+            },
+        )
+        text = result.content[0].text
+        assert "Search Failed" not in text
+
+
+@pytest.mark.asyncio
+async def test_search_notes_metadata_filters_as_string(mcp_server, app, test_project):
+    """search_notes should accept metadata_filters as a JSON string via MCP protocol."""
+    async with Client(mcp_server) as client:
+        await client.call_tool(
+            "write_note",
+            {
+                "project": test_project.name,
+                "title": "Metadata Coerce Test",
+                "directory": "test",
+                "content": "# Test\nMetadata content for coercion",
+            },
+        )
+
+        result = await client.call_tool(
+            "search_notes",
+            {
+                "project": test_project.name,
+                "query": "metadata",
+                "metadata_filters": '{"type": "note"}',
+            },
+        )
+        text = result.content[0].text
+        assert "Search Failed" not in text
+
+
+@pytest.mark.asyncio
+async def test_write_note_metadata_as_string(mcp_server, app, test_project):
+    """write_note should accept metadata as a JSON string via MCP protocol."""
+    async with Client(mcp_server) as client:
+        result = await client.call_tool(
+            "write_note",
+            {
+                "project": test_project.name,
+                "title": "String Metadata Note",
+                "directory": "test",
+                "content": "# Test\nWith string metadata",
+                "metadata": '{"priority": "high"}',
+            },
+        )
+        text = result.content[0].text
+        assert "Created note" in text or "Updated note" in text
+
+
+@pytest.mark.asyncio
+async def test_canvas_nodes_edges_as_string(mcp_server, app, test_project):
+    """canvas should accept nodes and edges as JSON strings via MCP protocol."""
+    import json
+
+    nodes = [
+        {
+            "id": "n1",
+            "type": "text",
+            "text": "Hello",
+            "x": 0,
+            "y": 0,
+            "width": 200,
+            "height": 100,
+        }
+    ]
+    edges = [
+        {"id": "e1", "fromNode": "n1", "toNode": "n1", "label": "self"}
+    ]
+
+    async with Client(mcp_server) as client:
+        result = await client.call_tool(
+            "canvas",
+            {
+                "project": test_project.name,
+                "title": "Coerce Canvas Test",
+                "directory": "test",
+                "nodes": json.dumps(nodes),
+                "edges": json.dumps(edges),
+            },
+        )
+        text = result.content[0].text
+        assert "Created" in text or "Updated" in text

tests/test_coerce.py

@@ -0,0 +1,58 @@
+"""Tests for coerce_list and coerce_dict utility functions.
+
+These must fail until the helpers are implemented in utils.py.
+"""
+
+
+from basic_memory.utils import coerce_list, coerce_dict
+
+
+class TestCoerceList:
+    """Tests for coerce_list."""
+
+    def test_none_passthrough(self):
+        assert coerce_list(None) is None
+
+    def test_native_list_passthrough(self):
+        assert coerce_list(["a", "b"]) == ["a", "b"]
+
+    def test_json_array_string(self):
+        assert coerce_list('["entity", "observation"]') == ["entity", "observation"]
+
+    def test_single_string_wrapped(self):
+        assert coerce_list("entity") == ["entity"]
+
+    def test_non_json_string_wrapped(self):
+        assert coerce_list("not-json") == ["not-json"]
+
+    def test_json_object_string_wrapped(self):
+        """A JSON object string is not a list, so wrap it."""
+        assert coerce_list('{"key": "val"}') == ['{"key": "val"}']
+
+    def test_int_passthrough(self):
+        """Non-string, non-None values pass through unchanged."""
+        assert coerce_list(42) == 42
+
+
+class TestCoerceDict:
+    """Tests for coerce_dict."""
+
+    def test_none_passthrough(self):
+        assert coerce_dict(None) is None
+
+    def test_native_dict_passthrough(self):
+        assert coerce_dict({"k": "v"}) == {"k": "v"}
+
+    def test_json_object_string(self):
+        assert coerce_dict('{"status": "draft"}') == {"status": "draft"}
+
+    def test_non_json_string_passthrough(self):
+        """Non-parseable strings pass through (Pydantic will reject them)."""
+        assert coerce_dict("not-json") == "not-json"
+
+    def test_json_array_string_passthrough(self):
+        """A JSON array string is not a dict, so pass through."""
+        assert coerce_dict('["a", "b"]') == '["a", "b"]'
+
+    def test_int_passthrough(self):
+        assert coerce_dict(42) == 42