Claude Desktop (Cowork), claude.ai and Claude Code: MCP tools with object parameters fail with `MCP error -32602` `"expected": "object", "received": "string"` (sent as a JSON string)

Context

Source(s): https://github.com/anthropics/claude-code/issues/25865 Status: reported working by source, not reproduced by us (workaround only; root fix not confirmed).

Problem

Every tool call that needs an object or array parameter fails, while tools with only string parameters work. With the Notion connector, updating a page, moving pages and creating a page under a parent all failed; only read tools such as search and fetch, and creating a page at the workspace root, worked. A value such as {"type": "page_id", "page_id": "<id>"} reached the server as the string "{\"type\": \"page_id\", \"page_id\": \"<id>\"}", so the server's schema check rejected it. A commenter saw the same pattern on three independent remote connectors, and another saw it in rare cases with a different product's MCP server, so it is not specific to Notion.

Error (redacted / normalized)

MCP error -32602: Invalid arguments for tool notion-update-page: [
  {
    "code": "invalid_type",
    "expected": "object",
    "received": "string",
    "path": ["data"],
    "message": "Expected object, received string"
  }
]

MCP error -32602: Invalid arguments for tool notion-create-pages: [ { "code": "invalid_type", "expected": "object", "received": "string", "path": ["parent"], "message": "Expected object, received string" } ]

Applies when

Claude Desktop in Cowork mode and local agent mode (macOS, 1.26832.0 in August), claude.ai web chat, Claude Code 2.1.51, claude-agent-sdk 0.1.37; reported February to October 2026.

Confirm it is this problem

1. Call a tool whose schema has an object or array parameter → expect: MCP error -32602 with "expected": "object", "received": "string" (or array).
2. If you run the server, log the raw argument → expect: a JSON-encoded string that decodes to the object you meant to send.

Not this when / boundary

The parameter is a number or boolean that arrived as a string; FixLore post fix/90 covers '1.57' is not of type 'number'. The string does not decode to valid JSON, so the model built a wrong value rather than an encoded one (our inference). A field is reported as Required (received: undefined) even though you sent it; one commenter saw that with Notion and the thread gives no separate fix for it.

Root cause

A collaborator said two separate bugs caused this. In Cowork, parameters declared with oneOf/anyOf or $ref reached the model without their type, so it sent objects and arrays as JSON strings; that was fixed in Claude Desktop 1.1.4498. In Claude Code, the model could emit a string for any parameter whose schema arrived without a type; from 2.1.246 the CLI repairs those values against the tool's schema before sending. The collaborator said local agent mode runs the Claude Code engine, and that a few less common schema shapes are not repaired yet.

Variables

<id>: a page id.

Fix

1. Update Claude Desktop to 1.1.4498 or later and Claude Code to 2.1.246 or later, then rerun confirm 1 → expect: per the collaborator, object parameters now arrive as objects. No user confirmed this in the thread before it was closed on 2026-10-18.
2. If you control the MCP server: when a parameter declared as an object or array arrives as a string, JSON-decode it and continue → expect: a commenter verified that a server doing this made real production writes work while every strict server failed.
3. Where a tool also offers a flat or scalar way to pass the same value, use that instead → expect: a commenter said scalar parameters still bind correctly.

How to verify

Rerun the call that failed → expect: no -32602 error, and the change shows up in the target app.

What did not work

One commenter found that Notion's comment tool, which takes page_id as a top-level string, still failed with body failed validation: body.create_comment.parent should be defined, instead was undefined, so a flat parameter did not always help.

Rollback

Remove the string-decoding fallback from your server once your clients send real objects.

Risks

In step 2, decode only parameters the schema declares as object or array, and validate the decoded value against the schema before using it (our inference). A commenter fell back to clicking through pages with browser automation; that is slow and easy to get wrong on large databases (our inference).

Verified scope

Claude Desktop before 1.1.4498 and Claude Code before 2.1.246, February to October 2026.

Becomes obsolete when

Users confirm the fixed versions. Open question: which schema shapes the collaborator meant are still not repaired.