[CRITICAL] MCP Parameter Serialization Bug in Both Claude Code and Claude Desktop

Update: Admin Privileges Test Results

Testing Environment: Claude Desktop running with Administrator privileges

Test Results

Ran comprehensive tests with admin privileges to rule out permission-related issues:

Session Start (Fresh)

1. ✅ mcp-db-server:get_database_info (no params) → SUCCESS
2. ❌ mcp-db-server:query with SQL parameter → FAILED ("Cannot convert undefined or null to object")
3. ❌ filesystem:list_directory with path parameter → FAILED
4. ❌ filesystem:list_allowed_directories (no params) → FAILED

Critical Discovery

Even more severe pattern observed:

• ✅ First parameter-less call: Works
• ❌ First parameter call attempt: Fails and corrupts entire MCP session
• ❌ ALL subsequent calls: Fail, even those without parameters

Conclusion

• Not a permission issue: Admin privileges do not resolve the problem
• Complete MCP system failure: First parameter call attempt breaks the entire MCP subsystem
• Session corruption: Once a parameter call fails, even parameter-less calls start failing
• Confirms fundamental architectural bug: This is a core MCP implementation defect

This makes the issue even more critical as it shows complete MCP system instability rather than just parameter serialization issues.

Impact Assessment

Severity: CRITICAL++

• Makes MCP completely unusable in production
• Single failed parameter call corrupts entire session
• Affects both Claude Code and Claude Desktop
• No viable workarounds beyond constant session restarts

Bump: can confirm, experiencing exactly the same. Tried multiple MCP servers, my own, filesystem, vs code, etc. Consistently get "Cannot convert undefined or null to object" when multiple calls are made in succession or when a call contains multiple parameters/arguments.

<img width="788" height="458" alt="Image" src="https://github.com/user-attachments/assets/e13891f9-cf8a-40ed-af75-0d52af624e3b" />

See issue https://github.com/anthropics/claude-code/issues/4188

"Cannot convert undefined or null to object" when multiple calls are made in succession or when a call contains multiple parameters/arguments.

Same issue here, first tool call works then all other tools from other MCPs returns the same error

Dost work even if the session is restarted. On Claude desktop

Same problem...

same here

Remote connectors and dtx also have that problems.. please fix it quickly.

Confirming this exact bug pattern.

I can reproduce the identical behaviour:

• First MCP call with parameters: SUCCESS
• Second MCP call with parameters: FAIL with "Cannot convert undefined or null to object"
• Calls without parameters: Always work

Additionally found that any first MCP call that returns an error (e.g., check a non-existent resource) immediately corrupts the client state, causing all subsequent parameterised calls to fail with the same JavaScript error.

Platform: Claude Desktop
Impact: Critical - makes MCP unusable for any multi-step workflows

I get this error: "Failed to call tool execute_sql: TypeError: Cannot convert undefined or null to object" when I try to use mysql_mcp_server. I’ve tried both Sonnet and Opus, and the issue is the same. Is this the same problem you’re experiencing?

Exactly the same @cris82mi. Not an issue with your MCP.

Same issue here.

@cris82mi Widespread issue. Suffering the same here.

Same here

Same issue with Jira MCP.

Exactly the same issue... Windows 11.

same issue

Can confirm on macOS Sequoia 15.5

Game breaker for me. I have several long running complex projects and Claude has free rein to read-write persistent memories to a directory I've given it.

Confirmed here to. Huge issue for me, killing several critical processes

same issue

I'll help you check the PostgreSQL database. Let me start by exploring the database structure to see what's available.Now let me check what tables and other objects are in the main user schema:Let me try a different approach and run a SQL query to check the database:Let me try checking the database health to see if there are any connectivity issues:I'm encountering connectivity issues with the PostgreSQL database - all the functions are returning "Cannot convert undefined or null to object" errors. This suggests the database connection might not be properly configured or the database might not be accessible.

Let me check if there are any database files or alternative data sources available:Let me try checking if there's a DuckDB database available:I'm unable to access the PostgreSQL database due to connectivity issues. All the database functions are returning "Cannot convert undefined or null to object" errors, which suggests:

1. Database Connection Issues: The PostgreSQL server might not be running or accessible
2. Configuration Problems: The database connection parameters might not be properly configured
3. MCP Server Issues: The Model Context Protocol servers that provide database access might not be properly connected

Troubleshooting steps you could try:

1. Check PostgreSQL Status: Verify that the PostgreSQL service is running
2. Verify Connection Settings: Ensure the database host, port, username, password, and database name are correctly configured
3. Network Connectivity: Check if there are any firewall or network issues preventing database access
4. MCP Configuration: Verify that the PostgreSQL MCP server is properly configured and connected

Would you like me to help you with any specific aspects once the database connectivity is restored? I can assist with:

• Database schema analysis
• Performance monitoring
• Query optimization
• Health checks
• Index recommendations

If you have any database dump files or SQL scripts available, I could also help analyze those while the connectivity issues are being resolved.

Same issue here and stuck since yesterday.

We're experiencing a similar problem. Several clients are considering switching from Claude to OpenAI or Gemini due to concerns about platform reliability.

I started experiencing this issue sometime yesterday. And am still experiencing it, even though the AI support agent assured me this had been fixed 🤣

When will it be fixed? Any updates please

Also experiencing this issue.

+1

Same for me.

A status update on any fixes would be appreciated...this issue breaks workflows and there is no work-around other than to wait for the fix to be implemented.

Would love any update from anthropic on this. Encountering this on both Claude Desktop and Claude Code, and is a production blocker. As a Max Subscriber, I am disappointed about how long this has lasted without being addressed.

LOL, i spend a whole day trying to debug my MCP services ><

Same issue here and stuck since yesterday.

Same for mee

Error executing code: Cannot convert undefined or null to object

Since I came in this morning. I spent $200/month for the pro max plan. I have lost a full day's work on this, for the current contract that is over $800 lost. How many people around the world, particularly people who bill for work by the day, have lost their income for the day?

What is Anthropic doing about this? There is no acknowledgement of the problem.
Go to their status page https://status.anthropic.com all you get is "Eleveated Errors on Sonnet 4"

Well, anthropic? You're losing my company as a customer.

agreed.. spent so much time trying to debug .. finally realized it is this. Lost a complete day of work too. Also have the pro plan.

Confirming this issue with ClaudeR on Windows
I can confirm this exact same pattern with the ClaudeR R package on Windows 11:
Setup:

ClaudeR 0.2.0 (R package for executing R code via MCP)
Windows 11 x64
Claude Desktop (latest version)

Exact same behavior:

First execute_r call: Always succeeds (simple or complex R commands)
Second execute_r call: Always fails with "Cannot convert undefined or null to object"
All subsequent calls: Continue failing until Claude Desktop restart

This started happening today - was working perfectly this morning, began failing this afternoon without any changes to my setup.
The ClaudeR MCP server appears to be running correctly (shows as connected), suggesting this is indeed a Claude Desktop client-side parameter serialization issue affecting all MCP servers requiring parameters.

Also happening on Claude Desktop for MacOS

agreed.. spent so much time trying to debug .. finally realized it is this. Lost a complete day of work too. Also have the pro plan.

....yes just spent hours thinking it was my error : (

I'm having the same issues on Claude Desktop for MacOS. Was working perfectly last night, this morning everything fails.

Facing same issue on Mac Claude desktop. Has anyone found any workaround?

+1

I just tried re-installing the desktop app — I had an older .dmg image from May 2025. Still no joy. My best guess is that this is a fault at the Anthropic server level, and not due to a botched app update.

Issue still open. It's impacting 99% of the open projects. An official ETA for resolution should be the normal support expected for a 200$/ mo service.

It's more than 12 hours that everything is unusable.

Atleast they have started to investigate the issue now:

https://status.anthropic.com/

+1

https://status.anthropic.com/ showing a fix - hard close of Claude Desktop resolves it

Hey all, appreciate the feedback here. It looks like this was due to an upstream API failure affecting all MCP tool calls. This should be resolved now (perhaps with a Claude Desktop restart). Please see https://status.anthropic.com/.

Reading this thread, it's clear there are a number of things we should follow up on. In particular, we can aim to provide better error messages, which can hopefully indicate if the error is server- or client-side. We also should have detected the issue sooner.

Apologies for the inconvenience - we do aim to make this better.

This bug is fixed, upgraded to v0.12.49!

Thank you for the quick fix! All good to go over here on Claude Desktop App for Windows

Reading this thread, it's clear there are a number of things we should follow up on. In particular, we can aim to provide better error messages, which can hopefully indicate if the error is server- or client-side.

@ollie-anthropic An often overlooked detail in try/catch hell. Every developer will relate. Thanks for the work.

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