JSON editor corrupts null values with cascading escape characters on nullable parameters

[gaoharimran29-glitch]

Describe the bug
When a tool exposes an optional nullable parameter (e.g. int | None = None in Python), the Inspector's JSON argument editor mishandles edits to the default null value. Instead of allowing normal deletion or replacement, each keystroke (e.g. Backspace) progressively re-escapes the value, corrupting it into invalid JSON. This appears to be a JSON editor/state-handling issue in the Inspector UI itself, not an issue with the underlying MCP server or tool schema.

To Reproduce
Steps to reproduce the behavior:

1. Create a tool with an optional nullable parameter:

   from fastmcp import FastMCP
   
   mcp = FastMCP(name="Demo")
   
   @mcp.tool
   def add_numbers(a: int, b: int | None = None) -> int:
        """
        Add two numbers and return the result
        """
        return a + b
   
   if __name__ == "__main__":
       mcp.run()

2. Launch the Inspector via fastmcp dev inspector file_name.py

3. Open the add_numbers tool in the Inspector UI

4. Observe the auto-generated JSON input:

   {
     "b": null
   }

5. Click into the null value field

6. Press Backspace repeatedly

7. Observe the value progressively accumulate escape characters instead of deleting cleanly:

   null → "nul" →  \"nul" → \\\"nul ...
   

   (see attached screen recording for the live progression)

Screen.Recording.2026-08-01.120224.mp4

Expected behavior
One of the following:

• Backspace/Delete should remove characters normally without introducing escape sequences.
• Clicking into a null value should allow direct replacement by typing a new value (e.g. an integer).
• Optional/nullable fields should be rendered with a more editor-friendly representation (e.g. an empty input with a placeholder of null, or a toggle for "no value") rather than as a literal, in-place-edited null token.

Logs

fastmcp dev inspector file_name.py
Starting MCP inspector...
MCP Inspector Web is up and running at:
   http://localhost:6274?MCP_INSPECTOR_API_TOKEN=<token>
   Sandbox (MCP Apps): http://localhost:29822/sandbox
   Auth token: <token>
Opening browser...

Additional context

• Changing the parameter to a non-nullable field with a non-null default (e.g. b: int = 30) reproduces normal, expected editor behavior — no escaping issue occurs. This suggests the bug is specific to how the editor handles/serializes null as the initial or current value, rather than an issue with numeric fields in general.
• This makes it difficult to test or interact with any tool that has optional nullable parameters through the Inspector's JSON editor, which is a common and idiomatic pattern in typed Python (and TypeScript) tool definitions.
• Screen recording attached showing the live escaping progression as Backspace is pressed repeatedly on the null value.
• Reproduction code (shown above) attached/included for convenience.

Environments:

• @modelcontextprotocol/inspector version: 2.0.0
• FastMCP Version : 3.4.5
• Python Version: 3.14.6
• Node Version: 22.19.0
• npm version: 10.9.3
• OS: Windows 11 Home Single Language, Version 10.0.26200 (Build 26200).
• Browser: Chrome Version 146.0.7680.80

---

[cliffhall]

Triage: Priority = Medium

Scored 8/16 with the priority rubric added to AGENTS.md in #1891, as part of the backlog triage in #1872. Added to project board #28 with Status Incoming — meaning filed and scored but not yet approved for work. A maintainer decides whether it moves to Todo.

Axis	Score	Reasoning
Severity / impact	3/5	Nullable parameters corrupt with cascading escapes, same editor and same mechanism as its siblings.
Urgency / staleness	4/5	Reproduces on the published v2.
Signal bonuses	+1	bug label (+1)
Total	8	Lands in the Medium band (6-8).

Related issues. These look connected and may be worth reading together:

• Fix JSON parameter editor escaping while typing #1853 — the escaping-while-typing report
• Backspace recursively escapes JSON tool inputs #1856 — the backspace variant
• Cannot correctly enter float parameters #1888 — float entry
• Tracking: JSON Schema Support in Inspector #1015 — JSON Schema tracking

Scores are a starting point, not a verdict — the rubric is meant to be overruled when it's plainly wrong, provided the reason is written down.