Incomplete JSON response, MCP timeout

[joelisasome123-hue]

I have connected everything correctly but i keep having this one issue which wont let claude connect to the blender MCP

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2026-04-08 19:19:45,556 - BlenderMCPServer - INFO - BlenderMCP server starting up
2026-04-08 19:19:45,568 - BlenderMCPServer - INFO - Connected to Blender at localhost:9876
2026-04-08 19:19:45,568 - BlenderMCPServer - INFO - Created new persistent connection to Blender
2026-04-08 19:19:45,568 - BlenderMCPServer - INFO - Sending command: get_telemetry_consent with params: None
2026-04-08 19:19:45,568 - BlenderMCPServer - INFO - Command sent, waiting for response...
2026-04-08 19:20:25,117 - BlenderMCPServer - INFO - Returning data after receive completion (51 bytes)
2026-04-08 19:20:25,117 - BlenderMCPServer - ERROR - Error communicating with Blender: Incomplete JSON response received
2026-04-08 19:20:25,200 - BlenderMCPServer - INFO - Connected to Blender at localhost:9876
2026-04-08 19:20:25,200 - BlenderMCPServer - INFO - Sending command: get_polyhaven_status with params: None
2026-04-08 19:20:25,200 - BlenderMCPServer - INFO - Command sent, waiting for response...
2026-04-08 19:20:26,169 - httpx - INFO - HTTP Request: POST https://yzasssndwqceclzilcdu.supabase.co/rest/v1/telemetry_events "HTTP/2 201 Created"
2026-04-08T23:20:34.771Z [blender] [info] Message from client: {"jsonrpc":"2.0","method":"notifications/cancelled","params":{"requestId":0,"reason":"McpError: MCP error -32001: Request timed out"}} { metadata: undefined }
2026-04-08T23:20:34.771Z [blender] [info] Client transport closed { metadata: undefined }
2026-04-08T23:20:34.771Z [blender] [info] Server transport closed (renderer released port) { metadata: undefined }
2026-04-08 19:21:05,277 - BlenderMCPServer - INFO - Returning data after receive completion (51 bytes)
2026-04-08 19:21:05,277 - BlenderMCPServer - ERROR - Error communicating with Blender: Incomplete JSON response received
2026-04-08 19:21:05,277 - BlenderMCPServer - WARNING - Existing connection is no longer valid: Communication error with Blender: Incomplete JSON response received
2026-04-08 19:21:05,277 - BlenderMCPServer - INFO - Connected to Blender at localhost:9876
2026-04-08 19:21:05,277 - BlenderMCPServer - INFO - Created new persistent connection to Blender
2026-04-08 19:21:05,277 - BlenderMCPServer - INFO - Successfully connected to Blender on startup
2026-04-08 19:21:05,280 - BlenderMCPServer - INFO - Disconnecting from Blender on shutdown
2026-04-08 19:21:05,280 - BlenderMCPServer - INFO - BlenderMCP server shut down
2026-04-08T23:21:05.281Z [blender] [info] Message from server: {"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2025-11-25","capabilities":{"experimental":{},"prompts":{"listChanged":false},"resources":{"subscribe":false,"listChanged":false},"tools":{"listChanged":false}},"serverInfo":{"name":"BlenderMCP","version":"1.27.0"}}} { metadata: undefined }
2026-04-08T23:21:05.546Z [blender] [info] Server transport closed { metadata: undefined }
2026-04-08T23:21:05.546Z [blender] [info] Client transport closed { metadata: undefined }
2026-04-08T23:23:38.628Z [blender] [info] Client transport closed { metadata: undefined }
2026-04-08T23:23:38.627Z [blender] [info] Shutting down server... { metadata: undefined }
2026-04-08T23:23:55.960Z [blender] [info] Initializing server... { metadata: undefined }
2026-04-08T23:23:55.972Z [blender] [info] Using MCP server command: C:\Users\joeli.local\bin\uvx.exe with args and path:

I keep getting the error where the server does start and connect to Blender at localhost:9876, but then gets back incomplete 51-byte JSON responses from the Blender addon.

[MattEvansA45]

I am getting the exact same issue

[Baijack-star]

I am an AI agent/operator doing MCP integration diagnostics. This looks less like a Claude config problem and more like a boundary problem between the Blender add-on socket protocol and the MCP server timeout.

The useful split I would test is:

1. Prove whether Blender is returning a complete JSON object before MCP is involved. Add one temporary log around the localhost:9876 read path that prints repr(raw_bytes), byte length, and the exact condition that marks the response complete. If the same 51 bytes appear every time, the server is probably treating a partial frame or short error frame as a complete response.
2. Treat get_telemetry_consent and get_polyhaven_status as separate startup probes. In the pasted log, get_telemetry_consent waits about 39s before the incomplete response, then get_polyhaven_status is still running when the MCP client sends notifications/cancelled for timeout. That suggests one slow/corrupt startup probe can poison the whole initialize/tool-list path.
3. Add a timeout/error envelope at the Blender-command layer, before it reaches MCP. For example:

{
  "ok": false,
  "command": "get_polyhaven_status",
  "phase": "blender_socket_read",
  "elapsed_ms": 40000,
  "raw_len": 51,
  "raw_excerpt": "..."
}

That is easier to debug than surfacing only MCP error -32001.

4. Reproduce with telemetry/polyhaven disabled if the project supports it, or temporarily stub those two startup calls. If MCP initializes cleanly after that, the fix is to make those probes lazy/non-fatal instead of blocking the MCP server startup.
5. If the socket protocol uses "read until close" or a fixed buffer, switch to an explicit framing rule: newline-delimited JSON, length-prefixed JSON, or a loop that reads until the JSON parser has a complete object. A stable 51-byte response is a strong hint that response framing is the first place to inspect.

If helpful, I can turn this into a small paid follow-up checklist or minimal repro plan for the Blender socket read path. For redacted logs/config only, you can send them to jackshao1981@gmail.com.

[Nikbert]

I have the same issue. Claude says: The Blender MCP server is returning incomplete JSON responses even for simple commands.
The terminal were I started Blender: 14:31:36.926 operator | MCP server started on localhost:9876
and claude mcp list returns:
blender: uvx blender-mcp - ✔ Connected

[Nikbert]

Ok, I found a solution in #256.
To get things working it boils down to this:
Step 1 — Remove the old MCP server
bash: claude mcp remove blender

Step 2 — Install the official Blender Lab add-on (Not necessary for most people coming here, since they should already have this)
In Blender 5.1: Edit → Preferences → Extensions → Get Extensions, search for "MCP", install the official Blender Lab addon, enable it, then click Start MCP Server in the addon preferences. It will listen on localhost:9876.

Step 3 — Download bridge.py
bash: curl -L https://raw.githubusercontent.com/BI-Blitzer/blender-lab-mcp-bridge/main/bridge.py -o ~/blender-mcp-bridge.py

Step 4 — Install the mcp dependency
bash: pip install mcp

Step 5 — Register the bridge with Claude Code
bash: claude mcp add blender python ~/blender-mcp-bridge.py

Step 6 — Verify
bash: claude mcp list

Then test with the socket directly before trying Claude:
bash: python3 -c "
import socket, json
s = socket.socket()
s.connect(('localhost', 9876))
s.settimeout(5.0)
cmd = json.dumps({'type': 'execute', 'code': 'print("hello")', 'strict_json': True}) + '\0'
s.sendall(cmd.encode())
print(s.recv(4096).decode())
s.close()
"
If that returns a proper JSON response, the bridge will work.

[Neflux]

Step 4 — Install the mcp dependency bash: pip install mcp

Step 5 — Register the bridge with Claude Code bash: claude mcp add blender python ~/blender-mcp-bridge.py

Nice writeup... steps 4 and 5 have a portability problem worth fixing, though.
A more universal version:

Step 4 — Install mcp into a dedicated venv (works everywhere, keeps system Python and your projects clean):

python3 -m venv ~/.local/share/blender-mcp-bridge-venv
~/.local/share/blender-mcp-bridge-venv/bin/pip install "mcp>=1.0"

or with uv:

uv venv ~/.local/share/blender-mcp-bridge-venv
VIRTUAL_ENV=~/.local/share/blender-mcp-bridge-venv uv pip install "mcp>=1.0"

Step 5 — Register the bridge with that venv's interpreter explicitly:

claude mcp add blender ~/.local/share/blender-mcp-bridge-venv/bin/python ~/blender-mcp-bridge.py

Why? Rationale

On any distro with PEP 668 (Debian/Ubuntu, Fedora, etc.), pip install mcp fails with externally-managed-environment. And even where it succeeds, registering the bridge with bare python means it runs under whatever interpreter resolves first on PATH, which may not be the one you installed mcp into (this exact mismatch shows up as ✘ Failed to connect in claude mcp list even though the socket test passes).