aws-pricing-mcp-server fails to connect on Windows (stdio JSON parse error)

[ManojRamani]

Description

The awslabs.aws-pricing-mcp-server fails to establish an MCP connection when running on Windows 11 via Claude Code (CLI). The server process starts successfully but the JSON-RPC handshake fails with a Pydantic validation error related to empty input parsing.

Environment

• OS: Windows 11 Enterprise 10.0.26100
• Shell: Git Bash / PowerShell
• MCP Client: Claude Code (CLI)
• Package versions tested: @latest (1.0.31), @1.0.28, @1.0.20
• Python: 3.12+ (via uvx)
• AWS credentials: Valid (other awslabs MCP servers connect fine with the same profile)

Steps to Reproduce

1. Configure ~/.claude/mcp.json:

{
  "mcpServers": {
    "aws-pricing-mcp-server": {
      "command": "uvx",
      "args": ["awslabs.aws-pricing-mcp-server@latest"],
      "env": {
        "AWS_PROFILE": "team-config",
        "AWS_REGION": "us-east-1"
      }
    }
  }
}

2. Launch Claude Code on Windows
3. The pricing server never appears in the available tools list

Error Output (manual launch)

Running manually in PowerShell:

$env:AWS_PROFILE="team-config"; $env:AWS_REGION="us-east-1"; uvx awslabs.aws-pricing-mcp-server@latest

Produces:

ERROR:mcp.server.lowlevel.server:Received exception from stream: 1 validation error for JSONRPCMessage
  Invalid JSON: EOF while parsing a value at line 2 column 0 [type=json_invalid, input_value='\n', input_type=str]
    For further information visit https://errors.pydantic.dev/2.13/v/json_invalid
{"method":"notifications/message","params":{"level":"error","logger":"mcp.server.exception_handler","data":"Internal Server Error"},"jsonrpc":"2.0"}

Expected Behavior

The server should connect and expose its pricing tools via MCP, as the other awslabs servers do (aws-api-mcp-server, aws-documentation-mcp-server, amazon-bedrock-agentcore-mcp-server) with the same configuration pattern on the same Windows machine.

Suspected Cause

Windows CRLF (\r\n) line endings on stdin may be causing the JSON-RPC message parser to fail. The error shows input_value='\n' suggesting the parser is receiving an empty/malformed line after splitting on \n but before stripping \r. The other awslabs servers likely handle this correctly.

Workaround

Using aws pricing get-products via the aws-api-mcp-server as a fallback for pricing queries.

[iperera97]

Does anyone solve this issue ?

[masamaru0513]

I could reproduce this on Windows.

Sending an initialize request terminated with \r\n (which is what PowerShell and a lot of Windows clients do) triggers the same error:

ERROR:mcp.server.lowlevel.server:Received exception from stream: 1 validation error for JSONRPCMessage
  Invalid JSON: EOF while parsing a value at line 2 column 0 [type=json_invalid, input_value='\n', input_type=str]

The initialize call itself actually succeeds, but then a leftover empty line gets fed into the parser and blows up.

This doesn't look like a bug in this server specifically, it's in mcp/server/stdio.py from the MCP Python SDK itself:

if not stdin:
    stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))
if not stdout:
    stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))

No newline="" on either wrapper, so Windows does its usual \n <-> \r\n translation and corrupts the NDJSON framing. Should affect any MCP server built on this SDK on Windows, not just aws-pricing-mcp-server — my guess is other servers just haven't hit the exact request pattern that triggers it.

Turns out this is already being tracked upstream:

• Windows: TextIOWrapper in stdio_server() emits CRLF instead of LF, corrupting newline-delimited JSON messages modelcontextprotocol/python-sdk#2433
• fix PR (still open): fix: disable newline translation in stdio TextIOWrapper to prevent CRLF on Windows modelcontextprotocol/python-sdk#2552 — adds newline="" to both wrappers
• there have been a few duplicate PRs for the same thing that got closed already (chore(aws-msk-mcp-server): Add new code owners for aws-msk-mcp-server #2497, RFC: AWS For SAP Management MCP Server #2743, fix(eks-mcp-server): prevent path traversal through app_name in generate_app_manifest #2757, chore(deps): update uv: bump cryptography from 45.0.6 to 46.0.7 in /src/aws-bedrock-custom-model-import-mcp-server #2908)

I patched newline="" in locally and the error goes away. Probably makes more sense to push on getting python-sdk#2552 merged than to patch around it here, since it's an SDK-level issue affecting every server in this repo on Windows.