Question

Issue Description

When using FastMCP's streamable_http_app(), requests to the /mcp endpoint result in a 307 Temporary Redirect to /mcp/, causing unnecessary latency and potential client compatibility issues.

Current Behavior

INFO:     127.0.0.1:58306 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
INFO:     127.0.0.1:58306 - "POST /mcp/ HTTP/1.1" 200 OK

## Code

```python
from mcp.server.fastmcp import FastMCP

mcp = FastMCP("My MCP Server")
app = mcp.streamable_http_app()

Root Cause

FastMCP uses Starlette's Router with redirect_slashes=True by default, which automatically redirects URLs without trailing slashes to URLs with trailing slashes using HTTP 307.

K8s Deployment Issues

In Kubernetes environments, this redirect behavior causes additional problems:

1. Ingress Controllers: Some ingress controllers (nginx, traefik) may not preserve request bodies during 307 redirects
2. Load Balancers: External load balancers may timeout or fail on redirect chains
3. Service Mesh: Istio/Linkerd may not handle redirects properly for streaming connections
4. TLS Termination: HTTPS redirects between services can cause certificate validation issues

Impact

1. Performance: Extra round-trip for every MCP request
2. Client Compatibility: Some HTTP clients may not handle 307 redirects properly
3. Debugging Confusion: Logs show double requests for every operation

Attempted Solutions

Solution 1: Disable redirect_slashes (Breaks Routing)

app = mcp.streamable_http_app()
app.router.redirect_slashes = False  # This breaks MCP routing entirely

Result: 404 Not Found for all MCP requests

Solution 2: Manual Route Configuration (Complex)

Requires understanding FastMCP's internal routing structure.

Proposed Solutions

Option 1: Add Configuration Parameter

app = mcp.streamable_http_app(redirect_slashes=False)

Option 2: Canonical Endpoint Documentation

Clearly document whether the canonical endpoint is /mcp or /mcp/ and ensure consistency.

Option 3: Handle Both Routes Explicitly

FastMCP could register both /mcp and /mcp/ routes to avoid redirects.

Environment

• mcp version:1.11.0
• Python version: 3.11.3
• HTTP Transport: SSE (Server-Sent Events)
• Framework: Starlette (via FastMCP)

Reproduction Steps

1. Create FastMCP server with streamable_http_app()
2. Make POST request to /mcp (without trailing slash)
3. Observe 307 redirect in logs
4. Request is automatically redirected to /mcp/

Request

Could FastMCP provide a way to either:

1. Disable the redirect behavior without breaking routing
2. Configure the canonical endpoint path
3. Handle both /mcp and /mcp/ without redirects

This would improve performance and reduce confusion for developers implementing MCP servers.

Additional Context

Environment

• mcp version:1.11.0
• Python version: 3.11.3
• HTTP Transport: SSE (Server-Sent Events)
• Framework: Starlette (via FastMCP)