from fastapi import FastAPI, Request
from fastapi.testclient import TestClient
from starlette.responses import RedirectResponse

# Configure root_path for deployment behind reverse proxy at /api
app = FastAPI(root_path="/api")

@app.get("/health")
async def health():
    """Route without trailing slash."""
    return {"status": "healthy"}

@app.get("/redirect-explicit")
async def redirect_explicit(request: Request):
    """Explicit redirect using url_for()."""
    url = request.url_for("health")
    return RedirectResponse(url=str(url))


# Test
client = TestClient(app)

# Test 1: Explicit redirect - WORKS ✅
response = client.get("/redirect-explicit", follow_redirects=False)
location = response.headers["location"]
print(f"Explicit redirect: {location}")
# Output: http://testserver/api/health ✅ Correct - includes /api

# Test 2: Automatic redirect (trailing slash) - BROKEN ❌
response = client.get("/health/", follow_redirects=False)
location = response.headers["location"]
print(f"Automatic redirect: {location}")
# Output: http://testserver/health ❌ Wrong - missing /api prefix!

# Request to /health/ should redirect to:
Location: http://example.com/api/health  # ✅ With /api prefix

# Request to /health/ redirects to:
Location: http://example.com/health  # ❌ Missing /api prefix


### Operating System

macOS

### Operating System Details

_No response_

### FastAPI Version

0.120.0

### Pydantic Version

2.12.3

### Python Version

Python 3.13.5

### Additional Context

## Probable Root Cause

The issue is in `starlette/routing.py` line 761:

```python
# When automatic redirect is needed:
redirect_url = URL(scope=redirect_scope)  # ← URL class ignores root_path
response = RedirectResponse(url=str(redirect_url))

class URL:
    def __init__(self, scope):
        path = scope["path"]  # ← Only reads path, ignores root_path
        url = f"{scheme}://{host}{path}"

# starlette/requests.py
def base_url(self) -> URL:
    path = self.scope.get("root_path", "")  # ← Reads root_path ✅