Fix 307 Temporary Redirect when use streamable_http #781

chi2liu · 2025-05-22T09:03:31Z

The default mount path of Streamable HTTP is /mcp, but in some environments, a 307 Temporary Redirect to /mcp/mcp/ will appear, causing the request to hang or fail. When the streamable_http path is set to /mcp/, everything works fine.

Code Analysis
Mount Path

In the server.py and simple-streamablehttp-stateless examples, the ASGI routes are Mount("/mcp", app=handle_streamable_http), that is, there is no trailing slash.
In the Settings class of server.py, the default value of streamable_http_path is "/mcp" (also without a slash).
Redirection Issue

Mount("/mcp", ...) route of Starlette/FastAPI, if /mcp is requested, will automatically 307 redirect to /mcp/ (with a slash).
But if the client requests /mcp/ and the server only mounts /mcp, a redirect chain of /mcp/mcp/ will appear in some environments, resulting in "unreachable".
This is related to the "strict slash matching" of ASGI routing.
Solution

Recommended practice: Always use /mcp/ as the mounting path (with a slash) and let the client request /mcp/, so there will be no redirection problem.
Or, when mounting /mcp, make sure the client only requests /mcp without a slash.

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

chi2liu · 2025-05-22T09:04:47Z

#732

chi2liu · 2025-05-22T09:09:01Z

when we set the default /mcp, the log will be, in some env will

INFO:     Started server process [55547]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8080 (Press CTRL+C to quit)
INFO:     10.183.170.230:53048 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
INFO:     10.183.170.230:53054 - "POST /mcp/ HTTP/1.1" 200 OK
INFO:     10.183.170.230:53048 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
INFO:     10.183.170.230:53054 - "POST /mcp/ HTTP/1.1" 202 Accepted
INFO:     10.183.170.230:53048 - "GET /mcp HTTP/1.1" 307 Temporary Redirect
INFO:     10.183.170.230:53054 - "GET /mcp/ HTTP/1.1" 200 OK
INFO:     10.183.170.230:34296 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
INFO:     10.183.170.230:34312 - "POST /mcp/ HTTP/1.1" 200 OK

in some env will be hanging, because 307 Temporary Redirect to /byoa/mcp/mcp/, this is unreachable

INFO:     Started server process [35]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8080 (Press CTRL+C to quit)
INFO:     100.67.31.17:58472 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
INFO:     100.67.30.95:57600 - "GET /mcp HTTP/1.1" 307 Temporary Redirect

when we set the streamable_http to '/mcp/', it will be good, as the log:

[05/22/25 01:52:28] INFO     Starting server "StatelessServer"...                                                                server.py:214
INFO:     Started server process [80154]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8080 (Press CTRL+C to quit)
INFO:     10.183.170.230:36166 - "POST /mcp/ HTTP/1.1" 200 OK
INFO:     10.183.170.230:36166 - "POST /mcp/ HTTP/1.1" 202 Accepted
INFO:     10.183.170.230:36182 - "GET /mcp/ HTTP/1.1" 200 OK
INFO:     10.183.170.230:33648 - "POST /mcp/ HTTP/1.1" 200 OK

chi2liu · 2025-05-22T09:09:57Z

@simonw @sheffler @comfuture @shimizukawa

chi2liu · 2025-05-22T09:24:05Z

@ihrpr

ihrpr

Thanks for identifying and reporting this redirect issue! However, I don't think changing the default
streamable_http_path is the right approach here.

The core issue: This change creates inconsistency across the codebase rather than solving the root problem.

All client code constructs URLs with /mcp (no trailing slash) - see examples/clients/simple-auth-client/main.py:344
All example servers mount at /mcp (no trailing slash) - see both simple-streamablehttp examples
All tests use /mcp (no trailing slash) consistently - see tests/shared/test_streamable_http.py:238
Only FastMCP would have a different default with this change

Problems with this approach:

Breaking consistency: Makes FastMCP behave differently from all examples and existing usage patterns
Incomplete fix: The examples and tests still use /mcp without trailing slash, so the inconsistency remains
Wrong direction: The entire ecosystem already standardized on /mcp - we shouldn't change that

Alterative:

Keep the default as /mcp and fix the ASGI mounting logic to handle redirects properly
Add logic to FastMCP to handle both /mcp and /mcp/ transparently at the mount level
Document the trailing slash behavior so users can choose the approach that works for their environment

The redirect issue you've identified is real and needs fixing, but changing the default path creates more problems than
it solves. Let's find a solution that maintains consistency while addressing the underlying ASGI routing behavior.

chi2liu · 2025-05-26T03:13:16Z

Thanks for identifying and reporting this redirect issue! However, I don't think changing the default streamable_http_path is the right approach here.

The core issue: This change creates inconsistency across the codebase rather than solving the root problem.

All client code constructs URLs with /mcp (no trailing slash) - see examples/clients/simple-auth-client/main.py:344

All example servers mount at /mcp (no trailing slash) - see both simple-streamablehttp examples

All tests use /mcp (no trailing slash) consistently - see tests/shared/test_streamable_http.py:238

Only FastMCP would have a different default with this change

Problems with this approach:

Breaking consistency: Makes FastMCP behave differently from all examples and existing usage patterns

Incomplete fix: The examples and tests still use /mcp without trailing slash, so the inconsistency remains

Wrong direction: The entire ecosystem already standardized on /mcp - we shouldn't change that

Alterative:

Keep the default as /mcp and fix the ASGI mounting logic to handle redirects properly

Add logic to FastMCP to handle both /mcp and /mcp/ transparently at the mount level

Document the trailing slash behavior so users can choose the approach that works for their environment

The redirect issue you've identified is real and needs fixing, but changing the default path creates more problems than it solves. Let's find a solution that maintains consistency while addressing the underlying ASGI routing behavior.

Thank you for your review and helpful suggestions.
@ihrpr The following changes have been made to completely solve the trailing slash compatibility issue:

Keep the default path as /mcp to avoid disrupting the existing ecosystem.
In the ASGI route registration logic of FastMCP, automatically mount handlers for both /mcp and /mcp/, and clients can access them in any form without redirection.

In this way, whether the client, example, or test uses /mcp or /mcp/, it can be seamlessly compatible without modifying the existing code. The problem has been completely solved.

vectorstain

GG

vectorstain

GG

src/mcp/server/fastmcp/server.py

vectorstain

GG

vectorstain · 2025-05-27T18:20:20Z

@chi2liu please update the branch with master and check again tests results

ihrpr

Thank you!

Please can you add tests to verify all works as expected?

…iu/python-sdk into fix-307-Temporary-Redirect

Kludex · 2025-05-28T08:33:59Z

Why are we even mounting applications? We can have a Router and set redirect_slashes to False.

…iu/python-sdk into fix-307-Temporary-Redirect

chi2liu · 2025-05-29T02:54:36Z

Why are we even mounting applications? We can have a Router and set redirect_slashes to False.

I have modified it according to your suggestion. Could you please review it? @Kludex

Kludex · 2025-05-29T07:27:39Z

src/mcp/server/fastmcp/server.py

@@ -618,7 +618,7 @@ async def run_streamable_http_async(self) -> None:
        import uvicorn

        starlette_app = self.streamable_http_app()
-
+        starlette_app.router.redirect_slashes = False


Hmmm... I think we can add redirect_slashes in the Starlette constructor on starlette's side.

But this part is fine.

Kludex · 2025-05-29T07:29:19Z

src/mcp/server/fastmcp/server.py

+        async def handle_streamable_http(request: Request) -> Response:
+            await self.session_manager.handle_request(
+                request.scope, request.receive, request._send
+            )
+            return Response()


This is not fine. Does handle_request needs to be an ASGI application?

@Kludex This could be a more suitable solution ? PR

Kludex · 2025-05-29T07:31:40Z

src/mcp/server/fastmcp/server.py

+        # Always register both /mcp and /mcp/ for full compatibility
+        _main_path = self.settings.streamable_http_path.removesuffix("/")
+        _alt_path = _main_path + "/"


I think the problem was the redirect, but if you have a 404 then it's not an issue anymore, and we shouldn't be needing this.

vectorstain · 2025-05-29T10:01:16Z

src/mcp/server/fastmcp/server.py

@@ -839,6 +839,7 @@ async def handle_streamable_http(
            routes=routes,
            middleware=middleware,
            lifespan=lambda app: self.session_manager.run(),
+            redirect_slashes=False,


Tests failed cause MCP server doesn't start.

return Starlette( TypeError: Starlette.__init__() got an unexpected keyword argument 'redirect_slashes'

You cant pass redirect_slashes to starlette. Pls review my proposed solution PR

That's what I said, we don't have it yet...

* Refactor FastMCP routing to use Router and streamline request handling * Add async support to token validation test and enhance metadata snapshot assertions * Fix tests * Fix tests --------- Co-authored-by: Vincenzo Maria Calandra <vincenzomariacalandra@MacBook-Pro-di-Vincenzo.local> Co-authored-by: 633WHU <cliu_whu@yeah.net>

Update streamable_http_path to /mcp/ to avoid 307 Temporary Redirect

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chi2liu mentioned this pull request May 22, 2025

307 Temporary Redirect #732

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Merge branch 'main' into fix-307-Temporary-Redirect

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chi2liu added 7 commits May 26, 2025 10:58

Streamable HTTP Trailing Slash Compatibility

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Streamable HTTP Trailing Slash Compatibility

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Streamable HTTP Trailing Slash Compatibility

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Merge branch 'main' into fix-307-Temporary-Redirect

900c2dd

Streamable HTTP Trailing Slash Compatibility

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Streamable HTTP Trailing Slash Compatibility

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Streamable HTTP Trailing Slash Compatibility

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vectorstain reviewed May 26, 2025

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Merge branch 'main' into fix-307-Temporary-Redirect

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chi2liu requested review from ihrpr and vectorstain May 27, 2025 01:37

vectorstain reviewed May 27, 2025

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src/mcp/server/fastmcp/server.py Outdated Show resolved Hide resolved

Clean logic (#3)

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chi2liu requested a review from vectorstain May 27, 2025 09:09

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Merge branch 'main' into fix-307-Temporary-Redirect

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chi2liu and others added 2 commits May 28, 2025 09:17

Merge branch 'main' into fix-307-Temporary-Redirect

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add unit test for Streamable HTTP Trailing Slash Compatibility

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chi2liu and others added 5 commits May 28, 2025 11:19

Update test_streamable_http.py

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add unit test for Streamable HTTP Trailing Slash Compatibility

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Merge branch 'fix-307-Temporary-Redirect' of https://github.com/chi2l…

09e403c

…iu/python-sdk into fix-307-Temporary-Redirect

add unit test for Streamable HTTP Trailing Slash Compatibility

a03c62e

add unit test for Streamable HTTP Trailing Slash Compatibility

10ccbbf

chiliu and others added 9 commits May 28, 2025 16:41

add unit test for Streamable HTTP Trailing Slash Compatibility

e102e7d

Merge branch 'main' into fix-307-Temporary-Redirect

a72ffe8

change Mount to Routers

6cf74f1

fix tests

2748a75

redirect_slashes to False

b726c86

Update server.py

3b03b83

fix typo

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Merge branch 'fix-307-Temporary-Redirect' of https://github.com/chi2l…

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…iu/python-sdk into fix-307-Temporary-Redirect

fix typo

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chi2liu requested a review from ihrpr May 29, 2025 02:52

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chiliu and others added 3 commits May 29, 2025 02:11

fix

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Update server.py

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chi2liu requested a review from Kludex May 29, 2025 09:13

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chi2liu and others added 2 commits May 29, 2025 22:18

Merge branch 'main' into fix-307-Temporary-Redirect

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chi2liu requested a review from vectorstain May 29, 2025 14:21

chi2liu added 2 commits May 29, 2025 22:21

Merge branch 'main' into fix-307-Temporary-Redirect

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Update server.py

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Fix 307 Temporary Redirect when use streamable_http #781

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Fix 307 Temporary Redirect when use streamable_http #781

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