write initialize error to stdout #95

wzxxing · 2025-11-25T16:44:42Z

Summary

Changes

Please provide a summary of what's being changed

When client fails, we want to the see the failure reason in the MCP client. This was possible with v1.0 where every request is independent, but with v1.1.1, to reuse the MCP session for every request, proxy takes an already connected mcp client, so the client initialization happens outside of the MCP client life cycle.

User experience

Please share what the user experience looks like before and after this change

The user should see a more detailed error message such as below, immediately after the connection is closed:

Failed to connect to MCP server "aws-local": MCP error -32001: Your AWS session token has expired. Please refresh your credentials and try again. If you're using temporary credentials, you may need to re-authenticate to obtain new credentials.

Previously v1.1.1 make the error message very unclear:

Failed to connect to MCP server "AWSMCPFromLatestPyPi": MCP error -32000: Connection closed

Testing

update MCP config as

"aws": {
      "command": "uvx",
      "args": [
        "--from",
        "git+https://github.com/aws/mcp-proxy-for-aws@wzxxing/write-last-error",
        "mcp-proxy-for-aws",
        "--service",
        "aws-mcp",
        "--profile",
        "mcp-tester",
        "--region",
        "us-east-1",
        "--log-level",
        "DEBUG",
        "<remote-mcp-server-url>"
      ]
    },

Expired credentials (Authn failure):

Credentials without permission (Authz faillure)

Tool call failure (Authz), normal without terminating the transport

Checklist

If your change doesn't seem to apply, please leave them unchecked.

I have reviewed the contributing guidelines
I have performed a self-review of this change
Changes have been tested
Changes are documented

Is this a breaking change? (Y/N)

Yes
No

Please add details about how this change was tested.

Did integration tests succeed?
If the feature is a new use case, is it necessary to add a new integration test case?

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

wzxxing · 2025-11-25T16:51:14Z

mcp_proxy_for_aws/server.py

+        logger.error('Failed to start server: %s', e)
+        cause = e.__cause__
+        if isinstance(cause, McpProxy) and cause.payload:
+            print(cause.payload, file=sys.stdout)


This is to print the initialization error to the process stdout so that the client can receive it and show the error message properly.

nit/qq: Maybe we want to do this also via the logger but point it to sys.stdout only in this case.

PCManticore · 2025-11-25T19:02:59Z

mcp_proxy_for_aws/exceptions.py

+    """Error which will be handled by the proxy writing the payload to stdout."""
+
+    def __init__(self, payload: str | None = None, *args: object) -> None:
+        """Consturct a MCP proxy error with payload."""


PCManticore · 2025-11-25T19:05:09Z

mcp_proxy_for_aws/sigv4_helper.py

+            if response.status_code == 401 or response.status_code == 403:
+                request = await response.request.aread()
+                request_body = json.loads(request.decode())
+                if isinstance(request_body, dict) and request_body.get('method') == 'initialize':


Why is this thrown only on initialize? I presume because other MCP operations would normally return 200 if they are implemented according to the spec. But we can't make that assumption especially as the proxy can be leveraged by any non-AWS service specific MCP server (for example an AgentCore Runtime MCP server would be feasible to use with the proxy).

would normally return 200 if they are implemented according to the spec.

Note, this is wrong. If done by spec, e.g. when a SessionId is expired the server must return 404 if its passed.
However I do agree this is not really respected properly 😓

arangatang · 2025-11-25T21:58:05Z

mcp_proxy_for_aws/server.py

+                name='MCP Proxy',
+                instructions=(
+                    'MCP Proxy for AWS Server that provides access to backend servers through a single interface. '
+                    'This proxy handles authentication and request routing to the appropriate backend services.'


nit: SigV4 authentication and we don't really do any routing.

mcp_proxy_for_aws/server.py

arangatang · 2025-11-25T22:01:17Z

mcp_proxy_for_aws/server.py

+        logger.error('Failed to start server: %s', e)
+        cause = e.__cause__
+        if isinstance(cause, McpProxy) and cause.payload:
+            print(cause.payload, file=sys.stdout)


nit/qq: Maybe we want to do this also via the logger but point it to sys.stdout only in this case.

arangatang · 2025-11-25T22:02:00Z

mcp_proxy_for_aws/server.py

+        cause = e.__cause__
+        if isinstance(cause, McpProxyError) and cause.payload:
+            print(cause.payload, file=sys.stdout)
+        raise e


nit: raise

Suggested change

raise e

raise

arangatang · 2025-11-25T22:06:40Z

mcp_proxy_for_aws/sigv4_helper.py

+            if response.status_code == 401 or response.status_code == 403:
+                request = await response.request.aread()
+                request_body = json.loads(request.decode())
+                if isinstance(request_body, dict) and request_body.get('method') == 'initialize':


would normally return 200 if they are implemented according to the spec.

Note, this is wrong. If done by spec, e.g. when a SessionId is expired the server must return 404 if its passed.
However I do agree this is not really respected properly 😓

arangatang · 2025-11-25T22:12:51Z

mcp_proxy_for_aws/sigv4_helper.py

            error_details = response.json()
            logger.log(log_level, 'HTTP %d Error Details: %s', response.status_code, error_details)
-        except Exception:
+            if response.status_code == 401 or response.status_code == 403:


Should we do this on any 4xx / 5xx errors per default and ignore failures for maybe GET requests?

e.g. allowlist for know "ok" failures like GET requests instad of this surgical way?
wdyt?

Move the error handling outside of sigv4 helper

the client

arangatang · 2025-11-26T14:14:20Z

mcp_proxy_for_aws/server.py

+    # line = sys.stdin.readline()
+    # logger.debug('First line from kiro %s', line)


Suggested change

# line = sys.stdin.readline()

# logger.debug('First line from kiro %s', line)

Co-authored-by: Leonardo Araneda Freccero <arangatang@users.noreply.github.com> Signed-off-by: wzxxing <169175349+wzxxing@users.noreply.github.com>

logging improvement

f1d647f

wzxxing force-pushed the wzxxing/write-last-error branch 2 times, most recently from 312c93d to 9b58283 Compare November 25, 2025 16:47

wzxxing commented Nov 25, 2025

View reviewed changes

wzxxing force-pushed the wzxxing/write-last-error branch from 9b58283 to 10b44b3 Compare November 25, 2025 17:04

wzxxing changed the title ~~write last error to stdout~~ write initialize error to stdout Nov 25, 2025

wzxxing force-pushed the wzxxing/write-last-error branch from 10b44b3 to 8d26bf9 Compare November 25, 2025 17:05

PCManticore reviewed Nov 25, 2025

View reviewed changes

arangatang reviewed Nov 25, 2025

View reviewed changes

wzxxing added 5 commits November 26, 2025 01:35

add uv.lock

309e9fa

fix: proxy must run as mpc stdio server explicitly

d63f7af

fix: on connection failure, write the mcp message to stdout

7313ce0

fix broken unit tests

0c8d480

add unit test for initialize

db7e54e

wzxxing force-pushed the wzxxing/write-last-error branch from 8d26bf9 to db7e54e Compare November 26, 2025 08:31

fix linter

b540a25

wzxxing marked this pull request as ready for review November 26, 2025 08:42

wzxxing requested a review from a team as a code owner November 26, 2025 08:42

wzxxing requested review from oleksii-k-amzn and zdwheels November 26, 2025 08:42

wzxxing force-pushed the wzxxing/write-last-error branch from b540a25 to d777d6d Compare November 26, 2025 08:42

wzxxing enabled auto-merge (squash) November 26, 2025 09:33

also write mcp error message in generic error

c6a9177

wzxxing force-pushed the wzxxing/write-last-error branch from d777d6d to c6a9177 Compare November 26, 2025 13:06

exit 1 instead of raise the unhandled exception, causing too much log in

7984f5e

the client

wzxxing requested review from PCManticore and arangatang November 26, 2025 13:32

arangatang previously approved these changes Nov 26, 2025

View reviewed changes

Apply suggestions from code review

bffd108

Co-authored-by: Leonardo Araneda Freccero <arangatang@users.noreply.github.com> Signed-off-by: wzxxing <169175349+wzxxing@users.noreply.github.com>

wzxxing dismissed arangatang’s stale review via bffd108 November 26, 2025 14:45

kyoncal approved these changes Nov 26, 2025

View reviewed changes

Merge branch 'main' into wzxxing/write-last-error

e05fb95

arangatang assigned wzxxing Nov 26, 2025

arangatang approved these changes Nov 26, 2025

View reviewed changes

wzxxing merged commit 9632822 into main Nov 26, 2025
7 checks passed

wzxxing deleted the wzxxing/write-last-error branch November 26, 2025 15:00

		# line = sys.stdin.readline()
		# logger.debug('First line from kiro %s', line)

write initialize error to stdout #95

write initialize error to stdout #95

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wzxxing commented Nov 25, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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wzxxing commented Nov 25, 2025 •

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