mcp-remote SSE error: Could not convert argument of type symbol to string
Connecting mcp-remote 0.1.18 to the Atlassian MCP SSE endpoint (mcp.atlassian.com/v1/sse) on Node 20.15.1 fails with "SSE error: Could not convert argument of type symbol to string." A direct curl to the same endpoint returns the expected event data, so the problem is inside mcp-remote's SSE/header handling. What causes this error and how can it be fixed?
Originally asked on GitHub: https://github.com/punkpeye/mcp-remote/issues/125
3 answers
The root cause is explained, and the error does not occur on newer Node.js versions; the maintainer is tracking the underlying bug in mcp-remote issue #157. A commenter reproduced the failure on Node 20.11.1 and Node 22.2.0, then bisected to find the error disappears starting with Node 22.3.0 and does not occur on Node 24.7.0. Based on the Node 22.3.0 changelog, the commenter suspected the switch to a new MessageEvent implementation (nodejs/node PR #52370). The maintainer later said this issue and #157 share the same root cause: header objects leaking symbol-keyed internals into the SSE fetch path, with the error wording varying by Node/undici version.
The workaround that worked in the thread is to upgrade Node.js to at least 22.3.0 (the reporter used Node 20.15.1, the commenter was on 20.11.1). The failing command for reference:
mcp-remote https://mcp.atlassian.com/v1/sse (opens external site) --auth atlassian --header "x-atlassian-mcp-target: jira" --debug
A direct curl to the same SSE endpoint returned the expected event data, so the endpoint was healthy; the failure happens inside mcp-remote's SSE/header handling on older Node versions. General advice: run MCP SSE clients on a current Node.js LTS, since this class of error is tied to the Node runtime rather than to the mcp.json config.
Did this work for you?What a GitHub user tried on GitHub (Didn't work)
Reporter ran mcp-remote 0.1.18 on Node 20.15.1 against the Atlassian SSE endpoint with --auth atlassian, the x-atlassian-mcp-target header, and --debug; it failed with the symbol-to-string SSE error. A direct curl to the same endpoint returned the expected event data.
Source: https://github.com/punkpeye/mcp-remote/issues/125 (opens external site)
Did this work for you?What @joshuadavid-wag tried on GitHub (Partly worked)
Commenter reproduced the error on Node 20.11.1, bisected Node versions, and found the error still fails on Node 22.2.0 but no longer occurs on Node 22.3.0 or Node 24.7.0, concluding that updating Node.js should fix it.
Source: https://github.com/punkpeye/mcp-remote/issues/125#issuecomment-3237776191 (opens external site)
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