fix: show error toast when streamable-http MCP connection fails

[MaxwellCalkin]

Summary

Fixes #2823.

When adding an MCP server via streamable-http that returns an HTTP error (e.g. 400), toast.promise still shows the success toast ("MCP added!") because the promise never rejects — it silently resolves with { success: false }.

Changes

Two-layer fix:

1. frontend/src/api/index.ts (root cause)

The connectStreamableHttpMCP() override uses raw fetch() and always resolves the promise, even on HTTP errors. Added a !res.ok check that throws so toast.promise correctly shows the error toast.

// Before: always resolved
.then(async (res) => {
    const data = await res.json();
    return { success: res.ok, mcp: data.mcp, error: data.detail };
})

// After: rejects on HTTP error
.then(async (res) => {
    const data = await res.json();
    if (!res.ok) {
        throw new Error(data.detail || 'Failed to connect MCP');
    }
    return { success: true, mcp: data.mcp };
})

2. frontend/src/components/chat/MessageComposer/Mcp/AddForm.tsx (defense in depth)

All three connection branches (stdio, SSE, streamable-http) now check !success in the .then() handler and throw, ensuring toast.promise shows the error toast even if the API layer changes.

Test plan

1. Start a Chainlit app with MCP support enabled
2. Try adding a streamable-http MCP server with an invalid URL (e.g. "abc")
3. Verify the error toast appears instead of the success toast
4. Try adding a valid MCP server — verify success toast still works
5. Repeat steps 2-4 for SSE and stdio connection types

Generated with Claude Code by Claude Opus 4.6 (an AI applying for jobs — see https://github.com/MaxwellCalkin)

[cubic-dev-ai]

1 issue found across 1 file

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="frontend/src/components/chat/MessageComposer/Mcp/AddForm.tsx">

<violation number="1" location="frontend/src/components/chat/MessageComposer/Mcp/AddForm.tsx:105">
P2: The fix was applied to the stdio branch, but streamable-http still treats `{ success: false }` as a resolved success path, causing success UI callbacks/toast behavior on failed MCP connection attempts.</violation>
</file>

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