fix(mcp): close existing client before reassignment to prevent leaks

[sauerdaniel]

Summary

Fix memory leak in MCP client management where existing clients are orphaned when replaced.

Fixes #8257
Relates to #7261
Relates to #3013

Problem

In MCP.add() and the OAuth finishAuth() flow, when a new MCP client is created and assigned to s.clients[name], any existing client at that key is silently overwritten:

// Before (leaks existing client)
s.clients[name] = result.mcpClient

The old client is never closed, leaving:

• Open connections/sockets
• Active event listeners
• Unreleased resources

This happens when:

• Reconnecting to an MCP server
• Completing OAuth flow for an already-connected server
• Re-initializing MCP configuration

Solution

Check for and close existing clients before reassignment in both locations:

// Close existing client if present to prevent memory leaks
const existingClient = s.clients[name]
if (existingClient) {
  await existingClient.close().catch((error) => {
    log.error("Failed to close existing MCP client", { name, error })
  })
}
s.clients[name] = result.mcpClient

Changes

• packages/opencode/src/mcp/index.ts: Added client cleanup in add() function
• packages/opencode/src/mcp/index.ts: Added client cleanup in finishAuth() flow

Testing

• ✅ Build passes for all platforms
• ✅ Full test suite passes (652 tests, 0 failures)
• Code-review verified to follow correct resource cleanup pattern

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

The search results show that PR #8253 (the current PR) is the only one directly addressing this specific issue. PR #7424 appears in the results as related work (force kill MCP processes), which the current PR already acknowledges as complementary.

No duplicate PRs found

[sauerdaniel]

Duplicate Analysis

The bot mentioned PR #7424 as potentially related since both modify mcp/index.ts. After investigation:

PR	What it does	Layer
#7424	Adds forceKillProcess() to kill orphaned MCP server child processes that don't terminate gracefully	OS process cleanup
#8253	Closes existing MCP client objects before reassigning s.clients[name]	JavaScript object cleanup

Verdict: Complementary, not duplicate.

These PRs address different cleanup layers:

• fix: force kill MCP server processes on dispose to prevent orphan processes #7424 handles the case where MCP server processes become orphaned at the OS level and need to be force-killed
• fix(mcp): close existing client before reassignment to prevent leaks #8253 handles the case where MCP client objects in JavaScript are overwritten without closing them first, leaking the previous client instance

Both fixes are valuable:

1. When add() or finishAuth() is called for an existing MCP name, the old client reference gets overwritten
2. Without fix(mcp): close existing client before reassignment to prevent leaks #8253, the old client object leaks (JS memory)
3. Without fix: force kill MCP server processes on dispose to prevent orphan processes #7424, if that client's server process doesn't terminate, it becomes a zombie (OS resources)

The fixes are orthogonal and both contribute to complete MCP resource cleanup.