v1.0.24 broke GitHub MCP for users that provide their own --mcp-config in claude_args

[awittzb]

Describe the bug

When using v1.0.24 with the Agent SDK path (now the default per #738), multiple --mcp-config flags in claude_args are not properly merged. Only the last --mcp-config value is kept, causing the action's built-in MCP servers (like github_comment) to be lost when a user provides any MCP servers themselves with --mcp-config.

This results in errors like:

Error: No such tool available: mcp__github_comment__update_claude_comment

Root Cause

In base-action/src/parse-sdk-options.ts, the parseClaudeArgsToExtraArgs function overwrites previous --mcp-config values instead of merging them:

// Line 37-42 in parse-sdk-options.ts
if (nextArg && !nextArg.startsWith("--")) {
  // For accumulating flags, join multiple values with commas
  if (ACCUMULATING_FLAGS.has(flag) && result[flag]) {
    result[flag] = `${result[flag]},${nextArg}`;
  } else {
    result[flag] = nextArg;  // <-- BUG: Overwrites previous mcp-config!
  }
}

The ACCUMULATING_FLAGS set only contains allowedTools and disallowedTools (added in PR #719), but not mcp-config.

To Reproduce

1. Use claude-code-action@v1.0.24 (or any version where Agent SDK is default)
2. Configure a workflow with user-provided --mcp-config in claude_args:

   claude_args: |
     --mcp-config /tmp/my-mcp-config.json

3. The action internally prepends its own --mcp-config with github_comment, github_ci, etc.
4. The SDK path parses claude_args and only keeps the last --mcp-config
5. Claude tries to use mcp__github_comment__update_claude_comment but the MCP server was never registered
6. Error: No such tool available: mcp__github_comment__update_claude_comment

Expected behavior

Multiple --mcp-config flags should be merged by combining their mcpServers objects, similar to how --allowedTools flags are accumulated. The CLI path handles this correctly by passing all flags through to the claude CLI.

Evidence from Job Logs

INPUT_CLAUDE_ARGS contains TWO --mcp-config flags:

--mcp-config '{"mcpServers": {"github_comment": {...}}}' ... --mcp-config /tmp/claude-mcp-config.json

But SDK options only shows ONE:

"extraArgs": {
  "mcp-config": "/tmp/claude-mcp-config.json"
}

The action's github_comment MCP server config is lost.

Workaround

1️⃣ Don't use agent SDK

Set USE_AGENT_SDK=false to force the CLI path:

- name: Run Claude Code
  uses: anthropics/claude-code-action@v1
  env:
    USE_AGENT_SDK: "false"
  with:
    # ... inputs

or

2️⃣ Pin the action to v1.0.23:

- name: Run Claude Code
  uses: anthropics/claude-code-action@v1.0.23
  with:
    # ... inputs

Any workflow using custom MCP servers via claude_args with --mcp-config is affected.

API Provider

• Anthropic First-Party API (default)
• AWS Bedrock (likely affected)
• GCP Vertex (likely affected)

Additional context

• Working version: v1.0.23 (CLI path was default)
• Broken version: v1.0.24 (SDK path is now default)
• Introduced by: Commit 79b343c - "feat: Make Agent SDK the default execution path (feat: Make Agent SDK the default execution path #738)"
• Similar fix exists: PR fix: accumulate multiple --allowedTools flags for Agent SDK #719 (05c95ae) fixed the same issue for --allowedTools flags

Proposed Fix

Merge multiple --mcp-config values by combining their mcpServers objects:

// Add special handling for mcp-config in parseClaudeArgsToExtraArgs or parseSdkOptions
function mergeMcpConfigs(configs: string[]): string {
  const merged = { mcpServers: {} };
  for (const config of configs) {
    const parsed = JSON.parse(config);
    Object.assign(merged.mcpServers, parsed.mcpServers || {});
  }
  return JSON.stringify(merged);
}

[ashwin-ant]

@claude can you fix this?

[claude]

Claude finished @ashwin-ant's task in 3m 27s —— View job • claude/issue-745-20251216-2048 • Create PR ➔

---

Fixing MCP Config Merging Issue

• Understand the issue and review affected code
• Implement fix for MCP config merging
• Add tests for the fix
• Run tests and verify fix
• Commit and push changes

Summary: Fixed the bug where multiple --mcp-config flags were overwriting each other instead of being merged.

Changes made to base-action/src/parse-sdk-options.ts:

• Added mcp-config to ACCUMULATING_FLAGS so multiple values are collected
• Changed delimiter from comma to null character (\x00) to avoid conflicts with JSON content in MCP configs
• Added mergeMcpConfigs() function that merges mcpServers objects from multiple inline JSON configs
• Updated parseSdkOptions() to merge MCP configs when multiple values are present

Tests added to base-action/test/parse-sdk-options.test.ts:

• Single mcp-config passthrough
• Multiple mcp-config flags merging
• Three mcp-config flags merging
• File path handling
• Mixed inline JSON and file paths
• Real-world scenario (action config + user config)

|

[ashwin-ant]

Just released https://github.com/anthropics/claude-code-action/releases/tag/v1.0.25, thank you for flagging!