feat: add mcp_config input that merges with existing mcp server

- Add mcp_config input parameter to action.yml 
- Modify prepareMcpConfig() to accept and merge additional config
- Provided config overrides built-in servers in case of naming collisions
- Pass MCP_CONFIG environment variable from action to prepare step

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

action.yml

@@ -39,6 +39,10 @@ inputs:
     description: "Direct instruction for Claude (bypasses normal trigger detection)"
     required: false
     default: ""
+  mcp_config:
+    description: "Additional MCP configuration (JSON string) that merges with the built-in GitHub MCP servers"
+    required: false
+    default: ""
 
   # Auth configuration
   anthropic_api_key:
@@ -92,6 +96,7 @@ runs:
         ALLOWED_TOOLS: ${{ inputs.allowed_tools }}
         CUSTOM_INSTRUCTIONS: ${{ inputs.custom_instructions }}
         DIRECT_PROMPT: ${{ inputs.direct_prompt }}
+        MCP_CONFIG: ${{ inputs.mcp_config }}
         OVERRIDE_GITHUB_TOKEN: ${{ inputs.github_token }}
         GITHUB_RUN_ID: ${{ github.run_id }}
 

src/entrypoints/prepare.ts

@@ -84,11 +84,13 @@ async function run() {
     );
 
     // Step 11: Get MCP configuration
+    const additionalMcpConfig = process.env.MCP_CONFIG || "";
     const mcpConfig = await prepareMcpConfig(
       githubToken,
       context.repository.owner,
       context.repository.repo,
       branchInfo.currentBranch,
+      additionalMcpConfig,
     );
     core.setOutput("mcp_config", mcpConfig);
   } catch (error) {

src/mcp/install-mcp-server.ts

@@ -5,9 +5,10 @@ export async function prepareMcpConfig(
   owner: string,
   repo: string,
   branch: string,
+  additionalMcpConfig?: string,
 ): Promise<string> {
   try {
-    const mcpConfig = {
+    const baseMcpConfig = {
       mcpServers: {
         github: {
           command: "docker",
@@ -40,7 +41,35 @@ export async function prepareMcpConfig(
       },
     };
 
-    return JSON.stringify(mcpConfig, null, 2);
+    // Merge with additional MCP config if provided
+    if (additionalMcpConfig && additionalMcpConfig.trim()) {
+      try {
+        const additionalConfig = JSON.parse(additionalMcpConfig);
+
+        // Merge mcpServers objects, with additional config overriding base config
+        if (additionalConfig.mcpServers) {
+          baseMcpConfig.mcpServers = {
+            ...baseMcpConfig.mcpServers,
+            ...additionalConfig.mcpServers,
+          };
+        }
+
+        // Merge any other top-level properties from additional config
+        const mergedConfig = {
+          ...baseMcpConfig,
+          ...additionalConfig,
+          mcpServers: baseMcpConfig.mcpServers, // Ensure mcpServers uses the merged version
+        };
+
+        return JSON.stringify(mergedConfig, null, 2);
+      } catch (parseError) {
+        core.warning(
+          `Failed to parse additional MCP config: ${parseError}. Using base config only.`,
+        );
+      }
+    }
+
+    return JSON.stringify(baseMcpConfig, null, 2);
   } catch (error) {
     core.setFailed(`Install MCP server failed with error: ${error}`);
     process.exit(1);