feat: add mcp_config input that merges with existing mcp server (#96)

* 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>

* refactor: improve MCP config validation and merging logic

- Add JSON validation to ensure parsed config is an object
- Simplify merge logic with explicit mcpServers merging
- Enhance error logging with config preview for debugging

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

* refactor: improve MCP config logging per review feedback

- Remove configPreview from error logging to avoid cluttering output
- Add informational log when merging MCP server configurations
- Simplify error message for failed config parsing

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

* test: add comprehensive unit tests for prepareMcpConfig

Add tests covering:
- Basic functionality with no additional config
- Valid JSON merging scenarios
- Invalid JSON handling
- Empty/null config handling
- Server name collision scenarios
- Complex nested configurations
- Environment variable handling

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

* docs: add mcp_config example with sequential-thinking server

- Add mcp_config to inputs table
- Add example section showing how to use mcp_config with sequential-thinking MCP server
- Include clear explanation that custom servers override built-in servers

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

* readme

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

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 +41,39 @@ 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);
+
+        // Validate that parsed JSON is an object
+        if (typeof additionalConfig !== "object" || additionalConfig === null) {
+          throw new Error("MCP config must be a valid JSON object");
+        }
+
+        core.info(
+          "Merging additional MCP server configuration with built-in servers",
+        );
+
+        // Merge configurations with user config overriding built-in servers
+        const mergedConfig = {
+          ...baseMcpConfig,
+          ...additionalConfig,
+          mcpServers: {
+            ...baseMcpConfig.mcpServers,
+            ...additionalConfig.mcpServers,
+          },
+        };
+
+        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);