feat: add mcp_config input variable to vscode-desktop-core module (#753)

## Description

Standardizes handling of `mcp` variables in VSCode Desktop-based
modules.
Made modular enough to pave the way for setting other config files than
`mcp_server.json` and `mcp.json`.

## Type of Change

- [ ] New module
- [ ] New template
- [ ] Bug fix
- [x] Feature/enhancement
- [ ] Documentation
- [ ] Other

## Testing & Validation

- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun fmt`)
- [x] Changes tested locally

---------

Co-authored-by: DevCats <christofer@coder.com>
Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

registry/coder/modules/vscode-desktop-core/README.md

@@ -16,7 +16,7 @@ The VSCode Desktop Core module is a building block for modules that need to expo
 ```tf
 module "vscode-desktop-core" {
   source  = "registry.coder.com/coder/vscode-desktop-core/coder"
-  version = "1.0.2"
+  version = "1.1.0"
 
   agent_id = var.agent_id
 

registry/coder/modules/vscode-desktop-core/main.test.ts

@@ -3,6 +3,11 @@ import {
   runTerraformApply,
   runTerraformInit,
   testRequiredVariables,
+  runContainer,
+  execContainer,
+  removeContainer,
+  findResourceInstance,
+  readFileContainer,
 } from "~test";
 
 // hardcoded coder_app name in main.tf
@@ -16,6 +21,7 @@ const defaultVariables = {
   coder_app_display_name: "VS Code Desktop",
 
   protocol: "vscode",
+  config_dir: "$HOME/.vscode",
 };
 
 describe("vscode-desktop-core", async () => {
@@ -134,4 +140,41 @@ describe("vscode-desktop-core", async () => {
       expect(coder_app?.instances[0].attributes.group).toBe("web-app-group");
     });
   });
+
+  it("writes mcp_config.json when mcp_config variable provided", async () => {
+    const id = await runContainer("alpine");
+
+    try {
+      const mcp_config = JSON.stringify({
+        servers: { demo: { url: "http://localhost:1234" } },
+      });
+
+      const state = await runTerraformApply(import.meta.dir, {
+        ...defaultVariables,
+
+        mcp_config,
+      });
+
+      const script = findResourceInstance(
+        state,
+        "coder_script",
+        "vscode-desktop-mcp",
+      ).script;
+
+      const resp = await execContainer(id, ["sh", "-c", script]);
+      if (resp.exitCode !== 0) {
+        console.log(resp.stdout);
+        console.log(resp.stderr);
+      }
+      expect(resp.exitCode).toBe(0);
+
+      const content = await readFileContainer(
+        id,
+        `${defaultVariables.config_dir.replace("$HOME", "/root")}/mcp_config.json`,
+      );
+      expect(content).toBe(mcp_config);
+    } finally {
+      await removeContainer(id);
+    }
+  }, 10000);
 });

registry/coder/modules/vscode-desktop-core/main.tf

@@ -26,11 +26,22 @@ variable "open_recent" {
   default     = false
 }
 
+variable "mcp_config" {
+  type        = map(any)
+  description = "MCP server configuration for the IDE. When set, writes mcp_config.json in var.config_dir."
+  default     = null
+}
+
 variable "protocol" {
   type        = string
   description = "The URI protocol the IDE."
 }
 
+variable "config_dir" {
+  type        = string
+  description = "The path of the IDE's configuration folder."
+}
+
 variable "coder_app_icon" {
   type        = string
   description = "The icon of the coder_app."
@@ -85,18 +96,33 @@ resource "coder_app" "vscode-desktop" {
     data.coder_workspace.me.access_url,
     "&token=$SESSION_TOKEN",
   ])
+}
 
-  /*
-    url = join("", [
-    "vscode://coder.coder-remote/open",
-    "?owner=${data.coder_workspace_owner.me.name}",
-    "&workspace=${data.coder_workspace.me.name}",
-    var.folder != "" ? join("", ["&folder=", var.folder]) : "",
-    var.open_recent ? "&openRecent" : "",
-    "&url=${data.coder_workspace.me.access_url}",
-    "&token=$SESSION_TOKEN",
-  ])
-  */
+resource "coder_script" "vscode-desktop-mcp" {
+  agent_id = var.agent_id
+  count    = var.mcp_config != null ? 1 : 0
+
+  icon         = var.coder_app_icon
+  display_name = "${var.coder_app_display_name} MCP"
+
+  run_on_start       = true
+  start_blocks_login = false
+
+  script = <<-EOT
+    #!/bin/sh
+    set -euo pipefail
+
+    IDE_CONFIG_FOLDER="${var.config_dir}"
+    IDE_MCP_CONFIG_PATH="$IDE_CONFIG_FOLDER/mcp_config.json"
+
+    mkdir -p "$IDE_CONFIG_FOLDER"
+
+    echo -n "${base64encode(jsonencode(var.mcp_config))}" | base64 -d > "$IDE_MCP_CONFIG_PATH"
+    chmod 600 "$IDE_MCP_CONFIG_PATH"
+
+    # Cursor/Windsurf use this config instead, no need for chmod as symlinks do not have modes
+    ln -s "$IDE_MCP_CONFIG_PATH" "$IDE_CONFIG_FOLDER/mcp.json"
+  EOT
 }
 
 output "ide_uri" {