Fixing up var sub for MCP in CCA (#60015)

Co-authored-by: Joe Clark <31087804+jc-clark@users.noreply.github.com>

Author: belaltaher8

content/copilot/how-tos/use-copilot-agents/coding-agent/extend-coding-agent-with-mcp.md

@@ -81,18 +81,27 @@ The configuration object can contain the following keys:
 **Local MCP specific keys**
 * `command` (`string`): Required. The command to run to start the MCP server.
 * `args` (`string[]`): Required. The arguments to pass to the `command`.
-* `env` (`object`): Optional. The environment variables to pass to the server. This object should map the name of the environment variable that should be exposed to your MCP server to either of the following:
-  * The name of a secret you have configured in your {% data variables.product.prodname_copilot_short %} environment, beginning with `COPILOT_MCP_`.
-  * The name of a variable you have configured in your {% data variables.product.prodname_copilot_short %} environment, beginning with `COPILOT_MCP_`.
+* `env` (`object`): Optional. The environment variables to pass to the server. This object should map the name of the environment variable that should be exposed to your MCP server to one of the following:
+  * A substitution reference to a secret or variable in your {% data variables.product.prodname_copilot_short %} environment, such as `$COPILOT_MCP_API_KEY` or `${COPILOT_MCP_API_KEY}`. Referenced names must start with `COPILOT_MCP_`.
+  * A literal string value.
 
 **Remote MCP specific keys**
 * `url` (`string`): Required. The MCP server's URL.
-* `headers` (`object`): Optional. The headers to attach to requests to the server. This object should map the name of header keys to either of the following:
-  * The name of a secret you have configured in your {% data variables.product.prodname_copilot_short %} environment, beginning with `COPILOT_MCP_` preceded by a `$`.
-  * The name of a variable you have configured in your {% data variables.product.prodname_copilot_short %} environment, beginning with `COPILOT_MCP_` preceded by a `$`.
-  * A string value.
+* `headers` (`object`): Optional. The headers to attach to requests to the server. This object should map the name of header keys to one of the following:
+  * A substitution reference to a secret or variable in your {% data variables.product.prodname_copilot_short %} environment, such as `$COPILOT_MCP_API_KEY` or `${COPILOT_MCP_API_KEY}`. Referenced names must start with `COPILOT_MCP_`.
+  * A literal string value.
 
-Note that all `string` and `string[]` fields besides `tools` & `type` support substitution with a variable or secret you have configured in your {% data variables.product.prodname_copilot_short %} environment, beginning with `COPILOT_MCP_` preceded by a `$`.
+Note that all `string` and `string[]` fields besides `tools` & `type` support substitution with a variable or secret you have configured in your {% data variables.product.prodname_copilot_short %} environment.
+
+### Variable substitution
+
+The following syntax patterns are supported for referencing environment variables configured in your {% data variables.product.prodname_copilot_short %} environment:
+
+| Syntax      | Example                 |
+| ----------- | ------------            |
+| `$VAR`      | `$COPILOT_MCP_API_KEY`  |
+| `${VAR}`    | `${COPILOT_MCP_API_KEY}`|
+| `${VAR:-default}` | `${COPILOT_MCP_API_KEY:-fallback_value}` |
 
 ## Example configurations
 
@@ -121,13 +130,11 @@ The [Sentry MCP server](https://github.com/getsentry/sentry-mcp) gives {% data v
       "args": ["@sentry/mcp-server@latest", "--host=$SENTRY_HOST"],
       "tools": ["get_issue_details", "get_issue_summary"],
       "env": {
-        // We can specify an environment variable value as
-        // a variable in your {% data variables.product.prodname_copilot_short %} environment
-        // where `COPILOT_MCP_SENTRY_HOST` = "https://contoso.sentry.io"...
-        "SENTRY_HOST": "COPILOT_MCP_SENTRY_HOST",
-        // or refer to a secret with a name starting with
-        // `COPILOT_MCP_`.
-        "SENTRY_ACCESS_TOKEN": "COPILOT_MCP_SENTRY_ACCESS_TOKEN"
+        // We can specify an environment variable value as a string...
+        "SENTRY_HOST": "https://contoso.sentry.io",
+        // or refer to a variable or secret in your {% data variables.product.prodname_copilot_short %} environment
+        // with a name starting with `COPILOT_MCP_`
+        "SENTRY_ACCESS_TOKEN": "$COPILOT_MCP_SENTRY_ACCESS_TOKEN"
       }
     }
   }
@@ -158,7 +165,7 @@ The [Notion MCP server](https://github.com/makenotion/notion-mcp-server) gives {
       "env": {
         // The value of the `COPILOT_MCP_NOTION_API_KEY` secret will be passed to the
         // server command as an environment variable called `NOTION_API_KEY`
-        "NOTION_API_KEY": "COPILOT_MCP_NOTION_API_KEY"
+        "NOTION_API_KEY": "$COPILOT_MCP_NOTION_API_KEY"
       },
       "tools": ["*"]
     }

content/copilot/reference/custom-agents-configuration.md

@@ -120,14 +120,14 @@ For compatibility, the `stdio` type used by Claude Code and {% data variables.pr
 
 Both the repository-level MCP JSON configuration and the {% data variables.copilot.copilot_custom_agent_short %} YAML configuration support the following syntax patterns:
 
-* `COPILOT_MCP_ENV_VAR_VALUE` - Environment variable only (not recommended due to header differences)
 * `$COPILOT_MCP_ENV_VAR_VALUE` - Environment variable and header
 * `${COPILOT_MCP_ENV_VAR_VALUE}` - Environment variable and header (Claude Code syntax)
+* `${COPILOT_MCP_ENV_VAR_VALUE:-default}` - Environment variable and header with default
 
 The {% data variables.copilot.copilot_custom_agent_short %} YAML configuration supports the following additional syntax patterns:
 
 * `{% raw %}${{ secrets.COPILOT_MCP_ENV_VAR_VALUE }}{% endraw %}` - Environment variable and header
-* `{% raw %}${{ var.COPILOT_MCP_ENV_VAR_VALUE }}{% endraw %}` - Environment variable and header
+* `{% raw %}${{ vars.COPILOT_MCP_ENV_VAR_VALUE }}{% endraw %}` - Environment variable and header
 
 ## Example {% data variables.copilot.agent_profile %} configurations