[MCP] Added support for --mcp-stdio flag to dab start

[anushakolan]

Why make this change?

• Add MCP stdio support to Data API Builder and wire it through both the engine and CLI so DAB can be used as a Model Context Protocol (MCP) server.
• Ensures MCP sessions can run under a specific DAB authorization role, making it possible to test and use MCP tools with permissions from dab-config.json.

What is this change?

Service entrypoint

• Detects --mcp-stdio early, configures stdin/stdout encodings, and redirects all non‑MCP output to STDERR to keep STDOUT clean for MCP JSON.
• Parses an optional role:<name> argument (e.g. role:anonymous, role:authenticated) and injects it into configuration as MCP:Role, defaulting to anonymous when omitted.
• In MCP stdio mode, forces Runtime:Host:Authentication:Provider = "Simulator" via in‑memory configuration so the requested role is always available during MCP sessions.
• Starts the full ASP.NET Core host, registers all MCP tools from DI, and runs the MCP stdio loop instead of the normal HTTP host.Run().

CLI Integration

• Adds --mcp-stdio to dab start to launch the engine in MCP stdio mode.
• Adds an optional positional role argument (e.g. role:anonymous) captured as StartOptions.McpRole.
• Keeps existing behavior for non‑MCP dab start unchanged.

Note

• ExecuteEntityTool now looks for MCP tool inputs under arguments (the standard MCP field) and falls back to the legacy parameters property only if arguments is missing. This aligns our server with how current MCP clients (like VS Code) actually send tool arguments, and preserves backward compatibility for any older clients that still use parameters.

How was this tested?

Integration-like manual testing via MCP clients against:

• Engine-based MCP server: dotnet Azure.DataApiBuilder.Service.dll --mcp-stdio role:authenticated.
• CLI-based MCP server: dab start --mcp-stdio role:authenticated.

Manual verification of all MCP tools:

• describe_entities shows correct entities and effective permissions for the active role.
• read_records, create_record, update_record, delete_record, execute_entity succeed when the role has the appropriate permissions.

Sample Request(s)

1. MCP server via CLI (dab)

{ "mcpServers": { "dab-with-exe": { "command": "C:\\DAB\\data-api-builder\\out\\publish\\Debug\\net8.0\\win-x64\\dab\\Microsoft.DataApiBuilder.exe", "args": ["start", "--mcp-stdio", "role:authenticated", "--config", "C:\\DAB\\data-api-builder\\dab-config.json"], "env": { "DAB_ENVIRONMENT": "Development" } } }
2. MCP server via engine DLL

{ "mcpServers": { "dab": { "command": "dotnet", "args": [ "C:\\DAB\\data-api-builder\\out\\publish\\Debug\\net8.0\\win-x64\\dab\\Azure.DataApiBuilder.Service.dll", "--mcp-stdio", "role:authenticated", "--config", "C:\\DAB\\data-api-builder\\dab-config.json" ], "type": "stdio" } } }

[anushakolan]

/azp run #Resolved

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

#Resolved

[anushakolan]

/azp run #Resolved

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

#Resolved

[anushakolan]

/azp run #Resolved

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

#Resolved

[azure-pipelines]

No pipelines are associated with this pull request.

#Resolved

[azure-pipelines]

No pipelines are associated with this pull request.

#Resolved

[azure-pipelines]

No pipelines are associated with this pull request.

#Resolved

[anushakolan]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

[aaronburtle]

Looks good!

[souvikghosh04]

Added some comments to have common role parsing and retrieval logic along with some refactoring/modularization.

[anushakolan]

Added some comments to have common role parsing and retrieval logic along with some refactoring/modularization.

Addressed all comments.

[anushakolan]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

[souvikghosh04]

Thanks for addressing the comments.