Notes

Check which features and MCP tools a sipnab build has

Feature updated

How to list the optional features a sipnab binary contains, and how that list decides which MCP tools the binary offers to an AI agent.

In short

A sipnab binary may include or leave out optional parts, called features.

  • sipnab --version, the MCP tool server_capabilities and the REST endpoint GET /v1/capabilities each list the features a binary contains. The three lists come from the same source, so they agree.
  • sipnab’s MCP server offers only the tools the binary can run. A build without the vcon feature does not offer export_vcon or validate_vcon.
  • A build without a feature refuses the command-line flags that need it, and the message names the feature.

This holds from sipnab 0.5.142.

Who this is for

  • Anyone who runs sipnab as an MCP server for an AI agent, especially from a build with a reduced feature set.
  • Anyone who needs to know which features a given sipnab binary has.

What it does

An MCP client is the AI agent’s side of the connection. It asks a server for its tools with a tools/list request. The answer is the whole of what the agent knows about what the server can do.

  • Tools follow the build.
    • sipnab registers export_vcon and validate_vcon only when the binary includes the vcon feature.
    • A build without it leaves both out of tools/list. A call to either one gets the MCP error tool not found.
  • Every surface names the same features.
    • --version prints them in build order.
    • server_capabilities returns them, in alphabetical order, in its features field.
    • GET /v1/capabilities on the REST API serves the same list.

Check the features of a binary

Run:

sipnab --version

sipnab 0.5.222 (16f7b0ea) features: native,tui,audio,tls,hep,api,mcp,mcp-http,metrics,wasm,plugins,bpf,vcon,opus,archive,g729
libpcap version 1.10.4 (with TPACKET_V3); alternate capture backends named: none

Check the features over MCP

This command runs from the root of a sipnab checkout. It sends the MCP handshake and one server_capabilities call on standard input, and uses jq to print the features field:

printf '%s\n' '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"demo","version":"1"}}}' '{"jsonrpc":"2.0","method":"notifications/initialized"}' '{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"server_capabilities","arguments":{}}}' | sipnab --mcp -N --quiet -I tests/pcap-samples/sip-488-codec-reject.pcapng 2>/dev/null | jq -c 'select(.id==2) | .result.structuredContent.features'

["api","archive","audio","bpf","g729","hep","mcp","mcp-http","metrics","native","opus","plugins","tls","tui","vcon","wasm"]

vcon is in the list, so tools/list on this binary includes the two vCon tools:

printf '%s\n' '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"demo","version":"1"}}}' '{"jsonrpc":"2.0","method":"notifications/initialized"}' '{"jsonrpc":"2.0","id":2,"method":"tools/list"}' | sipnab --mcp -N --quiet -I tests/pcap-samples/sip-488-codec-reject.pcapng 2>/dev/null | jq -r 'select(.id==2) | .result.tools[].name | select(. == "export_vcon" or . == "validate_vcon")'

export_vcon
validate_vcon

On a build without vcon, the same command prints nothing.

Read the refusal from a build without a feature

A build made with the default features has no vcon, mcp, hep, tls or api. Its --version line reads:

sipnab 0.5.222 (16f7b0ea) features: native,tui,audio,metrics

On that build, a vCon export flag stops the run with exit status 2:

sipnab -N -q -I tests/pcap-samples/sip-488-codec-reject.pcapng --export-vcon-when "response_code == 488" --export-vcon-dir vcons --no-cli-print

2026-10-16T11:48:08.362440Z ERROR sipnab: --export-vcon-when, --export-vcon-dir need the 'vcon' Cargo feature, which this build does not carry. Rebuild with --features vcon (or --features full); `sipnab --version` lists the features a binary was built with

--mcp on the same build stops with:

2026-10-16T11:49:14.538075Z ERROR sipnab::app::bootstrap: --mcp requires the 'mcp' feature (not compiled in)

To get the feature, install a build that carries it, or build sipnab with --features full. The Feature flags table lists what each feature adds.

Limits

  • Build features and the --mcp-tools option are separate.
    • --mcp-tools selects which registered tools the server offers. An example is only the vcon bundle.
    • It does not change which features the binary contains, and it cannot add a tool whose feature the build lacks.
  • Before sipnab 0.5.142, a build without vcon listed export_vcon and validate_vcon and refused every call to them, and server_capabilities did not name vcon, bpf or wasm. Upgrade to get an accurate list.

Terms used here

  • MCP: Model Context Protocol, the interface sipnab offers to AI agents.
  • Tool: one operation an MCP server offers, such as export_vcon.
  • tools/list: the MCP request a client sends to learn which tools a server offers.
  • Feature: an optional part of sipnab that a build includes or leaves out.
  • Build: one compiled sipnab binary with a particular set of features.
  • vCon: a standard JSON format for a record of a conversation.
  • Exit status: the number a program returns when it ends. 0 means success.
  • jq: a command-line program that reads JSON and prints selected fields.