feat: add optional Langfuse tracing for MCP tool calls

[adalton]

Summary

• Add Langfuse tracing integration to the mcp-acp server (RHOAIENG-48700)
• Every MCP tool call produces a Langfuse trace with tool name, filtered args, duration, and status
• HTTP requests to the ACP public API appear as nested child spans with method, path, and status code
• Silently no-ops when Langfuse credentials are not configured or MCP_ACP_TRACING_ENABLED=false

Changes

File	What
pyproject.toml	Add langfuse>=2.0.0 dependency
src/mcp_acp/tracing.py	New — Langfuse v3 SDK client singleton, trace_tool_call + trace_http_request async context managers, no-op fallbacks, flush/shutdown lifecycle
src/mcp_acp/server.py	Wrap call_tool() in trace_tool_call, flush tracing on shutdown
src/mcp_acp/client.py	Wrap _request() and _request_text() in trace_http_request
src/mcp_acp/settings.py	Add missing _acpctl_config_path() (pre-existing bug fix)
tests/test_tracing.py	New — 25 unit tests covering all tracing paths
scripts/test_tracing_e2e.py	New — E2E test script against live ACP cluster

Configuration

Opt-in via environment variables (Langfuse SDK reads these automatically):

export LANGFUSE_PUBLIC_KEY="pk-lf-..."
export LANGFUSE_SECRET_KEY="sk-lf-..."
export LANGFUSE_BASE_URL="https://us.cloud.langfuse.com"

Kill switch: MCP_ACP_TRACING_ENABLED=false disables tracing even with credentials present.

Test plan

• 25 unit tests pass (uv run pytest tests/test_tracing.py)
• All 53 existing tests pass (uv run pytest tests/)
• Ruff lint/format clean
• E2E verified against live kind cluster with Langfuse Cloud — traces visible in dashboard
• Verified silent no-op when credentials are not set
• Verified kill switch disables tracing with credentials present
• Verify CI passes

🤖 Generated with Claude Code

[adalton]

Testing

  ============================================================
  MCP-ACP Tracing End-to-End Test
  ============================================================
    Langfuse credentials: configured
    Tracing enabled:      True
    Kill switch:          MCP_ACP_TRACING_ENABLED=true

  [OK] MCP server started and initialized
  [OK] list_tools: 26 tools available

  --- Tool: acp_whoami ---
    Response: Configuration Status:

  Token Configured: Yes
  Cluster: kind
  Server: http://172.19.0.2:31648
  Project: ambient-code


  --- Tool: acp_list_clusters ---
    Response: Configured Clusters (default: kind):

  - kind [DEFAULT]
    Server: http://172.19.0.2:31648
    Description: Local kind development cluster
    Default Project: ambient-code



  --- Tool: acp_list_sessions ---
    Response: Found 0 session(s)

  Sessions:


  --- Tool: acp_get_session (expect error) ---
  {"method": "GET", "path": "/v1/sessions/nonexistent-session-xyz", "status_code": 404, "error": "Session not found", "event": "api_request_failed", "logger":
  "mcp_acp.client", "level": "warning", "timestamp": "2026-03-05T20:14:42.306721Z"}
  {"tool": "acp_get_session", "elapsed_seconds": 0.01, "error": "Session not found", "event": "tool_validation_error", "logger": "__main__", "level": "warning",
  "timestamp": "2026-03-05T20:14:42.306905Z"}
    Response: Validation Error: Session not found
    Got expected error: True

  --- Tool: acp_create_session (dry_run) ---
    Response: DRY RUN MODE - No changes made

  Would create session with custom prompt

  Manifest:
  {
    "task": "Hello from tracing e2e test",
    "model": "claude-sonnet-4",
    "displayName": "tracing-e2e-test"
  }

  --- Tool: acp_bulk_delete_sessions (expect validation error) ---
  {"tool": "acp_bulk_delete_sessions", "elapsed_seconds": 0.0, "error": "Bulk delete sessions requires confirm=true. Use dry_run=true to preview first.", "event":
  "tool_validation_error", "logger": "__main__", "level": "warning", "timestamp": "2026-03-05T20:14:42.315672Z"}
    Response: Validation Error: Bulk delete sessions requires confirm=true. Use dry_run=true to preview first.
    Got expected validation error: True

  ============================================================
  RESULTS SUMMARY
  ============================================================
    [PASS] list_tools
    [PASS] acp_whoami
    [PASS] acp_list_clusters
    [PASS] acp_list_sessions
    [PASS] acp_get_session_error
    [PASS] acp_create_session_dry_run
    [PASS] acp_bulk_validation_error

    7/7 tests passed

    Check your Langfuse dashboard for traces:
      https://us.cloud.langfuse.com
    Expected traces:
      - acp_whoami
      - acp_list_clusters
      - acp_list_sessions        (with child span: GET /v1/sessions)
      - acp_get_session          (with error status)
      - acp_create_session       (dry_run, no HTTP span)
      - acp_bulk_delete_sessions (validation_error, no HTTP span)