ACP Inspector

ACP Inspector is an F# implementation of the Agent Client Protocol (ACP) plus a validation / sentinel layer.

It aims to give you:

• Protocol holon: pure F# types and rules that model "what ACP means".
• Runtime holon: transports, clients, and agents (IO, processes, stdio wiring).
• Sentinel holon: stateful validation and assurance over ACP traffic.

Normative behavior follows the published ACP spec and schema:

• Spec source of truth (GitHub): https://github.com/agentclientprotocol/agent-client-protocol
• Overview/intro (website): https://agentclientprotocol.com/overview/introduction

Implementation targets:

• ACP schema: Acp.Domain.Spec.Schema (currently 0.10.5)
• Negotiated major protocolVersion: Acp.Domain.PrimitivesAndParties.ProtocolVersion.current (currently 1)

This repo adds an architecture-first view on assurance and observability.

Naming: Product = ACP Inspector, repo = ACP-inspector, CLI tool = acp-inspector, SDK package = ACP.Inspector.

What is ACP?

ACP (Agent Client Protocol) is a JSON-RPC 2.0 protocol that defines how a client and an agent establish a session, exchange prompt turns, call tools, and stream updates. The spec is versioned and evolves; ACP Inspector targets the published schema and keeps draft features behind a gate.

Problem Statement

ACP implementations can drift or accept malformed messages, making production debugging and compliance hard. Teams need a way to validate ACP traffic at boundaries and inspect traces without building their own protocol model, state machine, and validation logic. ACP Inspector provides the typed core, runtime adapters, and a CLI inspector to close that gap.

3-line CLI Quick Start

dotnet build cli/apps/ACP.Cli/ACP.Cli.fsproj -c Release
dotnet run --project cli/apps/ACP.Cli -- inspect trace.jsonl
dotnet run --project cli/apps/ACP.Cli -- --help

Quick Start Commands

Unified CLI Tool

The ACP Inspector CLI (acp-inspector) provides a unified interface for ACP protocol inspection, validation, replay, analysis, and benchmarking:

# Build the CLI
dotnet build cli/apps/ACP.Cli/ACP.Cli.fsproj -c Release

# Inspect trace files (full validation with colored output)
dotnet run --project cli/apps/ACP.Cli -- inspect trace.jsonl
dotnet run --project cli/apps/ACP.Cli -- inspect --raw trace.jsonl  # Raw JSON output

# Validate messages from stdin (real-time validation)
cat messages.json | dotnet run --project cli/apps/ACP.Cli -- validate --direction c2a

# Interactive replay (step through traces)
dotnet run --project cli/apps/ACP.Cli -- replay --interactive trace.jsonl
dotnet run --project cli/apps/ACP.Cli -- replay --stop-at 5 trace.jsonl  # Stop at frame 5

# Statistical analysis
dotnet run --project cli/apps/ACP.Cli -- analyze trace.jsonl

# Benchmark performance
dotnet run --project cli/apps/ACP.Cli -- benchmark --mode throughput --count 1000
dotnet run --project cli/apps/ACP.Cli -- benchmark --mode cold-start
dotnet run --project cli/apps/ACP.Cli -- benchmark --mode roundtrip
dotnet run --project cli/apps/ACP.Cli -- benchmark --mode codec
dotnet run --project cli/apps/ACP.Cli -- benchmark --mode tokens --tokens 1000000
dotnet run --project cli/apps/ACP.Cli -- benchmark --mode raw-json

# Show version
dotnet run --project cli/apps/ACP.Cli -- --version

# Get help
dotnet run --project cli/apps/ACP.Cli -- --help
dotnet run --project cli/apps/ACP.Cli -- inspect --help
dotnet run --project cli/apps/ACP.Cli -- validate --help
dotnet run --project cli/apps/ACP.Cli -- replay --help
dotnet run --project cli/apps/ACP.Cli -- analyze --help
dotnet run --project cli/apps/ACP.Cli -- benchmark --help

Commands Overview:

• inspect - Full Inspector validation from JSONL trace files with detailed findings
• validate - Real-time validation of ACP messages from stdin
• replay - Interactive stepping through trace files with validation
• analyze - Statistical analysis: method counts, timing, session stats
• benchmark - Performance testing: cold-start, throughput, codec, tokens

Build SDK

dotnet build runtime/src/ACP.fsproj

Run All Tests

dotnet test sentinel/tests/ACP.Tests.fsproj -c Release

Run Specific Test Category

# Protocol/runtime/sentinel tests
dotnet test sentinel/tests/ACP.Tests.fsproj --filter "FullyQualifiedName~TraceReplay"

# SDK component tests
dotnet test sentinel/tests/ACP.Tests.fsproj --filter "FullyQualifiedName~Transport|FullyQualifiedName~Connection|FullyQualifiedName~SessionState|FullyQualifiedName~ToolCalls|FullyQualifiedName~Permissions"

Format Code with Fantomas

# Format all code
dotnet tool restore && dotnet fantomas protocol/src runtime/src sentinel/src sentinel/tests cli/src cli/apps

# Check formatting only
dotnet tool restore && dotnet fantomas protocol/src runtime/src sentinel/src sentinel/tests cli/src cli/apps --check

Demo (CLI sample trace)

Run a full end-to-end demo with the bundled trace files:

dotnet build cli/apps/ACP.Cli/ACP.Cli.fsproj -c Release
dotnet run --project cli/apps/ACP.Cli -c Release -- inspect cli/examples/cli-demo/demo-session.jsonl
dotnet run --project cli/apps/ACP.Cli -c Release -- replay --stop-at 5 cli/examples/cli-demo/demo-session.jsonl
dotnet run --project cli/apps/ACP.Cli -c Release -- analyze cli/examples/cli-demo/demo-session.jsonl
cat cli/examples/cli-demo/single-message.json | dotnet run --project cli/apps/ACP.Cli -c Release -- validate --direction c2a

Or run the same checks via script:

bash cli/scripts/cli-smoke.sh

Documentation

• Tooling (paths, build/run commands): docs/tooling/acp-inspector.md
• FPF (external): https://github.com/ailev/FPF — Note: FPF is fetched daily to /tmp/FPF-YYYYMMDD for evaluation. No local copy is maintained in this repo.
• FPF Alignment: docs/reports/fpf-alignment-evaluation-20260106.md
• SDK docs entrypoint: docs/index.md
• SDK comparison: docs/SDK-COMPARISON.md
• ACP RFD tracker: docs/ACP-RFD-TRACKER.md
• Glossary: docs/glossary.md
• Examples: cli/examples/

Glossary

• ACP: Agent Client Protocol, a JSON-RPC 2.0 protocol for client/agent sessions, prompts, tool calls, and streaming updates.
• Inspector: CLI mode that reads ACP traces and reports validation findings.
• Sentinel: Validation layer that checks ACP traffic and emits findings.
• Lane: A validation category used to group findings.
• Finding: A validation result with severity and context.
• Holon: A coarse-grained layer in this repo (protocol, runtime, sentinel).

Who benefits (and how)

• Agent platform teams — drop-in ACP reference that keeps protocol drift low while you ship fast; the sentinel layer catches malformed turns before they hit production.
• Runtime integrators — Acp.RuntimeAdapter bridges ACP to your process/stdio boundary;
• Risk, SRE, and governance — validation lanes plus golden tests (sentinel/tests/) give repeatable evidence for change control, regressions, and incident postmortems.
• Enterprise engineering & compliance — typed protocol core + auditable validation findings reduce vendor risk, ease security reviews, and support regulated change windows.
• Applied AI researchers & prototypers — a fully typed F# core and UTS let you explore ACP variants with safety rails and auditable deductions.

Typical scenarios

• You need to enforce ACP correctness at the IO boundary of an LLM agent runner and want ready-made validateInbound / validateOutbound gates.
• You're adding a new tool/capability to an ACP agent and want protocol-safe fixtures plus validation findings instead of hand-rolled checks.
• You're mirroring ACP into another language/runtime and need a canonical model + tests to prevent semantic drift.
• You're onboarding teammates or stakeholders and need short, high-signal explainers that match the implementation.

60-second getting started

1. Prereqs: .NET 10 SDK. From repo root: dotnet build sentinel/src/ACP.fsproj (restores + builds).
2. Quick probe via F# Interactive (from sentinel/src/ after build):

#r "bin/Debug/net10.0/ACP.Inspector.dll" // built output of sentinel/src/ACP.fsproj
open Acp
open Acp.Domain
open Acp.Domain.PrimitivesAndParties
open Acp.Domain.Capabilities
open Acp.Domain.Messaging
open Acp.RuntimeAdapter

let session = SessionId "demo-001"

let inbound =
    RuntimeAdapter.validateInbound session None
        { rawByteLength = None
          message =
            Message.FromClient(
                ClientToAgentMessage.Initialize
                    { protocolVersion = ProtocolVersion.current
                      clientCapabilities =
                        { fs = { readTextFile = true; writeTextFile = false }
                          terminal = false }
                      clientInfo = None }) }
        false

let outbound =
    RuntimeAdapter.validateOutbound session None
        { rawByteLength = None
          message =
            Message.FromAgent(
                AgentToClientMessage.InitializeResult
                    { protocolVersion = ProtocolVersion.current
                      agentCapabilities =
                        { loadSession = true
                          mcpCapabilities = { http = false; sse = false }
                          promptCapabilities = { audio = false; image = false; embeddedContext = false } }
                      agentInfo = None
                      authMethods = [] }) }
        false

printfn "Inbound findings: %A" inbound.findings
printfn "Outbound findings: %A" outbound.findings

3. Want to wire it into a runtime?

Tooling Reference (Holon 2)

• protocol/src/Acp.Domain.fs — F# domain model: protocol versions, sessions, capabilities, messages, tool calls, etc.
• protocol/src/Acp.Protocol.fs — protocol state machine (initialize → sessions → prompt turns → updates → cancel).
• sentinel/src/Acp.Validation.fs — validation lanes/findings, protocol-error bridge, runWithValidation helper.
• sentinel/src/Acp.RuntimeAdapter.fs — runtime boundary: validateInbound/validateOutbound with profile-aware checks.
• sentinel/tests/ — protocol/runtime/sentinel tests

SDK Modules

The SDK provides runtime components for building ACP clients and agents, comparable to the official TypeScript and Python SDKs.

Transport Layer (Acp.Transport)

Abstractions for bidirectional communication:

open Acp.Transport

// In-memory transport for testing
let transport = MemoryTransport()
do! transport.SendAsync("message")
let! msg = transport.ReceiveAsync()

// Duplex pair for client-agent testing
let clientTransport, agentTransport = DuplexTransport.CreatePair()

// Stdio transport for process communication
let stdioTransport = StdioTransport(stdin, stdout)

Connection Layer (Acp.Connection)

High-level client and agent connections with JSON-RPC correlation:

open Acp.Connection

// Client side
let client = ClientConnection(transport)
let! initResult = client.InitializeAsync({ clientName = "MyClient"; clientVersion = "1.0" })
let! sessionResult = client.NewSessionAsync({})
let! promptResult = client.PromptAsync({ sessionId = sid; prompt = [...]; expectedTurnId = None })

// Agent side
let handlers = {
    onInitialize = fun p -> task { return Ok { agentName = "MyAgent"; agentVersion = "1.0" } }
    onNewSession = fun p -> task { return Ok { sessionId = ...; modes = [...]; currentModeId = ... } }
    onPrompt = fun p -> task { return Ok { sessionId = p.sessionId; outputTurnId = ... } }
    onCancel = fun p -> Task.FromResult()
    onSetMode = fun p -> task { return Ok { currentModeId = p.modeId } }
}
let agent = AgentConnection(transport, handlers)
let! _ = agent.StartListening()

Session State (Acp.Contrib.SessionState)

Accumulates streaming session notifications into coherent snapshots:

open Acp.Contrib.SessionState

let accumulator = SessionAccumulator()

// Subscribe to updates
let unsubscribe = accumulator.Subscribe(fun snapshot notification ->
    printfn "Tool calls: %d" snapshot.toolCalls.Count)

// Apply notifications as they arrive
let snapshot = accumulator.Apply(notification)

// Access current state
let toolCalls = snapshot.toolCalls
let currentMode = snapshot.currentModeId
let messages = snapshot.agentMessages

Tool Call Tracking (Acp.Contrib.ToolCalls)

Tracks tool call lifecycle with status filtering:

open Acp.Contrib.ToolCalls

let tracker = ToolCallTracker()

// Apply session updates
tracker.Apply(notification)

// Query by status
let pending = tracker.Pending()
let inProgress = tracker.InProgress()
let completed = tracker.Completed()
let failed = tracker.Failed()

// Check if work is ongoing
if tracker.HasInProgress() then ...

// Get specific tool call
match tracker.TryGet("tool-call-id") with
| Some tc -> printfn "Status: %A" tc.status
| None -> ()

Permission Handling (Acp.Contrib.Permissions)

Manages permission requests with auto-response rules:

open Acp.Contrib.Permissions

let broker = PermissionBroker()

// Add auto-response rules
broker.AddAutoRule(fun req ->
    req.toolCall.kind = Some ToolKind.Read, "allow-always") |> ignore

// Enqueue permission request
let requestId = broker.Enqueue(permissionRequest)

// Respond manually
match broker.Respond(requestId, "allow-once") with
| Ok response -> printfn "Allowed"
| Error e -> printfn "Error: %s" e

// Or wait async
let! outcome = broker.WaitForResponseAsync(requestId)

// Check pending
for pending in broker.PendingRequests() do
    printfn "Pending: %s" pending.request.toolCall.title

Examples

See the cli/examples/ directory for complete integration demos:

• BasicClientAgent.fsx - Client-agent communication
• SessionStateTracking.fsx - Notification accumulation
• ToolCallTracking.fsx - Tool call lifecycle
• PermissionHandling.fsx - Permission request/response
• FullIntegration.fsx - Complete SDK integration

---

How to work in this repo

• Treat the ACP spec as normative. If this repo and the published spec disagree, the spec wins; open an issue and tag the discrepancy.

  • Spec source of truth (GitHub): https://github.com/agentclientprotocol/agent-client-protocol
  • Overview/intro (website): https://agentclientprotocol.com/overview/introduction

• Keep holons separate. Avoid mixing protocol types, runtime IO concerns, and sentinel rules in the same module.

• Prefer idiomatic F#. Use discriminated unions and records, composition over inheritance, and Result-based error handling.

• Follow the F# style guide. See STYLEGUIDE.md for the canonical conventions used in this repo.

• Format with Fantomas. This repo uses Fantomas as the formatter (and as a formatting "linter" in --check mode):

  • Format: dotnet tool restore && dotnet fantomas protocol/src runtime/src sentinel/src sentinel/tests cli/src cli/apps
  • Check only: dotnet tool restore && dotnet fantomas protocol/src runtime/src sentinel/src sentinel/tests cli/src cli/apps --check
  • Note: sentinel/tests/golden/ is ignored via .fantomasignore (it contains intentionally-invalid F# samples).

• Optional: enable repo git hooks. This repo includes .githooks/pre-commit (Fantomas auto-format + restage) and .githooks/pre-push (blocks direct pushes to master/main):

  • One-time setup: git config core.hooksPath .githooks

• No Python in this repo. Do not add Python source/config/scripts; this project is .NET-only. (Enforced by tests.)

• Document spec grey areas. When the ACP spec is ambiguous, document assumptions in comments and mark them for later verification.