Toolwall Proxy Implementation

Overview

The toolwall-proxy crate implements a synchronous, blocking MCP stdio proxy that sits between MCP clients and servers, intercepting and evaluating tool calls against a policy engine.

Philosophy: Correctness and clarity over concurrency. MVP prioritizes boring, reliable, auditable proxying.

Architecture

    Client stdin          Server process            Server stdout
         |                    |                             |
         v                    v                             v
    [Client In] ------> [Proxy Loop] ------> [Server Out] ------> [Server]
                             |
                             |
                        Policy Engine
                        (intercept tools/call)
        Audit Writer <--------+
     (write events)           |
                              v
                         [Client Out]

Key Components

1. Frame (`frame.rs`)

Handles line-delimited JSON-RPC message serialization
JsonRpcFrame: Wraps serde_json::Value with helper methods
FrameReader/FrameWriter: Async I/O wrappers (future-proof but currently unused)
Provides:
- method(): Extract RPC method name
- id(): Extract request ID
- params(): Extract request arguments
- error_response(): Create well-formed JSON-RPC error responses

2. Error (`error.rs`)

ProxyError enum for all proxy-related errors
Maps to standard From implementations for ergonomic error handling
Covers I/O, JSON decode, malformed calls, process spawn, policy evaluation

3. Interceptor (`interceptor.rs`)

intercept_tools_call(): Policy evaluation for tools/call requests
- Validates request structure (params.name, params.arguments)
- Evaluates policy engine for allow/deny/approval
- Returns None if allowed (forward unchanged), Some(error_frame) if denied
extract_tool_list(): Parse tools/list responses
- Extracts tool array from result
- Handles error responses gracefully (returns empty list)

4. Proxy (`proxy.rs`)

McpProxy: Main orchestrator
- Spawns server process with configured command/args
- Implements blocking sync proxy loop
- Intercepts client requests, evaluates policy, forwards or denies
- Writes audit events for all decisions

Proxy Loop Logic:

Loop:
  1. Read line from client stdin
  2. If tools/call request:
     - Evaluate policy
     - If denied: send error response to client, continue
     - If allowed: continue to forward
  3. Forward request to server stdin
  4. Read response from server stdout
  5. Forward response to client stdout
  6. Repeat

Security Properties

1. Fail-Closed on Malformed Requests

Invalid JSON-RPC frames: Parse error → deny with error response
Missing params.name: Error response
Invalid policy effect: Unknown effects treated as deny
Protects against crash attacks or parser exploits

2. Policy Evaluation

Deny beats Allow beats Approval (priority order)
All tool calls evaluated against policy engine rules
Invalid globs in policy cause parse failure (fail-closed at config time)

3. No Secret Leakage

Error messages are generic (e.g., "malformed tool call")
Request/response content never exposed to client
Audit events redacted before writing via AuditWriter

4. Audit Trail

Every tools/call decision logged:
- Allow: tool name, server, decision
- Deny: tool name, server, decision, reason
Immutable append-only JSONL format
Events redacted before writing

Integration

Policy Engine

let policy = Arc::new(PolicyEngine::from_toml_str(policy_str)?);
let result = policy.evaluate(&server_name, &tool_name, &args);

Shared via Arc for safe reference in proxy loop
Evaluation is deterministic and side-effect-free

Audit Writer

let audit = Arc::new(AuditWriter::new(Path::new(".toolwall/audit.jsonl")));
audit.append_event(&event)?;

Redacts secrets before writing
Flushes after each event for durability
Shared via Arc for safe reference in proxy loop

Frame Handling

let frame = JsonRpcFrame::from_str(line)?;
if frame.method() == Some("tools/call") {
    // intercept
}

##Testing

Unit Tests (5 tests)

Test	Coverage
`frame::tests::test_frame_from_string`	JSON-RPC parsing
`frame::tests::test_error_response`	Error frame creation
`interceptor::tests::test_extract_tool_list_valid`	Tools list extraction
`interceptor::tests::test_extract_tool_list_error_response`	Error response handling
`proxy::tests::test_proxy_creation`	Proxy instantiation

All pass ✅

Integration Testing (Future)

Fake MCP servers that emit tool calls
Verify denied vs allowed decisions
Verify audit log written
Verify error responses match JSON-RPC spec

Known Limitations & Future Work

MVP Limitations

Synchronous blocking loop - Processes one request/response at a time
- Sufficient for MVP; full async with tokio::select! for full concurrency
No tools/list interception - Server metadata passes through unchanged
- Will implement fingerprinting & drift detection in next phase
No approval workflow - Approval requests return "not implemented" errors
- MVP denies; future will run interactive approval flow
No redaction in proxy - Payloads pass through; only audit events redacted
- Correct behavior; secrets redacted before logging, not in proxy
No connection pooling - One server per proxy instance
- Acceptable for MVP; each client gets own proxy process

Future Enhancements

Full async with tokio::select! for bidirectional proxying
tools/list interception for fingerprinting
Drift detection against baseline fingerprints
Metadata scanning for poisoned descriptions
Interactive approval workflow
Log rotation and tamper detection
Metrics and observability (Prometheus)

Code Quality

Metric	Status
Clippy warnings	✅ 0 (after fixes)
Format	✅ cargo fmt
Unit tests	✅ 5/5 passing
Workspace tests	✅ 29/29 passing
Documentation	✅ Module-level + inline comments
Error handling	✅ No unwraps in library code

Deployment

Running the Proxy

// Create config
let config = ProxyConfig {
    server_name: ServerName("filesystem".into()),
    server_command: "npx".into(),
    server_args: vec![
        "-y".into(),
        "@modelcontextprotocol/server-filesystem".into(),
        ".".into(),
    ],
    session_id: SessionId::default(),
    audit_path: ".toolwall/audit.jsonl".into(),
};

// Create policy
let policy = Arc::new(
    PolicyEngine::from_toml_str(policy_str)?
);

// Create audit writer
let audit = Arc::new(AuditWriter::new(Path::new(".toolwall/audit.jsonl")));

// Create and run proxy
let proxy = McpProxy::new(config, policy, audit);
proxy.run()?;

Example Flow

Client → Proxy:

{"jsonrpc":"2.0","method":"tools/call","id":1,"params":{"name":"read_file","arguments":{"path":"~/.aws/credentials"}}}

Proxy evaluates: read_file on filesystem with path ~/.aws/credentials

Policy has rule: protected_paths = ["~/.aws/**"]
Decision: DENY

Proxy → Client (error response):

{"jsonrpc":"2.0","id":1,"error":{"code":-32603,"message":"tool call denied: path matches protected secret"}}

Audit log entry:

{"event_id":"...","timestamp":"2026-06-01T...","session_id":"...","server":"filesystem","method":"tools/call","tool_name":"read_file","decision":"Deny","reason":"path matches protected secret"}

Security Analysis

Attack Scenarios Covered

Scenario	Mitigation
Malicious server sends huge JSON-RPC message	Frame parser requires line-delimited; redaction limits to 1MB
Client requests secret path	Policy engine blocks; audit logged; error returned
Unknown tool effect in policy	Fails closed as deny
Client tries path traversal via policy	Path validation in fingerprint/cli blocks absolute paths and `..`
Server crashes mid-request	Client sees error response; audit event already written (durability)
Invalid JSON-RPC frame	Parse error treated as deny; error response sent

Assumptions

Client is trusted - Proxy doesn't validate JSON-RPC structure beyond what's needed for interception
Server is untrusted - All server output treated as hostile (but metadata pass-through for MVP)
OS permissions enforced - Config/audit files created with 0600, assumed OS protects file access
Local execution - Proxy runs on same machine as client; no network isolation assumed
Policy is correct - Proxy enforces policy as written; policy correctness is user responsibility

Conclusion

toolwall-proxy is a clean, boring, blocking MCP proxy that enforces policy and writes audit trails. It prioritizes:

✅ Correctness - All tool calls evaluated before forwarding
✅ Auditing - Every decision logged to append-only JSONL
✅ Security - Fail-closed, no secret leakage, redacted logs
✅ Clarity - Synchronous, blocking loop easy to reason about
✅ Testability - Isolated components, unit tested

Ready for integration testing and future concurrency enhancements.

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Toolwall Proxy Implementation

Overview

Architecture

Key Components

1. Frame (`frame.rs`)

2. Error (`error.rs`)

3. Interceptor (`interceptor.rs`)

4. Proxy (`proxy.rs`)

Security Properties

1. Fail-Closed on Malformed Requests

2. Policy Evaluation

3. No Secret Leakage

4. Audit Trail

Integration

Policy Engine

Audit Writer

Frame Handling

Unit Tests (5 tests)

Integration Testing (Future)

Known Limitations & Future Work

MVP Limitations

Future Enhancements

Code Quality

Deployment

Running the Proxy

Example Flow

Security Analysis

Attack Scenarios Covered

Assumptions

Conclusion

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File metadata and controls

Toolwall Proxy Implementation

Overview

Architecture

Key Components

1. Frame (frame.rs)

2. Error (error.rs)

3. Interceptor (interceptor.rs)

4. Proxy (proxy.rs)

Security Properties

1. Fail-Closed on Malformed Requests

2. Policy Evaluation

3. No Secret Leakage

4. Audit Trail

Integration

Policy Engine

Audit Writer

Frame Handling

Unit Tests (5 tests)

Integration Testing (Future)

Known Limitations & Future Work

MVP Limitations

Future Enhancements

Code Quality

Deployment

Running the Proxy

Example Flow

Security Analysis

Attack Scenarios Covered

Assumptions

Conclusion

1. Frame (`frame.rs`)

2. Error (`error.rs`)

3. Interceptor (`interceptor.rs`)

4. Proxy (`proxy.rs`)