feat: extract mcp-over-acp proxy (#146)

* Remove html_panel and file_comment metacapabilities from P/ACP design doc

* Step 1/1b: Remove McpAcpTransport, rename acp_url → acp_id

- Remove McpAcpTransport struct and MetaCapability impl from capabilities.rs
  (keep MetaCapability/MetaCapabilityExt traits for proxy capability)
- Rename McpConnectRequest.acp_url to acp_id across all source and tests
- Update snapshot tests for schema changes (mcpCapabilities.acp, auth removal)
- Switch agent-client-protocol-schema to path dependency

* Step 2: Extract MCP bridging into agent-client-protocol-polyfill crate

New crate src/agent-client-protocol-polyfill with mcp_over_acp module:
- McpOverAcpPolyfill proxy (ConnectTo<Conductor>)
- BridgeConnectionActor, BridgeListeners, BridgeConnection
- HTTP and stdio bridge transports
- Uses own BridgeMessage enum instead of ConductorMessage

Conductor bridge code still present (removed in Step 3).

* Step 3: Remove MCP bridging from conductor (clean break)

- Remove McpBridgeMode enum and mcp_bridge_mode parameter
- Delete conductor/mcp_bridge module (actor, http, stdio)
- Remove bridge fields from ConductorResponder
- Remove bridge ConductorMessage variants and handling
- Remove NewSessionRequest MCP server transformation
- Update all 16 test files to remove McpBridgeMode
- Mark 13 bridge-dependent tests as #[ignore]

* Step 4: Update markdown docs (_meta.symposium.mcp_acp_transport → mcpCapabilities.acp)

Replace all _meta.symposium.mcp_acp_transport references with
mcpCapabilities.acp in:
- md/proxying-acp.md (8 references)
- md/protocol.md (3 references)
- md/conductor.md (2 references)
- md/mcp-bridge.md (1 reference)

* Add acp_id(), fix 13 tests with polyfill, remove conductor mcp CLI

- Add McpConnectionTo::acp_id(), deprecate acp_url()
- Remove orphaned 'conductor mcp $port' CLI subcommand and mcp_bridge.rs
- Fix all 13 previously-ignored tests to use McpOverAcpPolyfill::http()
- Fix BridgeResponder bug: route through BridgeMessage::ConnectionEstablished
  instead of creating new HashMap in on_receiving_result callback
- Auto-update 2 snapshot tests for new proxy in chain

* Add breaking change entries to CHANGELOGs

- agent-client-protocol: removed McpAcpTransport, renamed acp_url→acp_id,
  added acp_id(), deprecated acp_url()
- agent-client-protocol-conductor: removed McpBridgeMode, removed
  conductor mcp CLI subcommand

* Enable unstable_mcp_over_acp feature on schema dependency

* Forward unstable_mcp_over_acp feature through SDK crate

Instead of unconditionally enabling unstable_mcp_over_acp on the schema
dependency, declare it as a feature on agent-client-protocol that
forwards to agent-client-protocol-schema/unstable_mcp_over_acp.

Conductor and polyfill crates opt in explicitly.

* Only enable unstable_mcp_over_acp in test builds

The conductor and polyfill don't reference McpServer::Acp or
McpCapabilities.acp in their source — only snapshot tests see the
field in serialized output. Move the feature to dev-dependencies.

* Address PR review feedback

- Restore doc comments in polyfill actor.rs and http.rs that were
  stripped during extraction from conductor
- Polyfill now intercepts InitializeProxyRequest and sets
  mcpCapabilities.acp = true in the response
- Re-enable unstable_mcp_over_acp feature for polyfill crate
  (it needs to set the acp field)
- Update snapshot tests to reflect acp: true

ProxiesAndAgent API redesign noted for separate PR.

* Trim unused deps

* test: refresh expect snapshots after schema 0.13.0 bump

- Fix two stale test references to the removed McpAcpTransport type
  (regression from "Step 1/1b") by switching to TestCapability
- Regenerate trace_* expect snapshots so they match the actual output
  under --all-features (adds auth capability fields exposed by
  unstable_auth_methods)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(polyfill): satisfy clippy under -D warnings

CI runs clippy with `-D warnings` and surfaced three lints in the
new mcp_over_acp module:

- derivable_impls: replace the manual `impl Default for BridgeMode`
  with `#[derive(Default)]` and `#[default]` on the `Http` variant
- must_use_candidate: add `#[must_use]` to `McpOverAcpPolyfill::http`
  and `::stdio` constructors

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Trip deps

* More

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

Cargo.lock

@@ -4,6 +4,7 @@ members = [
     "src/agent-client-protocol-conductor",
     "src/agent-client-protocol-cookbook",
     "src/agent-client-protocol-derive",
+    "src/agent-client-protocol-polyfill",
     "src/agent-client-protocol-rmcp",
     "src/agent-client-protocol-test",
     "src/agent-client-protocol-trace-viewer",
@@ -50,7 +51,7 @@ tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 
 # MCP SDK
-rmcp = { version = "1.6.0", features = ["server", "transport-io", "schemars"] }
+rmcp = { version = "1.2.0", features = ["server", "transport-io", "schemars"] }
 
 # CLI parsing
 clap = { version = "4.5", features = ["derive"] }

Cargo.toml

@@ -93,12 +93,12 @@ See [Proxy Mode](#proxy-mode) below for hierarchical chain details.
 
 When components provide MCP servers with ACP transport (`"url": "acp:$UUID"`):
 
-**If agent has `mcp_acp_transport` capability:**
+**If agent has `mcpCapabilities.acp` capability:**
 
 - Pass through MCP server declarations unchanged
 - Agent handles `_mcp/*` messages natively
 
-**If agent lacks `mcp_acp_transport` capability:**
+**If agent lacks `mcpCapabilities.acp` capability:**
 
 - Bind TCP port for each ACP-transport MCP server
 - Transform MCP server spec to use `conductor mcp $port`

md/conductor.md

@@ -53,7 +53,7 @@ sequenceDiagram
 
     Proxy->>Conductor: session/new {<br/>  mcp_servers: [{<br/>    name: "research-tools",<br/>    url: "acp:uuid-123"<br/>  }]<br/>}
 
-    Note over Conductor: Detects acp: transport<br/>Agent lacks mcp_acp_transport capability
+    Note over Conductor: Detects acp: transport<br/>Agent lacks mcpCapabilities.acp
 
     Conductor->>Conductor: Bind TCP listener on port 54321
 

md/mcp-bridge.md

@@ -286,21 +286,23 @@ Send an MCP notification over the ACP connection. Bidirectional like `_mcp/reque
 }
 ```
 
-### Agent Capability: `mcp_acp_transport`
+### Agent Capability: `mcpCapabilities.acp`
 
 Agents that natively support MCP-over-ACP declare this capability:
 
 ```json
 {
-  "_meta": {
-    "mcp_acp_transport": true
+  "agentCapabilities": {
+    "mcpCapabilities": {
+      "acp": true
+    }
   }
 }
 ```
 
 **Conductor behavior:**
 
-- If the agent has `mcp_acp_transport: true`, conductor passes MCP server declarations through unchanged
+- If the agent has `mcpCapabilities.acp: true`, conductor passes MCP server declarations through unchanged
 - If the agent lacks this capability, conductor performs **bridging adaptation**:
   1. Binds a TCP port (e.g., `localhost:54321`)
   2. Transforms MCP server to use `conductor mcp PORT` command with stdio transport

md/protocol.md

@@ -108,7 +108,7 @@ P/ACP's orchestrator is called the **Conductor** (binary name: `conductor`). The
 
 **Key adaptation: MCP Bridge**
 
-- If the agent supports `mcp_acp_transport`, conductor passes MCP servers with ACP transport through unchanged
+- If the agent supports `mcpCapabilities.acp`, conductor passes MCP servers with ACP transport through unchanged
 - If not, conductor spawns `conductor mcp $port` processes to bridge between stdio (MCP) and ACP messages
 - Components can provide MCP servers without requiring agent modifications
 - See "MCP Bridge" section in Implementation Details for full protocol
@@ -216,9 +216,7 @@ An P/ACP-aware editor provides the following capability during ACP initializatio
 /// supports symposium proxy initialization.
 "_meta": {
     "symposium": {
-        "version": "1.0",
-        "html_panel": true,      // or false, if this is the ToEditor proxy
-        "file_comment": true,    // or false, if this is the ToEditor proxy
+        "version": "1.0"
     }
 }
 ```
@@ -358,26 +356,28 @@ Components declare MCP servers with ACP transport by using the HTTP MCP server f
 
 The `acp:$UUID` URL signals ACP transport. The component generates the UUID to identify which component handles calls to this MCP server.
 
-#### Agent Capability: `mcp_acp_transport`
+#### Agent Capability: `mcpCapabilities.acp`
 
 Agents that natively support MCP-over-ACP declare this capability:
 
 ```json
 {
-  "_meta": {
-    "mcp_acp_transport": true
+  "agentCapabilities": {
+    "mcpCapabilities": {
+      "acp": true
+    }
   }
 }
 ```
 
 **Conductor behavior:**
 
-- If the final agent has `mcp_acp_transport: true`, conductor passes MCP server declarations through unchanged
+- If the final agent has `mcpCapabilities.acp: true`, conductor passes MCP server declarations through unchanged
 - If the final agent lacks this capability, conductor performs **bridging adaptation**:
   1. Binds a fresh TCP port (e.g., `localhost:54321`)
   2. Transforms the MCP server declaration to use `conductor mcp $port` as the command
   3. Spawns `conductor mcp $port` which connects back via TCP and bridges to ACP messages
-  4. Always advertises `mcp_acp_transport: true` to intermediate components
+  4. Always advertises `mcpCapabilities.acp: true` to intermediate components
 
 #### Bridging Transformation Example
 
@@ -393,7 +393,7 @@ Agents that natively support MCP-over-ACP declare this capability:
 }
 ```
 
-**Transformed spec (passed to agent without `mcp_acp_transport`):**
+**Transformed spec (passed to agent without `mcpCapabilities.acp`):**
 
 ```json
 {
@@ -588,12 +588,12 @@ These extensions are beyond the scope of this initial RFD and will be defined as
 **Phase 2: Conductor Agent Mode - MCP Detection & Bridging**
 
 - [ ] Detect `"transport": "http", "url": "acp:$UUID"` MCP servers in initialization
-- [ ] Check final agent for `mcp_acp_transport` capability
+- [ ] Check final agent for `mcpCapabilities.acp` capability
 - [ ] Bind ephemeral TCP ports when bridging needed
 - [ ] Transform MCP server specs to use `conductor mcp $port`
 - [ ] Spawn `conductor mcp $port` subprocess per ACP-transport MCP server
 - [ ] Store mapping: `UUID → TCP port → bridge process`
-- [ ] Always advertise `mcp_acp_transport: true` to intermediate components
+- [ ] Always advertise `mcpCapabilities.acp: true` to intermediate components
 - [ ] Integration test: full chain with MCP bridging
 
 **Phase 3: `_mcp/*` Message Routing**

md/proxying-acp.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- **Removed `McpBridgeMode`** and the `mcp_bridge_mode` parameter from `ConductorImpl::new`, `new_agent`, and `new_proxy`. MCP-over-ACP bridging is no longer built into the conductor. Use `agent-client-protocol-polyfill::mcp_over_acp::McpOverAcpPolyfill` as a proxy in the chain instead.
+- **Removed `conductor mcp $port` CLI subcommand.** The stdio↔TCP bridge subprocess is no longer needed.
+
 ## [0.11.1](https://github.com/agentclientprotocol/rust-sdk/compare/agent-client-protocol-conductor-v0.11.0...agent-client-protocol-conductor-v0.11.1) - 2026-04-21
 
 ### Other

src/agent-client-protocol-conductor/CHANGELOG.md

@@ -18,11 +18,9 @@ path = "src/main.rs"
 test-support = []
 
 [dependencies]
-agent-client-protocol.workspace = true
+agent-client-protocol = { workspace = true }
 agent-client-protocol-trace-viewer.workspace = true
 anyhow.workspace = true
-async-stream.workspace = true
-axum.workspace = true
 chrono.workspace = true
 clap.workspace = true
 futures.workspace = true
@@ -31,20 +29,21 @@ rustc-hash.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 strip-ansi-escapes.workspace = true
-thiserror.workspace = true
 tokio.workspace = true
 tokio-util.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 uuid.workspace = true
 
 [dev-dependencies]
+agent-client-protocol = { workspace = true, features = ["unstable_mcp_over_acp"] }
 agent-client-protocol-test.workspace = true
 yopo.workspace = true
 expect-test.workspace = true
 regex.workspace = true
 rmcp = { workspace = true, features = ["client", "server", "transport-io", "transport-child-process"] }
 schemars.workspace = true
+agent-client-protocol-polyfill = { path = "../agent-client-protocol-polyfill" }
 
 [lints]
 workspace = true