agentclientprotocol · nikomatsakis · Apr 30, 2026 · Apr 30, 2026 · Apr 30, 2026 · Apr 30, 2026
diff --git a/Cargo.lock b/Cargo.lock
diff --git a/Cargo.toml b/Cargo.toml
@@ -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",

diff --git a/md/conductor.md b/md/conductor.md
@@ -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`

diff --git a/md/mcp-bridge.md b/md/mcp-bridge.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
 

diff --git a/md/protocol.md b/md/protocol.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

diff --git a/md/proxying-acp.md b/md/proxying-acp.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**

diff --git a/src/agent-client-protocol-conductor/CHANGELOG.md b/src/agent-client-protocol-conductor/CHANGELOG.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

diff --git a/src/agent-client-protocol-conductor/Cargo.toml b/src/agent-client-protocol-conductor/Cargo.toml
@@ -21,8 +21,6 @@ test-support = []
 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