feat(acp): Add /agent slash command for agent switching

codex-rs/acp/docs.md

@@ -27,9 +27,11 @@ The ACP registry in `@/codex-rs/acp/src/registry.rs` is **model-centric** rather
   - `provider_info`: Embedded `AcpProviderInfo` with provider configuration (name, retry settings, timeouts)
 - Model names are normalized to lowercase for case-insensitive matching (e.g., "Gemini-2.5-Flash" → "gemini-2.5-flash")
 - Uses exact matching only (no prefix matching) - each model must be explicitly registered
-- The `provider_slug` field enables future optimization to determine when existing subprocess can be reused vs when new one must be spawned when switching models
+- The `provider_slug` field enables determining when existing subprocess can be reused vs when new one must be spawned when switching models
 - Claude ACP is registered for both "claude" and "claude-acp" model names, using `npx @zed-industries/claude-code-acp` command with no arguments
+- `list_available_agents()` returns all available ACP agent configurations for UI presentation (used by the `/agent` picker)
 - Unit test `test_get_claude_model_config()` verifies Claude ACP registry configuration
+- Unit tests `test_list_available_agents_*` verify agent enumeration and uniqueness
 
 ### Embedded Provider Info
 

codex-rs/acp/src/lib.rs

@@ -16,6 +16,7 @@ pub use connection::ApprovalRequest;
 pub use registry::AcpAgentConfig;
 pub use registry::AcpProviderInfo;
 pub use registry::get_agent_config;
+pub use registry::list_available_agents;
 pub use tracing_setup::init_file_tracing;
 pub use translator::TranslatedEvent;
 pub use translator::translate_session_update;

codex-rs/acp/src/registry.rs

@@ -136,6 +136,48 @@ pub fn get_agent_config(model_name: &str) -> Result<AcpAgentConfig> {
     }
 }
 
+/// Returns all available ACP agent configurations.
+///
+/// This is used by the agent picker UI to show available agents.
+/// Each agent has a unique provider_slug for identification.
+pub fn list_available_agents() -> Vec<AcpAgentConfig> {
+    vec![
+        // Mock agent - uses resolved binary path
+        get_agent_config("mock-model").unwrap_or_else(|_| AcpAgentConfig {
+            provider_slug: "mock-acp".to_string(),
+            command: "mock_acp_agent".to_string(),
+            args: vec![],
+            provider_info: AcpProviderInfo {
+                name: "Mock ACP".to_string(),
+                ..Default::default()
+            },
+        }),
+        // Gemini ACP agent
+        AcpAgentConfig {
+            provider_slug: "gemini-acp".to_string(),
+            command: "npx".to_string(),
+            args: vec![
+                "@google/gemini-cli".to_string(),
+                "--experimental-acp".to_string(),
+            ],
+            provider_info: AcpProviderInfo {
+                name: "Gemini ACP".to_string(),
+                ..Default::default()
+            },
+        },
+        // Claude ACP agent
+        AcpAgentConfig {
+            provider_slug: "claude-acp".to_string(),
+            command: "npx".to_string(),
+            args: vec!["@zed-industries/claude-code-acp".to_string()],
+            provider_info: AcpProviderInfo {
+                name: "Claude ACP".to_string(),
+                ..Default::default()
+            },
+        },
+    ]
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -230,4 +272,56 @@ mod tests {
             "Error message should contain original input"
         );
     }
+
+    #[test]
+    fn test_list_available_agents_returns_all_agents() {
+        let agents = list_available_agents();
+
+        // Should return at least the known agents
+        assert!(
+            agents.len() >= 3,
+            "Should have at least 3 agents (mock, gemini, claude), got: {}",
+            agents.len()
+        );
+
+        // Should include mock-model agent
+        let mock_agent = agents
+            .iter()
+            .find(|a| a.provider_slug == "mock-acp")
+            .expect("Should include mock-acp agent");
+        assert!(
+            mock_agent.command.contains("mock_acp_agent"),
+            "Mock agent command should contain mock_acp_agent"
+        );
+
+        // Should include gemini agent
+        let gemini_agent = agents
+            .iter()
+            .find(|a| a.provider_slug == "gemini-acp")
+            .expect("Should include gemini-acp agent");
+        assert_eq!(gemini_agent.command, "npx");
+        assert_eq!(gemini_agent.provider_info.name, "Gemini ACP");
+
+        // Should include claude agent
+        let claude_agent = agents
+            .iter()
+            .find(|a| a.provider_slug == "claude-acp")
+            .expect("Should include claude-acp agent");
+        assert_eq!(claude_agent.command, "npx");
+        assert_eq!(claude_agent.provider_info.name, "Claude ACP");
+    }
+
+    #[test]
+    fn test_list_available_agents_has_unique_provider_slugs() {
+        let agents = list_available_agents();
+        let mut seen_slugs = std::collections::HashSet::new();
+
+        for agent in &agents {
+            assert!(
+                seen_slugs.insert(&agent.provider_slug),
+                "Duplicate provider_slug found: {}",
+                agent.provider_slug
+            );
+        }
+    }
 }

codex-rs/tui-pty-e2e/Cargo.toml

@@ -17,3 +17,4 @@ libc = "0.2"
 
 [dev-dependencies]
 tempfile = "3"
+regex = "1"

codex-rs/tui-pty-e2e/docs.md

@@ -29,6 +29,8 @@ The main API provides:
 - `wait_for_text(needle, timeout)` - Poll screen until text appears
 - `wait_for(predicate, timeout)` - Poll screen until condition matches
 - `screen_contents()` - Get current terminal screen as string
+- `acp_log_path()` - Get path to the `.codex-acp.log` file for this session
+- `read_acp_log()` - Read the ACP tracing log contents (useful for verifying subprocess behavior)
 
 **Debugging Aids:**
 
@@ -145,6 +147,7 @@ This delay allows the PTY subprocess time to process input and update the displa
 | `@/codex-rs/tui-pty-e2e/tests/input_handling.rs` | Text editing, backspace, Ctrl-C clearing, arrow key navigation with snapshot testing |
 | `@/codex-rs/tui-pty-e2e/tests/streaming.rs` | Prompt submission with timing delays, agent response streaming |
 | `@/codex-rs/tui-pty-e2e/tests/acp_mode.rs` | ACP mode startup, response flow, and approval bridging - validates TUI works with ACP wire API and mock agent; includes test for permission request display |
+| `@/codex-rs/tui-pty-e2e/tests/agent_switching.rs` | ACP agent/model switching: `/agent` picker display, pending selection behavior, subprocess lifecycle during picker navigation, `/model` disabled state in ACP mode |
 | `@/codex-rs/tui-pty-e2e/tests/live_acp.rs` | Live authenticated ACP tests for Gemini and Claude with real API connections (opt-in, marked `#[ignore]`) |
 
 **Snapshot Files:**
@@ -232,6 +235,19 @@ Tests control mock agent behavior via environment variables:
 
 See `@/codex-rs/mock-acp-agent/docs.md` for full list of env vars.
 
+**Agent Switching Tests:**
+
+The `agent_switching.rs` tests verify the ACP agent/model picker feature:
+- Uses `extract_mock_agent_pids_from_log()` to parse PIDs from ACP tracing logs
+- `process_exists_and_not_zombie()` verifies subprocess lifecycle via `/proc/<pid>/status`
+- Tests verify the "pending selection" pattern where agent changes are deferred until prompt submission
+- Key test cases:
+  - `/agent` picker displays available agents and marks current
+  - Selecting an agent does NOT immediately spawn new subprocess
+  - Prompt submission with pending selection triggers actual switch
+  - Picker navigation during active streaming does not kill subprocess
+  - `/model` command shows disabled state in ACP mode
+
 **Binary Discovery:**
 
 `codex_binary_path()` locates the compiled binary:

codex-rs/tui-pty-e2e/src/lib.rs

@@ -530,6 +530,22 @@ name = "Mock ACP provider for tests"
         self.writer.write_all(&key.to_escape_sequence())?;
         self.writer.flush()
     }
+
+    /// Get the path to the ACP log file for this session.
+    /// Returns None if no temp directory was created.
+    pub fn acp_log_path(&self) -> Option<std::path::PathBuf> {
+        self._temp_dir
+            .as_ref()
+            .map(|dir| dir.path().join(".codex-acp.log"))
+    }
+
+    /// Read the ACP log contents.
+    /// Returns empty string if log doesn't exist or temp dir not available.
+    pub fn read_acp_log(&self) -> String {
+        self.acp_log_path()
+            .and_then(|path| std::fs::read_to_string(path).ok())
+            .unwrap_or_default()
+    }
 }
 
 /// Sandbox policy for codex session