Disable multi-agent tools in side conversations

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -3237,7 +3237,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional client-supplied analytics source classification for this forked thread."
+          "description": "Optional client-supplied source classification for this forked thread. Threads marked as `side` do not receive multi-agent collaboration tools."
         }
       },
       "required": [
@@ -3797,6 +3797,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],
@@ -3934,7 +3935,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional client-supplied analytics source classification for this thread."
+          "description": "Optional client-supplied source classification for this thread. Threads marked as `side` do not receive multi-agent collaboration tools."
         }
       },
       "type": "object"

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -3463,7 +3463,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -4617,6 +4617,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -15879,7 +15879,7 @@
                 "type": "null"
               }
             ],
-            "description": "Optional analytics source classification for this thread."
+            "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
           },
           "turns": {
             "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -16095,7 +16095,7 @@
                 "type": "null"
               }
             ],
-            "description": "Optional client-supplied analytics source classification for this forked thread."
+            "description": "Optional client-supplied source classification for this forked thread. Threads marked as `side` do not receive multi-agent collaboration tools."
           }
         },
         "required": [
@@ -18059,6 +18059,7 @@
       "ThreadSource": {
         "enum": [
           "user",
+          "side",
           "subagent",
           "memory_consolidation"
         ],
@@ -18197,7 +18198,7 @@
                 "type": "null"
               }
             ],
-            "description": "Optional client-supplied analytics source classification for this thread."
+            "description": "Optional client-supplied source classification for this thread. Threads marked as `side` do not receive multi-agent collaboration tools."
           }
         },
         "title": "ThreadStartParams",

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -13696,7 +13696,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -13912,7 +13912,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional client-supplied analytics source classification for this forked thread."
+          "description": "Optional client-supplied source classification for this forked thread. Threads marked as `side` do not receive multi-agent collaboration tools."
         }
       },
       "required": [
@@ -15876,6 +15876,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],
@@ -16014,7 +16015,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional client-supplied analytics source classification for this thread."
+          "description": "Optional client-supplied source classification for this thread. Threads marked as `side` do not receive multi-agent collaboration tools."
         }
       },
       "title": "ThreadStartParams",

codex-rs/app-server-protocol/schema/json/v2/ThreadForkParams.json

@@ -75,6 +75,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],
@@ -173,7 +174,7 @@
           "type": "null"
         }
       ],
-      "description": "Optional client-supplied analytics source classification for this forked thread."
+      "description": "Optional client-supplied source classification for this forked thread. Threads marked as `side` do not receive multi-agent collaboration tools."
     }
   },
   "required": [

codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json

@@ -1084,7 +1084,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1832,6 +1832,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadListResponse.json

@@ -899,7 +899,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1647,6 +1647,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -899,7 +899,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1647,6 +1647,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json

@@ -899,7 +899,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1647,6 +1647,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadResumeResponse.json

@@ -1084,7 +1084,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1832,6 +1832,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadRollbackResponse.json

@@ -899,7 +899,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1647,6 +1647,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadStartParams.json

@@ -109,6 +109,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],
@@ -253,7 +254,7 @@
           "type": "null"
         }
       ],
-      "description": "Optional client-supplied analytics source classification for this thread."
+      "description": "Optional client-supplied source classification for this thread. Threads marked as `side` do not receive multi-agent collaboration tools."
     }
   },
   "title": "ThreadStartParams",

codex-rs/app-server-protocol/schema/json/v2/ThreadStartResponse.json

@@ -1084,7 +1084,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1832,6 +1832,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -899,7 +899,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1647,6 +1647,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/json/v2/ThreadUnarchiveResponse.json

@@ -899,7 +899,7 @@
               "type": "null"
             }
           ],
-          "description": "Optional analytics source classification for this thread."
+          "description": "Optional source classification for this thread. `side` threads do not receive multi-agent collaboration tools."
         },
         "turns": {
           "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
@@ -1647,6 +1647,7 @@
     "ThreadSource": {
       "enum": [
         "user",
+        "side",
         "subagent",
         "memory_consolidation"
       ],

codex-rs/app-server-protocol/schema/typescript/v2/Thread.ts

@@ -62,7 +62,8 @@ cliVersion: string,
  */
 source: SessionSource,
 /**
- * Optional analytics source classification for this thread.
+ * Optional source classification for this thread. `side` threads do not receive multi-agent
+ * collaboration tools.
  */
 threadSource: ThreadSource | null,
 /**

codex-rs/app-server-protocol/schema/typescript/v2/ThreadForkParams.ts

@@ -25,6 +25,7 @@ model?: string | null, modelProvider?: string | null, serviceTier?: string | nul
  * and subsequent turns.
  */
 approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, baseInstructions?: string | null, developerInstructions?: string | null, ephemeral?: boolean, /**
- * Optional client-supplied analytics source classification for this forked thread.
+ * Optional client-supplied source classification for this forked thread. Threads marked as
+ * `side` do not receive multi-agent collaboration tools.
  */
 threadSource?: ThreadSource | null};

codex-rs/app-server-protocol/schema/typescript/v2/ThreadSource.ts

@@ -2,4 +2,4 @@
 
 // This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
 
-export type ThreadSource = "user" | "subagent" | "memory_consolidation";
+export type ThreadSource = "user" | "side" | "subagent" | "memory_consolidation";

codex-rs/app-server-protocol/schema/typescript/v2/ThreadStartParams.ts

@@ -14,6 +14,7 @@ export type ThreadStartParams = {model?: string | null, modelProvider?: string |
  * and subsequent turns.
  */
 approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, serviceName?: string | null, baseInstructions?: string | null, developerInstructions?: string | null, personality?: Personality | null, ephemeral?: boolean | null, sessionStartSource?: ThreadStartSource | null, /**
- * Optional client-supplied analytics source classification for this thread.
+ * Optional client-supplied source classification for this thread. Threads marked as `side`
+ * do not receive multi-agent collaboration tools.
  */
 threadSource?: ThreadSource | null};

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -138,7 +138,8 @@ pub struct ThreadStartParams {
     pub ephemeral: Option<bool>,
     #[ts(optional = nullable)]
     pub session_start_source: Option<ThreadStartSource>,
-    /// Optional client-supplied analytics source classification for this thread.
+    /// Optional client-supplied source classification for this thread. Threads marked as `side`
+    /// do not receive multi-agent collaboration tools.
     #[ts(optional = nullable)]
     pub thread_source: Option<ThreadSource>,
     /// Optional sticky environments for this thread.
@@ -533,7 +534,8 @@ pub struct ThreadForkParams {
     pub developer_instructions: Option<String>,
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub ephemeral: bool,
-    /// Optional client-supplied analytics source classification for this forked thread.
+    /// Optional client-supplied source classification for this forked thread. Threads marked as
+    /// `side` do not receive multi-agent collaboration tools.
     #[ts(optional = nullable)]
     pub thread_source: Option<ThreadSource>,
     /// When true, return only thread metadata and live fork state without

codex-rs/app-server-protocol/src/protocol/v2/thread_data.rs

@@ -66,6 +66,7 @@ impl From<SessionSource> for CoreSessionSource {
 #[ts(rename_all = "snake_case", export_to = "v2/")]
 pub enum ThreadSource {
     User,
+    Side,
     Subagent,
     MemoryConsolidation,
 }
@@ -74,6 +75,7 @@ impl From<CoreThreadSource> for ThreadSource {
     fn from(value: CoreThreadSource) -> Self {
         match value {
             CoreThreadSource::User => ThreadSource::User,
+            CoreThreadSource::Side => ThreadSource::Side,
             CoreThreadSource::Subagent => ThreadSource::Subagent,
             CoreThreadSource::MemoryConsolidation => ThreadSource::MemoryConsolidation,
         }
@@ -84,6 +86,7 @@ impl From<ThreadSource> for CoreThreadSource {
     fn from(value: ThreadSource) -> Self {
         match value {
             ThreadSource::User => CoreThreadSource::User,
+            ThreadSource::Side => CoreThreadSource::Side,
             ThreadSource::Subagent => CoreThreadSource::Subagent,
             ThreadSource::MemoryConsolidation => CoreThreadSource::MemoryConsolidation,
         }
@@ -132,7 +135,8 @@ pub struct Thread {
     pub cli_version: String,
     /// Origin of the thread (CLI, VSCode, codex exec, codex app-server, etc.).
     pub source: SessionSource,
-    /// Optional analytics source classification for this thread.
+    /// Optional source classification for this thread. `side` threads do not receive multi-agent
+    /// collaboration tools.
     pub thread_source: Option<ThreadSource>,
     /// Optional random unique nickname assigned to an AgentControl-spawned sub-agent.
     pub agent_nickname: Option<String>,

codex-rs/app-server/README.md

@@ -132,7 +132,7 @@ Example with notification opt-out:
 
 - `thread/start` — create a new thread; emits `thread/started` (including the current `thread.status`) and auto-subscribes you to turn/item events for that thread. When the request includes a `cwd` and the resolved sandbox is `workspace-write` or full access, app-server also marks that project as trusted in the user `config.toml`. Pass `sessionStartSource: "clear"` when starting a replacement thread after clearing the current session so `SessionStart` hooks receive `source: "clear"` instead of the default `"startup"`. Experimental `runtimeWorkspaceRoots` replaces the thread-scoped runtime workspace roots used to materialize `:workspace_roots`; paths must be absolute. For permissions, prefer experimental `permissions` profile selection by id; the legacy `sandbox` shorthand is still accepted but cannot be combined with `permissions`. Experimental `environments` selects the sticky execution environments for turns on the thread; omit it to use the server default, pass `[]` to disable environments, or pass explicit environment ids with per-environment `cwd`.
 - `thread/resume` — reopen an existing thread by id so subsequent `turn/start` calls append to it. Accepts the same permission override rules as `thread/start`.
-- `thread/fork` — fork an existing thread into a new thread id by copying the stored history; if the source thread is currently mid-turn, the fork records the same interruption marker as `turn/interrupt` instead of inheriting an unmarked partial turn suffix. The returned `thread.forkedFromId` points at the source thread when known. Accepts `ephemeral: true` for an in-memory temporary fork, emits `thread/started` (including the current `thread.status`), and auto-subscribes you to turn/item events for the new thread. Experimental clients can pass `excludeTurns: true` when they plan to page fork history via `thread/turns/list` instead of receiving the full turn array immediately. Accepts the same permission override rules as `thread/start`.
+- `thread/fork` — fork an existing thread into a new thread id by copying the stored history; if the source thread is currently mid-turn, the fork records the same interruption marker as `turn/interrupt` instead of inheriting an unmarked partial turn suffix. The returned `thread.forkedFromId` points at the source thread when known. Accepts `ephemeral: true` for an in-memory temporary fork, emits `thread/started` (including the current `thread.status`), and auto-subscribes you to turn/item events for the new thread. Set `threadSource` to `side` for an isolated side conversation; side threads do not receive multi-agent collaboration tools. Experimental clients can pass `excludeTurns: true` when they plan to page fork history via `thread/turns/list` instead of receiving the full turn array immediately. Accepts the same permission override rules as `thread/start`.
 - `thread/start`, `thread/resume`, and `thread/fork` responses include the legacy `sandbox` compatibility projection. Experimental clients can read `runtimeWorkspaceRoots` for the thread-scoped runtime roots and `activePermissionProfile` for the named or implicit built-in profile identity/provenance when known.
 - `thread/list` — page through stored rollouts; supports cursor-based pagination and optional `modelProviders`, `sourceKinds`, `archived`, `cwd`, and `searchTerm` filters. Each returned `thread` includes `status` (`ThreadStatus`), defaulting to `notLoaded` when the thread is not currently loaded. Subagent threads also include `parentThreadId` when the immediate control/spawn parent is known.
 - `thread/loaded/list` — list the thread ids currently loaded in memory.

codex-rs/core/src/tools/spec_plan.rs

@@ -64,6 +64,7 @@ use codex_protocol::openai_models::ToolMode;
 use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
+use codex_protocol::protocol::ThreadSource;
 use codex_tools::DiscoverableTool;
 use codex_tools::ResponsesApiNamespace;
 use codex_tools::ResponsesApiNamespaceTool;
@@ -302,6 +303,9 @@ fn multi_agent_v2_enabled(turn_context: &TurnContext) -> bool {
 }
 
 fn collab_tools_enabled(turn_context: &TurnContext) -> bool {
+    if turn_context.thread_source == Some(ThreadSource::Side) {
+        return false;
+    }
     match turn_context.multi_agent_version {
         MultiAgentVersion::Disabled => false,
         MultiAgentVersion::V1 => !exceeds_thread_spawn_depth_limit(

codex-rs/core/src/tools/spec_plan_tests.rs

@@ -17,6 +17,7 @@ use codex_protocol::openai_models::ToolMode;
 use codex_protocol::openai_models::WebSearchToolType;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
+use codex_protocol::protocol::ThreadSource;
 use codex_tools::DiscoverablePluginInfo;
 use codex_tools::DiscoverableTool;
 use codex_tools::ResponsesApiNamespaceTool;
@@ -1043,6 +1044,42 @@ async fn multi_agent_feature_selects_one_agent_tool_family() {
     );
 }
 
+#[tokio::test]
+async fn side_threads_do_not_expose_multi_agent_tools() {
+    let v1 = probe(|turn| {
+        set_features(turn, &[Feature::Collab, Feature::SpawnCsv]);
+        set_feature(turn, Feature::MultiAgentV2, /*enabled*/ false);
+        turn.thread_source = Some(ThreadSource::Side);
+    })
+    .await;
+    v1.assert_visible_lacks(&[MULTI_AGENT_V1_NAMESPACE, "spawn_agents_on_csv"]);
+    v1.assert_registered_lacks(&[
+        &ToolName::namespaced(MULTI_AGENT_V1_NAMESPACE, "spawn_agent").to_string(),
+        &ToolName::namespaced(MULTI_AGENT_V1_NAMESPACE, "send_input").to_string(),
+        &ToolName::namespaced(MULTI_AGENT_V1_NAMESPACE, "resume_agent").to_string(),
+        &ToolName::namespaced(MULTI_AGENT_V1_NAMESPACE, "wait_agent").to_string(),
+        &ToolName::namespaced(MULTI_AGENT_V1_NAMESPACE, "close_agent").to_string(),
+        "spawn_agents_on_csv",
+    ]);
+
+    let v2 = probe(|turn| {
+        set_features(turn, &[Feature::MultiAgentV2, Feature::SpawnCsv]);
+        turn.thread_source = Some(ThreadSource::Side);
+    })
+    .await;
+    let v2_tools = [
+        "spawn_agent",
+        "send_message",
+        "followup_task",
+        "wait_agent",
+        "interrupt_agent",
+        "list_agents",
+        "spawn_agents_on_csv",
+    ];
+    v2.assert_visible_lacks(&v2_tools);
+    v2.assert_registered_lacks(&v2_tools);
+}
+
 #[tokio::test]
 async fn multi_agent_v2_message_schemas_are_encrypted() {
     let plan = probe(|turn| {

codex-rs/core/tests/suite/tools.rs

@@ -7,6 +7,7 @@ use std::time::Instant;
 
 use anyhow::Context;
 use anyhow::Result;
+use codex_core::StartThreadOptions;
 use codex_core::sandboxing::SandboxPermissions;
 use codex_features::Feature;
 use codex_protocol::models::PermissionProfile;
@@ -16,6 +17,11 @@ use codex_protocol::permissions::FileSystemSandboxEntry;
 use codex_protocol::permissions::FileSystemSandboxPolicy;
 use codex_protocol::permissions::NetworkSandboxPolicy;
 use codex_protocol::protocol::AskForApproval;
+use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::InitialHistory;
+use codex_protocol::protocol::Op;
+use codex_protocol::protocol::ThreadSource;
+use codex_protocol::user_input::UserInput;
 use core_test_support::assert_regex_match;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -30,6 +36,8 @@ use core_test_support::skip_if_no_network;
 use core_test_support::skip_if_sandbox;
 use core_test_support::test_codex::local;
 use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
+use pretty_assertions::assert_eq;
 use regex_lite::Regex;
 use serde_json::Value;
 use serde_json::json;
@@ -51,6 +59,85 @@ fn tool_names(body: &Value) -> Vec<String> {
         .unwrap_or_default()
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn side_threads_omit_multi_agent_tools_from_requests() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let response_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_assistant_message("msg-1", "done"),
+            ev_completed("resp-1"),
+        ]),
+    )
+    .await;
+    let mut builder = test_codex().with_config(|config| {
+        config
+            .features
+            .enable(Feature::MultiAgentV2)
+            .expect("multi-agent v2 should enable for test");
+        config
+            .features
+            .enable(Feature::SpawnCsv)
+            .expect("spawn CSV should enable for test");
+    });
+    let test = builder.build(&server).await?;
+    let side_thread = test
+        .thread_manager
+        .start_thread_with_options(StartThreadOptions {
+            config: test.config.clone(),
+            initial_history: InitialHistory::New,
+            session_source: None,
+            thread_source: Some(ThreadSource::Side),
+            dynamic_tools: Vec::new(),
+            metrics_service_name: None,
+            parent_trace: None,
+            environments: Vec::new(),
+        })
+        .await?;
+
+    side_thread
+        .thread
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "which tools are available?".to_string(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            responsesapi_client_metadata: None,
+            additional_context: Default::default(),
+            thread_settings: Default::default(),
+        })
+        .await?;
+    wait_for_event(&side_thread.thread, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let tools = tool_names(&response_mock.single_request().body_json());
+    assert!(tools.contains(&"update_plan".to_string()));
+    let present_multi_agent_tools = tools
+        .into_iter()
+        .filter(|tool| {
+            matches!(
+                tool.as_str(),
+                "spawn_agent"
+                    | "send_message"
+                    | "followup_task"
+                    | "wait_agent"
+                    | "interrupt_agent"
+                    | "list_agents"
+                    | "spawn_agents_on_csv"
+            )
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(present_multi_agent_tools, Vec::<String>::new());
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn empty_turn_environments_omits_environment_backed_tools() -> Result<()> {
     skip_if_no_network!(Ok(()));

codex-rs/protocol/src/protocol.rs

@@ -2534,6 +2534,7 @@ pub enum SessionSource {
 #[ts(rename_all = "snake_case")]
 pub enum ThreadSource {
     User,
+    Side,
     Subagent,
     MemoryConsolidation,
 }
@@ -2542,6 +2543,7 @@ impl ThreadSource {
     pub fn as_str(self) -> &'static str {
         match self {
             ThreadSource::User => "user",
+            ThreadSource::Side => "side",
             ThreadSource::Subagent => "subagent",
             ThreadSource::MemoryConsolidation => "memory_consolidation",
         }
@@ -2560,6 +2562,7 @@ impl FromStr for ThreadSource {
     fn from_str(value: &str) -> Result<Self, Self::Err> {
         match value {
             "user" => Ok(ThreadSource::User),
+            "side" => Ok(ThreadSource::Side),
             "subagent" => Ok(ThreadSource::Subagent),
             "memory_consolidation" => Ok(ThreadSource::MemoryConsolidation),
             other => Err(format!("unknown thread source: {other}")),

codex-rs/tui/src/app.rs

@@ -126,6 +126,7 @@ use codex_app_server_protocol::ThreadItem;
 use codex_app_server_protocol::ThreadLoadedListParams;
 use codex_app_server_protocol::ThreadMemoryMode;
 use codex_app_server_protocol::ThreadRollbackResponse;
+use codex_app_server_protocol::ThreadSource;
 use codex_app_server_protocol::ThreadStartSource;
 use codex_app_server_protocol::Turn;
 use codex_app_server_protocol::TurnError as AppServerTurnError;
@@ -960,7 +961,7 @@ impl App {
                     &[("source", "cli_subcommand")],
                 );
                 let forked = app_server
-                    .fork_thread(config.clone(), target_session.thread_id)
+                    .fork_thread(config.clone(), target_session.thread_id, ThreadSource::User)
                     .await
                     .map_err(|err| session_start_error("fork", &target_session, err))?;
                 let init = crate::chatwidget::ChatWidgetInit {

codex-rs/tui/src/app/event_dispatch.rs

@@ -144,7 +144,10 @@ impl App {
                 if let Some(thread_id) = self.chat_widget.thread_id() {
                     self.refresh_in_memory_config_from_disk_best_effort("forking the thread")
                         .await;
-                    match app_server.fork_thread(self.config.clone(), thread_id).await {
+                    match app_server
+                        .fork_thread(self.config.clone(), thread_id, ThreadSource::User)
+                        .await
+                    {
                         Ok(forked) => {
                             self.shutdown_current_thread(app_server).await;
                             match self

codex-rs/tui/src/app/side.rs

@@ -564,7 +564,10 @@ impl App {
             .await;
 
         let fork_config = self.side_fork_config();
-        match app_server.fork_thread(fork_config, parent_thread_id).await {
+        match app_server
+            .fork_thread(fork_config, parent_thread_id, ThreadSource::Side)
+            .await
+        {
             Ok(forked) => {
                 let child_thread_id = forked.session.thread_id;
                 let channel = self.ensure_thread_channel(child_thread_id);

codex-rs/tui/src/app_server_session.rs

@@ -445,6 +445,7 @@ impl AppServerSession {
         &mut self,
         config: Config,
         thread_id: ThreadId,
+        thread_source: ThreadSource,
     ) -> Result<AppServerStartedThread> {
         let request_id = self.next_request_id();
         let session_config = self.session_config_with_effective_service_tier(&config);
@@ -457,6 +458,7 @@ impl AppServerSession {
                     thread_id,
                     self.thread_params_mode(),
                     self.remote_cwd_override.as_deref(),
+                    thread_source,
                 ),
             })
             .await
@@ -1461,6 +1463,7 @@ fn thread_fork_params_from_config(
     thread_id: ThreadId,
     thread_params_mode: ThreadParamsMode,
     remote_cwd_override: Option<&std::path::Path>,
+    thread_source: ThreadSource,
 ) -> ThreadForkParams {
     let permissions = permissions_selection_from_config(&config, thread_params_mode);
     let sandbox = permissions
@@ -1490,7 +1493,7 @@ fn thread_fork_params_from_config(
             config.developer_instructions.clone(),
         ),
         ephemeral: config.ephemeral,
-        thread_source: Some(ThreadSource::User),
+        thread_source: Some(thread_source),
         ..ThreadForkParams::default()
     }
 }
@@ -2016,6 +2019,7 @@ mod tests {
             thread_id,
             ThreadParamsMode::Remote,
             /*remote_cwd_override*/ None,
+            ThreadSource::User,
         );
 
         assert_eq!(start.cwd, None);
@@ -2133,6 +2137,7 @@ mod tests {
             thread_id,
             ThreadParamsMode::Remote,
             Some(remote_cwd.as_path()),
+            ThreadSource::User,
         );
 
         assert_eq!(start.cwd.as_deref(), Some("repo/on/server"));
@@ -2184,6 +2189,7 @@ mod tests {
             thread_id,
             ThreadParamsMode::Embedded,
             /*remote_cwd_override*/ None,
+            ThreadSource::User,
         );
 
         let expected_service_tier = Some(Some(ServiceTier::Fast.request_value().to_string()));
@@ -2238,6 +2244,7 @@ mod tests {
             thread_id,
             ThreadParamsMode::Embedded,
             /*remote_cwd_override*/ None,
+            ThreadSource::Side,
         );
 
         assert_eq!(params.base_instructions.as_deref(), Some("Base override."));
@@ -2245,6 +2252,7 @@ mod tests {
             params.developer_instructions.as_deref(),
             Some("Developer override.")
         );
+        assert_eq!(params.thread_source, Some(ThreadSource::Side));
     }
 
     #[tokio::test]
@@ -2271,6 +2279,7 @@ mod tests {
             thread_id,
             ThreadParamsMode::Embedded,
             /*remote_cwd_override*/ None,
+            ThreadSource::User,
         );
 
         assert_eq!(control_start.developer_instructions, None);
@@ -2300,6 +2309,7 @@ mod tests {
             thread_id,
             ThreadParamsMode::Embedded,
             /*remote_cwd_override*/ None,
+            ThreadSource::User,
         );
         let expected = format!(
             "Developer override.\n\n{}",