diff --git a/codex-rs/core/src/agent/role.rs b/codex-rs/core/src/agent/role.rs
index 24766a9ac6..f37618ab87 100644
--- a/codex-rs/core/src/agent/role.rs
+++ b/codex-rs/core/src/agent/role.rs
@@ -130,7 +130,7 @@ pub(crate) fn default_fork_context_for_role(config: &Config, role_name: Option<&
     let role_name = role_name.unwrap_or(DEFAULT_ROLE_NAME);
     resolve_role_config(config, role_name)
         .and_then(|role| role.fork_context)
-        .unwrap_or(false)
+        .unwrap_or(true)
 }
 
 fn preservation_policy(config: &Config, role_layer_toml: &TomlValue) -> (bool, bool) {
diff --git a/codex-rs/core/src/agent/role_tests.rs b/codex-rs/core/src/agent/role_tests.rs
index 198709b580..23a0281b40 100644
--- a/codex-rs/core/src/agent/role_tests.rs
+++ b/codex-rs/core/src/agent/role_tests.rs
@@ -72,6 +72,86 @@ async fn apply_role_returns_error_for_unknown_role() {
     assert_eq!(err, "unknown agent_type 'missing-role'");
 }
 
+#[tokio::test]
+async fn default_fork_context_for_role_defaults_unspecified_custom_roles_to_true() {
+    let (_home, mut config) = test_config_with_cli_overrides(Vec::new()).await;
+    config.agent_roles.insert(
+        "custom".to_string(),
+        AgentRoleConfig {
+            description: Some("Custom role".to_string()),
+            config_file: None,
+            nickname_candidates: None,
+            fork_context: None,
+        },
+    );
+
+    assert!(default_fork_context_for_role(&config, Some("custom")));
+}
+
+#[tokio::test]
+async fn default_fork_context_for_role_defaults_discovered_role_files_to_true() {
+    let codex_home = TempDir::new().expect("create temp dir");
+    let repo_root = TempDir::new().expect("create temp dir");
+    let nested_cwd = repo_root.path().join("packages").join("app");
+    fs::create_dir_all(repo_root.path().join(".git")).expect("create git dir");
+    fs::create_dir_all(&nested_cwd).expect("create nested cwd");
+
+    let workspace_key = repo_root.path().to_string_lossy().replace('\\', "\\\\");
+    tokio::fs::write(
+        codex_home.path().join(CONFIG_TOML_FILE),
+        format!(
+            r#"[projects."{workspace_key}"]
+trust_level = "trusted"
+"#
+        ),
+    )
+    .await
+    .expect("write config");
+
+    let agents_dir = repo_root.path().join(".codex").join("agents");
+    tokio::fs::create_dir_all(&agents_dir)
+        .await
+        .expect("create agents dir");
+    tokio::fs::write(
+        agents_dir.join("custom.toml"),
+        r#"
+name = "custom"
+description = "Custom role"
+developer_instructions = "Stay focused"
+"#,
+    )
+    .await
+    .expect("write role file");
+
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .harness_overrides(ConfigOverrides {
+            cwd: Some(nested_cwd),
+            ..Default::default()
+        })
+        .build()
+        .await
+        .expect("load config");
+
+    assert!(default_fork_context_for_role(&config, Some("custom")));
+}
+
+#[tokio::test]
+async fn default_fork_context_for_role_uses_explicit_custom_role_override() {
+    let (_home, mut config) = test_config_with_cli_overrides(Vec::new()).await;
+    config.agent_roles.insert(
+        "custom".to_string(),
+        AgentRoleConfig {
+            description: Some("Custom role".to_string()),
+            config_file: None,
+            nickname_candidates: None,
+            fork_context: Some(false),
+        },
+    );
+
+    assert!(!default_fork_context_for_role(&config, Some("custom")));
+}
+
 #[tokio::test]
 #[ignore = "No role requiring it for now"]
 async fn apply_explorer_role_sets_model_and_adds_session_flags_layer() {
diff --git a/codex-rs/tools/src/agent_tool.rs b/codex-rs/tools/src/agent_tool.rs
index 1a1eeb19dc..f26f779630 100644
--- a/codex-rs/tools/src/agent_tool.rs
+++ b/codex-rs/tools/src/agent_tool.rs
@@ -601,7 +601,7 @@ fn spawn_agent_common_properties_v1(agent_type_description: &str) -> BTreeMap<St
             "model".to_string(),
             JsonSchema::String {
                 description: Some(
-                    "Optional model override for the new agent. Replaces the inherited model."
+                    "Optional model override for the new agent. Replaces the inherited model only when fork_context is false; forked children always inherit the parent model."
                         .to_string(),
                 ),
             },
@@ -610,7 +610,7 @@ fn spawn_agent_common_properties_v1(agent_type_description: &str) -> BTreeMap<St
             "reasoning_effort".to_string(),
             JsonSchema::String {
                 description: Some(
-                    "Optional reasoning effort override for the new agent. Replaces the inherited reasoning effort."
+                    "Optional reasoning effort override for the new agent. Replaces the inherited reasoning effort only when fork_context is false; forked children always inherit the parent reasoning effort."
                         .to_string(),
                 ),
             },
@@ -645,7 +645,7 @@ fn spawn_agent_common_properties_v2(agent_type_description: &str) -> BTreeMap<St
             "model".to_string(),
             JsonSchema::String {
                 description: Some(
-                    "Optional model override for the new agent. Replaces the inherited model."
+                    "Optional model override for the new agent. Replaces the inherited model only when fork_turns is `none`; forked children always inherit the parent model."
                         .to_string(),
                 ),
             },
@@ -654,7 +654,7 @@ fn spawn_agent_common_properties_v2(agent_type_description: &str) -> BTreeMap<St
             "reasoning_effort".to_string(),
             JsonSchema::String {
                 description: Some(
-                    "Optional reasoning effort override for the new agent. Replaces the inherited reasoning effort."
+                    "Optional reasoning effort override for the new agent. Replaces the inherited reasoning effort only when fork_turns is `none`; forked children always inherit the parent reasoning effort."
                         .to_string(),
                 ),
             },
diff --git a/codex-rs/tools/src/agent_tool_tests.rs b/codex-rs/tools/src/agent_tool_tests.rs
index fc1de7bc76..074621f73c 100644
--- a/codex-rs/tools/src/agent_tool_tests.rs
+++ b/codex-rs/tools/src/agent_tool_tests.rs
@@ -94,6 +94,58 @@ fn spawn_agent_tool_v1_keeps_legacy_fork_context_field() {
 
     assert!(properties.contains_key("fork_context"));
     assert!(!properties.contains_key("fork_turns"));
+    assert_eq!(
+        properties.get("model"),
+        Some(&JsonSchema::String {
+            description: Some(
+                "Optional model override for the new agent. Replaces the inherited model only when fork_context is false; forked children always inherit the parent model."
+                    .to_string(),
+            ),
+        })
+    );
+    assert_eq!(
+        properties.get("reasoning_effort"),
+        Some(&JsonSchema::String {
+            description: Some(
+                "Optional reasoning effort override for the new agent. Replaces the inherited reasoning effort only when fork_context is false; forked children always inherit the parent reasoning effort."
+                    .to_string(),
+            ),
+        })
+    );
+}
+
+#[test]
+fn spawn_agent_tool_v2_documents_that_forked_children_ignore_model_overrides() {
+    let tool = create_spawn_agent_tool_v2(SpawnAgentToolOptions {
+        available_models: &[],
+        agent_type_description: "role help".to_string(),
+    });
+
+    let ToolSpec::Function(ResponsesApiTool { parameters, .. }) = tool else {
+        panic!("spawn_agent should be a function tool");
+    };
+    let JsonSchema::Object { properties, .. } = parameters else {
+        panic!("spawn_agent should use object params");
+    };
+
+    assert_eq!(
+        properties.get("model"),
+        Some(&JsonSchema::String {
+            description: Some(
+                "Optional model override for the new agent. Replaces the inherited model only when fork_turns is `none`; forked children always inherit the parent model."
+                    .to_string(),
+            ),
+        })
+    );
+    assert_eq!(
+        properties.get("reasoning_effort"),
+        Some(&JsonSchema::String {
+            description: Some(
+                "Optional reasoning effort override for the new agent. Replaces the inherited reasoning effort only when fork_turns is `none`; forked children always inherit the parent reasoning effort."
+                    .to_string(),
+            ),
+        })
+    );
 }
 
 #[test]