diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/ThreadForkParams.ts b/codex-rs/app-server-protocol/schema/typescript/v2/ThreadForkParams.ts
index a7ba31180..952da565a 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/ThreadForkParams.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/ThreadForkParams.ts
@@ -27,7 +27,7 @@ model?: string | null, modelProvider?: string | null, serviceTier?: ServiceTier
  * Override where approval requests are routed for review on this thread
  * and subsequent turns.
  */
-approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, baseInstructions?: string | null, developerInstructions?: string | null, ephemeral?: boolean, /**
+approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, baseInstructions?: string | null | null, developerInstructions?: string | null | null, ephemeral?: boolean, /**
  * If true, persist additional rollout EventMsg variants required to
  * reconstruct a richer thread history on subsequent resume/fork/read.
  */
diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/ThreadResumeParams.ts b/codex-rs/app-server-protocol/schema/typescript/v2/ThreadResumeParams.ts
index 770344de8..fcde0ae50 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/ThreadResumeParams.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/ThreadResumeParams.ts
@@ -36,7 +36,7 @@ model?: string | null, modelProvider?: string | null, serviceTier?: ServiceTier
  * Override where approval requests are routed for review on this thread
  * and subsequent turns.
  */
-approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, baseInstructions?: string | null, developerInstructions?: string | null, personality?: Personality | null, /**
+approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, baseInstructions?: string | null | null, developerInstructions?: string | null | null, personality?: Personality | null, /**
  * If true, persist additional rollout EventMsg variants required to
  * reconstruct a richer thread history on subsequent resume/fork/read.
  */
diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/ThreadStartParams.ts b/codex-rs/app-server-protocol/schema/typescript/v2/ThreadStartParams.ts
index 61f501ad6..c7b27823f 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/ThreadStartParams.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/ThreadStartParams.ts
@@ -12,7 +12,7 @@ export type ThreadStartParams = {model?: string | null, modelProvider?: string |
  * Override where approval requests are routed for review on this thread
  * 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, /**
+approvalsReviewer?: ApprovalsReviewer | null, sandbox?: SandboxMode | null, config?: { [key in string]?: JsonValue } | null, serviceName?: string | null, baseInstructions?: string | null | null, developerInstructions?: string | null | null, personality?: Personality | null, ephemeral?: boolean | null, /**
  * If true, opt into emitting raw Responses API items on the event stream.
  * This is for internal use only (e.g. Codex Cloud).
  */
diff --git a/codex-rs/app-server-protocol/src/protocol/v2.rs b/codex-rs/app-server-protocol/src/protocol/v2.rs
index 61b7db45e..3562d9cda 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2.rs
@@ -2603,10 +2603,22 @@ pub struct ThreadStartParams {
     pub config: Option<HashMap<String, JsonValue>>,
     #[ts(optional = nullable)]
     pub service_name: Option<String>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub developer_instructions: Option<String>,
+    pub developer_instructions: Option<Option<String>>,
     #[ts(optional = nullable)]
     pub personality: Option<Personality>,
     #[ts(optional = nullable)]
@@ -2721,10 +2733,22 @@ pub struct ThreadResumeParams {
     pub sandbox: Option<SandboxMode>,
     #[ts(optional = nullable)]
     pub config: Option<HashMap<String, serde_json::Value>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub developer_instructions: Option<String>,
+    pub developer_instructions: Option<Option<String>>,
     #[ts(optional = nullable)]
     pub personality: Option<Personality>,
     /// If true, persist additional rollout EventMsg variants required to
@@ -2798,10 +2822,22 @@ pub struct ThreadForkParams {
     pub sandbox: Option<SandboxMode>,
     #[ts(optional = nullable)]
     pub config: Option<HashMap<String, serde_json::Value>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub developer_instructions: Option<String>,
+    pub developer_instructions: Option<Option<String>>,
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub ephemeral: bool,
     /// If true, persist additional rollout EventMsg variants required to
@@ -8325,6 +8361,35 @@ mod tests {
         assert_eq!(serialized_without_override.get("serviceTier"), None);
     }
 
+    #[test]
+    fn thread_start_params_preserve_explicit_null_instructions() {
+        let params: ThreadStartParams = serde_json::from_value(json!({
+            "baseInstructions": null,
+            "developerInstructions": null,
+        }))
+        .expect("params should deserialize");
+        assert_eq!(params.base_instructions, Some(None));
+        assert_eq!(params.developer_instructions, Some(None));
+
+        let serialized = serde_json::to_value(&params).expect("params should serialize");
+        assert_eq!(
+            serialized.get("baseInstructions"),
+            Some(&serde_json::Value::Null)
+        );
+        assert_eq!(
+            serialized.get("developerInstructions"),
+            Some(&serde_json::Value::Null)
+        );
+
+        let serialized_without_override =
+            serde_json::to_value(ThreadStartParams::default()).expect("params should serialize");
+        assert_eq!(serialized_without_override.get("baseInstructions"), None);
+        assert_eq!(
+            serialized_without_override.get("developerInstructions"),
+            None
+        );
+    }
+
     #[test]
     fn turn_start_params_preserve_explicit_null_service_tier() {
         let params: TurnStartParams = serde_json::from_value(json!({
diff --git a/codex-rs/app-server/src/codex_message_processor.rs b/codex-rs/app-server/src/codex_message_processor.rs
index 920fcbece..91641898d 100644
--- a/codex-rs/app-server/src/codex_message_processor.rs
+++ b/codex-rs/app-server/src/codex_message_processor.rs
@@ -2473,8 +2473,8 @@ impl CodexMessageProcessor {
         approval_policy: Option<codex_app_server_protocol::AskForApproval>,
         approvals_reviewer: Option<codex_app_server_protocol::ApprovalsReviewer>,
         sandbox: Option<SandboxMode>,
-        base_instructions: Option<String>,
-        developer_instructions: Option<String>,
+        base_instructions: Option<Option<String>>,
+        developer_instructions: Option<Option<String>>,
         personality: Option<Personality>,
     ) -> ConfigOverrides {
         ConfigOverrides {
@@ -4363,6 +4363,13 @@ impl CodexMessageProcessor {
             developer_instructions,
             /*personality*/ None,
         );
+        if typesafe_overrides.base_instructions.is_none()
+            && let Ok(history) = RolloutRecorder::get_rollout_history(&rollout_path).await
+            && let Some(base_instructions) = history.get_base_instructions()
+        {
+            typesafe_overrides.base_instructions =
+                Some(base_instructions.map(|base_instructions| base_instructions.text));
+        }
         typesafe_overrides.ephemeral = ephemeral.then_some(true);
         // Derive a Config using the same logic as new conversation, honoring overrides if provided.
         let cloud_requirements = self.current_cloud_requirements();
diff --git a/codex-rs/app-server/tests/suite/v2/thread_fork.rs b/codex-rs/app-server/tests/suite/v2/thread_fork.rs
index 9907fc4b1..b4e80d781 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_fork.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_fork.rs
@@ -26,6 +26,8 @@ use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
 use codex_config::types::AuthCredentialsStoreMode;
 use codex_login::REFRESH_TOKEN_URL_OVERRIDE_ENV_VAR;
+use core_test_support::responses;
+use core_test_support::skip_if_no_network;
 use pretty_assertions::assert_eq;
 use serde_json::Value;
 use serde_json::json;
@@ -183,6 +185,153 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_fork_honors_explicit_null_thread_instructions() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let response_mock =
+        responses::mount_sse_sequence(&server, vec![body.clone(), body.clone(), body]).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let conversation_id = create_fake_rollout(
+        codex_home.path(),
+        "2025-01-05T12-00-00",
+        "2025-01-05T12:00:00Z",
+        "Saved user message",
+        Some("mock_provider"),
+        /*git_info*/ None,
+    )?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let disabled_instruction_config = json!({
+        "include_permissions_instructions": false,
+        "include_apps_instructions": false,
+        "include_environment_context": false,
+        "features.apps": false,
+        "features.plugins": false,
+        "features.codex_hooks": false,
+        "skills.bundled.enabled": false,
+    });
+
+    let fork_params = [
+        (
+            json!({
+                "threadId": conversation_id.clone(),
+                "config": disabled_instruction_config.clone(),
+            }),
+            /*expect_instructions*/ true,
+        ),
+        (
+            json!({
+                "threadId": conversation_id.clone(),
+                "config": disabled_instruction_config.clone(),
+                "baseInstructions": null,
+                "developerInstructions": null,
+            }),
+            /*expect_instructions*/ false,
+        ),
+    ];
+
+    let mut forked_thread_ids = Vec::new();
+    for (params, _expect_instructions) in fork_params {
+        let fork_id = mcp.send_raw_request("thread/fork", Some(params)).await?;
+        let fork_resp: JSONRPCResponse = timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_response_message(RequestId::Integer(fork_id)),
+        )
+        .await??;
+        let ThreadForkResponse { thread, .. } = to_response::<ThreadForkResponse>(fork_resp)?;
+        forked_thread_ids.push(thread.id.clone());
+
+        let turn_id = mcp
+            .send_turn_start_request(TurnStartParams {
+                thread_id: thread.id,
+                input: vec![UserInput::Text {
+                    text: "continue".to_string(),
+                    text_elements: Vec::new(),
+                }],
+                ..Default::default()
+            })
+            .await?;
+        let turn_resp: JSONRPCResponse = timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_response_message(RequestId::Integer(turn_id)),
+        )
+        .await??;
+        let _: TurnStartResponse = to_response::<TurnStartResponse>(turn_resp)?;
+        timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_notification_message("turn/completed"),
+        )
+        .await??;
+    }
+
+    let refork_id = mcp
+        .send_raw_request(
+            "thread/fork",
+            Some(json!({
+                "threadId": forked_thread_ids[1].clone(),
+                "config": disabled_instruction_config.clone(),
+            })),
+        )
+        .await?;
+    let refork_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(refork_id)),
+    )
+    .await??;
+    let ThreadForkResponse { thread, .. } = to_response::<ThreadForkResponse>(refork_resp)?;
+    let turn_id = mcp
+        .send_turn_start_request(TurnStartParams {
+            thread_id: thread.id,
+            input: vec![UserInput::Text {
+                text: "continue again".to_string(),
+                text_elements: Vec::new(),
+            }],
+            ..Default::default()
+        })
+        .await?;
+    let turn_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(turn_id)),
+    )
+    .await??;
+    let _: TurnStartResponse = to_response::<TurnStartResponse>(turn_resp)?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("turn/completed"),
+    )
+    .await??;
+
+    let requests = response_mock.requests();
+    assert_eq!(requests.len(), 3);
+    for (request, expect_instructions) in requests.into_iter().zip([true, false, false]) {
+        let payload = request.body_json();
+        assert_eq!(
+            payload.get("instructions").is_some(),
+            expect_instructions,
+            "unexpected instructions field in payload: {payload:?}"
+        );
+        let developer_texts = request.message_input_texts("developer");
+        assert!(
+            developer_texts.iter().all(|text| !text.is_empty()),
+            "did not expect empty developer instruction messages: {developer_texts:?}"
+        );
+    }
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_fork_tracks_thread_initialized_analytics() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;
diff --git a/codex-rs/app-server/tests/suite/v2/turn_start.rs b/codex-rs/app-server/tests/suite/v2/turn_start.rs
index 59591dd0d..e8f3e1df2 100644
--- a/codex-rs/app-server/tests/suite/v2/turn_start.rs
+++ b/codex-rs/app-server/tests/suite/v2/turn_start.rs
@@ -152,6 +152,103 @@ async fn turn_start_sends_originator_header() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn turn_start_honors_explicit_null_thread_instructions() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let response_mock = responses::mount_sse_sequence(&server, vec![body.clone(), body]).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri(), "never", &BTreeMap::new())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let disabled_instruction_config = json!({
+        "include_permissions_instructions": false,
+        "include_apps_instructions": false,
+        "include_environment_context": false,
+        "features.apps": false,
+        "features.plugins": false,
+        "features.codex_hooks": false,
+        "skills.bundled.enabled": false,
+    });
+
+    let thread_start_params = [
+        (
+            json!({
+                "model": "mock-model",
+                "config": disabled_instruction_config.clone(),
+            }),
+            /*expect_instructions*/ true,
+        ),
+        (
+            json!({
+                "model": "mock-model",
+                "config": disabled_instruction_config.clone(),
+                "baseInstructions": null,
+                "developerInstructions": null,
+            }),
+            /*expect_instructions*/ false,
+        ),
+    ];
+
+    for (params, _expect_instructions) in thread_start_params {
+        let thread_req = mcp.send_raw_request("thread/start", Some(params)).await?;
+        let thread_resp: JSONRPCResponse = timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_response_message(RequestId::Integer(thread_req)),
+        )
+        .await??;
+        let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(thread_resp)?;
+
+        let turn_req = mcp
+            .send_turn_start_request(TurnStartParams {
+                thread_id: thread.id,
+                input: vec![V2UserInput::Text {
+                    text: "Hello".to_string(),
+                    text_elements: Vec::new(),
+                }],
+                ..Default::default()
+            })
+            .await?;
+        timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_response_message(RequestId::Integer(turn_req)),
+        )
+        .await??;
+        timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_notification_message("turn/completed"),
+        )
+        .await??;
+    }
+
+    let requests = response_mock.requests();
+    assert_eq!(requests.len(), 2);
+    for (request, expect_instructions) in requests.into_iter().zip([true, false]) {
+        let payload = request.body_json();
+        assert_eq!(
+            payload.get("instructions").is_some(),
+            expect_instructions,
+            "unexpected instructions field in payload: {payload:?}"
+        );
+        let developer_texts = request.message_input_texts("developer");
+        assert!(
+            developer_texts.iter().all(|text| !text.is_empty()),
+            "did not expect empty developer instruction messages: {developer_texts:?}"
+        );
+    }
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn turn_start_emits_user_message_item_with_text_elements() -> Result<()> {
     let responses = vec![create_final_assistant_message_sse_response("Done")?];
diff --git a/codex-rs/codex-api/src/common.rs b/codex-rs/codex-api/src/common.rs
index 39fb976e6..a2a2c1d47 100644
--- a/codex-rs/codex-api/src/common.rs
+++ b/codex-rs/codex-api/src/common.rs
@@ -153,7 +153,8 @@ impl From<VerbosityConfig> for OpenAiVerbosity {
 #[derive(Debug, Serialize, Clone, PartialEq)]
 pub struct ResponsesApiRequest {
     pub model: String,
-    pub instructions: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub instructions: Option<String>,
     pub input: Vec<ResponseItem>,
     pub tools: Vec<serde_json::Value>,
     pub tool_choice: String,
@@ -196,7 +197,8 @@ impl From<&ResponsesApiRequest> for ResponseCreateWsRequest {
 #[derive(Debug, Serialize)]
 pub struct ResponseCreateWsRequest {
     pub model: String,
-    pub instructions: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub instructions: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub previous_response_id: Option<String>,
     pub input: Vec<ResponseItem>,
diff --git a/codex-rs/codex-api/tests/clients.rs b/codex-rs/codex-api/tests/clients.rs
index ba0f41678..2f3bc17e2 100644
--- a/codex-rs/codex-api/tests/clients.rs
+++ b/codex-rs/codex-api/tests/clients.rs
@@ -266,7 +266,7 @@ async fn streaming_client_retries_on_transport_error() -> Result<()> {
 
     let request = ResponsesApiRequest {
         model: "gpt-test".into(),
-        instructions: "Say hi".into(),
+        instructions: Some("Say hi".into()),
         input: Vec::new(),
         tools: Vec::new(),
         tool_choice: "auto".into(),
@@ -302,7 +302,7 @@ async fn azure_default_store_attaches_ids_and_headers() -> Result<()> {
 
     let request = ResponsesApiRequest {
         model: "gpt-test".into(),
-        instructions: "Say hi".into(),
+        instructions: Some("Say hi".into()),
         input: vec![ResponseItem::Message {
             id: Some("msg_1".into()),
             role: "user".into(),
diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index d77e36870..b77dda2b7 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -399,7 +399,11 @@ impl ModelClient {
             ApiCompactClient::new(transport, client_setup.api_provider, client_setup.api_auth)
                 .with_telemetry(Some(request_telemetry));
 
-        let instructions = prompt.base_instructions.text.clone();
+        let instructions = prompt
+            .base_instructions
+            .as_ref()
+            .map(|base_instructions| base_instructions.text.clone())
+            .unwrap_or_default();
         let input = prompt.get_formatted_input();
         let tools = create_tools_json_for_responses_api(&prompt.tools)?;
         let reasoning = Self::build_reasoning(model_info, effort, summary);
@@ -755,7 +759,10 @@ impl ModelClientSession {
         summary: ReasoningSummaryConfig,
         service_tier: Option<ServiceTier>,
     ) -> Result<ResponsesApiRequest> {
-        let instructions = &prompt.base_instructions.text;
+        let instructions = prompt
+            .base_instructions
+            .as_ref()
+            .map(|base_instructions| base_instructions.text.clone());
         let input = prompt.get_formatted_input();
         let tools = create_tools_json_for_responses_api(&prompt.tools)?;
         let default_reasoning_effort = model_info.default_reasoning_level;
@@ -794,7 +801,7 @@ impl ModelClientSession {
         let prompt_cache_key = Some(self.client.state.conversation_id.to_string());
         let request = ResponsesApiRequest {
             model: model_info.slug.clone(),
-            instructions: instructions.clone(),
+            instructions,
             input,
             tools,
             tool_choice: "auto".to_string(),
diff --git a/codex-rs/core/src/client_common.rs b/codex-rs/core/src/client_common.rs
index cb50ccac3..828839ae7 100644
--- a/codex-rs/core/src/client_common.rs
+++ b/codex-rs/core/src/client_common.rs
@@ -23,7 +23,7 @@ pub const REVIEW_EXIT_INTERRUPTED_TMPL: &str =
     include_str!("../templates/review/exit_interrupted.xml");
 
 /// API request payload for a single model turn
-#[derive(Default, Debug, Clone)]
+#[derive(Debug, Clone)]
 pub struct Prompt {
     /// Conversation context input items.
     pub input: Vec<ResponseItem>,
@@ -35,7 +35,7 @@ pub struct Prompt {
     /// Whether parallel tool calls are permitted for this prompt.
     pub(crate) parallel_tool_calls: bool,
 
-    pub base_instructions: BaseInstructions,
+    pub base_instructions: Option<BaseInstructions>,
 
     /// Optionally specify the personality of the model.
     pub personality: Option<Personality>,
@@ -44,6 +44,19 @@ pub struct Prompt {
     pub output_schema: Option<Value>,
 }
 
+impl Default for Prompt {
+    fn default() -> Self {
+        Self {
+            input: Vec::new(),
+            tools: Vec::new(),
+            parallel_tool_calls: false,
+            base_instructions: Some(BaseInstructions::default()),
+            personality: None,
+            output_schema: None,
+        }
+    }
+}
+
 impl Prompt {
     pub(crate) fn get_formatted_input(&self) -> Vec<ResponseItem> {
         let mut input = self.input.clone();
diff --git a/codex-rs/core/src/client_common_tests.rs b/codex-rs/core/src/client_common_tests.rs
index 16a37796a..968703046 100644
--- a/codex-rs/core/src/client_common_tests.rs
+++ b/codex-rs/core/src/client_common_tests.rs
@@ -14,7 +14,7 @@ fn serializes_text_verbosity_when_set() {
     let tools: Vec<serde_json::Value> = vec![];
     let req = ResponsesApiRequest {
         model: "gpt-5.1".to_string(),
-        instructions: "i".to_string(),
+        instructions: Some("i".to_string()),
         input,
         tools,
         tool_choice: "auto".to_string(),
@@ -57,7 +57,7 @@ fn serializes_text_schema_with_strict_format() {
 
     let req = ResponsesApiRequest {
         model: "gpt-5.1".to_string(),
-        instructions: "i".to_string(),
+        instructions: Some("i".to_string()),
         input,
         tools,
         tool_choice: "auto".to_string(),
@@ -94,7 +94,7 @@ fn omits_text_when_not_set() {
     let tools: Vec<serde_json::Value> = vec![];
     let req = ResponsesApiRequest {
         model: "gpt-5.1".to_string(),
-        instructions: "i".to_string(),
+        instructions: Some("i".to_string()),
         input,
         tools,
         tool_choice: "auto".to_string(),
@@ -116,7 +116,7 @@ fn omits_text_when_not_set() {
 fn serializes_flex_service_tier_when_set() {
     let req = ResponsesApiRequest {
         model: "gpt-5.1".to_string(),
-        instructions: "i".to_string(),
+        instructions: Some("i".to_string()),
         input: vec![],
         tools: vec![],
         tool_choice: "auto".to_string(),
diff --git a/codex-rs/core/src/codex.rs b/codex-rs/core/src/codex.rs
index 423c62848..237b77e38 100644
--- a/codex-rs/core/src/codex.rs
+++ b/codex-rs/core/src/codex.rs
@@ -581,11 +581,15 @@ impl Codex {
         let model_info = models_manager
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;
-        let base_instructions = config
-            .base_instructions
-            .clone()
-            .or_else(|| conversation_history.get_base_instructions().map(|s| s.text))
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality));
+        let base_instructions = match config.base_instructions.clone() {
+            Some(base_instructions) => base_instructions,
+            None => conversation_history
+                .get_base_instructions()
+                .map(|base_instructions| {
+                    base_instructions.map(|base_instructions| base_instructions.text)
+                })
+                .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
+        };
 
         // Respect thread-start tools. When missing (resumed/forked threads), read from the db
         // first, then fall back to rollout-file tools.
@@ -1106,7 +1110,7 @@ pub(crate) struct SessionConfiguration {
     personality: Option<Personality>,
 
     /// Base instructions for the session.
-    base_instructions: String,
+    base_instructions: Option<String>,
 
     /// Compact prompt override.
     compact_prompt: Option<String>,
@@ -1545,9 +1549,10 @@ impl Session {
                         conversation_id,
                         forked_from_id,
                         session_source,
-                        BaseInstructions {
-                            text: session_configuration.base_instructions.clone(),
-                        },
+                        session_configuration
+                            .base_instructions
+                            .clone()
+                            .map(|text| BaseInstructions { text }),
                         session_configuration.dynamic_tools.clone(),
                         if session_configuration.persist_extended_history {
                             EventPersistenceMode::Extended
@@ -2109,8 +2114,9 @@ impl Session {
                 ));
             }
         }
-        sess.schedule_startup_prewarm(session_configuration.base_instructions.clone())
-            .await;
+        if let Some(base_instructions) = session_configuration.base_instructions.clone() {
+            sess.schedule_startup_prewarm(base_instructions).await;
+        }
         let session_start_source = match &initial_history {
             InitialHistory::Resumed(_) => codex_hooks::SessionStartSource::Resume,
             InitialHistory::New | InitialHistory::Forked(_) => {
@@ -2212,11 +2218,13 @@ impl Session {
         state.history.estimate_token_count(turn_context)
     }
 
-    pub(crate) async fn get_base_instructions(&self) -> BaseInstructions {
+    pub(crate) async fn get_base_instructions(&self) -> Option<BaseInstructions> {
         let state = self.state.lock().await;
-        BaseInstructions {
-            text: state.session_configuration.base_instructions.clone(),
-        }
+        state
+            .session_configuration
+            .base_instructions
+            .clone()
+            .map(|text| BaseInstructions { text })
     }
 
     // Merges connector IDs into the session-level explicit connector selection.
@@ -3620,7 +3628,11 @@ impl Session {
                 state.reference_context_item(),
                 state.previous_turn_settings(),
                 state.session_configuration.collaboration_mode.clone(),
-                state.session_configuration.base_instructions.clone(),
+                state
+                    .session_configuration
+                    .base_instructions
+                    .clone()
+                    .unwrap_or_default(),
                 state.session_configuration.session_source.clone(),
             )
         };
@@ -3861,7 +3873,13 @@ impl Session {
 
     pub(crate) async fn recompute_token_usage(&self, turn_context: &TurnContext) {
         let history = self.clone_history().await;
-        let base_instructions = self.get_base_instructions().await;
+        let empty_base_instructions = BaseInstructions {
+            text: String::new(),
+        };
+        let base_instructions = self
+            .get_base_instructions()
+            .await
+            .unwrap_or(empty_base_instructions);
         let Some(estimated_total_tokens) =
             history.estimate_token_count_with_base_instructions(&base_instructions)
         else {
@@ -6555,7 +6573,7 @@ pub(crate) fn build_prompt(
     input: Vec<ResponseItem>,
     router: &ToolRouter,
     turn_context: &TurnContext,
-    base_instructions: BaseInstructions,
+    base_instructions: Option<BaseInstructions>,
 ) -> Prompt {
     let deferred_dynamic_tools = turn_context
         .dynamic_tools
diff --git a/codex-rs/core/src/codex_tests.rs b/codex-rs/core/src/codex_tests.rs
index cf7c23be9..b0437edbf 100644
--- a/codex-rs/core/src/codex_tests.rs
+++ b/codex-rs/core/src/codex_tests.rs
@@ -591,11 +591,15 @@ async fn get_base_instructions_no_user_content() {
 
         {
             let mut state = session.state.lock().await;
-            state.session_configuration.base_instructions = model_info.base_instructions.clone();
+            state.session_configuration.base_instructions =
+                Some(model_info.base_instructions.clone());
         }
 
         let base_instructions = session.get_base_instructions().await;
-        assert_eq!(base_instructions.text, model_info.base_instructions);
+        assert_eq!(
+            base_instructions.expect("base instructions").text,
+            model_info.base_instructions
+        );
     }
 }
 
@@ -1091,7 +1095,7 @@ async fn recompute_token_usage_uses_session_base_instructions() {
     let override_instructions = "SESSION_OVERRIDE_INSTRUCTIONS_ONLY".repeat(120);
     {
         let mut state = session.state.lock().await;
-        state.session_configuration.base_instructions = override_instructions.clone();
+        state.session_configuration.base_instructions = Some(override_instructions.clone());
     }
 
     let item = user_message("hello");
@@ -1855,7 +1859,7 @@ async fn set_rate_limits_retains_previous_credits() {
         base_instructions: config
             .base_instructions
             .clone()
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+            .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
         compact_prompt: config.compact_prompt.clone(),
         approval_policy: config.permissions.approval_policy.clone(),
         approvals_reviewer: config.approvals_reviewer,
@@ -1957,7 +1961,7 @@ async fn set_rate_limits_updates_plan_type_when_present() {
         base_instructions: config
             .base_instructions
             .clone()
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+            .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
         compact_prompt: config.compact_prompt.clone(),
         approval_policy: config.permissions.approval_policy.clone(),
         approvals_reviewer: config.approvals_reviewer,
@@ -2222,7 +2226,7 @@ async fn attach_rollout_recorder(session: &Arc<Session>) -> PathBuf {
             ThreadId::default(),
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
@@ -2306,7 +2310,7 @@ pub(crate) async fn make_session_configuration_for_tests() -> SessionConfigurati
         base_instructions: config
             .base_instructions
             .clone()
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+            .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
         compact_prompt: config.compact_prompt.clone(),
         approval_policy: config.permissions.approval_policy.clone(),
         approvals_reviewer: config.approvals_reviewer,
@@ -2572,7 +2576,7 @@ async fn session_new_fails_when_zsh_fork_enabled_without_zsh_path() {
         base_instructions: config
             .base_instructions
             .clone()
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+            .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
         compact_prompt: config.compact_prompt.clone(),
         approval_policy: config.permissions.approval_policy.clone(),
         approvals_reviewer: config.approvals_reviewer,
@@ -2675,7 +2679,7 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
         base_instructions: config
             .base_instructions
             .clone()
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+            .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
         compact_prompt: config.compact_prompt.clone(),
         approval_policy: config.permissions.approval_policy.clone(),
         approvals_reviewer: config.approvals_reviewer,
@@ -3515,7 +3519,7 @@ pub(crate) async fn make_session_and_context_with_dynamic_tools_and_rx(
         base_instructions: config
             .base_instructions
             .clone()
-            .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+            .unwrap_or_else(|| Some(model_info.get_model_instructions(config.personality))),
         compact_prompt: config.compact_prompt.clone(),
         approval_policy: config.permissions.approval_policy.clone(),
         approvals_reviewer: config.approvals_reviewer,
@@ -4264,7 +4268,7 @@ async fn record_context_updates_and_set_reference_context_item_persists_baseline
             ThreadId::default(),
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
@@ -4361,7 +4365,7 @@ async fn record_context_updates_and_set_reference_context_item_persists_full_rei
             ThreadId::default(),
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
diff --git a/codex-rs/core/src/compact_remote.rs b/codex-rs/core/src/compact_remote.rs
index 5bc7944d2..640ca3c1f 100644
--- a/codex-rs/core/src/compact_remote.rs
+++ b/codex-rs/core/src/compact_remote.rs
@@ -76,10 +76,16 @@ async fn run_remote_compact_task_inner_impl(
         .await;
     let mut history = sess.clone_history().await;
     let base_instructions = sess.get_base_instructions().await;
+    let token_count_base_instructions =
+        base_instructions
+            .clone()
+            .unwrap_or_else(|| BaseInstructions {
+                text: String::new(),
+            });
     let deleted_items = trim_function_call_history_to_fit_context_window(
         &mut history,
         turn_context.as_ref(),
-        &base_instructions,
+        &token_count_base_instructions,
     );
     if deleted_items > 0 {
         info!(
@@ -127,8 +133,13 @@ async fn run_remote_compact_task_inner_impl(
         )
         .or_else(|err| async {
             let total_usage_breakdown = sess.get_total_token_usage_breakdown().await;
+            let base_instruction_text = prompt
+                .base_instructions
+                .as_ref()
+                .map(|base_instructions| base_instructions.text.as_str())
+                .unwrap_or("");
             let compact_request_log_data =
-                build_compact_request_log_data(&prompt.input, &prompt.base_instructions.text);
+                build_compact_request_log_data(&prompt.input, base_instruction_text);
             log_remote_compact_failure(
                 turn_context,
                 &compact_request_log_data,
diff --git a/codex-rs/core/src/config/mod.rs b/codex-rs/core/src/config/mod.rs
index 3c2d80464..20d3dd2f1 100644
--- a/codex-rs/core/src/config/mod.rs
+++ b/codex-rs/core/src/config/mod.rs
@@ -243,7 +243,7 @@ pub struct Config {
     pub user_instructions: Option<String>,
 
     /// Base instructions override.
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
 
     /// Developer instructions override injected as a separate message.
     pub developer_instructions: Option<String>,
@@ -687,7 +687,7 @@ impl Config {
             model_context_window: self.model_context_window,
             model_auto_compact_token_limit: self.model_auto_compact_token_limit,
             tool_output_token_limit: self.tool_output_token_limit,
-            base_instructions: self.base_instructions.clone(),
+            base_instructions: self.base_instructions.clone().flatten(),
             personality_enabled: self.features.enabled(Feature::Personality),
             model_supports_reasoning_summaries: self.model_supports_reasoning_summaries,
             model_catalog: self.model_catalog.clone(),
@@ -1200,8 +1200,8 @@ pub struct ConfigOverrides {
     pub js_repl_node_path: Option<PathBuf>,
     pub js_repl_node_module_dirs: Option<Vec<PathBuf>>,
     pub zsh_path: Option<PathBuf>,
-    pub base_instructions: Option<String>,
-    pub developer_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    pub developer_instructions: Option<Option<String>>,
     pub personality: Option<Personality>,
     pub compact_prompt: Option<String>,
     pub include_apply_patch_tool: Option<bool>,
@@ -1760,8 +1760,9 @@ impl Config {
             .or(cfg.model_instructions_file.as_ref());
         let file_base_instructions =
             Self::try_read_non_empty_file(model_instructions_path, "model instructions file")?;
-        let base_instructions = base_instructions.or(file_base_instructions);
-        let developer_instructions = developer_instructions.or(cfg.developer_instructions);
+        let base_instructions = base_instructions.or_else(|| file_base_instructions.map(Some));
+        let developer_instructions =
+            developer_instructions.unwrap_or_else(|| cfg.developer_instructions.clone());
         let include_permissions_instructions = config_profile
             .include_permissions_instructions
             .or(cfg.include_permissions_instructions)
diff --git a/codex-rs/core/src/config_loader/tests.rs b/codex-rs/core/src/config_loader/tests.rs
index 7978f37ec..629730cbf 100644
--- a/codex-rs/core/src/config_loader/tests.rs
+++ b/codex-rs/core/src/config_loader/tests.rs
@@ -905,7 +905,7 @@ model_instructions_file = "child.txt"
         .await?;
 
     assert_eq!(
-        config.base_instructions.as_deref(),
+        config.base_instructions.as_ref().and_then(Option::as_deref),
         Some("child instructions")
     );
 
@@ -941,7 +941,7 @@ async fn cli_override_model_instructions_file_sets_base_instructions() -> std::i
         .await?;
 
     assert_eq!(
-        config.base_instructions.as_deref(),
+        config.base_instructions.as_ref().and_then(Option::as_deref),
         Some("cli override instructions")
     );
 
diff --git a/codex-rs/core/src/guardian/review_session.rs b/codex-rs/core/src/guardian/review_session.rs
index bc24974c8..368b812e5 100644
--- a/codex-rs/core/src/guardian/review_session.rs
+++ b/codex-rs/core/src/guardian/review_session.rs
@@ -137,7 +137,7 @@ impl GuardianReviewSessionReuseKey {
             model_reasoning_summary: spawn_config.model_reasoning_summary,
             permissions: spawn_config.permissions.clone(),
             developer_instructions: spawn_config.developer_instructions.clone(),
-            base_instructions: spawn_config.base_instructions.clone(),
+            base_instructions: spawn_config.base_instructions.clone().flatten(),
             user_instructions: spawn_config.user_instructions.clone(),
             compact_prompt: spawn_config.compact_prompt.clone(),
             cwd: spawn_config.cwd.to_path_buf(),
diff --git a/codex-rs/core/src/memories/phase1.rs b/codex-rs/core/src/memories/phase1.rs
index 37cb0e9f4..0b8cdcdad 100644
--- a/codex-rs/core/src/memories/phase1.rs
+++ b/codex-rs/core/src/memories/phase1.rs
@@ -336,9 +336,9 @@ mod job {
             }],
             tools: Vec::new(),
             parallel_tool_calls: false,
-            base_instructions: BaseInstructions {
+            base_instructions: Some(BaseInstructions {
                 text: phase_one::PROMPT.to_string(),
-            },
+            }),
             personality: None,
             output_schema: Some(output_schema()),
         };
diff --git a/codex-rs/core/src/session_startup_prewarm.rs b/codex-rs/core/src/session_startup_prewarm.rs
index 397ffc949..708401aed 100644
--- a/codex-rs/core/src/session_startup_prewarm.rs
+++ b/codex-rs/core/src/session_startup_prewarm.rs
@@ -217,9 +217,9 @@ async fn schedule_startup_prewarm_inner(
         Vec::new(),
         startup_router.as_ref(),
         startup_turn_context.as_ref(),
-        BaseInstructions {
+        Some(BaseInstructions {
             text: base_instructions,
-        },
+        }),
     );
     let startup_turn_metadata_header = startup_turn_context
         .turn_metadata_state
diff --git a/codex-rs/core/src/tasks/review.rs b/codex-rs/core/src/tasks/review.rs
index a1cc07110..834cf20b4 100644
--- a/codex-rs/core/src/tasks/review.rs
+++ b/codex-rs/core/src/tasks/review.rs
@@ -112,7 +112,7 @@ async fn start_review_conversation(
     let _ = sub_agent_config.features.disable(Feature::Collab);
 
     // Set explicit review rubric for the sub-agent
-    sub_agent_config.base_instructions = Some(crate::REVIEW_PROMPT.to_string());
+    sub_agent_config.base_instructions = Some(Some(crate::REVIEW_PROMPT.to_string()));
     sub_agent_config.permissions.approval_policy = Constrained::allow_only(AskForApproval::Never);
 
     let model = config
diff --git a/codex-rs/core/src/tools/handlers/agent_jobs.rs b/codex-rs/core/src/tools/handlers/agent_jobs.rs
index fe30f3d75..837b654ed 100644
--- a/codex-rs/core/src/tools/handlers/agent_jobs.rs
+++ b/codex-rs/core/src/tools/handlers/agent_jobs.rs
@@ -535,7 +535,7 @@ async fn build_runner_options(
     let max_concurrency =
         normalize_concurrency(requested_concurrency, turn.config.agent_max_threads);
     let base_instructions = session.get_base_instructions().await;
-    let spawn_config = build_agent_spawn_config(&base_instructions, turn.as_ref())?;
+    let spawn_config = build_agent_spawn_config(base_instructions.as_ref(), turn.as_ref())?;
     Ok(JobRunnerOptions {
         max_concurrency,
         spawn_config,
diff --git a/codex-rs/core/src/tools/handlers/multi_agents/spawn.rs b/codex-rs/core/src/tools/handlers/multi_agents/spawn.rs
index 8e4bfb5b5..a2482ea6a 100644
--- a/codex-rs/core/src/tools/handlers/multi_agents/spawn.rs
+++ b/codex-rs/core/src/tools/handlers/multi_agents/spawn.rs
@@ -59,8 +59,10 @@ impl ToolHandler for Handler {
                 .into(),
             )
             .await;
-        let mut config =
-            build_agent_spawn_config(&session.get_base_instructions().await, turn.as_ref())?;
+        let mut config = build_agent_spawn_config(
+            session.get_base_instructions().await.as_ref(),
+            turn.as_ref(),
+        )?;
         apply_requested_spawn_agent_model_overrides(
             &session,
             turn.as_ref(),
diff --git a/codex-rs/core/src/tools/handlers/multi_agents_common.rs b/codex-rs/core/src/tools/handlers/multi_agents_common.rs
index 2078c229b..4205c4bb4 100644
--- a/codex-rs/core/src/tools/handlers/multi_agents_common.rs
+++ b/codex-rs/core/src/tools/handlers/multi_agents_common.rs
@@ -201,11 +201,12 @@ pub(crate) fn parse_collab_input(
 /// skipping this helper and cloning stale config state directly can send the child agent out with
 /// the wrong provider or runtime policy.
 pub(crate) fn build_agent_spawn_config(
-    base_instructions: &BaseInstructions,
+    base_instructions: Option<&BaseInstructions>,
     turn: &TurnContext,
 ) -> Result<Config, FunctionCallError> {
     let mut config = build_agent_shared_config(turn)?;
-    config.base_instructions = Some(base_instructions.text.clone());
+    config.base_instructions =
+        Some(base_instructions.map(|base_instructions| base_instructions.text.clone()));
     Ok(config)
 }
 
diff --git a/codex-rs/core/src/tools/handlers/multi_agents_tests.rs b/codex-rs/core/src/tools/handlers/multi_agents_tests.rs
index eab43f899..03f4b0780 100644
--- a/codex-rs/core/src/tools/handlers/multi_agents_tests.rs
+++ b/codex-rs/core/src/tools/handlers/multi_agents_tests.rs
@@ -3209,9 +3209,9 @@ async fn build_agent_spawn_config_uses_turn_context_values() {
         .set(AskForApproval::OnRequest)
         .expect("approval policy set");
 
-    let config = build_agent_spawn_config(&base_instructions, &turn).expect("spawn config");
+    let config = build_agent_spawn_config(Some(&base_instructions), &turn).expect("spawn config");
     let mut expected = (*turn.config).clone();
-    expected.base_instructions = Some(base_instructions.text);
+    expected.base_instructions = Some(Some(base_instructions.text));
     expected.model = Some(turn.model_info.slug.clone());
     expected.model_provider = turn.provider.clone();
     expected.model_reasoning_effort = turn.reasoning_effort;
@@ -3247,7 +3247,7 @@ async fn build_agent_spawn_config_preserves_base_user_instructions() {
         text: "base".to_string(),
     };
 
-    let config = build_agent_spawn_config(&base_instructions, &turn).expect("spawn config");
+    let config = build_agent_spawn_config(Some(&base_instructions), &turn).expect("spawn config");
 
     assert_eq!(config.user_instructions, base_config.user_instructions);
 }
@@ -3256,7 +3256,7 @@ async fn build_agent_spawn_config_preserves_base_user_instructions() {
 async fn build_agent_resume_config_clears_base_instructions() {
     let (_session, mut turn) = make_session_and_context().await;
     let mut base_config = (*turn.config).clone();
-    base_config.base_instructions = Some("caller-base".to_string());
+    base_config.base_instructions = Some(Some("caller-base".to_string()));
     turn.config = Arc::new(base_config);
     turn.approval_policy
         .set(AskForApproval::OnRequest)
diff --git a/codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs b/codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs
index 77c48af9e..a1cdb121d 100644
--- a/codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs
+++ b/codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs
@@ -68,8 +68,10 @@ impl ToolHandler for Handler {
                 .into(),
             )
             .await;
-        let mut config =
-            build_agent_spawn_config(&session.get_base_instructions().await, turn.as_ref())?;
+        let mut config = build_agent_spawn_config(
+            session.get_base_instructions().await.as_ref(),
+            turn.as_ref(),
+        )?;
         apply_requested_spawn_agent_model_overrides(
             &session,
             turn.as_ref(),
diff --git a/codex-rs/core/tests/suite/client.rs b/codex-rs/core/tests/suite/client.rs
index 8e719460e..cb97e4a3b 100644
--- a/codex-rs/core/tests/suite/client.rs
+++ b/codex-rs/core/tests/suite/client.rs
@@ -921,7 +921,7 @@ async fn includes_base_instructions_override_in_request() {
     let mut builder = test_codex()
         .with_auth(CodexAuth::from_api_key("Test API Key"))
         .with_config(|config| {
-            config.base_instructions = Some("test instructions".to_string());
+            config.base_instructions = Some(Some("test instructions".to_string()));
         });
     let codex = builder
         .build(&server)
@@ -953,6 +953,47 @@ async fn includes_base_instructions_override_in_request() {
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn omits_explicit_null_base_instructions_from_request() {
+    skip_if_no_network!();
+    // Mock server
+    let server = MockServer::start().await;
+    let resp_mock = mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp1"), ev_completed("resp1")]),
+    )
+    .await;
+
+    let mut builder = test_codex()
+        .with_auth(CodexAuth::from_api_key("Test API Key"))
+        .with_config(|config| {
+            config.base_instructions = Some(None);
+        });
+    let codex = builder
+        .build(&server)
+        .await
+        .expect("create new conversation")
+        .codex;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "hello".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await
+        .unwrap();
+
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
+
+    let request = resp_mock.single_request();
+    let request_body = request.body_json();
+
+    assert_eq!(request_body.get("instructions"), None);
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn chatgpt_auth_sends_correct_request() {
     skip_if_no_network!();
diff --git a/codex-rs/core/tests/suite/client_websockets.rs b/codex-rs/core/tests/suite/client_websockets.rs
index d348d4a02..0a1195a11 100755
--- a/codex-rs/core/tests/suite/client_websockets.rs
+++ b/codex-rs/core/tests/suite/client_websockets.rs
@@ -1654,9 +1654,9 @@ fn prompt_with_input(input: Vec<ResponseItem>) -> Prompt {
 
 fn prompt_with_input_and_instructions(input: Vec<ResponseItem>, instructions: &str) -> Prompt {
     let mut prompt = prompt_with_input(input);
-    prompt.base_instructions = BaseInstructions {
+    prompt.base_instructions = Some(BaseInstructions {
         text: instructions.to_string(),
-    };
+    });
     prompt
 }
 
diff --git a/codex-rs/core/tests/suite/compact_remote.rs b/codex-rs/core/tests/suite/compact_remote.rs
index 0069c5fb8..dd99e0f39 100644
--- a/codex-rs/core/tests/suite/compact_remote.rs
+++ b/codex-rs/core/tests/suite/compact_remote.rs
@@ -869,7 +869,7 @@ async fn remote_compact_trim_estimate_uses_session_base_instructions() -> Result
                 let override_base_instructions = override_base_instructions.clone();
                 move |config| {
                     config.model_context_window = Some(override_context_window);
-                    config.base_instructions = Some(override_base_instructions);
+                    config.base_instructions = Some(Some(override_base_instructions));
                 }
             }),
     )
diff --git a/codex-rs/core/tests/suite/personality.rs b/codex-rs/core/tests/suite/personality.rs
index 172738ffd..8a0cbdcb6 100644
--- a/codex-rs/core/tests/suite/personality.rs
+++ b/codex-rs/core/tests/suite/personality.rs
@@ -66,7 +66,7 @@ async fn base_instructions_override_disables_personality_template() {
         .enable(Feature::Personality)
         .expect("test config should allow feature update");
     config.personality = Some(Personality::Friendly);
-    config.base_instructions = Some("override instructions".to_string());
+    config.base_instructions = Some(Some("override instructions".to_string()));
 
     let model_info =
         codex_core::test_support::construct_model_info_offline("gpt-5.2-codex", &config);
diff --git a/codex-rs/core/tests/suite/rollout_list_find.rs b/codex-rs/core/tests/suite/rollout_list_find.rs
index 7445a4ba0..93edd1621 100644
--- a/codex-rs/core/tests/suite/rollout_list_find.rs
+++ b/codex-rs/core/tests/suite/rollout_list_find.rs
@@ -173,7 +173,7 @@ async fn find_locates_rollout_file_written_by_recorder() -> std::io::Result<()>
             thread_id,
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
diff --git a/codex-rs/core/tests/suite/stream_error_allows_next_turn.rs b/codex-rs/core/tests/suite/stream_error_allows_next_turn.rs
index 19d9d27cf..c5aa57ce7 100644
--- a/codex-rs/core/tests/suite/stream_error_allows_next_turn.rs
+++ b/codex-rs/core/tests/suite/stream_error_allows_next_turn.rs
@@ -84,7 +84,7 @@ async fn continue_after_stream_error() {
 
     let TestCodex { codex, .. } = test_codex()
         .with_config(move |config| {
-            config.base_instructions = Some("You are a helpful assistant".to_string());
+            config.base_instructions = Some(Some("You are a helpful assistant".to_string()));
             config.model_provider = provider;
         })
         .build(&server)
diff --git a/codex-rs/mcp-server/src/codex_tool_config.rs b/codex-rs/mcp-server/src/codex_tool_config.rs
index f83fd4fd5..341f0ac4b 100644
--- a/codex-rs/mcp-server/src/codex_tool_config.rs
+++ b/codex-rs/mcp-server/src/codex_tool_config.rs
@@ -179,8 +179,8 @@ impl CodexToolCallParam {
             codex_self_exe: arg0_paths.codex_self_exe.clone(),
             codex_linux_sandbox_exe: arg0_paths.codex_linux_sandbox_exe.clone(),
             main_execve_wrapper_exe: arg0_paths.main_execve_wrapper_exe.clone(),
-            base_instructions,
-            developer_instructions,
+            base_instructions: base_instructions.map(Some),
+            developer_instructions: developer_instructions.map(Some),
             compact_prompt,
             ..Default::default()
         };
diff --git a/codex-rs/protocol/src/lib.rs b/codex-rs/protocol/src/lib.rs
index de580de6f..e78bfe6e4 100644
--- a/codex-rs/protocol/src/lib.rs
+++ b/codex-rs/protocol/src/lib.rs
@@ -23,4 +23,5 @@ pub mod plan_tool;
 pub mod protocol;
 pub mod request_permissions;
 pub mod request_user_input;
+mod serde_helpers;
 pub mod user_input;
diff --git a/codex-rs/protocol/src/protocol.rs b/codex-rs/protocol/src/protocol.rs
index 7116878b4..05926a954 100644
--- a/codex-rs/protocol/src/protocol.rs
+++ b/codex-rs/protocol/src/protocol.rs
@@ -48,6 +48,8 @@ use crate::plan_tool::UpdatePlanArgs;
 use crate::request_permissions::RequestPermissionsEvent;
 use crate::request_permissions::RequestPermissionsResponse;
 use crate::request_user_input::RequestUserInputResponse;
+use crate::serde_helpers::deserialize_double_option;
+use crate::serde_helpers::serialize_double_option;
 use crate::user_input::UserInput;
 use codex_git_utils::GitSha;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -2332,7 +2334,7 @@ impl InitialHistory {
         }
     }
 
-    pub fn get_base_instructions(&self) -> Option<BaseInstructions> {
+    pub fn get_base_instructions(&self) -> Option<Option<BaseInstructions>> {
         // TODO: SessionMeta should (in theory) always be first in the history, so we can probably only check the first item?
         match self {
             InitialHistory::New => None,
@@ -2540,7 +2542,13 @@ pub struct SessionMeta {
     /// base_instructions for the session. This *should* always be present when creating a new session,
     /// but may be missing for older sessions. If not present, fall back to rendering the base_instructions
     /// from ModelsManager.
-    pub base_instructions: Option<BaseInstructions>,
+    #[serde(
+        default,
+        deserialize_with = "deserialize_double_option",
+        serialize_with = "serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
+    pub base_instructions: Option<Option<BaseInstructions>>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub dynamic_tools: Option<Vec<DynamicToolSpec>>,
     #[serde(skip_serializing_if = "Option::is_none")]
diff --git a/codex-rs/protocol/src/serde_helpers.rs b/codex-rs/protocol/src/serde_helpers.rs
new file mode 100644
index 000000000..0e35ebdba
--- /dev/null
+++ b/codex-rs/protocol/src/serde_helpers.rs
@@ -0,0 +1,23 @@
+use serde::Deserialize;
+use serde::Deserializer;
+use serde::Serialize;
+use serde::Serializer;
+
+pub fn deserialize_double_option<'de, T, D>(deserializer: D) -> Result<Option<Option<T>>, D::Error>
+where
+    T: Deserialize<'de>,
+    D: Deserializer<'de>,
+{
+    serde_with::rust::double_option::deserialize(deserializer)
+}
+
+pub fn serialize_double_option<T, S>(
+    value: &Option<Option<T>>,
+    serializer: S,
+) -> Result<S::Ok, S::Error>
+where
+    T: Serialize,
+    S: Serializer,
+{
+    serde_with::rust::double_option::serialize(value, serializer)
+}
diff --git a/codex-rs/rollout/src/recorder.rs b/codex-rs/rollout/src/recorder.rs
index f39c38af6..aa7b485b0 100644
--- a/codex-rs/rollout/src/recorder.rs
+++ b/codex-rs/rollout/src/recorder.rs
@@ -81,7 +81,7 @@ pub enum RolloutRecorderParams {
         conversation_id: ThreadId,
         forked_from_id: Option<ThreadId>,
         source: SessionSource,
-        base_instructions: BaseInstructions,
+        base_instructions: Option<BaseInstructions>,
         dynamic_tools: Vec<DynamicToolSpec>,
         event_persistence_mode: EventPersistenceMode,
     },
@@ -110,7 +110,7 @@ impl RolloutRecorderParams {
         conversation_id: ThreadId,
         forked_from_id: Option<ThreadId>,
         source: SessionSource,
-        base_instructions: BaseInstructions,
+        base_instructions: Option<BaseInstructions>,
         dynamic_tools: Vec<DynamicToolSpec>,
         event_persistence_mode: EventPersistenceMode,
     ) -> Self {
diff --git a/codex-rs/rollout/src/recorder_tests.rs b/codex-rs/rollout/src/recorder_tests.rs
index 163c8a1ee..6ee55672e 100644
--- a/codex-rs/rollout/src/recorder_tests.rs
+++ b/codex-rs/rollout/src/recorder_tests.rs
@@ -73,7 +73,7 @@ async fn recorder_materializes_only_after_explicit_persist() -> std::io::Result<
             thread_id,
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
@@ -166,7 +166,7 @@ async fn metadata_irrelevant_events_touch_state_db_updated_at() -> std::io::Resu
             thread_id,
             /*forked_from_id*/ None,
             SessionSource::Cli,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
diff --git a/codex-rs/rollout/src/tests.rs b/codex-rs/rollout/src/tests.rs
index e647a54cd..aeda089a2 100644
--- a/codex-rs/rollout/src/tests.rs
+++ b/codex-rs/rollout/src/tests.rs
@@ -977,7 +977,7 @@ async fn test_get_thread_contents() {
 }
 
 #[tokio::test]
-async fn test_base_instructions_missing_in_meta_defaults_to_null() {
+async fn test_base_instructions_missing_in_meta_stays_missing() {
     let temp = TempDir::new().unwrap();
     let home = temp.path();
 
@@ -1011,10 +1011,7 @@ async fn test_base_instructions_missing_in_meta_defaults_to_null() {
         .await
         .expect("session meta head");
     let first = head.first().expect("first head entry");
-    assert_eq!(
-        first.get("base_instructions"),
-        Some(&serde_json::Value::Null)
-    );
+    assert_eq!(first.get("base_instructions"), None);
 }
 
 #[tokio::test]