preserve model-derived instruction provenance

codex-rs/thread-store/src/in_memory.rs

@@ -178,7 +178,7 @@ impl ThreadStore for InMemoryThreadStore {
             source: params.source.clone(),
             thread_source: params.thread_source,
             model_provider: Some(params.metadata.model_provider.clone()),
-            base_instructions: Some(params.base_instructions.clone()),
+            base_instructions: params.base_instructions.clone(),
             dynamic_tools: (!params.dynamic_tools.is_empty()).then(|| params.dynamic_tools.clone()),
             memory_mode: matches!(params.metadata.memory_mode, ThreadMemoryMode::Disabled)
                 .then_some("disabled".to_string()),

codex-rs/thread-store/src/local/mod.rs

@@ -1018,7 +1018,7 @@ mod tests {
             parent_thread_id: None,
             source: SessionSource::Exec,
             thread_source: None,
-            base_instructions: BaseInstructions::default(),
+            base_instructions: Some(BaseInstructions::default()),
             dynamic_tools: Vec::new(),
             multi_agent_version: None,
             metadata: thread_metadata(),

codex-rs/thread-store/src/types.rs

@@ -75,8 +75,11 @@ pub struct CreateThreadParams {
     pub source: SessionSource,
     /// Optional analytics source classification for this thread.
     pub thread_source: Option<ThreadSource>,
-    /// Base instructions persisted in session metadata.
-    pub base_instructions: BaseInstructions,
+    /// Fixed base instructions persisted in session metadata.
+    ///
+    /// `None` leaves model-derived instructions unresolved so the current model catalog can rebuild
+    /// them when the thread is resumed.
+    pub base_instructions: Option<BaseInstructions>,
     /// Dynamic tools available to the thread at startup.
     pub dynamic_tools: Vec<DynamicToolSpec>,
     /// Multi-agent runtime selected when the thread was created.