feat: enable parallel tool calls (#6796)

codex-rs/core/src/codex.rs

@@ -1934,12 +1934,30 @@ async fn run_turn(
         .client
         .get_model_family()
         .supports_parallel_tool_calls;
-    let parallel_tool_calls = model_supports_parallel;
+
+    // TODO(jif) revert once testing phase is done.
+    let parallel_tool_calls = model_supports_parallel
+        && sess
+            .state
+            .lock()
+            .await
+            .session_configuration
+            .features
+            .enabled(Feature::ParallelToolCalls);
+    let mut base_instructions = turn_context.base_instructions.clone();
+    if parallel_tool_calls {
+        static INSTRUCTIONS: &str = include_str!("../templates/parallel/instructions.md");
+        static INSERTION_SPOT: &str = "## Editing constraints";
+        base_instructions
+            .as_mut()
+            .map(|base| base.replace(INSERTION_SPOT, INSTRUCTIONS));
+    }
+
     let prompt = Prompt {
         input,
         tools: router.specs(),
         parallel_tool_calls,
-        base_instructions_override: turn_context.base_instructions.clone(),
+        base_instructions_override: base_instructions,
         output_schema: turn_context.final_output_json_schema.clone(),
     };
 

codex-rs/core/src/features.rs

@@ -50,6 +50,8 @@ pub enum Feature {
     RemoteCompaction,
     /// Enable the default shell tool.
     ShellTool,
+    /// Allow model to call multiple tools in parallel (only for models supporting it).
+    ParallelToolCalls,
 }
 
 impl Feature {
@@ -313,6 +315,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Experimental,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::ParallelToolCalls,
+        key: "parallel",
+        stage: Stage::Experimental,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ShellTool,
         key: "shell_tool",

codex-rs/core/src/model_family.rs

@@ -178,6 +178,7 @@ pub fn find_family_for_model(slug: &str) -> Option<ModelFamily> {
             base_instructions: GPT_5_CODEX_INSTRUCTIONS.to_string(),
             apply_patch_tool_type: Some(ApplyPatchToolType::Freeform),
             shell_type: if cfg!(windows) { ConfigShellToolType::ShellCommand } else { ConfigShellToolType::Default },
+            supports_parallel_tool_calls: true,
             support_verbosity: false,
         )
     } else if slug.starts_with("gpt-5.1") {
@@ -189,6 +190,7 @@ pub fn find_family_for_model(slug: &str) -> Option<ModelFamily> {
             default_verbosity: Some(Verbosity::Low),
             base_instructions: GPT_5_1_INSTRUCTIONS.to_string(),
             default_reasoning_effort: Some(ReasoningEffort::Medium),
+            supports_parallel_tool_calls: true,
         )
     } else if slug.starts_with("gpt-5") {
         model_family!(

codex-rs/core/templates/parallel/instructions.md

@@ -0,0 +1,14 @@
+## Exploration and reading files
+
+- **Think first.** Before any tool call, decide ALL files/resources you will need.
+- **Batch everything.** If you need multiple files (even from different places), read them together.
+- **multi_tool_use.parallel** Use `multi_tool_use.parallel` to parallelize tool calls and only this.
+- **Only make sequential calls if you truly cannot know the next file without seeing a result first.**
+- **Workflow:** (a) plan all needed reads → (b) issue one parallel batch → (c) analyze results → (d) repeat if new, unpredictable reads arise.
+
+**Additional notes**:
+* Always maximize parallelism. Never read files one-by-one unless logically unavoidable.
+* This concern every read/list/search operations including, but not only, `cat`, `rg`, `sed`, `ls`, `git show`, `nl`, `wc`, ...
+* Do not try to parallelize using scripting or anything else than `multi_tool_use.parallel`.
+
+## Editing constraints