feat(personality) introduce model_personality config (#9459)

## Summary
Introduces the concept of a config model_personality. I would consider
this an MVP for testing out the feature. There are a number of
follow-ups to this PR:

- More sophisticated templating with validation
- In-product experience to manage this

## Testing
- [x] Testing locally

codex-rs/core/src/client_common.rs

@@ -1,4 +1,5 @@
 use crate::client_common::tools::ToolSpec;
+use crate::config::types::Personality;
 use crate::error::Result;
 pub use codex_api::common::ResponseEvent;
 use codex_protocol::models::BaseInstructions;
@@ -35,6 +36,9 @@ pub struct Prompt {
 
     pub base_instructions: BaseInstructions,
 
+    /// Optionally specify the personality of the model.
+    pub personality: Option<Personality>,
+
     /// Optional the output schema for the model's response.
     pub output_schema: Option<Value>,
 }

codex-rs/core/src/codex.rs

@@ -2974,6 +2974,7 @@ async fn run_sampling_request(
         tools: router.specs(),
         parallel_tool_calls: model_supports_parallel,
         base_instructions,
+        personality: None,
         output_schema: turn_context.final_output_json_schema.clone(),
     };
 

codex-rs/core/src/compact_remote.rs

@@ -55,6 +55,7 @@ async fn run_remote_compact_task_inner_impl(
         tools: vec![],
         parallel_tool_calls: false,
         base_instructions: sess.get_base_instructions().await,
+        personality: None,
         output_schema: None,
     };
 

codex-rs/core/src/config/mod.rs

@@ -11,6 +11,7 @@ use crate::config::types::Notifications;
 use crate::config::types::OtelConfig;
 use crate::config::types::OtelConfigToml;
 use crate::config::types::OtelExporterKind;
+use crate::config::types::Personality;
 use crate::config::types::SandboxWorkspaceWrite;
 use crate::config::types::ScrollInputMode;
 use crate::config::types::ShellEnvironmentPolicy;
@@ -127,6 +128,9 @@ pub struct Config {
     /// Info needed to make an API request to the model.
     pub model_provider: ModelProviderInfo,
 
+    /// Optionally specify the personality of the model
+    pub model_personality: Option<Personality>,
+
     /// Approval policy for executing commands.
     pub approval_policy: Constrained<AskForApproval>,
 
@@ -890,6 +894,10 @@ pub struct ConfigToml {
     /// Override to force-enable reasoning summaries for the configured model.
     pub model_supports_reasoning_summaries: Option<bool>,
 
+    /// EXPERIMENTAL
+    /// Optionally specify a personality for the model
+    pub model_personality: Option<Personality>,
+
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: Option<String>,
 
@@ -1474,6 +1482,7 @@ impl Config {
             notify: cfg.notify,
             user_instructions,
             base_instructions,
+            model_personality: config_profile.model_personality.or(cfg.model_personality),
             developer_instructions,
             compact_prompt,
             // The config.toml omits "_mode" because it's a config file. However, "_mode"
@@ -3605,6 +3614,7 @@ model_verbosity = "high"
                 model_reasoning_summary: ReasoningSummary::Detailed,
                 model_supports_reasoning_summaries: None,
                 model_verbosity: None,
+                model_personality: None,
                 chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
                 base_instructions: None,
                 developer_instructions: None,
@@ -3692,6 +3702,7 @@ model_verbosity = "high"
             model_reasoning_summary: ReasoningSummary::default(),
             model_supports_reasoning_summaries: None,
             model_verbosity: None,
+            model_personality: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             base_instructions: None,
             developer_instructions: None,
@@ -3794,6 +3805,7 @@ model_verbosity = "high"
             model_reasoning_summary: ReasoningSummary::default(),
             model_supports_reasoning_summaries: None,
             model_verbosity: None,
+            model_personality: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             base_instructions: None,
             developer_instructions: None,
@@ -3882,6 +3894,7 @@ model_verbosity = "high"
             model_reasoning_summary: ReasoningSummary::Detailed,
             model_supports_reasoning_summaries: None,
             model_verbosity: Some(Verbosity::High),
+            model_personality: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             base_instructions: None,
             developer_instructions: None,

codex-rs/core/src/config/profile.rs

@@ -3,6 +3,7 @@ use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
 
+use crate::config::types::Personality;
 use crate::protocol::AskForApproval;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
@@ -24,6 +25,7 @@ pub struct ConfigProfile {
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
     pub model_verbosity: Option<Verbosity>,
+    pub model_personality: Option<Personality>,
     pub chatgpt_base_url: Option<String>,
     pub experimental_instructions_file: Option<AbsolutePathBuf>,
     pub experimental_compact_prompt_file: Option<AbsolutePathBuf>,

codex-rs/core/src/config/types.rs

@@ -748,6 +748,14 @@ impl Default for ShellEnvironmentPolicy {
     }
 }
 
+#[derive(Default, Debug, Clone, Eq, PartialEq, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "kebab-case")]
+pub enum Personality {
+    Friendly,
+    #[default]
+    Pragmatic,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

codex-rs/core/src/models_manager/model_info.rs

@@ -9,6 +9,7 @@ use codex_protocol::openai_models::TruncationMode;
 use codex_protocol::openai_models::TruncationPolicyConfig;
 
 use crate::config::Config;
+use crate::config::types::Personality;
 use crate::truncate::approx_bytes_for_tokens;
 use tracing::warn;
 
@@ -88,6 +89,20 @@ pub(crate) fn with_config_overrides(mut model: ModelInfo, config: &Config) -> Mo
     }
     if let Some(base_instructions) = &config.base_instructions {
         model.base_instructions = base_instructions.clone();
+    } else if model.slug.contains("gpt-5.2-codex")
+        && let Some(personality) = &config.model_personality
+    {
+        let template = include_str!(
+            "../../templates/model_instructions/gpt-5.2-codex_instructions_template.md"
+        );
+        let personality_message = match personality {
+            Personality::Friendly => include_str!("../../templates/personalities/friendly.md"),
+            Personality::Pragmatic => {
+                include_str!("../../templates/personalities/pragmatic.md")
+            }
+        };
+        let instructions = template.replace("{{ personality_message }}", personality_message);
+        model.base_instructions = instructions;
     }
     model
 }