diff --git a/codex-rs/app-server/src/lib.rs b/codex-rs/app-server/src/lib.rs
index 52ea9325f1..f7d108f55f 100644
--- a/codex-rs/app-server/src/lib.rs
+++ b/codex-rs/app-server/src/lib.rs
@@ -215,6 +215,23 @@ pub async fn run_main(
         .await
     {
         Ok(config) => {
+            let effective_toml = config.config_layer_stack.effective_config();
+            match effective_toml.try_into() {
+                Ok(config_toml) => {
+                    if let Err(err) = codex_core::personality_migration::maybe_migrate_personality(
+                        &config.codex_home,
+                        &config_toml,
+                    )
+                    .await
+                    {
+                        warn!(error = %err, "Failed to run personality migration");
+                    }
+                }
+                Err(err) => {
+                    warn!(error = %err, "Failed to deserialize config for personality migration");
+                }
+            }
+
             let auth_manager = AuthManager::shared(
                 config.codex_home.clone(),
                 false,
diff --git a/codex-rs/core/src/lib.rs b/codex-rs/core/src/lib.rs
index ba47838e1f..75b53624f4 100644
--- a/codex-rs/core/src/lib.rs
+++ b/codex-rs/core/src/lib.rs
@@ -48,6 +48,7 @@ mod message_history;
 mod model_provider_info;
 pub mod parse_command;
 pub mod path_utils;
+pub mod personality_migration;
 pub mod powershell;
 mod proposed_plan_parser;
 pub mod sandboxing;
diff --git a/codex-rs/core/src/personality_migration.rs b/codex-rs/core/src/personality_migration.rs
new file mode 100644
index 0000000000..cfe17c1711
--- /dev/null
+++ b/codex-rs/core/src/personality_migration.rs
@@ -0,0 +1,265 @@
+use crate::config::ConfigToml;
+use crate::config::edit::ConfigEditsBuilder;
+use crate::rollout::ARCHIVED_SESSIONS_SUBDIR;
+use crate::rollout::SESSIONS_SUBDIR;
+use crate::rollout::list::ThreadListConfig;
+use crate::rollout::list::ThreadListLayout;
+use crate::rollout::list::ThreadSortKey;
+use crate::rollout::list::get_threads_in_root;
+use crate::state_db;
+use codex_protocol::config_types::Personality;
+use codex_protocol::protocol::SessionSource;
+use std::io;
+use std::path::Path;
+use tokio::fs::OpenOptions;
+use tokio::io::AsyncWriteExt;
+
+pub const PERSONALITY_MIGRATION_FILENAME: &str = ".personality_migration";
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PersonalityMigrationStatus {
+    SkippedMarker,
+    SkippedExplicitPersonality,
+    SkippedNoSessions,
+    Applied,
+}
+
+pub async fn maybe_migrate_personality(
+    codex_home: &Path,
+    config_toml: &ConfigToml,
+) -> io::Result<PersonalityMigrationStatus> {
+    let marker_path = codex_home.join(PERSONALITY_MIGRATION_FILENAME);
+    if tokio::fs::try_exists(&marker_path).await? {
+        return Ok(PersonalityMigrationStatus::SkippedMarker);
+    }
+
+    let config_profile = config_toml
+        .get_config_profile(None)
+        .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))?;
+    if config_toml.model_personality.is_some() || config_profile.model_personality.is_some() {
+        create_marker(&marker_path).await?;
+        return Ok(PersonalityMigrationStatus::SkippedExplicitPersonality);
+    }
+
+    let model_provider_id = config_profile
+        .model_provider
+        .or_else(|| config_toml.model_provider.clone())
+        .unwrap_or_else(|| "openai".to_string());
+
+    if !has_recorded_sessions(codex_home, model_provider_id.as_str()).await? {
+        create_marker(&marker_path).await?;
+        return Ok(PersonalityMigrationStatus::SkippedNoSessions);
+    }
+
+    ConfigEditsBuilder::new(codex_home)
+        .set_model_personality(Some(Personality::Pragmatic))
+        .apply()
+        .await
+        .map_err(|err| {
+            io::Error::other(format!("failed to persist personality migration: {err}"))
+        })?;
+
+    create_marker(&marker_path).await?;
+    Ok(PersonalityMigrationStatus::Applied)
+}
+
+async fn has_recorded_sessions(codex_home: &Path, default_provider: &str) -> io::Result<bool> {
+    let allowed_sources: &[SessionSource] = &[];
+
+    if let Some(state_db_ctx) = state_db::open_if_present(codex_home, default_provider).await
+        && let Some(ids) = state_db::list_thread_ids_db(
+            Some(state_db_ctx.as_ref()),
+            codex_home,
+            1,
+            None,
+            ThreadSortKey::CreatedAt,
+            allowed_sources,
+            None,
+            false,
+            "personality_migration",
+        )
+        .await
+        && !ids.is_empty()
+    {
+        return Ok(true);
+    }
+
+    let sessions = get_threads_in_root(
+        codex_home.join(SESSIONS_SUBDIR),
+        1,
+        None,
+        ThreadSortKey::CreatedAt,
+        ThreadListConfig {
+            allowed_sources,
+            model_providers: None,
+            default_provider,
+            layout: ThreadListLayout::NestedByDate,
+        },
+    )
+    .await?;
+    if !sessions.items.is_empty() {
+        return Ok(true);
+    }
+
+    let archived_sessions = get_threads_in_root(
+        codex_home.join(ARCHIVED_SESSIONS_SUBDIR),
+        1,
+        None,
+        ThreadSortKey::CreatedAt,
+        ThreadListConfig {
+            allowed_sources,
+            model_providers: None,
+            default_provider,
+            layout: ThreadListLayout::Flat,
+        },
+    )
+    .await?;
+    Ok(!archived_sessions.items.is_empty())
+}
+
+async fn create_marker(marker_path: &Path) -> io::Result<()> {
+    match OpenOptions::new()
+        .create_new(true)
+        .write(true)
+        .open(marker_path)
+        .await
+    {
+        Ok(mut file) => file.write_all(b"v1\n").await,
+        Err(err) if err.kind() == io::ErrorKind::AlreadyExists => Ok(()),
+        Err(err) => Err(err),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use codex_protocol::ThreadId;
+    use codex_protocol::protocol::EventMsg;
+    use codex_protocol::protocol::RolloutItem;
+    use codex_protocol::protocol::RolloutLine;
+    use codex_protocol::protocol::SessionMeta;
+    use codex_protocol::protocol::SessionMetaLine;
+    use codex_protocol::protocol::SessionSource;
+    use codex_protocol::protocol::UserMessageEvent;
+    use pretty_assertions::assert_eq;
+    use tempfile::TempDir;
+    use tokio::io::AsyncWriteExt;
+
+    const TEST_TIMESTAMP: &str = "2025-01-01T00-00-00";
+
+    async fn read_config_toml(codex_home: &Path) -> io::Result<ConfigToml> {
+        let contents = tokio::fs::read_to_string(codex_home.join("config.toml")).await?;
+        toml::from_str(&contents).map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
+    }
+
+    async fn write_session_with_user_event(codex_home: &Path) -> io::Result<()> {
+        let thread_id = ThreadId::new();
+        let dir = codex_home
+            .join(SESSIONS_SUBDIR)
+            .join("2025")
+            .join("01")
+            .join("01");
+        tokio::fs::create_dir_all(&dir).await?;
+        let file_path = dir.join(format!("rollout-{TEST_TIMESTAMP}-{thread_id}.jsonl"));
+        let mut file = tokio::fs::File::create(&file_path).await?;
+
+        let session_meta = SessionMetaLine {
+            meta: SessionMeta {
+                id: thread_id,
+                forked_from_id: None,
+                timestamp: TEST_TIMESTAMP.to_string(),
+                cwd: std::path::PathBuf::from("."),
+                originator: "test_originator".to_string(),
+                cli_version: "test_version".to_string(),
+                source: SessionSource::Cli,
+                model_provider: None,
+                base_instructions: None,
+                dynamic_tools: None,
+            },
+            git: None,
+        };
+        let meta_line = RolloutLine {
+            timestamp: TEST_TIMESTAMP.to_string(),
+            item: RolloutItem::SessionMeta(session_meta),
+        };
+        let user_event = RolloutLine {
+            timestamp: TEST_TIMESTAMP.to_string(),
+            item: RolloutItem::EventMsg(EventMsg::UserMessage(UserMessageEvent {
+                message: "hello".to_string(),
+                images: None,
+                local_images: Vec::new(),
+                text_elements: Vec::new(),
+            })),
+        };
+
+        file.write_all(format!("{}\n", serde_json::to_string(&meta_line)?).as_bytes())
+            .await?;
+        file.write_all(format!("{}\n", serde_json::to_string(&user_event)?).as_bytes())
+            .await?;
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn applies_when_sessions_exist_and_no_personality() -> io::Result<()> {
+        let temp = TempDir::new()?;
+        write_session_with_user_event(temp.path()).await?;
+
+        let config_toml = ConfigToml::default();
+        let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+        assert_eq!(status, PersonalityMigrationStatus::Applied);
+        assert!(temp.path().join(PERSONALITY_MIGRATION_FILENAME).exists());
+
+        let persisted = read_config_toml(temp.path()).await?;
+        assert_eq!(persisted.model_personality, Some(Personality::Pragmatic));
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn skips_when_marker_exists() -> io::Result<()> {
+        let temp = TempDir::new()?;
+        create_marker(&temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?;
+
+        let config_toml = ConfigToml::default();
+        let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+        assert_eq!(status, PersonalityMigrationStatus::SkippedMarker);
+        assert!(!temp.path().join("config.toml").exists());
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn skips_when_personality_explicit() -> io::Result<()> {
+        let temp = TempDir::new()?;
+        ConfigEditsBuilder::new(temp.path())
+            .set_model_personality(Some(Personality::Friendly))
+            .apply()
+            .await
+            .map_err(|err| io::Error::other(format!("failed to write config: {err}")))?;
+
+        let config_toml = read_config_toml(temp.path()).await?;
+        let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+        assert_eq!(
+            status,
+            PersonalityMigrationStatus::SkippedExplicitPersonality
+        );
+        assert!(temp.path().join(PERSONALITY_MIGRATION_FILENAME).exists());
+
+        let persisted = read_config_toml(temp.path()).await?;
+        assert_eq!(persisted.model_personality, Some(Personality::Friendly));
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn skips_when_no_sessions() -> io::Result<()> {
+        let temp = TempDir::new()?;
+        let config_toml = ConfigToml::default();
+        let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+        assert_eq!(status, PersonalityMigrationStatus::SkippedNoSessions);
+        assert!(temp.path().join(PERSONALITY_MIGRATION_FILENAME).exists());
+        assert!(!temp.path().join("config.toml").exists());
+        Ok(())
+    }
+}
diff --git a/codex-rs/core/tests/suite/mod.rs b/codex-rs/core/tests/suite/mod.rs
index 0c3f5bf1a2..f34ca09b40 100644
--- a/codex-rs/core/tests/suite/mod.rs
+++ b/codex-rs/core/tests/suite/mod.rs
@@ -49,6 +49,7 @@ mod otel;
 mod pending_input;
 mod permissions_messages;
 mod personality;
+mod personality_migration;
 mod prompt_caching;
 mod quota_exceeded;
 mod read_file;
diff --git a/codex-rs/core/tests/suite/personality_migration.rs b/codex-rs/core/tests/suite/personality_migration.rs
new file mode 100644
index 0000000000..4431db2472
--- /dev/null
+++ b/codex-rs/core/tests/suite/personality_migration.rs
@@ -0,0 +1,154 @@
+use codex_core::ARCHIVED_SESSIONS_SUBDIR;
+use codex_core::SESSIONS_SUBDIR;
+use codex_core::config::ConfigToml;
+use codex_core::personality_migration::PERSONALITY_MIGRATION_FILENAME;
+use codex_core::personality_migration::PersonalityMigrationStatus;
+use codex_core::personality_migration::maybe_migrate_personality;
+use codex_protocol::ThreadId;
+use codex_protocol::config_types::Personality;
+use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::RolloutItem;
+use codex_protocol::protocol::RolloutLine;
+use codex_protocol::protocol::SessionMeta;
+use codex_protocol::protocol::SessionMetaLine;
+use codex_protocol::protocol::SessionSource;
+use codex_protocol::protocol::UserMessageEvent;
+use pretty_assertions::assert_eq;
+use std::io;
+use std::path::Path;
+use tempfile::TempDir;
+use tokio::io::AsyncWriteExt;
+
+const TEST_TIMESTAMP: &str = "2025-01-01T00-00-00";
+
+async fn read_config_toml(codex_home: &Path) -> io::Result<ConfigToml> {
+    let contents = tokio::fs::read_to_string(codex_home.join("config.toml")).await?;
+    toml::from_str(&contents).map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
+}
+
+async fn write_session_with_user_event(codex_home: &Path) -> io::Result<()> {
+    let thread_id = ThreadId::new();
+    let dir = codex_home
+        .join(SESSIONS_SUBDIR)
+        .join("2025")
+        .join("01")
+        .join("01");
+    write_rollout_with_user_event(&dir, thread_id).await
+}
+
+async fn write_archived_session_with_user_event(codex_home: &Path) -> io::Result<()> {
+    let thread_id = ThreadId::new();
+    let dir = codex_home.join(ARCHIVED_SESSIONS_SUBDIR);
+    write_rollout_with_user_event(&dir, thread_id).await
+}
+
+async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::Result<()> {
+    tokio::fs::create_dir_all(&dir).await?;
+    let file_path = dir.join(format!("rollout-{TEST_TIMESTAMP}-{thread_id}.jsonl"));
+    let mut file = tokio::fs::File::create(&file_path).await?;
+
+    let session_meta = SessionMetaLine {
+        meta: SessionMeta {
+            id: thread_id,
+            forked_from_id: None,
+            timestamp: TEST_TIMESTAMP.to_string(),
+            cwd: std::path::PathBuf::from("."),
+            originator: "test_originator".to_string(),
+            cli_version: "test_version".to_string(),
+            source: SessionSource::Cli,
+            model_provider: None,
+            base_instructions: None,
+            dynamic_tools: None,
+        },
+        git: None,
+    };
+    let meta_line = RolloutLine {
+        timestamp: TEST_TIMESTAMP.to_string(),
+        item: RolloutItem::SessionMeta(session_meta),
+    };
+    let user_event = RolloutLine {
+        timestamp: TEST_TIMESTAMP.to_string(),
+        item: RolloutItem::EventMsg(EventMsg::UserMessage(UserMessageEvent {
+            message: "hello".to_string(),
+            images: None,
+            local_images: Vec::new(),
+            text_elements: Vec::new(),
+        })),
+    };
+
+    let meta_json = serde_json::to_string(&meta_line)?;
+    file.write_all(format!("{meta_json}\n").as_bytes()).await?;
+    let user_json = serde_json::to_string(&user_event)?;
+    file.write_all(format!("{user_json}\n").as_bytes()).await?;
+    Ok(())
+}
+
+#[tokio::test]
+async fn migration_marker_exists_no_sessions_no_change() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    let marker_path = temp.path().join(PERSONALITY_MIGRATION_FILENAME);
+    tokio::fs::write(&marker_path, "v1\n").await?;
+
+    let status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::SkippedMarker);
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join("config.toml")).await?,
+        false
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn no_marker_no_sessions_no_change() -> io::Result<()> {
+    let temp = TempDir::new()?;
+
+    let status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::SkippedNoSessions);
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        true
+    );
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join("config.toml")).await?,
+        false
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn no_marker_sessions_sets_personality() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_session_with_user_event(temp.path()).await?;
+
+    let status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::Applied);
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        true
+    );
+
+    let persisted = read_config_toml(temp.path()).await?;
+    assert_eq!(persisted.model_personality, Some(Personality::Pragmatic));
+    Ok(())
+}
+
+#[tokio::test]
+async fn no_marker_archived_sessions_sets_personality() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_archived_session_with_user_event(temp.path()).await?;
+
+    let status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::Applied);
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        true
+    );
+
+    let persisted = read_config_toml(temp.path()).await?;
+    assert_eq!(persisted.model_personality, Some(Personality::Pragmatic));
+    Ok(())
+}
diff --git a/codex-rs/tui/src/lib.rs b/codex-rs/tui/src/lib.rs
index 82a06a7032..dc29030434 100644
--- a/codex-rs/tui/src/lib.rs
+++ b/codex-rs/tui/src/lib.rs
@@ -209,6 +209,13 @@ pub async fn run_main(
         }
     };
 
+    if let Err(err) =
+        codex_core::personality_migration::maybe_migrate_personality(&codex_home, &config_toml)
+            .await
+    {
+        tracing::warn!(error = %err, "failed to run personality migration");
+    }
+
     let cloud_auth_manager = AuthManager::shared(
         codex_home.to_path_buf(),
         false,