diff --git a/crates/tui/src/runtime_api.rs b/crates/tui/src/runtime_api.rs
index 8602f5746..7cc3820bf 100644
--- a/crates/tui/src/runtime_api.rs
+++ b/crates/tui/src/runtime_api.rs
@@ -35,6 +35,7 @@ use crate::automation_manager::{
 };
 use crate::config::{Config, DEFAULT_TEXT_MODEL};
 use crate::mcp::{McpConfig, McpPool};
+use crate::models::{ContentBlock, Message};
 use crate::runtime_threads::{
     CompactThreadRequest, CreateThreadRequest, ExternalApprovalDecision, RuntimeThreadManager,
     RuntimeThreadManagerConfig, SharedRuntimeThreadManager, StartTurnRequest, SteerTurnRequest,
@@ -514,7 +515,10 @@ pub async fn run_http_server(
 
 pub fn build_router(state: RuntimeApiState) -> Router {
     let api_routes = Router::new()
-        .route("/v1/sessions", get(list_sessions))
+        .route(
+            "/v1/sessions",
+            get(list_sessions).post(create_session_from_thread),
+        )
         .route("/v1/sessions/{id}", get(get_session).delete(delete_session))
         .route(
             "/v1/sessions/{id}/resume-thread",
@@ -857,6 +861,148 @@ async fn delete_session(
     Ok(StatusCode::NO_CONTENT)
 }
 
+/// Request body for creating a session from a thread
+#[derive(Debug, Deserialize)]
+struct CreateSessionRequest {
+    /// Thread ID to save as session
+    thread_id: String,
+    /// Optional custom title (defaults to thread title or first message)
+    #[serde(default)]
+    title: Option<String>,
+}
+
+/// Response for session creation
+#[derive(Debug, Serialize)]
+struct CreateSessionResponse {
+    session_id: String,
+    thread_id: String,
+    message_count: usize,
+    title: String,
+}
+
+/// Save a thread as a session for cross-workspace resumption
+async fn create_session_from_thread(
+    State(state): State<RuntimeApiState>,
+    Json(req): Json<CreateSessionRequest>,
+) -> Result<(StatusCode, Json<CreateSessionResponse>), ApiError> {
+    // Get thread detail
+    let detail = state
+        .runtime_threads
+        .get_thread_detail(&req.thread_id)
+        .await
+        .map_err(|e| {
+            // Distinguish between "not found" and internal errors
+            let msg = e.to_string();
+            if msg.contains("not found") || msg.contains("no rows") {
+                ApiError::not_found(format!("Thread {} not found", req.thread_id))
+            } else {
+                ApiError::internal(format!("Failed to get thread {}: {}", req.thread_id, e))
+            }
+        })?;
+
+    let thread = detail.thread;
+
+    // Extract messages from items (UserMessage and AgentMessage items)
+    // Iterate directly over items to preserve all messages, including
+    // multiple user/agent messages within a single turn (e.g. steered turns)
+    let mut messages: Vec<Message> = Vec::new();
+
+    for item in &detail.items {
+        match item.kind {
+            TurnItemKind::UserMessage => {
+                let text = item.detail.clone().unwrap_or_else(|| item.summary.clone());
+                if !text.trim().is_empty() {
+                    messages.push(Message {
+                        role: "user".to_string(),
+                        content: vec![ContentBlock::Text {
+                            text,
+                            cache_control: None,
+                        }],
+                    });
+                }
+            }
+            TurnItemKind::AgentMessage => {
+                let text = item.detail.clone().unwrap_or_else(|| item.summary.clone());
+                if !text.trim().is_empty() {
+                    messages.push(Message {
+                        role: "assistant".to_string(),
+                        content: vec![ContentBlock::Text {
+                            text,
+                            cache_control: None,
+                        }],
+                    });
+                }
+            }
+            _ => {}
+        }
+    }
+
+    // Calculate total tokens from turns
+    let total_tokens: u64 = detail
+        .turns
+        .iter()
+        .filter_map(|t| t.usage.as_ref())
+        .map(|u| u.input_tokens as u64 + u.output_tokens as u64)
+        .sum();
+
+    // Determine title
+    let title = req.title.clone().unwrap_or_else(|| {
+        thread.title.clone().unwrap_or_else(|| {
+            // Fallback: first user message
+            messages
+                .iter()
+                .find(|m| m.role == "user")
+                .and_then(|m| {
+                    m.content.iter().find_map(|block| match block {
+                        ContentBlock::Text { text, .. } => Some(truncate_text(text, 50)),
+                        _ => None,
+                    })
+                })
+                .unwrap_or_else(|| "Saved Session".to_string())
+        })
+    });
+
+    // Create session using session_manager helper
+    let manager = SessionManager::new(state.sessions_dir.clone())
+        .map_err(|e| ApiError::internal(format!("Failed to open sessions dir: {e}")))?;
+
+    let session_id = uuid::Uuid::new_v4().to_string();
+    // Convert thread system_prompt (String) to SystemPrompt enum
+    let system_prompt = thread
+        .system_prompt
+        .as_ref()
+        .map(|s| crate::models::SystemPrompt::Text(s.clone()));
+
+    let session = crate::session_manager::create_saved_session_with_id_and_mode(
+        session_id.clone(),
+        &messages,
+        &thread.model,
+        &thread.workspace,
+        total_tokens,
+        system_prompt.as_ref(),
+        Some(&thread.mode),
+    );
+
+    // Override title if provided
+    let mut session = session;
+    session.metadata.title = title.clone();
+
+    // Save session
+    manager
+        .save_session(&session)
+        .map_err(|e| ApiError::internal(format!("Failed to save session: {e}")))?;
+
+    Ok((
+        StatusCode::CREATED,
+        Json(CreateSessionResponse {
+            session_id,
+            thread_id: thread.id,
+            message_count: messages.len(),
+            title,
+        }),
+    ))
+}
+
 fn session_to_detail(session: SavedSession) -> SessionDetailResponse {
     let messages: Vec<serde_json::Value> = session
         .messages