diff --git a/Cargo.lock b/Cargo.lock
index 4b2961547..06a4843ed 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -142,6 +142,7 @@ dependencies = [
  "arc-workflows",
  "axum",
  "base64",
+ "chrono",
  "clap",
  "dirs",
  "http-body-util",
@@ -308,6 +309,7 @@ dependencies = [
 name = "arc-types"
 version = "0.1.0"
 dependencies = [
+ "chrono",
  "prettyplease",
  "schemars 0.8.22",
  "serde",
diff --git a/apps/arc-web/app/routes/session-detail.tsx b/apps/arc-web/app/routes/session-detail.tsx
index 440e86527..cf8609283 100644
--- a/apps/arc-web/app/routes/session-detail.tsx
+++ b/apps/arc-web/app/routes/session-detail.tsx
@@ -31,21 +31,25 @@ export async function loader({ request, params }: Route.LoaderArgs) {
     model: apiSession.model,
     created_at: apiSession.created_at,
     updated_at: apiSession.updated_at,
-    turns: apiSession.turns.map((t) => {
-      if (t.kind === "tool" && t.tools) {
-        return {
-          kind: "tool" as const,
-          tools: t.tools.map((tu) => ({
-            id: tu.id,
-            toolName: tu.tool_name,
-            input: tu.input,
-            result: tu.result,
-            isError: tu.is_error,
-            durationMs: tu.duration_ms,
-          })),
-        };
+    turns: apiSession.turns.map((t): Turn => {
+      switch (t.kind) {
+        case "tool":
+          return {
+            kind: "tool",
+            tools: t.tools.map((tu) => ({
+              id: tu.id,
+              toolName: tu.tool_name,
+              input: tu.input,
+              result: tu.result,
+              isError: tu.is_error,
+              durationMs: tu.duration_ms,
+            })),
+          };
+        case "user":
+          return { kind: "user", content: t.content, created_at: t.created_at };
+        case "assistant":
+          return { kind: "assistant", content: t.content };
       }
-      return { kind: t.kind as "user" | "assistant", content: t.content ?? "", created_at: t.created_at };
     }),
   };
   const sessionGroups = groupSessionsByDate(
diff --git a/crates/arc-api/Cargo.toml b/crates/arc-api/Cargo.toml
index bb0d0928e..835dc34b9 100644
--- a/crates/arc-api/Cargo.toml
+++ b/crates/arc-api/Cargo.toml
@@ -15,6 +15,7 @@ arc-llm = { path = "../arc-llm" }
 arc-util = { path = "../arc-util" }
 arc-db = { path = "../arc-db" }
 arc-types = { path = "../arc-types" }
+chrono.workspace = true
 axum = "0.8"
 dirs.workspace = true
 sqlx.workspace = true
diff --git a/crates/arc-api/src/demo/mod.rs b/crates/arc-api/src/demo/mod.rs
index a0f8ee553..83f71d3ca 100644
--- a/crates/arc-api/src/demo/mod.rs
+++ b/crates/arc-api/src/demo/mod.rs
@@ -524,7 +524,7 @@ pub async fn send_message_stub(
     State(_state): State<Arc<AppState>>,
     Path(_id): Path<String>,
 ) -> Response {
-    (StatusCode::OK, Json(serde_json::json!({"accepted": true}))).into_response()
+    (StatusCode::ACCEPTED, Json(serde_json::json!({"accepted": true}))).into_response()
 }
 
 pub async fn session_events_stub(
@@ -2366,48 +2366,61 @@ mod retros {
 
 mod sessions {
     use arc_types::*;
+    use chrono::{DateTime, Utc};
+
+    fn ts(s: &str) -> DateTime<Utc> {
+        s.parse().unwrap()
+    }
 
     pub fn list_items() -> Vec<SessionListItem> {
         vec![
             SessionListItem {
                 id: "s1".into(),
                 title: "Add rate limiting to auth endpoints".into(),
-                created_at: "2026-03-06T14:30:00Z".into(),
+                created_at: ts("2026-03-06T14:30:00Z"),
+                updated_at: ts("2026-03-06T15:45:00Z"),
             },
             SessionListItem {
                 id: "s2".into(),
                 title: "Fix config parsing for nested values".into(),
-                created_at: "2026-03-06T12:30:00Z".into(),
+                created_at: ts("2026-03-06T12:30:00Z"),
+                updated_at: ts("2026-03-06T13:15:00Z"),
             },
             SessionListItem {
                 id: "s3".into(),
                 title: "Migrate to React Router v7".into(),
-                created_at: "2026-03-05T10:00:00Z".into(),
+                created_at: ts("2026-03-05T10:00:00Z"),
+                updated_at: ts("2026-03-05T11:30:00Z"),
             },
             SessionListItem {
                 id: "s4".into(),
                 title: "Add dark mode toggle".into(),
-                created_at: "2026-03-05T09:00:00Z".into(),
+                created_at: ts("2026-03-05T09:00:00Z"),
+                updated_at: ts("2026-03-05T09:45:00Z"),
             },
             SessionListItem {
                 id: "s5".into(),
                 title: "Update OpenAPI spec for v3".into(),
-                created_at: "2026-03-05T08:00:00Z".into(),
+                created_at: ts("2026-03-05T08:00:00Z"),
+                updated_at: ts("2026-03-05T08:30:00Z"),
             },
             SessionListItem {
                 id: "s6".into(),
                 title: "Terraform module for Redis cluster".into(),
-                created_at: "2026-03-03T15:00:00Z".into(),
+                created_at: ts("2026-03-03T15:00:00Z"),
+                updated_at: ts("2026-03-03T16:00:00Z"),
             },
             SessionListItem {
                 id: "s7".into(),
                 title: "Add pipeline event types".into(),
-                created_at: "2026-03-01T11:00:00Z".into(),
+                created_at: ts("2026-03-01T11:00:00Z"),
+                updated_at: ts("2026-03-01T12:00:00Z"),
             },
             SessionListItem {
                 id: "s8".into(),
                 title: "Implement webhook retry logic".into(),
-                created_at: "2026-02-28T09:00:00Z".into(),
+                created_at: ts("2026-02-28T09:00:00Z"),
+                updated_at: ts("2026-02-28T10:00:00Z"),
             },
         ]
     }
@@ -2415,50 +2428,50 @@ mod sessions {
     pub fn detail(id: &str) -> Option<SessionDetail> {
         match id {
             "s1" => Some(SessionDetail {
-                id: "s1".into(), title: "Add rate limiting to auth endpoints".into(), model: "Opus 4.6".into(), created_at: "2026-03-06T14:30:00Z".into(), updated_at: "2026-03-06T15:45:00Z".into(),
+                id: "s1".into(), title: "Add rate limiting to auth endpoints".into(), model: "Opus 4.6".into(), created_at: ts("2026-03-06T14:30:00Z"), updated_at: ts("2026-03-06T15:45:00Z"),
                 turns: vec![
-                    SessionTurn { kind: SessionTurnKind::User, content: Some("Add rate limiting to the auth endpoints. We're getting hit with brute force attempts on /api/auth/login and /api/auth/register. Use a sliding window approach with Redis, 10 requests per minute per IP.".into()), created_at: Some("2026-02-28T10:00:00Z".into()), tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("I'll implement sliding window rate limiting using Redis. Let me first look at the existing auth routes and middleware setup.".into()), created_at: None, tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Tool, content: None, created_at: None, tools: vec![
+                    SessionTurn::UserTurn(UserTurn { kind: UserTurnKind::User, content: "Add rate limiting to the auth endpoints. We're getting hit with brute force attempts on /api/auth/login and /api/auth/register. Use a sliding window approach with Redis, 10 requests per minute per IP.".into(), created_at: ts("2026-02-28T10:00:00Z") }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "I'll implement sliding window rate limiting using Redis. Let me first look at the existing auth routes and middleware setup.".into() }),
+                    SessionTurn::ToolTurn(ToolTurn { kind: ToolTurnKind::Tool, tools: vec![
                         ToolUse { id: "toolu_03".into(), tool_name: "read_file".into(), input: r#"{ "path": "src/routes/auth.ts" }"#.into(), result: "import { Router } from \"express\";\nimport { loginHandler } from \"../handlers/auth\";\nimport { registerHandler } from \"../handlers/auth\";\n\nconst router = Router();\nrouter.post(\"/login\", loginHandler);\nrouter.post(\"/register\", registerHandler);\n\nexport default router;".into(), is_error: false, duration_ms: Some(52) },
                         ToolUse { id: "toolu_04".into(), tool_name: "read_file".into(), input: r#"{ "path": "src/middleware/index.ts" }"#.into(), result: "export { authMiddleware } from \"./auth\";\nexport { corsMiddleware } from \"./cors\";\nexport { loggingMiddleware } from \"./logging\";".into(), is_error: false, duration_ms: Some(35) },
                         ToolUse { id: "toolu_05".into(), tool_name: "search_files".into(), input: r#"{ "pattern": "redis", "path": "src/" }"#.into(), result: "src/lib/redis.ts:1: import Redis from \"ioredis\";\nsrc/lib/redis.ts:3: export const redis = new Redis(process.env.REDIS_URL);".into(), is_error: false, duration_ms: Some(180) },
-                    ] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("Good \u{2014} there's already a Redis client at `src/lib/redis.ts`. I'll create a rate limiting middleware using the sliding window algorithm and apply it to the auth routes.\n\nHere's the plan:\n1. Create `src/middleware/rate-limit.ts` with sliding window logic\n2. Apply it to `/login` and `/register` routes\n3. Return `429 Too Many Requests` when the limit is exceeded".into()), created_at: None, tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Tool, content: None, created_at: None, tools: vec![
+                    ] }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "Good \u{2014} there's already a Redis client at `src/lib/redis.ts`. I'll create a rate limiting middleware using the sliding window algorithm and apply it to the auth routes.\n\nHere's the plan:\n1. Create `src/middleware/rate-limit.ts` with sliding window logic\n2. Apply it to `/login` and `/register` routes\n3. Return `429 Too Many Requests` when the limit is exceeded".into() }),
+                    SessionTurn::ToolTurn(ToolTurn { kind: ToolTurnKind::Tool, tools: vec![
                         ToolUse { id: "toolu_06".into(), tool_name: "write_file".into(), input: r#"{ "path": "src/middleware/rate-limit.ts" }"#.into(), result: "File written: src/middleware/rate-limit.ts (47 lines)".into(), is_error: false, duration_ms: Some(62) },
                         ToolUse { id: "toolu_07".into(), tool_name: "edit_file".into(), input: r#"{ "path": "src/routes/auth.ts", "operations": [{ "type": "insert", "line": 3 }] }"#.into(), result: "Applied 1 edit to src/routes/auth.ts".into(), is_error: false, duration_ms: Some(41) },
                         ToolUse { id: "toolu_08".into(), tool_name: "edit_file".into(), input: r#"{ "path": "src/middleware/index.ts", "operations": [{ "type": "append" }] }"#.into(), result: "Applied 1 edit to src/middleware/index.ts".into(), is_error: false, duration_ms: Some(55) },
-                    ] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("Done. I've created the rate limiter and wired it up:\n\n- **`src/middleware/rate-limit.ts`** \u{2014} Sliding window rate limiter using Redis sorted sets. Each request adds a timestamped entry, expired entries are pruned, and the count is checked against the limit.\n- **`src/routes/auth.ts`** \u{2014} Applied `rateLimit({ windowMs: 60_000, max: 10 })` to both `/login` and `/register`.\n- **`src/middleware/index.ts`** \u{2014} Re-exported the new middleware.\n\nWhen the limit is exceeded, clients get a `429` response with a `Retry-After` header indicating seconds until the window resets.".into()), created_at: None, tools: vec![] },
+                    ] }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "Done. I've created the rate limiter and wired it up:\n\n- **`src/middleware/rate-limit.ts`** \u{2014} Sliding window rate limiter using Redis sorted sets. Each request adds a timestamped entry, expired entries are pruned, and the count is checked against the limit.\n- **`src/routes/auth.ts`** \u{2014} Applied `rateLimit({ windowMs: 60_000, max: 10 })` to both `/login` and `/register`.\n- **`src/middleware/index.ts`** \u{2014} Re-exported the new middleware.\n\nWhen the limit is exceeded, clients get a `429` response with a `Retry-After` header indicating seconds until the window resets.".into() }),
                 ],
             }),
             "s2" => Some(SessionDetail {
-                id: "s2".into(), title: "Fix config parsing for nested values".into(), model: "Sonnet 4.6".into(), created_at: "2026-03-06T12:30:00Z".into(), updated_at: "2026-03-06T13:15:00Z".into(),
+                id: "s2".into(), title: "Fix config parsing for nested values".into(), model: "Sonnet 4.6".into(), created_at: ts("2026-03-06T12:30:00Z"), updated_at: ts("2026-03-06T13:15:00Z"),
                 turns: vec![
-                    SessionTurn { kind: SessionTurnKind::User, content: Some("The CLI crashes when parsing nested TOML config values like [database.connection]. Can you debug and fix this?".into()), created_at: Some("2026-02-28T10:00:00Z".into()), tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("Let me look at the config parser to understand how nested keys are handled.".into()), created_at: None, tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Tool, content: None, created_at: None, tools: vec![
+                    SessionTurn::UserTurn(UserTurn { kind: UserTurnKind::User, content: "The CLI crashes when parsing nested TOML config values like [database.connection]. Can you debug and fix this?".into(), created_at: ts("2026-02-28T10:00:00Z") }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "Let me look at the config parser to understand how nested keys are handled.".into() }),
+                    SessionTurn::ToolTurn(ToolTurn { kind: ToolTurnKind::Tool, tools: vec![
                         ToolUse { id: "toolu_09".into(), tool_name: "read_file".into(), input: r#"{ "path": "src/config/parser.ts" }"#.into(), result: "export function parseConfig(raw: string): Record<string, string> {\n  const result: Record<string, string> = {};\n  for (const line of raw.split(\"\\n\")) {\n    const match = line.match(/^(\\w+)\\s*=\\s*(.+)$/);\n    if (match) result[match[1]] = match[2].trim();\n  }\n  return result;\n}".into(), is_error: false, duration_ms: Some(67) },
-                    ] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("Found the issue. The parser uses a flat `Record<string, string>` and the regex `^(\\w+)` only matches simple keys \u{2014} it can't handle TOML section headers like `[database.connection]` or dotted keys like `database.host`.\n\nI'll rewrite the parser to support nested sections and dotted keys, producing a properly nested object.".into()), created_at: None, tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Tool, content: None, created_at: None, tools: vec![
+                    ] }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "Found the issue. The parser uses a flat `Record<string, string>` and the regex `^(\\w+)` only matches simple keys \u{2014} it can't handle TOML section headers like `[database.connection]` or dotted keys like `database.host`.\n\nI'll rewrite the parser to support nested sections and dotted keys, producing a properly nested object.".into() }),
+                    SessionTurn::ToolTurn(ToolTurn { kind: ToolTurnKind::Tool, tools: vec![
                         ToolUse { id: "toolu_10".into(), tool_name: "write_file".into(), input: r#"{ "path": "src/config/parser.ts" }"#.into(), result: "File written: src/config/parser.ts (62 lines)".into(), is_error: false, duration_ms: Some(78) },
                         ToolUse { id: "toolu_11".into(), tool_name: "run_command".into(), input: r#"{ "command": "npm test -- --grep 'config parser'" }"#.into(), result: "PASS src/config/__tests__/parser.test.ts\n  config parser\n    \u{2713} parses flat keys (2ms)\n    \u{2713} parses section headers (1ms)\n    \u{2713} parses dotted keys (1ms)\n    \u{2713} handles nested sections (1ms)\n\nTest Suites: 1 passed\nTests: 4 passed".into(), is_error: false, duration_ms: Some(2150) },
-                    ] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("Fixed. The parser now:\n\n1. Tracks the current section header (e.g. `[database.connection]`)\n2. Constructs nested objects from dotted paths\n3. Handles both `key = value` within sections and bare `dotted.key = value` syntax\n\nAll 4 tests pass, including the new nested section case.".into()), created_at: None, tools: vec![] },
+                    ] }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "Fixed. The parser now:\n\n1. Tracks the current section header (e.g. `[database.connection]`)\n2. Constructs nested objects from dotted paths\n3. Handles both `key = value` within sections and bare `dotted.key = value` syntax\n\nAll 4 tests pass, including the new nested section case.".into() }),
                 ],
             }),
             "s3" => Some(SessionDetail {
-                id: "s3".into(), title: "Migrate to React Router v7".into(), model: "Opus 4.6".into(), created_at: "2026-03-05T10:00:00Z".into(), updated_at: "2026-03-05T11:30:00Z".into(),
+                id: "s3".into(), title: "Migrate to React Router v7".into(), model: "Opus 4.6".into(), created_at: ts("2026-03-05T10:00:00Z"), updated_at: ts("2026-03-05T11:30:00Z"),
                 turns: vec![
-                    SessionTurn { kind: SessionTurnKind::User, content: Some("Help me migrate our app from React Router v6 to v7. We're using createBrowserRouter with data loaders.".into()), created_at: Some("2026-02-26T10:00:00Z".into()), tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("I'll audit your current router setup and identify what needs to change for v7. Let me scan the codebase.".into()), created_at: None, tools: vec![] },
-                    SessionTurn { kind: SessionTurnKind::Tool, content: None, created_at: None, tools: vec![
+                    SessionTurn::UserTurn(UserTurn { kind: UserTurnKind::User, content: "Help me migrate our app from React Router v6 to v7. We're using createBrowserRouter with data loaders.".into(), created_at: ts("2026-02-26T10:00:00Z") }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "I'll audit your current router setup and identify what needs to change for v7. Let me scan the codebase.".into() }),
+                    SessionTurn::ToolTurn(ToolTurn { kind: ToolTurnKind::Tool, tools: vec![
                         ToolUse { id: "toolu_12".into(), tool_name: "search_files".into(), input: r#"{ "pattern": "createBrowserRouter" }"#.into(), result: "src/main.tsx:5: import { createBrowserRouter } from \"react-router-dom\";\nsrc/main.tsx:8: const router = createBrowserRouter([".into(), is_error: false, duration_ms: Some(220) },
                         ToolUse { id: "toolu_13".into(), tool_name: "read_file".into(), input: r#"{ "path": "package.json" }"#.into(), result: "\"react-router-dom\": \"^6.22.0\"".into(), is_error: false, duration_ms: Some(30) },
-                    ] },
-                    SessionTurn { kind: SessionTurnKind::Assistant, content: Some("You're on React Router 6.22. The migration to v7 involves:\n\n1. Replace `react-router-dom` with `react-router` (single package)\n2. Update `createBrowserRouter` import path\n3. Migrate data loaders to the new `Route.LoaderArgs` type\n4. Update any `useLoaderData` calls to be type-safe\n\nLet me make these changes.".into()), created_at: None, tools: vec![] },
+                    ] }),
+                    SessionTurn::AssistantTurn(AssistantTurn { kind: AssistantTurnKind::Assistant, content: "You're on React Router 6.22. The migration to v7 involves:\n\n1. Replace `react-router-dom` with `react-router` (single package)\n2. Update `createBrowserRouter` import path\n3. Migrate data loaders to the new `Route.LoaderArgs` type\n4. Update any `useLoaderData` calls to be type-safe\n\nLet me make these changes.".into() }),
                 ],
             }),
             _ => None,
diff --git a/crates/arc-types/Cargo.toml b/crates/arc-types/Cargo.toml
index 2e31d253e..da4aa03cb 100644
--- a/crates/arc-types/Cargo.toml
+++ b/crates/arc-types/Cargo.toml
@@ -9,6 +9,7 @@ description = "Generated Rust types from the Arc API OpenAPI spec"
 doctest = false
 
 [dependencies]
+chrono = { workspace = true, features = ["serde"] }
 serde.workspace = true
 serde_json.workspace = true
 
diff --git a/openapi/arc-api.yaml b/openapi/arc-api.yaml
index 74fd8e694..3900a0425 100644
--- a/openapi/arc-api.yaml
+++ b/openapi/arc-api.yaml
@@ -763,13 +763,7 @@ paths:
       summary: Retrieve Session
       description: Returns the full session detail including all conversation turns.
       parameters:
-        - name: id
-          in: path
-          required: true
-          description: Unique session identifier.
-          schema:
-            type: string
-          example: s1
+        - $ref: "#/components/parameters/SessionId"
       responses:
         "200":
           description: Session detail
@@ -791,13 +785,7 @@ paths:
       summary: Send Session Message
       description: Append a user message to an existing session. The server will process it and produce assistant and tool turns asynchronously via the event stream.
       parameters:
-        - name: id
-          in: path
-          required: true
-          description: Unique session identifier.
-          schema:
-            type: string
-          example: s1
+        - $ref: "#/components/parameters/SessionId"
       requestBody:
         required: true
         content:
@@ -805,19 +793,12 @@ paths:
             schema:
               $ref: "#/components/schemas/SendMessageRequest"
       responses:
-        "200":
-          description: Message accepted
+        "202":
+          description: Message accepted for processing
           content:
             application/json:
               schema:
-                type: object
-                properties:
-                  accepted:
-                    type: boolean
-                    description: Whether the message was accepted for processing.
-                    example: true
-                required:
-                  - accepted
+                $ref: "#/components/schemas/SendMessageResponse"
         "404":
           description: Session not found
           content:
@@ -832,13 +813,7 @@ paths:
       summary: Stream Session Events
       description: Opens a server-sent event (SSE) stream for real-time session updates. Events include new assistant turns, tool invocations, and completion signals.
       parameters:
-        - name: id
-          in: path
-          required: true
-          description: Unique session identifier.
-          schema:
-            type: string
-          example: s1
+        - $ref: "#/components/parameters/SessionId"
       responses:
         "200":
           description: Server-sent event stream
@@ -1080,6 +1055,15 @@ components:
       schema:
         type: string
 
+    SessionId:
+      name: id
+      in: path
+      required: true
+      description: Unique session identifier.
+      schema:
+        type: string
+      example: s1
+
     PageLimit:
       name: page[limit]
       in: query
@@ -2091,6 +2075,7 @@ components:
         - id
         - title
         - created_at
+        - updated_at
       properties:
         id:
           type: string
@@ -2102,35 +2087,78 @@ components:
           example: Add rate limiting to auth endpoints
         created_at:
           type: string
-          description: ISO 8601 timestamp when the session was created.
+          format: date-time
+          description: Timestamp when the session was created.
           example: "2026-03-06T14:30:00Z"
+        updated_at:
+          type: string
+          format: date-time
+          description: Timestamp when the session was last updated (e.g. new turn added).
+          example: "2026-03-06T15:45:00Z"
 
 
     SessionTurn:
       description: A single turn in a session conversation — a user message, assistant response, or tool invocation block.
+      discriminator:
+        propertyName: kind
+        mapping:
+          user: "#/components/schemas/UserTurn"
+          assistant: "#/components/schemas/AssistantTurn"
+          tool: "#/components/schemas/ToolTurn"
+      oneOf:
+        - $ref: "#/components/schemas/UserTurn"
+        - $ref: "#/components/schemas/AssistantTurn"
+        - $ref: "#/components/schemas/ToolTurn"
+
+    UserTurn:
+      description: A user message turn.
       type: object
       required:
         - kind
+        - content
+        - created_at
       properties:
         kind:
           type: string
-          description: The type of turn.
-          enum:
-            - user
-            - assistant
-            - tool
-          example: user
+          enum: [user]
         content:
           type: string
-          description: Text content of the turn. Present for user and assistant turns, absent for tool turns.
+          description: Text content of the user message.
           example: Add rate limiting to the auth endpoints using a sliding window approach with Redis.
         created_at:
           type: string
-          description: ISO 8601 timestamp when the turn was created. Typically present for user turns.
+          format: date-time
+          description: Timestamp when the turn was created.
           example: "2026-02-28T10:00:00Z"
+
+    AssistantTurn:
+      description: An assistant response turn.
+      type: object
+      required:
+        - kind
+        - content
+      properties:
+        kind:
+          type: string
+          enum: [assistant]
+        content:
+          type: string
+          description: Text content of the assistant response.
+          example: I'll implement sliding window rate limiting using Redis.
+
+    ToolTurn:
+      description: A tool invocation turn.
+      type: object
+      required:
+        - kind
+        - tools
+      properties:
+        kind:
+          type: string
+          enum: [tool]
         tools:
           type: array
-          description: Tool invocations for this turn. Present only when kind is "tool".
+          description: Tool invocations for this turn.
           items:
             $ref: "#/components/schemas/ToolUse"
 
@@ -2159,11 +2187,13 @@ components:
           example: Opus 4.6
         created_at:
           type: string
-          description: ISO 8601 timestamp when the session was created.
+          format: date-time
+          description: Timestamp when the session was created.
           example: "2026-03-06T14:30:00Z"
         updated_at:
           type: string
-          description: ISO 8601 timestamp when the session was last updated (e.g. new turn added).
+          format: date-time
+          description: Timestamp when the session was last updated (e.g. new turn added).
           example: "2026-03-06T15:45:00Z"
         turns:
           type: array
@@ -2175,9 +2205,9 @@ components:
       description: Request body for starting a new session.
       type: object
       required:
-        - prompt
+        - content
       properties:
-        prompt:
+        content:
           type: string
           description: The initial user message to start the session.
           example: Add rate limiting to the auth endpoints using a sliding window approach with Redis, 10 requests per minute per IP.
@@ -2199,7 +2229,8 @@ components:
           example: s42
         created_at:
           type: string
-          description: ISO 8601 timestamp when the session was created.
+          format: date-time
+          description: Timestamp when the session was created.
           example: "2026-03-06T16:00:00Z"
 
     SendMessageRequest:
@@ -2213,6 +2244,17 @@ components:
           description: The user message text.
           example: Can you also add a bypass for internal health-check IPs?
 
+    SendMessageResponse:
+      description: Acknowledgement that the message was accepted for asynchronous processing.
+      type: object
+      required:
+        - accepted
+      properties:
+        accepted:
+          type: boolean
+          description: Whether the message was accepted for processing.
+          example: true
+
     # ── Insights Schemas ────────────────────────────────────────────────
 
     SavedQuery:
diff --git a/packages/arc-api-client/src/.openapi-generator/FILES b/packages/arc-api-client/src/.openapi-generator/FILES
index 79c109885..bbd7cd112 100644
--- a/packages/arc-api-client/src/.openapi-generator/FILES
+++ b/packages/arc-api-client/src/.openapi-generator/FILES
@@ -19,6 +19,7 @@ index.ts
 models/aggregate-usage.ts
 models/api-question-option.ts
 models/api-question.ts
+models/assistant-turn.ts
 models/branch.ts
 models/cancel-run200-response.ts
 models/check-run-status.ts
@@ -75,7 +76,7 @@ models/run-verification.ts
 models/save-query-request.ts
 models/saved-query.ts
 models/send-message-request.ts
-models/send-session-message200-response.ts
+models/send-message-response.ts
 models/session-detail.ts
 models/session-list-item.ts
 models/session-turn.ts
@@ -92,11 +93,13 @@ models/steer-request.ts
 models/steer-run200-response.ts
 models/submit-answer-request.ts
 models/submit-answer-response.ts
+models/tool-turn.ts
 models/tool-use.ts
 models/usage-by-model.ts
 models/usage-stage.ts
 models/usage-totals.ts
 models/user-response.ts
+models/user-turn.ts
 models/verification-category.ts
 models/verification-control.ts
 models/verification-detail-response.ts
diff --git a/packages/arc-api-client/src/api/sessions-api.ts b/packages/arc-api-client/src/api/sessions-api.ts
index 16c93466c..750c5a438 100644
--- a/packages/arc-api-client/src/api/sessions-api.ts
+++ b/packages/arc-api-client/src/api/sessions-api.ts
@@ -32,7 +32,7 @@ import type { PaginatedSessionList } from '../models';
 // @ts-ignore
 import type { SendMessageRequest } from '../models';
 // @ts-ignore
-import type { SendSessionMessage200Response } from '../models';
+import type { SendMessageResponse } from '../models';
 // @ts-ignore
 import type { SessionDetail } from '../models';
 /**
@@ -314,7 +314,7 @@ export const SessionsApiFp = function(configuration?: Configuration) {
          * @param {*} [options] Override http request option.
          * @throws {RequiredError}
          */
-        async sendSessionMessage(id: string, sendMessageRequest: SendMessageRequest, options?: RawAxiosRequestConfig): Promise<(axios?: AxiosInstance, basePath?: string) => AxiosPromise<SendSessionMessage200Response>> {
+        async sendSessionMessage(id: string, sendMessageRequest: SendMessageRequest, options?: RawAxiosRequestConfig): Promise<(axios?: AxiosInstance, basePath?: string) => AxiosPromise<SendMessageResponse>> {
             const localVarAxiosArgs = await localVarAxiosParamCreator.sendSessionMessage(id, sendMessageRequest, options);
             const localVarOperationServerIndex = configuration?.serverIndex ?? 0;
             const localVarOperationServerBasePath = operationServerMap['SessionsApi.sendSessionMessage']?.[localVarOperationServerIndex]?.url;
@@ -381,7 +381,7 @@ export const SessionsApiFactory = function (configuration?: Configuration, baseP
          * @param {*} [options] Override http request option.
          * @throws {RequiredError}
          */
-        sendSessionMessage(id: string, sendMessageRequest: SendMessageRequest, options?: RawAxiosRequestConfig): AxiosPromise<SendSessionMessage200Response> {
+        sendSessionMessage(id: string, sendMessageRequest: SendMessageRequest, options?: RawAxiosRequestConfig): AxiosPromise<SendMessageResponse> {
             return localVarFp.sendSessionMessage(id, sendMessageRequest, options).then((request) => request(axios, basePath));
         },
         /**
diff --git a/packages/arc-api-client/src/models/assistant-turn.ts b/packages/arc-api-client/src/models/assistant-turn.ts
new file mode 100644
index 000000000..ba78a039e
--- /dev/null
+++ b/packages/arc-api-client/src/models/assistant-turn.ts
@@ -0,0 +1,34 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Arc Run API
+ * HTTP API for managing Arc workflow run executions.
+ *
+ * The version of the OpenAPI document: 0.1.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+
+/**
+ * An assistant response turn.
+ */
+export interface AssistantTurn {
+    'kind': AssistantTurnKindEnum;
+    /**
+     * Text content of the assistant response.
+     */
+    'content': string;
+}
+
+export const AssistantTurnKindEnum = {
+    ASSISTANT: 'assistant'
+} as const;
+
+export type AssistantTurnKindEnum = typeof AssistantTurnKindEnum[keyof typeof AssistantTurnKindEnum];
+
+
diff --git a/packages/arc-api-client/src/models/create-session-request.ts b/packages/arc-api-client/src/models/create-session-request.ts
index 2a1028542..a879efcdb 100644
--- a/packages/arc-api-client/src/models/create-session-request.ts
+++ b/packages/arc-api-client/src/models/create-session-request.ts
@@ -21,7 +21,7 @@ export interface CreateSessionRequest {
     /**
      * The initial user message to start the session.
      */
-    'prompt': string;
+    'content': string;
     /**
      * LLM model to use. If omitted, the server default is used.
      */
diff --git a/packages/arc-api-client/src/models/create-session-response.ts b/packages/arc-api-client/src/models/create-session-response.ts
index 1d0938e67..aa4afd22e 100644
--- a/packages/arc-api-client/src/models/create-session-response.ts
+++ b/packages/arc-api-client/src/models/create-session-response.ts
@@ -23,7 +23,7 @@ export interface CreateSessionResponse {
      */
     'id': string;
     /**
-     * ISO 8601 timestamp when the session was created.
+     * Timestamp when the session was created.
      */
     'created_at': string;
 }
diff --git a/packages/arc-api-client/src/models/index.ts b/packages/arc-api-client/src/models/index.ts
index 86c20dfc7..aac00499e 100644
--- a/packages/arc-api-client/src/models/index.ts
+++ b/packages/arc-api-client/src/models/index.ts
@@ -1,6 +1,7 @@
 export * from './aggregate-usage';
 export * from './api-question';
 export * from './api-question-option';
+export * from './assistant-turn';
 export * from './branch';
 export * from './cancel-run200-response';
 export * from './check-run';
@@ -56,7 +57,7 @@ export * from './run-verification-control';
 export * from './save-query-request';
 export * from './saved-query';
 export * from './send-message-request';
-export * from './send-session-message200-response';
+export * from './send-message-response';
 export * from './session-detail';
 export * from './session-list-item';
 export * from './session-turn';
@@ -73,11 +74,13 @@ export * from './steer-request';
 export * from './steer-run200-response';
 export * from './submit-answer-request';
 export * from './submit-answer-response';
+export * from './tool-turn';
 export * from './tool-use';
 export * from './usage-by-model';
 export * from './usage-stage';
 export * from './usage-totals';
 export * from './user-response';
+export * from './user-turn';
 export * from './verification-category';
 export * from './verification-control';
 export * from './verification-detail-response';
diff --git a/packages/arc-api-client/src/models/send-session-message200-response.ts b/packages/arc-api-client/src/models/send-message-response.ts
similarity index 77%
rename from packages/arc-api-client/src/models/send-session-message200-response.ts
rename to packages/arc-api-client/src/models/send-message-response.ts
index f59e634d4..3c0029a2e 100644
--- a/packages/arc-api-client/src/models/send-session-message200-response.ts
+++ b/packages/arc-api-client/src/models/send-message-response.ts
@@ -14,7 +14,10 @@
 
 
 
-export interface SendSessionMessage200Response {
+/**
+ * Acknowledgement that the message was accepted for asynchronous processing.
+ */
+export interface SendMessageResponse {
     /**
      * Whether the message was accepted for processing.
      */
diff --git a/packages/arc-api-client/src/models/session-detail.ts b/packages/arc-api-client/src/models/session-detail.ts
index 66fbdd085..b3f2187ab 100644
--- a/packages/arc-api-client/src/models/session-detail.ts
+++ b/packages/arc-api-client/src/models/session-detail.ts
@@ -34,11 +34,11 @@ export interface SessionDetail {
      */
     'model': string;
     /**
-     * ISO 8601 timestamp when the session was created.
+     * Timestamp when the session was created.
      */
     'created_at': string;
     /**
-     * ISO 8601 timestamp when the session was last updated (e.g. new turn added).
+     * Timestamp when the session was last updated (e.g. new turn added).
      */
     'updated_at': string;
     /**
diff --git a/packages/arc-api-client/src/models/session-list-item.ts b/packages/arc-api-client/src/models/session-list-item.ts
index 26dfcaa5f..18aca7551 100644
--- a/packages/arc-api-client/src/models/session-list-item.ts
+++ b/packages/arc-api-client/src/models/session-list-item.ts
@@ -27,8 +27,12 @@ export interface SessionListItem {
      */
     'title': string;
     /**
-     * ISO 8601 timestamp when the session was created.
+     * Timestamp when the session was created.
      */
     'created_at': string;
+    /**
+     * Timestamp when the session was last updated (e.g. new turn added).
+     */
+    'updated_at': string;
 }
 
diff --git a/packages/arc-api-client/src/models/session-turn.ts b/packages/arc-api-client/src/models/session-turn.ts
index 20d21f64f..621d1fbc8 100644
--- a/packages/arc-api-client/src/models/session-turn.ts
+++ b/packages/arc-api-client/src/models/session-turn.ts
@@ -13,38 +13,23 @@
  */
 
 
+// May contain unused imports in some cases
+// @ts-ignore
+import type { AssistantTurn } from './assistant-turn';
+// May contain unused imports in some cases
+// @ts-ignore
+import type { ToolTurn } from './tool-turn';
 // May contain unused imports in some cases
 // @ts-ignore
 import type { ToolUse } from './tool-use';
+// May contain unused imports in some cases
+// @ts-ignore
+import type { UserTurn } from './user-turn';
 
 /**
+ * @type SessionTurn
  * A single turn in a session conversation — a user message, assistant response, or tool invocation block.
  */
-export interface SessionTurn {
-    /**
-     * The type of turn.
-     */
-    'kind': SessionTurnKindEnum;
-    /**
-     * Text content of the turn. Present for user and assistant turns, absent for tool turns.
-     */
-    'content'?: string;
-    /**
-     * ISO 8601 timestamp when the turn was created. Typically present for user turns.
-     */
-    'created_at'?: string;
-    /**
-     * Tool invocations for this turn. Present only when kind is \"tool\".
-     */
-    'tools'?: Array<ToolUse>;
-}
-
-export const SessionTurnKindEnum = {
-    USER: 'user',
-    ASSISTANT: 'assistant',
-    TOOL: 'tool'
-} as const;
-
-export type SessionTurnKindEnum = typeof SessionTurnKindEnum[keyof typeof SessionTurnKindEnum];
+export type SessionTurn = { kind: 'assistant' } & AssistantTurn | { kind: 'tool' } & ToolTurn | { kind: 'user' } & UserTurn;
 
 
diff --git a/packages/arc-api-client/src/models/tool-turn.ts b/packages/arc-api-client/src/models/tool-turn.ts
new file mode 100644
index 000000000..0fb31f42f
--- /dev/null
+++ b/packages/arc-api-client/src/models/tool-turn.ts
@@ -0,0 +1,37 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Arc Run API
+ * HTTP API for managing Arc workflow run executions.
+ *
+ * The version of the OpenAPI document: 0.1.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+// May contain unused imports in some cases
+// @ts-ignore
+import type { ToolUse } from './tool-use';
+
+/**
+ * A tool invocation turn.
+ */
+export interface ToolTurn {
+    'kind': ToolTurnKindEnum;
+    /**
+     * Tool invocations for this turn.
+     */
+    'tools': Array<ToolUse>;
+}
+
+export const ToolTurnKindEnum = {
+    TOOL: 'tool'
+} as const;
+
+export type ToolTurnKindEnum = typeof ToolTurnKindEnum[keyof typeof ToolTurnKindEnum];
+
+
diff --git a/packages/arc-api-client/src/models/user-turn.ts b/packages/arc-api-client/src/models/user-turn.ts
new file mode 100644
index 000000000..732ea2170
--- /dev/null
+++ b/packages/arc-api-client/src/models/user-turn.ts
@@ -0,0 +1,38 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Arc Run API
+ * HTTP API for managing Arc workflow run executions.
+ *
+ * The version of the OpenAPI document: 0.1.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+
+/**
+ * A user message turn.
+ */
+export interface UserTurn {
+    'kind': UserTurnKindEnum;
+    /**
+     * Text content of the user message.
+     */
+    'content': string;
+    /**
+     * Timestamp when the turn was created.
+     */
+    'created_at': string;
+}
+
+export const UserTurnKindEnum = {
+    USER: 'user'
+} as const;
+
+export type UserTurnKindEnum = typeof UserTurnKindEnum[keyof typeof UserTurnKindEnum];
+
+