[codex-analytics] add compaction analytics event (#17155)

- event for compaction analytics
- introduces thread-connection and thread metadata caches for data
denormalization, expected to be useful for denormalization onto core
emitted events in general
- threads analytics event client into core (mirrors approved
implementation in #16640)
- denormalizes key thread metadata: thread_source, subagent_source,
parent_thread_id, as well as app-server client and runtime metadata)
- compaction strategy defaults to memento, forward compatible with
expected prefill_compaction strategy

1. Manual standalone compact, local
`INFO | 2026-04-09 17:35:50 | codex_backend.routers.analytics_events |
analytics_events.track_analytics_events:526 | Tracked
codex_compaction_event event params={'thread_id':
'019d74d0-5cfb-70c0-bef9-165c3bf9b2df', 'turn_id':
'019d74d0-d7f6-7c81-acc6-aae2030243d6', 'product_surface': 'codex',
'app_server_client': {'product_client_id': 'CODEX_CLI', 'client_name':
'codex-tui', 'client_version': '0.0.0', 'rpc_transport': 'in_process',
'experimental_api_enabled': True}, 'runtime': {'codex_rs_version':
'0.0.0', 'runtime_os': 'macos', 'runtime_os_version': '26.4.0',
'runtime_arch': 'aarch64'}, 'trigger': 'manual', 'reason':
'user_requested', 'implementation': 'responses', 'phase':
'standalone_turn', 'strategy': 'memento', 'status': 'completed',
'active_context_tokens_before': 20170, 'active_context_tokens_after':
4830, 'started_at': 1775781337, 'completed_at': 1775781350,
'thread_source': 'user', 'subagent_source': None, 'parent_thread_id':
None, 'error': None, 'duration_ms': 13524} | `

2. Auto pre-turn compact, local
`INFO | 2026-04-09 17:37:30 | codex_backend.routers.analytics_events |
analytics_events.track_analytics_events:526 | Tracked
codex_compaction_event event params={'thread_id':
'019d74d2-45ef-71d1-9c93-23cc0c13d988', 'turn_id':
'019d74d2-7b42-7372-9f0e-c0da3f352328', 'product_surface': 'codex',
'app_server_client': {'product_client_id': 'CODEX_CLI', 'client_name':
'codex-tui', 'client_version': '0.0.0', 'rpc_transport': 'in_process',
'experimental_api_enabled': True}, 'runtime': {'codex_rs_version':
'0.0.0', 'runtime_os': 'macos', 'runtime_os_version': '26.4.0',
'runtime_arch': 'aarch64'}, 'trigger': 'auto', 'reason':
'context_limit', 'implementation': 'responses', 'phase': 'pre_turn',
'strategy': 'memento', 'status': 'completed',
'active_context_tokens_before': 20063, 'active_context_tokens_after':
4822, 'started_at': 1775781444, 'completed_at': 1775781449,
'thread_source': 'user', 'subagent_source': None, 'parent_thread_id':
None, 'error': None, 'duration_ms': 5497} | `

3. Auto mid-turn compact, local
`INFO | 2026-04-09 17:38:28 | codex_backend.routers.analytics_events |
analytics_events.track_analytics_events:526 | Tracked
codex_compaction_event event params={'thread_id':
'019d74d3-212f-7a20-8c0a-4816a978675e', 'turn_id':
'019d74d3-3ee1-7462-89f6-2ffbeefcd5e3', 'product_surface': 'codex',
'app_server_client': {'product_client_id': 'CODEX_CLI', 'client_name':
'codex-tui', 'client_version': '0.0.0', 'rpc_transport': 'in_process',
'experimental_api_enabled': True}, 'runtime': {'codex_rs_version':
'0.0.0', 'runtime_os': 'macos', 'runtime_os_version': '26.4.0',
'runtime_arch': 'aarch64'}, 'trigger': 'auto', 'reason':
'context_limit', 'implementation': 'responses', 'phase': 'mid_turn',
'strategy': 'memento', 'status': 'completed',
'active_context_tokens_before': 20325, 'active_context_tokens_after':
14641, 'started_at': 1775781500, 'completed_at': 1775781508,
'thread_source': 'user', 'subagent_source': None, 'parent_thread_id':
None, 'error': None, 'duration_ms': 7507} | `

4. Remote /responses/compact, manual standalone
`INFO | 2026-04-09 17:40:20 | codex_backend.routers.analytics_events |
analytics_events.track_analytics_events:526 | Tracked
codex_compaction_event event params={'thread_id':
'019d74d4-7a11-78a1-89f7-0535a1149416', 'turn_id':
'019d74d4-e087-7183-9c20-b1e40b7578c0', 'product_surface': 'codex',
'app_server_client': {'product_client_id': 'CODEX_CLI', 'client_name':
'codex-tui', 'client_version': '0.0.0', 'rpc_transport': 'in_process',
'experimental_api_enabled': True}, 'runtime': {'codex_rs_version':
'0.0.0', 'runtime_os': 'macos', 'runtime_os_version': '26.4.0',
'runtime_arch': 'aarch64'}, 'trigger': 'manual', 'reason':
'user_requested', 'implementation': 'responses_compact', 'phase':
'standalone_turn', 'strategy': 'memento', 'status': 'completed',
'active_context_tokens_before': 23461, 'active_context_tokens_after':
6171, 'started_at': 1775781601, 'completed_at': 1775781620,
'thread_source': 'user', 'subagent_source': None, 'parent_thread_id':
None, 'error': None, 'duration_ms': 18971} | `

codex-rs/analytics/src/facts.rs

@@ -64,6 +64,69 @@ pub struct SubAgentThreadStartedInput {
     pub created_at: u64,
 }
 
+#[derive(Clone, Copy, Debug, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CompactionTrigger {
+    Manual,
+    Auto,
+}
+
+#[derive(Clone, Copy, Debug, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CompactionReason {
+    UserRequested,
+    ContextLimit,
+    ModelDownshift,
+}
+
+#[derive(Clone, Copy, Debug, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CompactionImplementation {
+    Responses,
+    ResponsesCompact,
+}
+
+#[derive(Clone, Copy, Debug, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CompactionPhase {
+    StandaloneTurn,
+    PreTurn,
+    MidTurn,
+}
+
+#[derive(Clone, Copy, Debug, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CompactionStrategy {
+    Memento,
+    PrefixCompaction,
+}
+
+#[derive(Clone, Copy, Debug, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CompactionStatus {
+    Completed,
+    Failed,
+    Interrupted,
+}
+
+#[derive(Clone)]
+pub struct CodexCompactionEvent {
+    pub thread_id: String,
+    pub turn_id: String,
+    pub trigger: CompactionTrigger,
+    pub reason: CompactionReason,
+    pub implementation: CompactionImplementation,
+    pub phase: CompactionPhase,
+    pub strategy: CompactionStrategy,
+    pub status: CompactionStatus,
+    pub error: Option<String>,
+    pub active_context_tokens_before: i64,
+    pub active_context_tokens_after: i64,
+    pub started_at: u64,
+    pub completed_at: u64,
+    pub duration_ms: Option<u64>,
+}
+
 #[allow(dead_code)]
 pub(crate) enum AnalyticsFact {
     Initialize {
@@ -90,6 +153,7 @@ pub(crate) enum AnalyticsFact {
 
 pub(crate) enum CustomAnalyticsFact {
     SubAgentThreadStarted(SubAgentThreadStartedInput),
+    Compaction(Box<CodexCompactionEvent>),
     SkillInvoked(SkillInvokedInput),
     AppMentioned(AppMentionedInput),
     AppUsed(AppUsedInput),