diff --git a/src/state.ts b/src/state.ts
new file mode 100644
index 0000000..8b3a2af
--- /dev/null
+++ b/src/state.ts
@@ -0,0 +1,32 @@
+/** How a skill was first observed in the session (SPEC §6.2). */
+export type SkillSource = "slash" | "skill-block" | "read";
+
+/** Compaction that triggered or skipped re-inject (SPEC §8). */
+export type CompactionSource = "auto" | "manual";
+
+/** pi-auto-compact delivery mode (SPEC §6.5, §16). */
+export type AutoCompactIntegration = "auto" | "defer" | "immediate" | "off";
+
+export interface TrackedSkill {
+	name: string;
+	filePath: string;
+	baseDir: string;
+	firstSeenAt: number;
+	lastSeenAt: number;
+	sources: SkillSource[];
+}
+
+export interface ExtensionState {
+	version: 1;
+	sessionOverride: boolean | null;
+	skills: TrackedSkill[];
+	lastCompactionSource: CompactionSource | null;
+	/** Skill names awaiting re-inject on the next before_agent_start (SPEC §6.5). */
+	pendingReinject: string[];
+}
+
+/** Runtime-only; not persisted via appendEntry (SPEC §6.1). */
+export interface RuntimeFlags {
+	autoCompactDetected: boolean;
+	autoCompactIntegration: AutoCompactIntegration;
+}