diff --git a/.changeset/calm-moves-cache.md b/.changeset/calm-moves-cache.md
new file mode 100644
index 0000000000..eb408e642a
--- /dev/null
+++ b/.changeset/calm-moves-cache.md
@@ -0,0 +1,5 @@
+---
+"@opencode-ai/core": patch
+---
+
+Preserve prompt cache prefixes when sessions move between locations with unchanged instructions.
diff --git a/AGENTS.md b/AGENTS.md
index be191e23dd..8b7fb11564 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -171,4 +171,4 @@ const table = sqliteTable("session", {
 - One step is one logical LLM call; its durable record covers only the model-visible span. Do not write "provider turn", and do not use bare "turn" for a single call: "turn" is reserved for the future assistant-turn unit containing all steps from prompt promotion until the session would go idle.
 - Keep EventV2 replay owner claims separate from clustered Session execution ownership.
 - Keep the Instructions algebra and built-ins in `src/instructions`; keep instruction producers with their observed domains, and keep Session History selection plus `InstructionState` and `InstructionEntry` persistence Session-owned. `InstructionDiscovery` observes ambient global and upward-project instructions. The runner composes built-ins, discovery, guidance, and entries explicitly in `loadInstructions`; there is no instruction registry.
-- `session.instructions.updated` stores only changed source keys and content hashes. Blob values live once in `instruction_blob`; `instruction_state` is a rebuildable fold cache, never primary state. Render initial instructions and chronological updates from values during request assembly. Completed compaction moves the instruction epoch; Session movement and committed revert clear it. Unavailable sources retain the last value and block only the initial complete delta.
+- `session.instructions.updated` stores only changed source keys and content hashes. Blob values live once in `instruction_blob`; `instruction_state` is a rebuildable fold cache, never primary state. Render initial instructions and chronological updates from values during request assembly. Completed compaction moves the instruction epoch; Session movement retains it so destination instruction changes are chronological, while committed revert clears it. Unavailable sources retain the last value and block only the initial complete delta.
diff --git a/packages/core/src/session/projector.ts b/packages/core/src/session/projector.ts
index 1393536df5..05b7f4561b 100644
--- a/packages/core/src/session/projector.ts
+++ b/packages/core/src/session/projector.ts
@@ -436,7 +436,6 @@ const layer = Layer.effectDiscard(
           .where(eq(SessionTable.id, event.data.sessionID))
           .run()
           .pipe(Effect.orDie)
-        yield* InstructionState.reset(db, event.data.sessionID)
       }),
     )
     // Sessions whose ownership came from the directory's previous resolution
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index a5cdbe1efc..f7531621e5 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -1204,12 +1204,17 @@ describe("SessionRunnerLLM", () => {
     }),
   )
 
-  it.effect("interrupts a source Location runner after a Session moves", () =>
+  it.effect("preserves instruction state and interrupts the source Location runner after a Session moves", () =>
     Effect.gen(function* () {
       const session = yield* setup
       const bus = yield* Bus.Service
       const { db } = yield* Database.Service
       yield* runPrompt(session, "First")
+      const instructionState = yield* db
+        .select()
+        .from(InstructionStateTable)
+        .where(eq(InstructionStateTable.session_id, sessionID))
+        .get()
 
       yield* bus.publish(SessionEvent.Moved, {
         sessionID,
@@ -1218,7 +1223,7 @@ describe("SessionRunnerLLM", () => {
       })
       expect(
         yield* db.select().from(InstructionStateTable).where(eq(InstructionStateTable.session_id, sessionID)).get(),
-      ).toBeUndefined()
+      ).toEqual(instructionState)
 
       yield* admit(session, "Second")
       const exit = yield* session.resume(sessionID).pipe(Effect.exit)