diff --git a/packages/core/src/session/runner/llm.ts b/packages/core/src/session/runner/llm.ts
index 07f50385092f..d07d878e3704 100644
--- a/packages/core/src/session/runner/llm.ts
+++ b/packages/core/src/session/runner/llm.ts
@@ -211,9 +211,10 @@ export const layer = Layer.effect(
       const model = yield* models.resolve(session)
       const entries = yield* SessionHistory.entriesForRunner(db, session.id, system.baselineSeq)
       const context = entries.map((entry) => entry.message)
+      const promptCacheKey = /^ses_[0-9a-f]{64}$/.test(session.id) ? session.id.slice(4) : session.id
       const request = LLM.request({
         model,
-        providerOptions: { openai: { promptCacheKey: session.id } },
+        providerOptions: { openai: { promptCacheKey } },
         system: [agent.info?.system, system.baseline]
           .filter((part): part is string => part !== undefined && part.length > 0)
           .map(SystemPart.make),
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index 7df44617b768..61a13a427e33 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -2824,6 +2824,42 @@ describe("SessionRunnerLLM", () => {
     }),
   )
 
+  it.effect("bounds external session prompt cache keys", () =>
+    Effect.gen(function* () {
+      yield* setup
+      const externalSessionID = SessionV2.ID.fromExternal({
+        namespace: "discord",
+        key: "thread-one",
+      })
+      const otherExternalSessionID = SessionV2.ID.fromExternal({
+        namespace: "discord",
+        key: "thread-two",
+      })
+      yield* insertSession(externalSessionID)
+      yield* insertSession(otherExternalSessionID)
+      const session = yield* SessionV2.Service
+      yield* session.prompt({
+        sessionID: externalSessionID,
+        prompt: new Prompt({ text: "Run external session" }),
+        resume: false,
+      })
+      yield* session.prompt({
+        sessionID: otherExternalSessionID,
+        prompt: new Prompt({ text: "Run other external session" }),
+        resume: false,
+      })
+
+      requests.length = 0
+      yield* session.resume(externalSessionID)
+      yield* session.resume(otherExternalSessionID)
+
+      const keys = requests.map((request) => request.providerOptions?.openai?.promptCacheKey)
+      expect(keys).toEqual([externalSessionID.slice(4), otherExternalSessionID.slice(4)])
+      expect(keys.every((key) => typeof key === "string" && key.length === 64)).toBe(true)
+      expect(keys[0]).not.toBe(keys[1])
+    }),
+  )
+
   it.effect("fans out one failed run and allows a later retry", () =>
     Effect.gen(function* () {
       yield* setup