diff --git a/packages/opencode/src/tool/truncate.ts b/packages/opencode/src/tool/truncate.ts
index 236f9b8a12..81fdfa2b3e 100644
--- a/packages/opencode/src/tool/truncate.ts
+++ b/packages/opencode/src/tool/truncate.ts
@@ -6,7 +6,6 @@ import type { Agent } from "../agent/agent"
 import { FSUtil } from "@opencode-ai/core/fs-util"
 import { evaluate } from "@/permission/evaluate"
 import { Config } from "@/config/config"
-import { Identifier } from "../id/id"
 import { ToolID } from "./schema"
 import { TRUNCATION_DIR } from "./truncation-dir"
 
@@ -52,17 +51,28 @@ export const layer = Layer.effect(
     const fs = yield* FSUtil.Service
 
     const cleanup = Effect.fn("Truncate.cleanup")(function* () {
-      const cutoff = Identifier.timestamp(
-        Identifier.create("tool", "ascending", Date.now() - Duration.toMillis(RETENTION)),
-      )
+      // altimate_change start — upstream_fix: age files by mtime, not decoded ID timestamps.
+      // Identifier packs `timestamp * 4096` into 48 bits, which wraps every
+      // 2^36 ms (~795 days) — the 26th wrap landed 2026-08-14T11:19:55Z, after
+      // which every new ID decoded as "ancient" and cleanup deleted files the
+      // moment they were written. File mtime has no wrap.
+      const cutoffMs = Date.now() - Duration.toMillis(RETENTION)
       const entries = yield* fs.readDirectory(TRUNCATION_DIR).pipe(
         Effect.map((all) => all.filter((name) => name.startsWith("tool_"))),
         Effect.catch(() => Effect.succeed([])),
       )
       for (const entry of entries) {
-        if (Identifier.timestamp(entry) >= cutoff) continue
-        yield* fs.remove(path.join(TRUNCATION_DIR, entry)).pipe(Effect.catch(() => Effect.void))
+        const file = path.join(TRUNCATION_DIR, entry)
+        // Stat through the INJECTED filesystem (FSUtil extends FileSystem) so
+        // custom/in-memory providers behave identically to the host FS.
+        const info = yield* fs.stat(file).pipe(Effect.catch(() => Effect.succeed(undefined)))
+        // Unstat-able file or absent mtime: keep it — deletion must fail safe.
+        const mtimeMs =
+          info && Option.isSome(info.mtime) ? info.mtime.value.getTime() : Number.POSITIVE_INFINITY
+        if (mtimeMs >= cutoffMs) continue
+        yield* fs.remove(file).pipe(Effect.catch(() => Effect.void))
       }
+      // altimate_change end
     })
 
     const write = Effect.fn("Truncate.write")(function* (text: string) {
diff --git a/packages/opencode/src/tool/truncation.ts b/packages/opencode/src/tool/truncation.ts
index 34370624a4..fbd92b1d7c 100644
--- a/packages/opencode/src/tool/truncation.ts
+++ b/packages/opencode/src/tool/truncation.ts
@@ -1,7 +1,6 @@
 import fs from "fs/promises"
 import path from "path"
 import { Global } from "../global"
-import { Identifier } from "../id/id"
 import { PermissionNext } from "../permission/next"
 import type { Agent } from "../agent/agent"
 import { Scheduler } from "../scheduler"
@@ -35,20 +34,24 @@ export namespace Truncate {
   }
 
   export async function cleanup() {
-    const cutoff = Identifier.timestamp(Identifier.create("tool", "ascending", Date.now() - RETENTION_MS))
+    // altimate_change start — upstream_fix: age files by mtime, not decoded ID
+    // timestamps. Identifier packs `timestamp * 4096` into 48 bits, wrapping
+    // every 2^36 ms (~795 days; the 26th wrap: 2026-08-14T11:19:55Z), after
+    // which every new ID decoded as "ancient" and cleanup deleted files the
+    // moment they were written. File mtime has no wrap. Stat failures keep
+    // the file — deletion must fail safe.
+    const cutoffMs = Date.now() - RETENTION_MS
     const entries = await Glob.scan("tool_*", { cwd: DIR, include: "file" }).catch(() => [] as string[])
     for (const entry of entries) {
-      // altimate_change start - tolerate stale/malformed tool-output files from older builds.
-      let timestamp: number
-      try {
-        timestamp = Identifier.timestamp(entry)
-      } catch {
-        continue
-      }
-      if (timestamp >= cutoff) continue
-      // altimate_change end
-      await fs.unlink(path.join(DIR, entry)).catch(() => {})
+      const file = path.join(DIR, entry)
+      const mtimeMs = await fs
+        .stat(file)
+        .then((st) => st.mtimeMs)
+        .catch(() => Number.POSITIVE_INFINITY)
+      if (mtimeMs >= cutoffMs) continue
+      await fs.unlink(file).catch(() => {})
     }
+    // altimate_change end
   }
 
   function hasTaskTool(agent?: Agent.Info): boolean {
diff --git a/packages/opencode/test/lib/filesystem.ts b/packages/opencode/test/lib/filesystem.ts
index 66f702ec3d..ba4eac1599 100644
--- a/packages/opencode/test/lib/filesystem.ts
+++ b/packages/opencode/test/lib/filesystem.ts
@@ -8,3 +8,12 @@ export const writeFileStringScoped = Effect.fn("test.writeFileStringScoped")(fun
   yield* Effect.addFinalizer(() => fs.remove(file, { force: true }).pipe(Effect.orDie))
   return file
 })
+
+/** Create a symlink whose removal is guaranteed by the test scope, even when
+ *  an assertion fails mid-test (finalizers run on scope close regardless). */
+export const symlinkScoped = Effect.fn("test.symlinkScoped")(function* (target: string, link: string) {
+  const fs = yield* FileSystem.FileSystem
+  yield* fs.symlink(target, link)
+  yield* Effect.addFinalizer(() => fs.remove(link, { force: true }).pipe(Effect.orDie))
+  return link
+})
diff --git a/packages/opencode/test/tool/truncation.test.ts b/packages/opencode/test/tool/truncation.test.ts
index 6e65b5f54c..450559c7d2 100644
--- a/packages/opencode/test/tool/truncation.test.ts
+++ b/packages/opencode/test/tool/truncation.test.ts
@@ -9,7 +9,7 @@ import { Identifier } from "../../src/id/id"
 import { Process } from "@/util/process"
 import path from "path"
 import { testEffect } from "../lib/effect"
-import { writeFileStringScoped } from "../lib/filesystem"
+import { symlinkScoped, writeFileStringScoped } from "../lib/filesystem"
 import { TestConfig } from "../fixture/config"
 
 const FIXTURES_DIR = path.join(import.meta.dir, "fixtures")
@@ -251,15 +251,33 @@ describe("Truncate", () => {
 
         yield* fs.makeDirectory(Truncate.DIR, { recursive: true })
 
-        const old = path.join(Truncate.DIR, Identifier.create("tool", "ascending", Date.now() - 10 * DAY_MS))
-        const recent = path.join(Truncate.DIR, Identifier.create("tool", "ascending", Date.now() - 3 * DAY_MS))
+        // Age is judged by file mtime (ID-embedded timestamps wrap every
+        // ~795 days — see Truncate.cleanup), so set mtimes explicitly.
+        const old = path.join(Truncate.DIR, Identifier.create("tool", "ascending"))
+        const recent = path.join(Truncate.DIR, Identifier.create("tool", "ascending"))
+        // Dangling symlink: listed by readDirectory, but stat fails — the
+        // fail-safe branch must KEEP it rather than delete on uncertainty.
+        const dangling = path.join(Truncate.DIR, Identifier.create("tool", "ascending"))
 
         yield* writeFileStringScoped(old, "old content")
         yield* writeFileStringScoped(recent, "recent content")
+        const nfs = yield* Effect.promise(() => import("node:fs/promises"))
+        // Scoped: the finalizer unlinks it even when an assertion fails —
+        // otherwise a failed expect would leak the link into the real data
+        // dir, where the fail-safe under test deliberately keeps it forever.
+        yield* symlinkScoped(path.join(Truncate.DIR, "nonexistent-target"), dangling)
+        const oldTime = new Date(Date.now() - 10 * DAY_MS)
+        const recentTime = new Date(Date.now() - 3 * DAY_MS)
+        yield* Effect.promise(() => nfs.utimes(old, oldTime, oldTime))
+        yield* Effect.promise(() => nfs.utimes(recent, recentTime, recentTime))
         yield* svc.cleanup()
 
         expect(yield* fs.exists(old)).toBe(false)
         expect(yield* fs.exists(recent)).toBe(true)
+        // lstat: fs.exists follows symlinks and would report false for a
+        // dangling link even when the link itself survived.
+        const danglingKept = yield* Effect.promise(() => nfs.lstat(dangling).then(() => true).catch(() => false))
+        expect(danglingKept).toBe(true)
       }),
     )
   })