diff --git a/apps/server/src/attachmentStore.ts b/apps/server/src/attachmentStore.ts
index 5d8ab2e90181b..616b0d90c17b2 100644
--- a/apps/server/src/attachmentStore.ts
+++ b/apps/server/src/attachmentStore.ts
@@ -13,7 +13,7 @@ import {
 import { inferImageExtension, SAFE_IMAGE_FILE_EXTENSIONS } from "./imageMime.ts";
 
 const ATTACHMENT_FILENAME_EXTENSIONS = [...SAFE_IMAGE_FILE_EXTENSIONS, ".bin"];
-const ATTACHMENT_ID_THREAD_SEGMENT_MAX_CHARS = 80;
+export const ATTACHMENT_ID_THREAD_SEGMENT_MAX_CHARS = 80;
 const ATTACHMENT_ID_THREAD_SEGMENT_PATTERN = "[a-z0-9_]+(?:-[a-z0-9_]+)*";
 const ATTACHMENT_ID_UUID_PATTERN = "[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}";
 const ATTACHMENT_ID_FILE_EXTENSION_PATTERN = "[a-z0-9]{1,10}";
diff --git a/apps/server/src/provider/EventNdjsonLogger.test.ts b/apps/server/src/provider/EventNdjsonLogger.test.ts
index c04150dce8df6..3cb7870f3f518 100644
--- a/apps/server/src/provider/EventNdjsonLogger.test.ts
+++ b/apps/server/src/provider/EventNdjsonLogger.test.ts
@@ -156,6 +156,39 @@ it.layer(NodeServices.layer)("EventNdjsonLogger", (it) => {
     }),
   );
 
+  it.effect("keeps long thread ids that share a prefix in separate files", () =>
+    Effect.gen(function* () {
+      const tempDir = NodeFS.mkdtempSync(NodePath.join(NodeOS.tmpdir(), "t3-provider-log-"));
+      const basePath = NodePath.join(tempDir, "events.log");
+      const prefix =
+        "thread:delegated-task:command%3Amcp%3A00000000-0000-4000-8000-000000000000%3Adelegate-task%3A";
+
+      try {
+        const store = yield* makeEventNdjsonLogStore(basePath, { batchWindowMs: 0 });
+        const logger = store.logger("native");
+        yield* logger.write({ id: "first" }, ThreadId.make(`${prefix}review-the-diff`));
+        yield* logger.write({ id: "second" }, ThreadId.make(`${prefix}write-the-tests`));
+        yield* store.close();
+
+        const files = NodeFS.readdirSync(tempDir);
+        assert.equal(files.length, 2);
+        assert.deepEqual(
+          files
+            .map((name) =>
+              NodeFS.readFileSync(NodePath.join(tempDir, name), "utf8")
+                .trim()
+                .split("\n")
+                .map((line) => parseLogLine(line).payload),
+            )
+            .toSorted(),
+          [['{"id":"first"}'], ['{"id":"second"}']],
+        );
+      } finally {
+        NodeFS.rmSync(tempDir, { recursive: true, force: true });
+      }
+    }),
+  );
+
   it.effect("keeps shared store views non-owning when one adapter closes", () =>
     Effect.gen(function* () {
       const tempDir = NodeFS.mkdtempSync(NodePath.join(NodeOS.tmpdir(), "t3-provider-log-"));
diff --git a/apps/server/src/provider/EventNdjsonLogger.ts b/apps/server/src/provider/EventNdjsonLogger.ts
index 0e8c9043dcf0f..6a1d3844b2373 100644
--- a/apps/server/src/provider/EventNdjsonLogger.ts
+++ b/apps/server/src/provider/EventNdjsonLogger.ts
@@ -1,5 +1,6 @@
 // @effect-diagnostics nodeBuiltinImport:off
 /** Native and canonical views share write state to avoid races on the same thread file. */
+import * as NodeCrypto from "node:crypto";
 import * as NodeFS from "node:fs";
 import * as NodePath from "node:path";
 
@@ -18,7 +19,10 @@ import * as Schema from "effect/Schema";
 import * as Scope from "effect/Scope";
 import * as SynchronizedRef from "effect/SynchronizedRef";
 
-import { toSafeThreadAttachmentSegment } from "../attachmentStore.ts";
+import {
+  ATTACHMENT_ID_THREAD_SEGMENT_MAX_CHARS,
+  toSafeThreadAttachmentSegment,
+} from "../attachmentStore.ts";
 import type * as ResourceAttribution from "../resourceTelemetry/ResourceAttribution.ts";
 
 const MEBIBYTE = 1024 * 1024;
@@ -145,8 +149,13 @@ function logWarning(message: string, context: Record<string, unknown>): Effect.E
 }
 
 function resolveThreadSegment(raw: string | null | undefined): string {
-  const normalized = typeof raw === "string" ? toSafeThreadAttachmentSegment(raw) : null;
-  return normalized ?? GLOBAL_THREAD_SEGMENT;
+  if (typeof raw !== "string") return GLOBAL_THREAD_SEGMENT;
+  const normalized = toSafeThreadAttachmentSegment(raw);
+  if (normalized === null) return GLOBAL_THREAD_SEGMENT;
+  // Delegated thread ids share a prefix longer than the segment, so ids that may be cut get a hash.
+  if (raw.length <= ATTACHMENT_ID_THREAD_SEGMENT_MAX_CHARS) return normalized;
+  const hash = NodeCrypto.createHash("sha256").update(raw).digest("hex").slice(0, 16);
+  return `${normalized}-${hash}`;
 }
 
 function resolveStreamLabel(stream: EventNdjsonStream): string {
@@ -725,12 +734,13 @@ export const makeEventNdjsonLogStore = Effect.fnUntraced(function* (
       const observedAt = yield* DateTime.now.pipe(Effect.map(DateTime.formatIso));
       const line = `[${observedAt}] ${resolveStreamLabel(stream)}: ${payload}\n`;
       const bytes = Buffer.byteLength(line);
+      const threadSegment = resolveThreadSegment(threadId);
       const action = yield* SynchronizedRef.modifyEffect(stateRef, (state) => {
         if (state.closed) {
           return Effect.succeed([{ flush: false }, state] as const);
         }
         const pending = state.pending;
-        pending.push({ stream, threadSegment: resolveThreadSegment(threadId), line, bytes });
+        pending.push({ stream, threadSegment, line, bytes });
         const pendingBytes = state.pendingBytes + bytes;
         const flush =
           resolved.batchWindowMs === 0 ||