diff --git a/packages/pi-fff/src/index.ts b/packages/pi-fff/src/index.ts
index 608d09fda..637a484e3 100644
--- a/packages/pi-fff/src/index.ts
+++ b/packages/pi-fff/src/index.ts
@@ -259,6 +259,13 @@ function buildAtCompletionValue(path: string): string {
   return path.includes(" ") ? `@"${path}"` : `@${path}`;
 }
 
+// heed refuses to reopen an LMDB env already open in the process, surfaced
+// through FileFinder.create() as this error string. Matched to fall back to a
+// transient (no-DB) finder for in-process subagents — see #760.
+function isEnvAlreadyOpenError(error: string): boolean {
+  return /environment already open/i.test(error);
+}
+
 function createFffMentionProvider(
   getItems: (query: string, signal: AbortSignal) => Promise<AutocompleteItem[]>,
 ): AutocompleteProvider {
@@ -396,7 +403,7 @@ export default function fffExtension(pi: ExtensionAPI) {
       }
 
       const { FileFinder } = await loadSdk();
-      const result = FileFinder.create({
+      let result = FileFinder.create({
         basePath: cwd,
         frecencyDbPath,
         historyDbPath,
@@ -405,6 +412,21 @@ export default function fffExtension(pi: ExtensionAPI) {
         enableFsRootScanning,
       });
 
+      // In-process subagents (createAgentSession) load this extension a second
+      // time in the same process. LMDB opens an env at most once per path per
+      // process, so the shared frecency/history DBs are already owned by the
+      // parent session's finder — reopening them fails. Fall back to a
+      // transient finder without persistent scoring, exactly like aux finders
+      // do (#700). See #760.
+      if (!result.ok && isEnvAlreadyOpenError(result.error)) {
+        result = FileFinder.create({
+          basePath: cwd,
+          aiMode: true,
+          enableHomeDirScanning,
+          enableFsRootScanning,
+        });
+      }
+
       if (!result.ok)
         throw new Error(`Failed to create FFF file finder: ${result.error}`);
 
diff --git a/packages/pi-fff/test/extension.test.ts b/packages/pi-fff/test/extension.test.ts
index eb977b9f9..7403bf921 100644
--- a/packages/pi-fff/test/extension.test.ts
+++ b/packages/pi-fff/test/extension.test.ts
@@ -49,10 +49,16 @@ function createMockFinder(): MockFinder {
   };
 }
 
+let createImpl: ((options: unknown) => unknown) | undefined;
+
 const finderModule = {
   FileFinder: {
     create: mock((options: unknown) => {
       createCalls.push(options);
+      if (createImpl) {
+        const override = createImpl(options);
+        if (override !== undefined) return override;
+      }
       const finder = createMockFinder();
       finders.push(finder);
       return { ok: true, value: finder };
@@ -163,6 +169,7 @@ beforeEach(() => {
   createCalls.length = 0;
   finders = [];
   mixedSearchImpl = undefined;
+  createImpl = undefined;
   scanProgressImpl = undefined;
   delete process.env.PI_FFF_MODE;
   delete process.env.FFF_ENABLE_HOME_SCAN;
@@ -256,6 +263,43 @@ describe("pi-fff autocomplete registration", () => {
     expect(opts.enableHomeDirScanning).toBe(false);
   });
 
+  // In-process subagents (createAgentSession) reopen the shared LMDB env,
+  // which fails. The main finder must fall back to a transient no-DB finder
+  // instead of throwing during session_start — see #760.
+  test("falls back to a transient finder when the LMDB env is already open", async () => {
+    process.env.FFF_FRECENCY_DB = "/tmp/fff-test/frecency";
+    process.env.FFF_HISTORY_DB = "/tmp/fff-test/history";
+    let firstCall = true;
+    createImpl = () => {
+      if (firstCall) {
+        firstCall = false;
+        return {
+          ok: false,
+          error:
+            "Failed to init frecency db: Failed to open frecency database env: environment already open in this program; close it to be able to open it again with different options",
+        };
+      }
+      return undefined;
+    };
+
+    const { ctx } = await start();
+
+    expect(ctx.ui.notify).not.toHaveBeenCalledWith(
+      expect.stringContaining("FFF init failed"),
+      "error",
+    );
+    expect(createCalls).toHaveLength(2);
+    const retry = createCalls[1] as {
+      frecencyDbPath?: string;
+      historyDbPath?: string;
+    };
+    expect(retry.frecencyDbPath).toBeUndefined();
+    expect(retry.historyDbPath).toBeUndefined();
+
+    delete process.env.FFF_FRECENCY_DB;
+    delete process.env.FFF_HISTORY_DB;
+  });
+
   test("session_start survives hosts without addAutocompleteProvider", async () => {
     const setup = createPi();
     const ctx = {