diff --git a/apps/server/src/services/environments/environment-provisioning-internal.ts b/apps/server/src/services/environments/environment-provisioning-internal.ts
index 89f4ded675..50fac9e1d4 100644
--- a/apps/server/src/services/environments/environment-provisioning-internal.ts
+++ b/apps/server/src/services/environments/environment-provisioning-internal.ts
@@ -8,6 +8,7 @@ import {
   getAppSettings,
   getEnvironment,
   getThread,
+  isSqliteUniqueConstraintOnColumns,
   listStoredThreadProvisioningRowsByProvisioningId,
   threads,
 } from "@bb/db";
@@ -586,19 +587,37 @@ export function settleEnvironmentProvisionCommandResult(
     .all();
 
   if (args.report.ok) {
-    recordProvisionedEnvironmentWorkspace(
-      args.deps.db,
-      args.deps.hub,
-      args.command.environmentId,
-      {
-        path: args.report.result.path,
-        isGitRepo: args.report.result.isGitRepo,
-        isWorktree: args.report.result.isWorktree,
-        branchName: args.report.result.branchName,
-        defaultBranch: args.report.result.defaultBranch,
-        ...resolveProvisionedEnvironmentBranchMetadata(args.command),
-      },
-    );
+    try {
+      recordProvisionedEnvironmentWorkspace(
+        args.deps.db,
+        args.deps.hub,
+        args.command.environmentId,
+        {
+          path: args.report.result.path,
+          isGitRepo: args.report.result.isGitRepo,
+          isWorktree: args.report.result.isWorktree,
+          branchName: args.report.result.branchName,
+          defaultBranch: args.report.result.defaultBranch,
+          ...resolveProvisionedEnvironmentBranchMetadata(args.command),
+        },
+      );
+    } catch (error) {
+      if (
+        error instanceof Error &&
+        isSqliteUniqueConstraintOnColumns(error, {
+          columnNames: ["project_id", "host_id", "path"],
+          indexName: "environments_project_host_path_idx",
+          tableName: "environments",
+        })
+      ) {
+        throw new ApiError(
+          409,
+          "invalid_request",
+          "Workspace path is already attached to another environment",
+        );
+      }
+      throw error;
+    }
     const provisionedOutcome =
       applyLoggedEnvironmentLifecycleEventInTransaction(args.deps, {
         environmentId: args.command.environmentId,
diff --git a/apps/server/test/environments/environment-provisioning.test.ts b/apps/server/test/environments/environment-provisioning.test.ts
index 56cc82b46d..7731ed420d 100644
--- a/apps/server/test/environments/environment-provisioning.test.ts
+++ b/apps/server/test/environments/environment-provisioning.test.ts
@@ -5,7 +5,10 @@ import {
   listEvents,
   threads,
 } from "@bb/db";
-import { systemThreadProvisioningEventDataSchema } from "@bb/domain";
+import {
+  systemErrorEventDataSchema,
+  systemThreadProvisioningEventDataSchema,
+} from "@bb/domain";
 import { describe, expect, it, vi } from "vitest";
 import { ApiError } from "../../src/errors.js";
 import {
@@ -334,6 +337,87 @@ describe("environment reprovisioning", () => {
     });
   });
 
+  it("reports a stable error when concurrent unmanaged provisions resolve to the same path", async () => {
+    await withTestHarness(async (harness) => {
+      const { host } = seedHostSession(harness.deps, {
+        id: "host-concurrent-unmanaged-path",
+      });
+      const { project } = seedProjectWithSource(harness.deps, {
+        hostId: host.id,
+        path: "/tmp/concurrent-unmanaged-project",
+      });
+      const createThread = (input: string) =>
+        createThreadFromRequest(harness.deps, {
+          startedOnBehalfOf: null,
+          environment: {
+            type: "host",
+            hostId: host.id,
+            workspace: {
+              type: "unmanaged",
+              path: "/tmp/concurrent-unmanaged-workspace",
+            },
+          },
+          input: textInput(input),
+          origin: "cli",
+          projectId: project.id,
+          providerId: "codex",
+        });
+
+      const firstThread = await createThread("first concurrent provision");
+      const secondThread = await createThread("second concurrent provision");
+      const firstProvision = await waitForQueuedCommand(
+        harness,
+        ({ command }) =>
+          command.type === "environment.provision" &&
+          command.initiator?.threadId === firstThread.id,
+      );
+      const secondProvision = await waitForQueuedCommand(
+        harness,
+        ({ command }) =>
+          command.type === "environment.provision" &&
+          command.initiator?.threadId === secondThread.id,
+      );
+      if (
+        firstProvision.command.type !== "environment.provision" ||
+        secondProvision.command.type !== "environment.provision"
+      ) {
+        throw new Error("Expected environment provision commands");
+      }
+      expect(firstProvision.command.environmentId).not.toBe(
+        secondProvision.command.environmentId,
+      );
+      expect(
+        getEnvironment(harness.db, firstProvision.command.environmentId)?.path,
+      ).toBeNull();
+      expect(
+        getEnvironment(harness.db, secondProvision.command.environmentId)?.path,
+      ).toBeNull();
+
+      const result = {
+        path: "/tmp/concurrent-unmanaged-workspace",
+        isGitRepo: true,
+        isWorktree: false,
+        branchName: "main",
+        defaultBranch: "main",
+        transcript: [],
+      };
+      await reportQueuedCommandSuccess(harness, firstProvision, result);
+      await reportQueuedCommandSuccess(harness, secondProvision, result);
+
+      const secondError = listEvents(harness.db, {
+        threadId: secondThread.id,
+      }).find((event) => event.type === "system/error");
+      expect(secondError).toBeDefined();
+      expect(
+        systemErrorEventDataSchema.parse(JSON.parse(secondError?.data ?? "{}")),
+      ).toMatchObject({
+        code: "thread_provisioning_failed",
+        message: "Provisioning thread failed",
+        detail: "Workspace path is already attached to another environment",
+      });
+    });
+  });
+
   it("finalizes a tombstoned thread instead of activating it when provisioning succeeds late", async () => {
     await withTestHarness(async (harness) => {
       const { host } = seedHostSession(harness.deps, {