diff --git a/.server-changes/env-not-found-404.md b/.server-changes/env-not-found-404.md
new file mode 100644
index 00000000000..89e50726606
--- /dev/null
+++ b/.server-changes/env-not-found-404.md
@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Dashboard runs, sessions, batches, and schedule-detail loaders now return 404 (or redirect to the user's home with a toast for missing projects) instead of 500 when a slug doesn't resolve.
diff --git a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.batches/route.tsx b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.batches/route.tsx
index 17dcfbc4619..47318edd355 100644
--- a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.batches/route.tsx
+++ b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.batches/route.tsx
@@ -54,6 +54,7 @@ import {
   v3BatchPath,
   v3BatchRunsPath,
 } from "~/utils/pathBuilder";
+import { throwNotFound } from "~/utils/httpErrors";
 
 export const meta: MetaFunction = () => {
   return [
@@ -74,7 +75,7 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
 
   const environment = await findEnvironmentBySlug(project.id, envParam, userId);
   if (!environment) {
-    throw new Error("Environment not found");
+    throwNotFound("Environment not found");
   }
 
   const url = new URL(request.url);
diff --git a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.runs._index/route.tsx b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.runs._index/route.tsx
index f555f98171e..d271e6f2b22 100644
--- a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.runs._index/route.tsx
+++ b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.runs._index/route.tsx
@@ -40,6 +40,7 @@ import { useOrganization } from "~/hooks/useOrganizations";
 import { useProject } from "~/hooks/useProject";
 import { useSearchParams } from "~/hooks/useSearchParam";
 import { useShortcutKeys } from "~/hooks/useShortcutKeys";
+import { redirectWithErrorMessage } from "~/models/message.server";
 import { findProjectBySlug } from "~/models/project.server";
 import { findEnvironmentBySlug } from "~/models/runtimeEnvironment.server";
 import { getRunFiltersFromRequest } from "~/presenters/RunFilters.server";
@@ -59,6 +60,7 @@ import {
   v3TestPath,
   v3TestTaskPath,
 } from "~/utils/pathBuilder";
+import { throwNotFound } from "~/utils/httpErrors";
 import { ListPagination } from "../../components/ListPagination";
 import { CreateBulkActionInspector } from "../resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.runs.bulkaction";
 import { Callout } from "~/components/primitives/Callout";
@@ -77,12 +79,12 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
 
   const project = await findProjectBySlug(organizationSlug, projectParam, userId);
   if (!project) {
-    throw new Error("Project not found");
+    return redirectWithErrorMessage("/", request, "Project not found");
   }
 
   const environment = await findEnvironmentBySlug(project.id, envParam, userId);
   if (!environment) {
-    throw new Error("Environment not found");
+    throwNotFound("Environment not found");
   }
 
   const filters = await getRunFiltersFromRequest(request);
diff --git a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.schedules.$scheduleParam/route.tsx b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.schedules.$scheduleParam/route.tsx
index f4e663b1b7d..a837274222b 100644
--- a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.schedules.$scheduleParam/route.tsx
+++ b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.schedules.$scheduleParam/route.tsx
@@ -55,6 +55,7 @@ import {
   v3SchedulePath,
   v3SchedulesPath,
 } from "~/utils/pathBuilder";
+import { throwNotFound } from "~/utils/httpErrors";
 import { DeleteTaskScheduleService } from "~/v3/services/deleteTaskSchedule.server";
 import { SetActiveOnTaskScheduleService } from "~/v3/services/setActiveOnTaskSchedule.server";
 
@@ -84,7 +85,7 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
   });
 
   if (!result) {
-    throw new Error("Schedule not found");
+    throwNotFound("Schedule not found");
   }
 
   return typedjson({ schedule: result.schedule });
diff --git a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions.$sessionParam/route.tsx b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions.$sessionParam/route.tsx
index c873dd9f406..688477281d6 100644
--- a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions.$sessionParam/route.tsx
+++ b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions.$sessionParam/route.tsx
@@ -51,6 +51,7 @@ import {
   v3RunsPath,
   v3SessionsPath,
 } from "~/utils/pathBuilder";
+import { throwNotFound } from "~/utils/httpErrors";
 
 const ParamsSchema = EnvironmentParamSchema.extend({
   sessionParam: z.string(),
@@ -71,7 +72,7 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
 
   const environment = await findEnvironmentBySlug(project.id, envParam, userId);
   if (!environment) {
-    throw new Error("Environment not found");
+    throwNotFound("Environment not found");
   }
 
   const presenter = new SessionPresenter($replica);
diff --git a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions._index/route.tsx b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions._index/route.tsx
index 99b0a96b5d1..8d2fa6f7961 100644
--- a/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions._index/route.tsx
+++ b/apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.sessions._index/route.tsx
@@ -19,6 +19,7 @@ import { SessionListPresenter } from "~/presenters/v3/SessionListPresenter.serve
 import { clickhouseClient } from "~/services/clickhouseInstance.server";
 import { requireUserId } from "~/services/session.server";
 import { docsPath, EnvironmentParamSchema } from "~/utils/pathBuilder";
+import { throwNotFound } from "~/utils/httpErrors";
 
 export const meta: MetaFunction = () => {
   return [
@@ -39,7 +40,7 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
 
   const environment = await findEnvironmentBySlug(project.id, envParam, userId);
   if (!environment) {
-    throw new Error("Environment not found");
+    throwNotFound("Environment not found");
   }
 
   const filters = getSessionFiltersFromRequest(request);
diff --git a/apps/webapp/app/utils/httpErrors.ts b/apps/webapp/app/utils/httpErrors.ts
index 5131730e3bb..2e41aa67eff 100644
--- a/apps/webapp/app/utils/httpErrors.ts
+++ b/apps/webapp/app/utils/httpErrors.ts
@@ -1,3 +1,7 @@
+export function throwNotFound(statusText: string): never {
+  throw new Response(undefined, { status: 404, statusText });
+}
+
 export function friendlyErrorDisplay(statusCode: number, statusText?: string) {
   switch (statusCode) {
     case 400:
diff --git a/apps/webapp/test/httpErrors.test.ts b/apps/webapp/test/httpErrors.test.ts
new file mode 100644
index 00000000000..dab90bdb024
--- /dev/null
+++ b/apps/webapp/test/httpErrors.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+import { throwNotFound } from "~/utils/httpErrors";
+
+describe("throwNotFound", () => {
+  it("throws a Response with status 404 and the provided statusText", () => {
+    let thrown: unknown;
+    try {
+      throwNotFound("Environment not found");
+    } catch (e) {
+      thrown = e;
+    }
+
+    expect(thrown).toBeInstanceOf(Response);
+    expect((thrown as Response).status).toBe(404);
+    expect((thrown as Response).statusText).toBe("Environment not found");
+  });
+
+  it("passes through whatever statusText the caller provides", () => {
+    let thrown: unknown;
+    try {
+      throwNotFound("Project not found");
+    } catch (e) {
+      thrown = e;
+    }
+
+    expect((thrown as Response).statusText).toBe("Project not found");
+  });
+});