diff --git a/.server-changes/remove-deprecated-realtime-stream-action.md b/.server-changes/remove-deprecated-realtime-stream-action.md
new file mode 100644
index 0000000000..3ad4f19d7f
--- /dev/null
+++ b/.server-changes/remove-deprecated-realtime-stream-action.md
@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Remove the deprecated realtime stream write endpoint used by retired v3 task clients.
diff --git a/apps/webapp/app/routes/realtime.v1.streams.$runId.$streamId.ts b/apps/webapp/app/routes/realtime.v1.streams.$runId.$streamId.ts
index 420368c630..895e336fd8 100644
--- a/apps/webapp/app/routes/realtime.v1.streams.$runId.$streamId.ts
+++ b/apps/webapp/app/routes/realtime.v1.streams.$runId.$streamId.ts
@@ -1,86 +1,15 @@
-import { type ActionFunctionArgs } from "@remix-run/server-runtime";
 import { z } from "zod";
 import { $replica } from "~/db.server";
 import { getRequestAbortSignal } from "~/services/httpAsyncStorage.server";
 import { getRealtimeStreamInstance } from "~/services/realtime/v1StreamsGlobal.server";
 import { anyResource, createLoaderApiRoute } from "~/services/routeBuilders/apiBuilder.server";
 import { runStore } from "~/v3/runStore.server";
-import { controlPlaneResolver } from "~/v3/runOpsMigration/controlPlaneResolver.server";
 
 const ParamsSchema = z.object({
   runId: z.string(),
   streamId: z.string(),
 });
 
-// Plain action for backwards compatibility with older clients that don't send auth headers
-export async function action({ request, params }: ActionFunctionArgs) {
-  const parsedParams = ParamsSchema.safeParse(params);
-
-  if (!parsedParams.success) {
-    return new Response("Invalid parameters", { status: 400 });
-  }
-
-  const { runId, streamId } = parsedParams.data;
-
-  // Look up the run without environment scoping for backwards compatibility
-  const run = await runStore.findRun(
-    {
-      friendlyId: runId,
-    },
-    {
-      select: {
-        id: true,
-        friendlyId: true,
-        streamBasinName: true,
-        runtimeEnvironmentId: true,
-      },
-    },
-    $replica
-  );
-
-  if (!run) {
-    return new Response("Run not found", { status: 404 });
-  }
-
-  const environment = await controlPlaneResolver.resolveAuthenticatedEnv(run.runtimeEnvironmentId);
-
-  if (!environment) {
-    return new Response("Run not found", { status: 404 });
-  }
-
-  // Extract client ID from header, default to "default" if not provided
-  const clientId = request.headers.get("X-Client-Id") || "default";
-  const streamVersion = request.headers.get("X-Stream-Version") || "v1";
-
-  if (!request.body) {
-    return new Response("No body provided", { status: 400 });
-  }
-
-  const resumeFromChunk = request.headers.get("X-Resume-From-Chunk");
-  let resumeFromChunkNumber: number | undefined = undefined;
-  if (resumeFromChunk) {
-    const parsed = parseInt(resumeFromChunk, 10);
-    if (isNaN(parsed) || parsed < 0) {
-      return new Response(`Invalid X-Resume-From-Chunk header value: ${resumeFromChunk}`, {
-        status: 400,
-      });
-    }
-    resumeFromChunkNumber = parsed;
-  }
-
-  const realtimeStream = getRealtimeStreamInstance(environment, streamVersion, {
-    run,
-  });
-
-  return realtimeStream.ingestData(
-    request.body,
-    run.friendlyId,
-    streamId,
-    clientId,
-    resumeFromChunkNumber
-  );
-}
-
 export const loader = createLoaderApiRoute(
   {
     params: ParamsSchema,