diff --git a/.server-changes/realtime-append-cap.md b/.server-changes/realtime-append-cap.md
new file mode 100644
index 0000000000..cfbd3b7fa1
--- /dev/null
+++ b/.server-changes/realtime-append-cap.md
@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Session `.in/append` returns readable 413s on oversize bodies (was failing browser fetches as opaque `TypeError: Failed to fetch`) and now rejects only records that would actually exceed S2's per-record ceiling, instead of guessing at a conservative pre-encoding cap.
diff --git a/apps/webapp/app/routes/realtime.v1.sessions.$session.$io.append.ts b/apps/webapp/app/routes/realtime.v1.sessions.$session.$io.append.ts
index 792b6480d8..dbdb9f47d5 100644
--- a/apps/webapp/app/routes/realtime.v1.sessions.$session.$io.append.ts
+++ b/apps/webapp/app/routes/realtime.v1.sessions.$session.$io.append.ts
@@ -27,12 +27,18 @@ const ParamsSchema = z.object({
 // POST: server-side append of a single record to a session channel. Mirrors
 // the existing /realtime/v1/streams/:runId/:target/:streamId/append route,
 // scoped to a Session primitive.
-// S2 enforces a 1 MiB per-record limit (metered as
-// `8 + 2*H + Σ(header name+value) + body`). We cap the raw HTTP body at
-// 512 KiB so the JSON wrapper (`{"data":"...","id":"..."}`), string
-// escaping, and any future per-record header additions all stay comfortably
-// below S2's ceiling. See https://s2.dev/docs/limits.
-const MAX_APPEND_BODY_BYTES = 1024 * 512;
+//
+// The HTTP body cap here is just a DoS pre-guard — set generously at
+// 1 MiB so we don't buffer arbitrarily large inputs before we can
+// compute the wrapped size. The actual S2 per-record limit (verified
+// empirically against cloud S2) is enforced precisely inside
+// `S2RealtimeStreams.#appendPartByName` — it throws
+// `S2RecordTooLargeError` (a `ServiceValidationError` with status
+// 413) when the metered record size would exceed S2's 1 MiB ceiling
+// after JSON wrapping. That lets legitimate bodies up to ~1023 KiB
+// raw through (ASCII or low-escape content) while still rejecting
+// pathological all-quote content that would double on wrap.
+const MAX_APPEND_BODY_BYTES = 1024 * 1024;
 
 const { action, loader } = createActionApiRoute(
   {
diff --git a/apps/webapp/app/routes/realtime.v1.streams.$runId.input.$streamId.ts b/apps/webapp/app/routes/realtime.v1.streams.$runId.input.$streamId.ts
index 335116043d..a404e6a76a 100644
--- a/apps/webapp/app/routes/realtime.v1.streams.$runId.input.$streamId.ts
+++ b/apps/webapp/app/routes/realtime.v1.streams.$runId.input.$streamId.ts
@@ -1,4 +1,5 @@
 import { json } from "@remix-run/server-runtime";
+import { tryCatch } from "@trigger.dev/core/utils";
 import { z } from "zod";
 import { $replica } from "~/db.server";
 import { getRequestAbortSignal } from "~/services/httpAsyncStorage.server";
@@ -13,6 +14,7 @@ import {
 } from "~/services/routeBuilders/apiBuilder.server";
 import { getRealtimeStreamInstance } from "~/services/realtime/v1StreamsGlobal.server";
 import { engine } from "~/v3/runEngine.server";
+import { ServiceValidationError } from "~/v3/services/common.server";
 
 const ParamsSchema = z.object({
   runId: z.string(),
@@ -77,13 +79,29 @@ const { action } = createActionApiRoute(
     const recordId = `inp_${crypto.randomUUID().replace(/-/g, "").slice(0, 12)}`;
     const record = JSON.stringify(body.data.data);
 
-    // Append the record to the per-stream S2 stream (auto-creates on first write)
-    await realtimeStream.appendPart(
-      record,
-      recordId,
-      run.friendlyId,
-      `$trigger.input:${params.streamId}`
+    // Append the record to the per-stream S2 stream (auto-creates on
+    // first write). `appendPart` can throw `S2RecordTooLargeError` (a
+    // `ServiceValidationError` with status 413) when the wrapped
+    // record exceeds S2's per-record ceiling — surface that as 413
+    // rather than letting it propagate to the apiBuilder catch-all
+    // as a generic 500.
+    const [appendError] = await tryCatch(
+      realtimeStream.appendPart(
+        record,
+        recordId,
+        run.friendlyId,
+        `$trigger.input:${params.streamId}`
+      )
     );
+    if (appendError) {
+      if (appendError instanceof ServiceValidationError) {
+        return json(
+          { ok: false, error: appendError.message },
+          { status: appendError.status ?? 422 }
+        );
+      }
+      throw appendError;
+    }
 
     // Check Redis cache for a linked .wait() waitpoint (fast, no DB hit if none)
     // Get first, complete, then delete — so the mapping survives if completeWaitpoint throws
diff --git a/apps/webapp/app/routes/resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.playground.realtime.v1.sessions.$session.$io.append.ts b/apps/webapp/app/routes/resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.playground.realtime.v1.sessions.$session.$io.append.ts
index 04d6ec2d8e..77f03f4ce5 100644
--- a/apps/webapp/app/routes/resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.playground.realtime.v1.sessions.$session.$io.append.ts
+++ b/apps/webapp/app/routes/resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.playground.realtime.v1.sessions.$session.$io.append.ts
@@ -24,10 +24,12 @@ const ParamsSchema = z.object({
   io: z.enum(["out", "in"]),
 });
 
-// S2 record body cap. Mirrors the public /realtime/v1/sessions/:s/:io/append
-// route — keep it well under S2's 1 MiB per-record limit so JSON wrapping,
-// string escaping, and any future per-record headers stay safe.
-const MAX_APPEND_BODY_BYTES = 1024 * 512;
+// HTTP body cap. Mirrors the public /realtime/v1/sessions/:s/:io/append
+// route — DoS pre-guard only. The actual S2 per-record limit is
+// enforced precisely by `S2RealtimeStreams.#appendPartByName`
+// (throws `S2RecordTooLargeError` with status 413 when the metered
+// record size would exceed S2's 1 MiB ceiling after JSON wrapping).
+const MAX_APPEND_BODY_BYTES = 1024 * 1024;
 
 // POST: Append a single record to a Session channel from the dashboard
 // playground. Mirrors the public `POST /realtime/v1/sessions/:session/:io/append`
diff --git a/apps/webapp/app/services/realtime/s2realtimeStreams.server.ts b/apps/webapp/app/services/realtime/s2realtimeStreams.server.ts
index 0706107144..b91f47c6b3 100644
--- a/apps/webapp/app/services/realtime/s2realtimeStreams.server.ts
+++ b/apps/webapp/app/services/realtime/s2realtimeStreams.server.ts
@@ -4,6 +4,36 @@ import { StreamIngestor, StreamRecord, StreamResponder, StreamResponseOptions }
 import { Logger, LogLevel } from "@trigger.dev/core/logger";
 import { headerValue } from "@trigger.dev/core/v3";
 import { randomUUID } from "node:crypto";
+import { ServiceValidationError } from "~/v3/services/common.server";
+
+// S2's per-record metered-size limit. Verified empirically against
+// cloud S2: append succeeds at metered=1048576 and 422s at 1048577
+// with `"record must have metered size less than 1 MiB"` (the "less
+// than" wording is slightly off — the boundary is inclusive).
+//
+// Metered size formula:
+//   metered = 8 (record overhead) + 2*H + Σ(header name + value) + body
+// where `body` is the unescaped record body length in UTF-8 bytes and
+// `H` is the number of S2 record headers.
+//
+// We attach no record headers (H=0), so the budget reduces to:
+//   8 + body ≤ 1048576  →  body ≤ 1048568
+export const S2_MAX_METERED_BYTES = 1024 * 1024; // 1 MiB
+export const S2_RECORD_BASE_OVERHEAD_BYTES = 8;
+
+/**
+ * Thrown when a record's metered size would exceed S2's hard per-record
+ * limit. Caught by the route handler and surfaced as 413.
+ */
+export class S2RecordTooLargeError extends ServiceValidationError {
+  constructor(public readonly meteredBytes: number) {
+    super(
+      `Record metered size ${meteredBytes} bytes exceeds the S2 per-record limit of ${S2_MAX_METERED_BYTES} bytes. Reduce tool-output size or split into smaller parts.`,
+      413
+    );
+    this.name = "S2RecordTooLargeError";
+  }
+}
 
 export type S2RealtimeStreamsOptions = {
   // S2
@@ -181,8 +211,14 @@ export class S2RealtimeStreams implements StreamResponder, StreamIngestor {
   async #appendPartByName(part: string, partId: string, s2Stream: string): Promise<void> {
     this.logger.debug(`S2 appending to stream`, { part, stream: s2Stream });
 
+    const recordBody = JSON.stringify({ data: part, id: partId });
+    const meteredBytes = Buffer.byteLength(recordBody, "utf8") + S2_RECORD_BASE_OVERHEAD_BYTES;
+    if (meteredBytes > S2_MAX_METERED_BYTES) {
+      throw new S2RecordTooLargeError(meteredBytes);
+    }
+
     const result = await this.s2Append(s2Stream, {
-      records: [{ body: JSON.stringify({ data: part, id: partId }) }],
+      records: [{ body: recordBody }],
     });
 
     this.logger.debug(`S2 append result`, { result });
diff --git a/apps/webapp/app/services/routeBuilders/apiBuilder.server.ts b/apps/webapp/app/services/routeBuilders/apiBuilder.server.ts
index a66de63e11..0c661a7e68 100644
--- a/apps/webapp/app/services/routeBuilders/apiBuilder.server.ts
+++ b/apps/webapp/app/services/routeBuilders/apiBuilder.server.ts
@@ -777,7 +777,11 @@ export function createActionApiRoute<
         const contentLength = request.headers.get("content-length");
 
         if (!contentLength || parseInt(contentLength) > maxContentLength) {
-          return json({ error: "Request body too large" }, { status: 413 });
+          return await wrapResponse(
+            request,
+            json({ error: "Request body too large" }, { status: 413 }),
+            corsStrategy !== "none"
+          );
         }
       }