diff --git a/src/adapters/cursor.ts b/src/adapters/cursor.ts
index e495f2a6713..5cfef1b8ada 100644
--- a/src/adapters/cursor.ts
+++ b/src/adapters/cursor.ts
@@ -48,6 +48,7 @@ import {
 import { runCursorTurnWithRetry } from "./cursor/transport-retry";
 import { cursorRequestHasShellAlias, cursorRequestUsesCodeMode } from "./cursor/tool-definitions";
 import {
+  CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES,
   CURSOR_ECHO_RETRY_CONTINUATION_TEXT,
   CURSOR_ROUTING_COMMENTARY_RETRY_TEXT,
   CursorEnvelopeEchoSniffer,
@@ -315,6 +316,8 @@ export function createCursorAdapter(provider: OcxProviderConfig, deps: CursorAda
             ? new CursorRoutingCommentarySniffer()
             : undefined;
           let guardHeld: AdapterEvent[] = [];
+          let guardHeldBytes = 0;
+          const guardEncoder = new TextEncoder();
           // Exactly-once observation: every client-bound text delta passes through here
           // exactly once — held deltas only on release, ordinary deltas at emit time.
           const emitTextObserved = (event: AdapterEvent): void => {
@@ -327,7 +330,49 @@ export function createCursorAdapter(provider: OcxProviderConfig, deps: CursorAda
               emitTextObserved(held);
             }
             guardHeld = [];
+            guardHeldBytes = 0;
           };
+          // A single frame can carry a multi-megabyte payload (the transport accepts up to the
+          // 16 MiB Cursor message bound), so the serialized size is projected — object overhead
+          // plus raw payload length — BEFORE any encoded copy exists. Escapes only inflate the
+          // exact figure, making the raw length a safe lower bound for the overflow decision.
+          const GUARD_EVENT_OVERHEAD_BYTES = 64;
+          const projectedGuardEventBytes = (event: AdapterEvent): number =>
+            GUARD_EVENT_OVERHEAD_BYTES
+            + (event.type === "text_delta"
+              ? Buffer.byteLength(event.text, "utf8")
+              : event.type === "thinking_delta"
+                ? Buffer.byteLength(event.thinking, "utf8")
+                : 0);
+          const holdGuardEvent = (event: AdapterEvent) => {
+            if (guardHeldBytes + projectedGuardEventBytes(event) > CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES) {
+              // Too large to retain even unescaped: settle the sniffers, release what was held,
+              // and pass this event through without ever encoding it.
+              echoSniffer?.finish();
+              routingCommentarySniffer?.finish();
+              releaseGuardHeld();
+              if (event.type !== "heartbeat") emittedOutput = true;
+              emitTextObserved(event);
+              return false;
+            }
+            guardHeld.push(event);
+            // Count the complete retained representation, including per-event overhead, so an
+            // upstream cannot evade the cap with empty or non-text reasoning frames.
+            guardHeldBytes += guardEncoder.encode(JSON.stringify(event)).byteLength;
+            if (guardHeldBytes <= CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES) return true;
+            echoSniffer?.finish();
+            routingCommentarySniffer?.finish();
+            releaseGuardHeld();
+            return false;
+          };
+          // Each sniffer settles from a bounded leading window (40 B / 512 B respectively), so
+          // feeding an oversized delta whole would retain megabytes it never inspects. The
+          // bounded prefix still covers every decision path — including marker prefixes and
+          // routing claims — while the tail falls through to the aggregate cap.
+          const ECHO_SNIFF_FEED_MAX_CHARS = 512;
+          const ROUTING_SNIFF_FEED_MAX_CHARS = 2048;
+          const boundedSniffText = (text: string, maxChars: number): string =>
+            text.length > maxChars ? text.slice(0, maxChars) : text;
           const guardsSettled = () =>
             (!echoSniffer || echoSniffer.settled)
             && (!routingCommentarySniffer || routingCommentarySniffer.settled);
@@ -368,27 +413,30 @@ export function createCursorAdapter(provider: OcxProviderConfig, deps: CursorAda
                 }
                 if (!guardsSettled()) {
                   if (event.type === "text_delta") {
-                    guardHeld.push(event);
+                    // Classify the delta before the aggregate-cap check: an oversized first
+                    // delta must still pass the armed sniffers (echo/hallucination detection is
+                    // prefix-based), so the cap cannot disarm them before they see the text.
                     if (echoSniffer && !echoSniffer.settled) {
-                      const decision = echoSniffer.feed(event.text);
+                      const decision = echoSniffer.feed(boundedSniffText(event.text, ECHO_SNIFF_FEED_MAX_CHARS));
                       if (decision.kind === "echo") {
                         guardHeld = [];
                         throw new CursorToolResultEchoError(decision.marker);
                       }
                     }
                     if (routingCommentarySniffer && !routingCommentarySniffer.settled) {
-                      const decision = routingCommentarySniffer.feed(event.text);
+                      const decision = routingCommentarySniffer.feed(boundedSniffText(event.text, ROUTING_SNIFF_FEED_MAX_CHARS));
                       if (decision.kind === "hallucination") {
                         guardHeld = [];
                         throw new CursorRoutingCommentaryError();
                       }
                     }
+                    if (!holdGuardEvent(event)) continue;
                     if (guardsSettled()) releaseGuardHeld();
                     continue;
                   } else if (event.type === "thinking_delta" || event.type === "heartbeat") {
                     // Reasoning before first text stays ordered; liveness still passes through.
                     if (event.type === "thinking_delta") {
-                      guardHeld.push(event);
+                      holdGuardEvent(event);
                       continue;
                     }
                   } else {
diff --git a/src/adapters/cursor/envelope-echo.ts b/src/adapters/cursor/envelope-echo.ts
index 4691731fa01..554423ad9db 100644
--- a/src/adapters/cursor/envelope-echo.ts
+++ b/src/adapters/cursor/envelope-echo.ts
@@ -76,7 +76,7 @@ export const MAX_MIDSTREAM_SCAN_LENGTH = 512 * 1024;
 const MAX_MIDSTREAM_FINDINGS = 8;
 const MAX_ROUTING_COMMENTARY_BYTES = 512;
 /** Aggregate quarantine cap: past this, flush and disarm. */
-const MAX_HOLD_BYTES = 8 * 1024;
+export const CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES = 8 * 1024;
 const encoder = new TextEncoder();
 
 export class CursorToolResultEchoError extends Error {
@@ -251,7 +251,11 @@ export class CursorEnvelopeEchoSniffer {
     const stillPrefix = ECHO_MARKERS.some(marker =>
       probe.length < marker.length && marker.startsWith(probe),
     );
-    if (stillPrefix && this.byteCount <= MAX_SNIFF_BYTES && this.buffered.length < MAX_HOLD_BYTES) {
+    if (
+      stillPrefix
+      && this.byteCount <= MAX_SNIFF_BYTES
+      && this.buffered.length < CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES
+    ) {
       return { kind: "hold" };
     }
     this.done = true;
@@ -316,7 +320,7 @@ export class CursorRoutingCommentarySniffer {
       && lineBreakCount < 2;
     if (
       this.byteCount < MAX_ROUTING_COMMENTARY_BYTES
-      && this.buffered.length < MAX_HOLD_BYTES
+      && this.buffered.length < CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES
       && (lineBreakCount === 0 || pendingFailureClaim)
       && (hasRoutingHint || this.byteCount < 64)
     ) {
diff --git a/structure/providers/cursor.md b/structure/providers/cursor.md
index 7252130843a..ce21712e910 100644
--- a/structure/providers/cursor.md
+++ b/structure/providers/cursor.md
@@ -192,6 +192,15 @@ Translated Chat request construction uses the [inline-image budget](../transport
 
 ## Mid-stream envelope echo
 
+Held quarantine output is bounded by the aggregate `CURSOR_OUTPUT_GUARD_MAX_HOLD_BYTES` (8 KiB)
+budget in `src/adapters/cursor.ts`. Text deltas are fed to the armed echo and
+routing-commentary sniffers BEFORE the cap check, so a single oversized first delta cannot
+disarm the guards without being classified; each sniffer reads only the bounded leading window
+its decision needs. Retained bytes are projected from payload length before any serialized
+copy exists, so a multi-megabyte frame cannot force a same-size encoded allocation. An event
+that cannot fit the remaining budget settles both sniffers, releases the held events, and is
+emitted directly.
+
 The prefix sniffer only watches the opening bytes of a turn. An external model that writes real
 prose first and then pastes a replayed `[Tool Result]` envelope defeats it, so that text reaches
 the client and is stored as assistant output. `CursorMidstreamEchoObserver` records those
diff --git a/tests/providers/cursor/cursor-envelope-echo-retry.test.ts b/tests/providers/cursor/cursor-envelope-echo-retry.test.ts
index daeeb77176a..88eaa515f03 100644
--- a/tests/providers/cursor/cursor-envelope-echo-retry.test.ts
+++ b/tests/providers/cursor/cursor-envelope-echo-retry.test.ts
@@ -313,6 +313,133 @@ describe("cursor external output quarantine + corrective retry (devlog 260826 ga
     expect(text).toBe("[note] leading bracket but not an envelope");
   });
 
+  test("reasoning-only quarantine is capped and disarms before unbounded retention", async () => {
+    let attempt = 0;
+    const factory = () => ({
+      async *run() {
+        attempt += 1;
+        for (let i = 0; i < 100; i += 1) {
+          yield { type: "thinking", thinking: "x".repeat(128) } satisfies CursorServerMessage;
+        }
+        // Once the aggregate hold cap flushes, later marker-like text is ordinary output rather
+        // than evidence for a retry whose preceding reasoning has already reached the client.
+        yield { type: "text", text: ECHO_TEXT } satisfies CursorServerMessage;
+        yield { type: "done", usage: { inputTokens: 1, outputTokens: 1 } } satisfies CursorServerMessage;
+      },
+      writeClient() {},
+    });
+    const adapter = createCursorAdapter(
+      { ...provider, apiKey: "cursor-token" },
+      { createTransport: factory as never },
+    );
+    const events: AdapterEvent[] = [];
+    await adapter.runTurn?.(
+      toolResultBody("cursor/kimi-k3"),
+      { headers: new Headers() },
+      event => events.push(event),
+    );
+
+    expect(attempt).toBe(1);
+    expect(events.filter(event => event.type === "thinking_delta")).toHaveLength(100);
+    expect(events.filter(event => event.type === "text_delta")).not.toHaveLength(0);
+  });
+
+  test("an oversized first text delta is still classified by the echo sniffer", async () => {
+    // One text delta larger than the aggregate hold cap whose leading bytes are the
+    // echoed envelope marker.
+    let attempt = 0;
+    const runRequests: CursorRunRequest[] = [];
+    const oversizedFactory = () => ({
+      async *run(request: CursorRunRequest) {
+        runRequests.push(request);
+        attempt += 1;
+        if (attempt === 1) {
+          yield { type: "text", text: ECHO_TEXT + "x".repeat(32 * 1024) } satisfies CursorServerMessage;
+          yield { type: "done", usage: { inputTokens: 1, outputTokens: 1 } } satisfies CursorServerMessage;
+          return;
+        }
+        yield { type: "text", text: "STATE A17" } satisfies CursorServerMessage;
+        yield { type: "done", usage: { inputTokens: 1, outputTokens: 1 } } satisfies CursorServerMessage;
+      },
+      writeClient() {},
+    });
+    const adapter = createCursorAdapter({ ...provider, apiKey: "cursor-token" }, { createTransport: oversizedFactory as never });
+    const events: AdapterEvent[] = [];
+    await adapter.runTurn?.(toolResultBody("cursor/kimi-k3"), { headers: new Headers() }, event => events.push(event));
+    expect(attempt).toBe(2);
+    const text = events.filter(e => e.type === "text_delta").map(e => (e as { text: string }).text).join("");
+    expect(text).toBe("STATE A17");
+  });
+
+  test("an oversized first text delta is still classified by the routing sniffer", async () => {
+    let attempt = 0;
+    const factory = () => ({
+      async *run() {
+        attempt += 1;
+        if (attempt === 1) {
+          // Routing claim padded past the 8 KiB aggregate cap in a single delta.
+          yield {
+            type: "text",
+            text: "네이티브 셸은 차단됐으니 exec_command 경로로 읽겠습니다. " + "x".repeat(32 * 1024),
+          } satisfies CursorServerMessage;
+          yield { type: "done", usage: { inputTokens: 1, outputTokens: 1 } } satisfies CursorServerMessage;
+          return;
+        }
+        yield { type: "text", text: "READ_OK" } satisfies CursorServerMessage;
+        yield { type: "done", usage: { inputTokens: 1, outputTokens: 1 } } satisfies CursorServerMessage;
+      },
+      writeClient() {},
+    });
+    const body = {
+      modelId: "cursor/kimi-k3-1m",
+      context: {
+        messages: [{ role: "user", content: "Read the file and report its first line.", timestamp: 1 }],
+        tools: [{
+          name: "exec",
+          description: "Run JavaScript code to orchestrate nested tool calls.",
+          parameters: {},
+          freeform: true,
+        }],
+      },
+      stream: false,
+      options: {},
+      _cursorConversationId: "cursor_routing_oversized",
+      _cursorIdentityScope: "acct-routing-commentary",
+    } as OcxParsedRequest;
+    const adapter = createCursorAdapter({ ...provider, apiKey: "cursor-token" }, { createTransport: factory as never });
+    const events: AdapterEvent[] = [];
+    await adapter.runTurn?.(body, { headers: new Headers() }, event => events.push(event));
+    expect(attempt).toBe(2);
+    const text = events.filter(e => e.type === "text_delta").map(e => (e as { text: string }).text).join("");
+    expect(text).toBe("READ_OK");
+  });
+
+  test("a single reasoning frame larger than the cap flushes without unbounded retention", async () => {
+    let attempt = 0;
+    const bigThinking = "y".repeat(64 * 1024);
+    const factory = () => ({
+      async *run() {
+        attempt += 1;
+        yield { type: "thinking", thinking: bigThinking } satisfies CursorServerMessage;
+        yield { type: "text", text: "post-thought answer" } satisfies CursorServerMessage;
+        yield { type: "done", usage: { inputTokens: 1, outputTokens: 1 } } satisfies CursorServerMessage;
+      },
+      writeClient() {},
+    });
+    const adapter = createCursorAdapter({ ...provider, apiKey: "cursor-token" }, { createTransport: factory as never });
+    const events: AdapterEvent[] = [];
+    await adapter.runTurn?.(
+      toolResultBody("cursor/kimi-k3"),
+      { headers: new Headers() },
+      event => events.push(event),
+    );
+    expect(attempt).toBe(1);
+    const thinking = events.filter(e => e.type === "thinking_delta").map(e => (e as { thinking: string }).thinking).join("");
+    expect(thinking).toBe(bigThinking);
+    const text = events.filter(e => e.type === "text_delta").map(e => (e as { text: string }).text).join("");
+    expect(text).toBe("post-thought answer");
+  });
+
   test("plain user turns (no trailing toolResult) never arm the sniffer", async () => {
     let attempt = 0;
     const factory = () => ({