diff --git a/extensions/acpx/src/pi-session-catalog-plugin.ts b/extensions/acpx/src/pi-session-catalog-plugin.ts
index 3f9d90c2340a..c00ec7e3b548 100644
--- a/extensions/acpx/src/pi-session-catalog-plugin.ts
+++ b/extensions/acpx/src/pi-session-catalog-plugin.ts
@@ -20,8 +20,8 @@ import type {
 } from "openclaw/plugin-sdk/session-catalog";
 import { isRecord } from "openclaw/plugin-sdk/string-coerce-runtime";
 import {
+  isExactPiSessionCursor,
   listLocalPiSessionPage,
-  optionalPiString,
   readLocalPiTranscriptPage,
   type PiSessionPage,
 } from "./pi-session-catalog.js";
@@ -35,7 +35,6 @@ const CAPABILITY = "pi-sessions";
 const LOCAL_HOST_ID = "gateway";
 const MAX_PAGE_LIMIT = 100;
 const MAX_HOSTS = 100;
-const MAX_CURSOR_LENGTH = 128;
 const NODE_TIMEOUT_MS = 20_000;
 const SESSION_ID_PATTERN = /^(?!-)[A-Za-z0-9._:-]{1,256}$/u;
 const TRANSCRIPT_ITEM_TYPES = new Set([
@@ -244,13 +243,17 @@ async function listPiNodeHost(
     };
   }
   try {
+    const cursor = query.cursors?.[hostId];
+    if (cursor !== undefined && !isExactPiSessionCursor(cursor)) {
+      throw new Error("cursor is invalid");
+    }
     const raw = await runtime.nodes.invoke({
       nodeId: node.nodeId,
       command: PI_SESSIONS_LIST_COMMAND,
       params: {
         ...(query.limitPerHost ? { limit: query.limitPerHost } : {}),
         ...(query.search ? { searchTerm: query.search } : {}),
-        ...(query.cursors?.[hostId] ? { cursor: query.cursors[hostId] } : {}),
+        ...(cursor !== undefined ? { cursor } : {}),
       },
       timeoutMs: NODE_TIMEOUT_MS,
       scopes: ["operator.write"],
@@ -283,11 +286,11 @@ function parseNodeSessionPage(value: unknown): PiSessionPage {
     throw new Error("Pi node returned an invalid session page");
   }
   const sessions = value.sessions;
-  const nextCursor = optionalPiString(value.nextCursor, MAX_CURSOR_LENGTH);
-  if (value.nextCursor !== undefined && !nextCursor) {
+  const nextCursor = value.nextCursor;
+  if (nextCursor !== undefined && !isExactPiSessionCursor(nextCursor)) {
     throw new Error("Pi node returned an invalid cursor");
   }
-  return { sessions, ...(nextCursor ? { nextCursor } : {}) };
+  return { sessions, ...(nextCursor !== undefined ? { nextCursor } : {}) };
 }
 
 function parseNodeTranscriptPage(value: unknown, threadId: string): SessionsCatalogReadResult {
@@ -300,15 +303,15 @@ function parseNodeTranscriptPage(value: unknown, threadId: string): SessionsCata
   ) {
     throw new Error("Pi node returned an invalid transcript page");
   }
-  const nextCursor = optionalPiString(value.nextCursor, MAX_CURSOR_LENGTH);
-  if (value.nextCursor !== undefined && !nextCursor) {
+  const nextCursor = value.nextCursor;
+  if (nextCursor !== undefined && !isExactPiSessionCursor(nextCursor)) {
     throw new Error("Pi node returned an invalid cursor");
   }
   return {
     hostId: LOCAL_HOST_ID,
     threadId,
     items: value.items,
-    ...(nextCursor ? { nextCursor } : {}),
+    ...(nextCursor !== undefined ? { nextCursor } : {}),
   };
 }
 
@@ -457,11 +460,15 @@ async function readPiTranscript(
   runtime: PluginRuntime,
   request: Parameters<SessionCatalogProvider["read"]>[0],
 ): Promise<SessionsCatalogReadResult> {
+  const cursor = request.cursor;
+  if (cursor !== undefined && !isExactPiSessionCursor(cursor)) {
+    throw new Error("cursor is invalid");
+  }
   if (request.hostId === LOCAL_HOST_ID) {
     return await readLocalPiTranscriptPage({
       threadId: request.threadId,
       ...(request.limit ? { limit: request.limit } : {}),
-      ...(request.cursor ? { cursor: request.cursor } : {}),
+      ...(cursor !== undefined ? { cursor } : {}),
     });
   }
   if (!request.hostId.startsWith("node:")) {
@@ -483,7 +490,7 @@ async function readPiTranscript(
     params: {
       threadId: request.threadId,
       ...(request.limit ? { limit: request.limit } : {}),
-      ...(request.cursor ? { cursor: request.cursor } : {}),
+      ...(cursor !== undefined ? { cursor } : {}),
     },
     timeoutMs: NODE_TIMEOUT_MS,
     scopes: ["operator.write"],
diff --git a/extensions/acpx/src/pi-session-catalog.test.ts b/extensions/acpx/src/pi-session-catalog.test.ts
index 91a74f4fe86b..92cbfce1acc6 100644
--- a/extensions/acpx/src/pi-session-catalog.test.ts
+++ b/extensions/acpx/src/pi-session-catalog.test.ts
@@ -261,6 +261,27 @@ describe("Pi session catalog", () => {
       cursor: latest.nextCursor,
     });
     expect(older.items.map((item) => item.type)).toEqual(["reasoning", "agentMessage"]);
+    const nonEmitted = Buffer.from(JSON.stringify({ offset: 2, extra: true }), "utf8").toString(
+      "base64url",
+    );
+    const unsafeOffset = Buffer.from(
+      JSON.stringify({ offset: Number.MAX_SAFE_INTEGER + 1 }),
+      "utf8",
+    ).toString("base64url");
+    for (const cursor of [
+      `${latest.nextCursor}$`,
+      `${latest.nextCursor}=`,
+      ` ${latest.nextCursor} `,
+      nonEmitted,
+      unsafeOffset,
+    ]) {
+      await expect(
+        readLocalPiTranscriptPage({
+          threadId: "pi-session",
+          cursor,
+        }),
+      ).rejects.toThrow("cursor is invalid");
+    }
     await expect(listLocalPiSessionPage({ cursor: " " })).rejects.toThrow("cursor is invalid");
     await expect(
       readLocalPiTranscriptPage({ threadId: "pi-session", cursor: 123 }),
@@ -913,5 +934,61 @@ describe("Pi session catalog", () => {
     await expect(catalog!.read({ hostId: "node:node-1", threadId: "pi-remote" })).rejects.toThrow(
       "invalid transcript page",
     );
+
+    invoke.mockClear();
+    await expect(
+      catalog!.read({ hostId: "node:node-1", threadId: "pi-remote", cursor: "" }),
+    ).rejects.toThrow("cursor is invalid");
+    await expect(
+      catalog!.list({
+        hostIds: ["node:node-1"],
+        cursors: { "node:node-1": "" },
+      }),
+    ).resolves.toEqual([
+      expect.objectContaining({
+        error: { code: "NODE_INVOKE_FAILED", message: expect.any(String) },
+      }),
+    ]);
+    expect(invoke).not.toHaveBeenCalled();
+
+    invoke.mockResolvedValueOnce({
+      payloadJSON: JSON.stringify({ sessions: [], nextCursor: " wrapped " }),
+    });
+    await expect(catalog!.list({ hostIds: ["node:node-1"] })).resolves.toEqual([
+      expect.objectContaining({
+        error: { code: "NODE_INVOKE_FAILED", message: expect.any(String) },
+      }),
+    ]);
+    invoke.mockResolvedValueOnce({
+      payloadJSON: JSON.stringify({
+        threadId: "pi-remote",
+        items: [],
+        nextCursor: " wrapped ",
+      }),
+    });
+    await expect(catalog!.read({ hostId: "node:node-1", threadId: "pi-remote" })).rejects.toThrow(
+      "invalid cursor",
+    );
+
+    const exactCursor = Buffer.from(JSON.stringify({ offset: 1 }), "utf8").toString("base64url");
+    invoke.mockResolvedValueOnce({ payloadJSON: JSON.stringify({ sessions: [] }) });
+    await catalog!.list({
+      hostIds: ["node:node-1"],
+      cursors: { "node:node-1": exactCursor },
+    });
+    expect(invoke).toHaveBeenLastCalledWith(
+      expect.objectContaining({ params: { cursor: exactCursor } }),
+    );
+    invoke.mockResolvedValueOnce({
+      payloadJSON: JSON.stringify({ threadId: "pi-remote", items: [] }),
+    });
+    await catalog!.read({
+      hostId: "node:node-1",
+      threadId: "pi-remote",
+      cursor: exactCursor,
+    });
+    expect(invoke).toHaveBeenLastCalledWith(
+      expect.objectContaining({ params: { threadId: "pi-remote", cursor: exactCursor } }),
+    );
   });
 });
diff --git a/extensions/acpx/src/pi-session-catalog.ts b/extensions/acpx/src/pi-session-catalog.ts
index 3c54aff3d771..e8073759b41a 100644
--- a/extensions/acpx/src/pi-session-catalog.ts
+++ b/extensions/acpx/src/pi-session-catalog.ts
@@ -19,7 +19,7 @@ const SESSION_ID_PATTERN = /^(?!-)[A-Za-z0-9._:-]{1,256}$/u;
 
 export type PiSessionPage = { sessions: SessionCatalogSession[]; nextCursor?: string };
 
-export function optionalPiString(value: unknown, maxLength: number): string | undefined {
+function optionalPiString(value: unknown, maxLength: number): string | undefined {
   if (typeof value !== "string") {
     return undefined;
   }
@@ -41,25 +41,52 @@ function encodeCursor(offset: number): string {
   return Buffer.from(JSON.stringify({ offset }), "utf8").toString("base64url");
 }
 
-function decodeCursor(value: unknown): number {
+function optionalRawCursor(value: unknown): string | undefined {
   if (value === undefined) {
-    return 0;
+    return undefined;
   }
-  const cursor = optionalPiString(value, MAX_CURSOR_LENGTH);
-  if (!cursor) {
+  if (typeof value !== "string" || value.length === 0 || value.length > MAX_CURSOR_LENGTH) {
     throw new Error("cursor is invalid");
   }
+  return value;
+}
+
+function decodeCursor(value: unknown): number {
+  const cursor = optionalRawCursor(value);
+  if (cursor === undefined) {
+    return 0;
+  }
   try {
-    const parsed = JSON.parse(Buffer.from(cursor, "base64url").toString("utf8")) as unknown;
-    if (!isRecord(parsed) || !Number.isInteger(parsed.offset) || Number(parsed.offset) < 0) {
+    const bytes = Buffer.from(cursor, "base64url");
+    if (bytes.toString("base64url") !== cursor) {
+      throw new Error("non-canonical base64url");
+    }
+    const parsed = JSON.parse(bytes.toString("utf8")) as unknown;
+    if (!isRecord(parsed) || !Number.isSafeInteger(parsed.offset) || Number(parsed.offset) < 0) {
       throw new Error("invalid offset");
     }
-    return Number(parsed.offset);
+    const offset = Number(parsed.offset);
+    if (encodeCursor(offset) !== cursor) {
+      throw new Error("non-canonical cursor payload");
+    }
+    return offset;
   } catch (error) {
     throw new Error("cursor is invalid", { cause: error });
   }
 }
 
+export function isExactPiSessionCursor(value: unknown): value is string {
+  if (typeof value !== "string") {
+    return false;
+  }
+  try {
+    decodeCursor(value);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 function truncateUtf8(text: string, maxBytes: number): string {
   if (Buffer.byteLength(text, "utf8") <= maxBytes) {
     return text;
@@ -162,10 +189,7 @@ function parseListParams(value: unknown): { searchTerm?: string; limit: number;
   if (value.searchTerm !== undefined && !searchTerm) {
     throw new Error("searchTerm is invalid");
   }
-  const cursor = optionalPiString(value.cursor, MAX_CURSOR_LENGTH);
-  if (value.cursor !== undefined && !cursor) {
-    throw new Error("cursor is invalid");
-  }
+  const cursor = optionalRawCursor(value.cursor);
   return {
     limit: boundedLimit(value.limit),
     ...(searchTerm ? { searchTerm } : {}),
@@ -185,10 +209,7 @@ function parseReadParams(value: unknown): { threadId: string; limit: number; cur
   if (!threadId || !SESSION_ID_PATTERN.test(threadId)) {
     throw new Error("threadId is invalid");
   }
-  const cursor = optionalPiString(value.cursor, MAX_CURSOR_LENGTH);
-  if (value.cursor !== undefined && !cursor) {
-    throw new Error("cursor is invalid");
-  }
+  const cursor = optionalRawCursor(value.cursor);
   return {
     threadId,
     limit: boundedLimit(value.limit),
diff --git a/extensions/opencode/session-catalog-plugin.ts b/extensions/opencode/session-catalog-plugin.ts
index cf2298ef9610..714c460ad952 100644
--- a/extensions/opencode/session-catalog-plugin.ts
+++ b/extensions/opencode/session-catalog-plugin.ts
@@ -31,14 +31,13 @@ import {
   openOpenCodeCatalogTerminal,
 } from "./session-catalog-terminal.js";
 import {
+  isExactOpenCodeSessionCursor,
   listLocalOpenCodeSessionPage,
-  optionalOpenCodeString,
   readLocalOpenCodeTranscriptPage,
   type OpenCodeSessionPage,
 } from "./session-catalog.js";
 
 const MAX_HOSTS = 100;
-const MAX_CURSOR_LENGTH = 128;
 const TRANSCRIPT_ITEM_TYPES = new Set([
   "userMessage",
   "agentMessage",
@@ -231,13 +230,17 @@ async function listOpenCodeNodeHost(
     };
   }
   try {
+    const cursor = query.cursors?.[hostId];
+    if (cursor !== undefined && !isExactOpenCodeSessionCursor(cursor)) {
+      throw new Error("cursor is invalid");
+    }
     const raw = await runtime.nodes.invoke({
       nodeId: node.nodeId,
       command: OPENCODE_SESSIONS_LIST_COMMAND,
       params: {
         ...(query.limitPerHost ? { limit: query.limitPerHost } : {}),
         ...(query.search ? { searchTerm: query.search } : {}),
-        ...(query.cursors?.[hostId] ? { cursor: query.cursors[hostId] } : {}),
+        ...(cursor !== undefined ? { cursor } : {}),
       },
       timeoutMs: NODE_TIMEOUT_MS,
       scopes: ["operator.write"],
@@ -273,11 +276,11 @@ function parseNodeSessionPage(value: unknown): OpenCodeSessionPage {
     throw new Error("OpenCode node returned an invalid session page");
   }
   const sessions = value.sessions;
-  const nextCursor = optionalOpenCodeString(value.nextCursor, MAX_CURSOR_LENGTH);
-  if (value.nextCursor !== undefined && !nextCursor) {
+  const nextCursor = value.nextCursor;
+  if (nextCursor !== undefined && !isExactOpenCodeSessionCursor(nextCursor)) {
     throw new Error("OpenCode node returned an invalid cursor");
   }
-  return { sessions, ...(nextCursor ? { nextCursor } : {}) };
+  return { sessions, ...(nextCursor !== undefined ? { nextCursor } : {}) };
 }
 
 function parseNodeTranscriptPage(value: unknown, threadId: string): SessionsCatalogReadResult {
@@ -290,15 +293,15 @@ function parseNodeTranscriptPage(value: unknown, threadId: string): SessionsCata
   ) {
     throw new Error("OpenCode node returned an invalid transcript page");
   }
-  const nextCursor = optionalOpenCodeString(value.nextCursor, MAX_CURSOR_LENGTH);
-  if (value.nextCursor !== undefined && !nextCursor) {
+  const nextCursor = value.nextCursor;
+  if (nextCursor !== undefined && !isExactOpenCodeSessionCursor(nextCursor)) {
     throw new Error("OpenCode node returned an invalid cursor");
   }
   return {
     hostId: LOCAL_HOST_ID,
     threadId,
     items: value.items,
-    ...(nextCursor ? { nextCursor } : {}),
+    ...(nextCursor !== undefined ? { nextCursor } : {}),
   };
 }
 
@@ -363,11 +366,15 @@ async function readOpenCodeTranscript(
   runtime: PluginRuntime,
   request: Parameters<SessionCatalogProvider["read"]>[0],
 ): Promise<SessionsCatalogReadResult> {
+  const cursor = request.cursor;
+  if (cursor !== undefined && !isExactOpenCodeSessionCursor(cursor)) {
+    throw new Error("cursor is invalid");
+  }
   if (request.hostId === LOCAL_HOST_ID) {
     return await readLocalOpenCodeTranscriptPage({
       threadId: request.threadId,
       ...(request.limit ? { limit: request.limit } : {}),
-      ...(request.cursor ? { cursor: request.cursor } : {}),
+      ...(cursor !== undefined ? { cursor } : {}),
     });
   }
   if (!request.hostId.startsWith("node:")) {
@@ -389,7 +396,7 @@ async function readOpenCodeTranscript(
     params: {
       threadId: request.threadId,
       ...(request.limit ? { limit: request.limit } : {}),
-      ...(request.cursor ? { cursor: request.cursor } : {}),
+      ...(cursor !== undefined ? { cursor } : {}),
     },
     timeoutMs: NODE_TIMEOUT_MS,
     scopes: ["operator.write"],
diff --git a/extensions/opencode/session-catalog.test.ts b/extensions/opencode/session-catalog.test.ts
index bffa750ef465..26edf2066cea 100644
--- a/extensions/opencode/session-catalog.test.ts
+++ b/extensions/opencode/session-catalog.test.ts
@@ -257,6 +257,27 @@ describe("OpenCode session catalog", () => {
         cursor: latest.nextCursor,
       });
       expect(older.items.map((item) => item.type)).toEqual(["reasoning", "agentMessage"]);
+      const nonEmitted = Buffer.from(JSON.stringify({ offset: 2, extra: true }), "utf8").toString(
+        "base64url",
+      );
+      const unsafeOffset = Buffer.from(
+        JSON.stringify({ offset: Number.MAX_SAFE_INTEGER + 1 }),
+        "utf8",
+      ).toString("base64url");
+      for (const cursor of [
+        `${latest.nextCursor}$`,
+        `${latest.nextCursor}=`,
+        ` ${latest.nextCursor} `,
+        nonEmitted,
+        unsafeOffset,
+      ]) {
+        await expect(
+          readLocalOpenCodeTranscriptPage({
+            threadId: "ses_test",
+            cursor,
+          }),
+        ).rejects.toThrow("cursor is invalid");
+      }
       await expect(listLocalOpenCodeSessionPage({ cursor: " " })).rejects.toThrow(
         "cursor is invalid",
       );
@@ -632,6 +653,62 @@ describe("OpenCode session catalog", () => {
     await expect(catalog!.read({ hostId: "node:node-1", threadId: "ses_remote" })).rejects.toThrow(
       "invalid transcript page",
     );
+
+    invoke.mockClear();
+    await expect(
+      catalog!.read({ hostId: "node:node-1", threadId: "ses_remote", cursor: "" }),
+    ).rejects.toThrow("cursor is invalid");
+    await expect(
+      catalog!.list({
+        hostIds: ["node:node-1"],
+        cursors: { "node:node-1": "" },
+      }),
+    ).resolves.toEqual([
+      expect.objectContaining({
+        error: { code: "NODE_INVOKE_FAILED", message: expect.any(String) },
+      }),
+    ]);
+    expect(invoke).not.toHaveBeenCalled();
+
+    invoke.mockResolvedValueOnce({
+      payloadJSON: JSON.stringify({ sessions: [], nextCursor: " wrapped " }),
+    });
+    await expect(catalog!.list({ hostIds: ["node:node-1"] })).resolves.toEqual([
+      expect.objectContaining({
+        error: { code: "NODE_INVOKE_FAILED", message: expect.any(String) },
+      }),
+    ]);
+    invoke.mockResolvedValueOnce({
+      payloadJSON: JSON.stringify({
+        threadId: "ses_remote",
+        items: [],
+        nextCursor: " wrapped ",
+      }),
+    });
+    await expect(catalog!.read({ hostId: "node:node-1", threadId: "ses_remote" })).rejects.toThrow(
+      "invalid cursor",
+    );
+
+    const exactCursor = Buffer.from(JSON.stringify({ offset: 1 }), "utf8").toString("base64url");
+    invoke.mockResolvedValueOnce({ payloadJSON: JSON.stringify({ sessions: [] }) });
+    await catalog!.list({
+      hostIds: ["node:node-1"],
+      cursors: { "node:node-1": exactCursor },
+    });
+    expect(invoke).toHaveBeenLastCalledWith(
+      expect.objectContaining({ params: { cursor: exactCursor } }),
+    );
+    invoke.mockResolvedValueOnce({
+      payloadJSON: JSON.stringify({ threadId: "ses_remote", items: [] }),
+    });
+    await catalog!.read({
+      hostId: "node:node-1",
+      threadId: "ses_remote",
+      cursor: exactCursor,
+    });
+    expect(invoke).toHaveBeenLastCalledWith(
+      expect.objectContaining({ params: { threadId: "ses_remote", cursor: exactCursor } }),
+    );
   });
 
   it.each(["stdout", "stderr"] as const)(
diff --git a/extensions/opencode/session-catalog.ts b/extensions/opencode/session-catalog.ts
index c7342ef33e84..7cf7150d6ce7 100644
--- a/extensions/opencode/session-catalog.ts
+++ b/extensions/opencode/session-catalog.ts
@@ -63,7 +63,7 @@ type OpenCodeReadParams = {
   cursor?: string;
 };
 
-export function optionalOpenCodeString(value: unknown, maxLength: number): string | undefined {
+function optionalOpenCodeString(value: unknown, maxLength: number): string | undefined {
   if (typeof value !== "string") {
     return undefined;
   }
@@ -85,25 +85,52 @@ function encodeCursor(offset: number): string {
   return Buffer.from(JSON.stringify({ offset }), "utf8").toString("base64url");
 }
 
-function decodeCursor(value: unknown): number {
+function optionalRawCursor(value: unknown): string | undefined {
   if (value === undefined) {
-    return 0;
+    return undefined;
   }
-  const cursor = optionalOpenCodeString(value, MAX_CURSOR_LENGTH);
-  if (!cursor) {
+  if (typeof value !== "string" || value.length === 0 || value.length > MAX_CURSOR_LENGTH) {
     throw new Error("cursor is invalid");
   }
+  return value;
+}
+
+function decodeCursor(value: unknown): number {
+  const cursor = optionalRawCursor(value);
+  if (cursor === undefined) {
+    return 0;
+  }
   try {
-    const parsed = JSON.parse(Buffer.from(cursor, "base64url").toString("utf8")) as unknown;
-    if (!isRecord(parsed) || !Number.isInteger(parsed.offset) || Number(parsed.offset) < 0) {
+    const bytes = Buffer.from(cursor, "base64url");
+    if (bytes.toString("base64url") !== cursor) {
+      throw new Error("non-canonical base64url");
+    }
+    const parsed = JSON.parse(bytes.toString("utf8")) as unknown;
+    if (!isRecord(parsed) || !Number.isSafeInteger(parsed.offset) || Number(parsed.offset) < 0) {
       throw new Error("invalid offset");
     }
-    return Number(parsed.offset);
+    const offset = Number(parsed.offset);
+    if (encodeCursor(offset) !== cursor) {
+      throw new Error("non-canonical cursor payload");
+    }
+    return offset;
   } catch (error) {
     throw new Error("cursor is invalid", { cause: error });
   }
 }
 
+export function isExactOpenCodeSessionCursor(value: unknown): value is string {
+  if (typeof value !== "string") {
+    return false;
+  }
+  try {
+    decodeCursor(value);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 function truncateUtf8(text: string, maxBytes: number): string {
   if (Buffer.byteLength(text, "utf8") <= maxBytes) {
     return text;
@@ -173,10 +200,7 @@ function parseListParams(
   if (value.searchTerm !== undefined && !searchTerm) {
     throw new Error("searchTerm is invalid");
   }
-  const cursor = optionalOpenCodeString(value.cursor, MAX_CURSOR_LENGTH);
-  if (value.cursor !== undefined && !cursor) {
-    throw new Error("cursor is invalid");
-  }
+  const cursor = optionalRawCursor(value.cursor);
   return {
     limit: boundedLimit(value.limit),
     ...(searchTerm ? { searchTerm } : {}),
@@ -198,10 +222,7 @@ function parseReadParams(
   if (!threadId || !SESSION_ID_PATTERN.test(threadId)) {
     throw new Error("threadId is invalid");
   }
-  const cursor = optionalOpenCodeString(value.cursor, MAX_CURSOR_LENGTH);
-  if (value.cursor !== undefined && !cursor) {
-    throw new Error("cursor is invalid");
-  }
+  const cursor = optionalRawCursor(value.cursor);
   return {
     threadId,
     limit: boundedLimit(value.limit),
diff --git a/src/acp/translator.lifecycle.test.ts b/src/acp/translator.lifecycle.test.ts
index 7d687a3256bd..b157693b5050 100644
--- a/src/acp/translator.lifecycle.test.ts
+++ b/src/acp/translator.lifecycle.test.ts
@@ -206,6 +206,11 @@ describe("acp translator stable lifecycle handlers", () => {
     ]);
     expect(second.sessions.map((session) => session.cwd)).toEqual(["/work/a", "/work/a"]);
     expect(second.nextCursor).toBeNull();
+    await expect(
+      agent.listSessions(
+        createListSessionsRequest({ cwd: "/work/a", cursor: ` ${first.nextCursor} ` }),
+      ),
+    ).rejects.toThrow("Invalid ACP session list cursor.");
     expect(request).toHaveBeenNthCalledWith(1, "sessions.list", {
       limit: 3,
       includeDerivedTitles: true,
diff --git a/src/acp/translator.session-list.test.ts b/src/acp/translator.session-list.test.ts
index f862eda39e29..572f19b8d5d0 100644
--- a/src/acp/translator.session-list.test.ts
+++ b/src/acp/translator.session-list.test.ts
@@ -29,6 +29,18 @@ describe("ACP translator session list helpers", () => {
     ).toThrow("Invalid ACP session list cursor offset.");
   });
 
+  it("rejects altered and non-emitted cursor spellings", () => {
+    const canonical = encodeListSessionsCursor({ offset: 25, cwd: "/tmp/work" });
+    const extraField = Buffer.from(
+      JSON.stringify({ v: 1, offset: 25, cwd: "/tmp/work", extra: true }),
+      "utf8",
+    ).toString("base64url");
+
+    for (const cursor of [`${canonical}$`, `${canonical}=`, ` ${canonical} `, extraField]) {
+      expect(() => decodeListSessionsCursor(cursor)).toThrow("Invalid ACP session list cursor.");
+    }
+  });
+
   it("clamps page size metadata to the bridge maximum", () => {
     expect(resolveListSessionsPageSize(null)).toBe(100);
     expect(resolveListSessionsPageSize({ limit: 2.9 })).toBe(2);
diff --git a/src/acp/translator.session-list.ts b/src/acp/translator.session-list.ts
index cff340c1e505..a81fc47fbde8 100644
--- a/src/acp/translator.session-list.ts
+++ b/src/acp/translator.session-list.ts
@@ -24,12 +24,16 @@ export function encodeListSessionsCursor(cursor: ListSessionsCursor): string {
 
 /** Decodes and validates an ACP session-list cursor, defaulting to the first page. */
 export function decodeListSessionsCursor(value: string | null | undefined): ListSessionsCursor {
-  if (!value) {
+  if (value === null || value === undefined) {
     return { offset: 0 };
   }
   let parsed: unknown;
   try {
-    parsed = JSON.parse(Buffer.from(value, "base64url").toString("utf8"));
+    const bytes = Buffer.from(value, "base64url");
+    if (bytes.toString("base64url") !== value) {
+      throw new Error("non-canonical base64url");
+    }
+    parsed = JSON.parse(bytes.toString("utf8"));
   } catch {
     throw new Error("Invalid ACP session list cursor.");
   }
@@ -42,17 +46,21 @@ export function decodeListSessionsCursor(value: string | null | undefined): List
   }
   if (
     typeof record.offset !== "number" ||
-    !Number.isInteger(record.offset) ||
+    !Number.isSafeInteger(record.offset) ||
     record.offset < 0 ||
     record.offset > ACP_LIST_SESSIONS_MAX_CURSOR_OFFSET
   ) {
     throw new Error("Invalid ACP session list cursor offset.");
   }
   const cwd = normalizeOptionalString(record.cwd);
-  return {
+  const cursor = {
     offset: record.offset,
     ...(cwd ? { cwd } : {}),
   };
+  if (encodeListSessionsCursor(cursor) !== value) {
+    throw new Error("Invalid ACP session list cursor.");
+  }
+  return cursor;
 }
 
 /** Throws when an ACP method receives a relative cwd filter/path. */
diff --git a/src/acp/translator.ts b/src/acp/translator.ts
index 07f89996952d..65606fac715d 100644
--- a/src/acp/translator.ts
+++ b/src/acp/translator.ts
@@ -471,7 +471,7 @@ export class AcpGatewayAgent implements Agent {
       assertAbsoluteCwd(requestedCwd, "session/list");
     }
     const fallbackCwd = requestedCwd ?? process.cwd();
-    const rawCursor = normalizeOptionalString(params.cursor);
+    const rawCursor = params.cursor;
     const cursor = decodeListSessionsCursor(rawCursor);
     if (rawCursor && cursor.cwd !== requestedCwd) {
       throw new Error("ACP session list cursor does not match the cwd filter.");