diff --git a/src/main/services/agent-attachment-tools.ts b/src/main/services/agent-attachment-tools.ts
index 7c5ae8e8..f15a5731 100644
--- a/src/main/services/agent-attachment-tools.ts
+++ b/src/main/services/agent-attachment-tools.ts
@@ -1,5 +1,5 @@
+import { readVerifiedFile } from "./files/file-object-store";
 import type { DatabaseSync } from "node:sqlite";
-import { readFileSync } from "node:fs";
 import { FILE_SCAN_STATE } from "../../shared/domain";
 import { requireString } from "./agent-tool-args";
 import type { AgentToolContext, AgentToolDeps } from "./agent-tools";
@@ -32,11 +32,11 @@ export function attachmentNote(db: DatabaseSync, messageId: string): string {
 
 export function readAttachmentTool(deps: AgentToolDeps, ctx: AgentToolContext, args: JsonRecord): string {
   const fileId = requireString(args.file_id, "file_id");
-  const file = deps.db.prepare("select name, storage_path, scan_state from file_objects where id = ? and org_id = ?")
-    .get(fileId, ctx.orgId) as { name: string; storage_path: string; scan_state: string } | undefined;
+  const file = deps.db.prepare("select name, storage_path, sha256, scan_state from file_objects where id = ? and org_id = ?")
+    .get(fileId, ctx.orgId) as { name: string; storage_path: string; sha256: string; scan_state: string } | undefined;
   if (!file) throw new Error(`No file with file_id "${fileId}".`);
   if (file.scan_state !== FILE_SCAN_STATE.APPROVED) throw new Error(`${file.name} was blocked by the file scan and cannot be read.`);
-  const bytes = readFileSync(file.storage_path);
+  const bytes = readVerifiedFile({ name: file.name, storagePath: file.storage_path, sha256: file.sha256 });
   const content = utf8Text(bytes);
   if (content === null) return `${file.name} is a binary file (${bytes.length} bytes); it has no text to read.`;
   return pageText(file.name, content, readPageArgs(args));
diff --git a/src/main/services/agent-task-handoff.ts b/src/main/services/agent-task-handoff.ts
index e9760245..29565d61 100644
--- a/src/main/services/agent-task-handoff.ts
+++ b/src/main/services/agent-task-handoff.ts
@@ -1,3 +1,4 @@
+import { readVerifiedFile } from "./files/file-object-store";
 import { createHash } from "node:crypto";
 import { existsSync, readFileSync } from "node:fs";
 import type { DatabaseSync } from "node:sqlite";
@@ -27,12 +28,12 @@ export function buildTaskHandoff(
   args: JsonRecord,
 ): TaskHandoff {
   const inputs = optionalStringArray(args.input_file_ids, "input_file_ids").map((fileId) => {
-    const file = db.prepare("select id, name from file_objects where id = ? and org_id = ?")
-      .get(fileId, orgId) as { id: string; name: string } | undefined;
+    const file = db.prepare("select id, name, storage_path, sha256 from file_objects where id = ? and org_id = ?")
+      .get(fileId, orgId) as { id: string; name: string; storage_path: string; sha256: string } | undefined;
     if (!file) throw new Error(`No file with file_id "${fileId}" exists in this company.`);
-    const sha256 = currentSha256(db, orgId, file.id);
-    if (!sha256) throw new Error(`File ${file.name} (file_id "${fileId}") is missing on disk.`);
-    return { fileId: file.id, name: file.name, sha256 };
+    // The handoff pins the recorded content; bytes that no longer match it are refused, not pinned as if they were.
+    readVerifiedFile({ name: file.name, storagePath: file.storage_path, sha256: file.sha256 });
+    return { fileId: file.id, name: file.name, sha256: file.sha256 };
   });
   return {
     sourceTaskId: source.taskId,
diff --git a/src/main/services/chat-service.ts b/src/main/services/chat-service.ts
index f31f34fc..a0229e53 100644
--- a/src/main/services/chat-service.ts
+++ b/src/main/services/chat-service.ts
@@ -1,3 +1,4 @@
+import { readVerifiedFile } from "./files/file-object-store";
 import type { DatabaseSync } from "node:sqlite";
 import type { CreateChatSpaceInput, DeleteMessageInput, EditMessageInput, MarkChatSpaceReadInput, SendMessageInput } from "../../shared/commands";
 import {
@@ -436,6 +437,8 @@ export class ChatService {
       if (!file) throw new Error("Attachment file was not found.");
       if (file.scan_state !== FILE_SCAN_STATE.APPROVED) throw new Error("Attachment file is not approved for handoff.");
       if (!this.senderCanReadFile(orgId, file, sender)) throw new Error("Attachment file access is required.");
+      // A handoff carries the recorded checksum, so the bytes must still be the recorded ones.
+      readVerifiedFile({ name: file.id, storagePath: file.storage_path, sha256: file.sha256 });
       return {
         fileId,
         checksum: file.sha256,
diff --git a/src/main/services/file-service.ts b/src/main/services/file-service.ts
index 533ba140..35a07263 100644
--- a/src/main/services/file-service.ts
+++ b/src/main/services/file-service.ts
@@ -7,7 +7,7 @@ import type { ImportFileInput, OpenFileInput } from "../../shared/commands";
 import { FILE_SCAN_STATE } from "../../shared/domain";
 import { createId } from "../../shared/ids";
 import { AuditService } from "./audit-service";
-import { fileKind, scanFileContent, upsertFileObject } from "./files/file-object-store";
+import { fileKind, readVerifiedFile, scanFileContent, upsertFileObject } from "./files/file-object-store";
 
 export class FileService {
   private readonly audit: AuditService;
@@ -77,10 +77,12 @@ export class FileService {
 
   async openFile(input: OpenFileInput): Promise<void> {
     const row = this.db.prepare(
-      "select storage_path as storagePath, scan_state as scanState from file_objects where id = ? and org_id = ?",
-    ).get(input.fileId, input.orgId) as { storagePath: string; scanState: string } | undefined;
+      "select name, storage_path as storagePath, sha256, scan_state as scanState from file_objects where id = ? and org_id = ?",
+    ).get(input.fileId, input.orgId) as { name: string; storagePath: string; sha256: string; scanState: string } | undefined;
     if (!row) throw new Error("File was not found.");
     if (row.scanState !== FILE_SCAN_STATE.APPROVED) throw new Error("Blocked files cannot be opened.");
+    // What opens must still be what was recorded; a file changed by another program is refused, not shown under the old record.
+    readVerifiedFile(row);
     const failure = await shell.openPath(row.storagePath);
     if (failure) throw new Error(failure);
   }
diff --git a/src/main/services/files/file-object-store.ts b/src/main/services/files/file-object-store.ts
index a5860fa1..717c1984 100644
--- a/src/main/services/files/file-object-store.ts
+++ b/src/main/services/files/file-object-store.ts
@@ -184,6 +184,17 @@ export function recordWorkspaceFile(db: DatabaseSync, input: {
   }));
 }
 
+/** A recorded file's bytes, refused when they are gone or no longer match the hash it was recorded with. */
+export function readVerifiedFile(file: { name: string; storagePath: string; sha256: string }): Buffer {
+  if (!existsSync(file.storagePath)) throw new Error(`${file.name} is no longer on disk.`);
+  const bytes = readFileSync(file.storagePath);
+  if (createHash("sha256").update(bytes).digest("hex") !== file.sha256) {
+    console.warn(`[files] ${file.storagePath} no longer matches its recorded hash ${file.sha256}`);
+    throw new Error(`${file.name} changed on disk since it was recorded; it is not the content that was handed over.`);
+  }
+  return bytes;
+}
+
 /** The content-addressed copy of a workspace file, written once; it keeps the extension so the file still opens with its app. */
 function keepCopy(workspaceRoot: string, source: string, sha256: string, bytes: Buffer): string {
   const copy = join(workspaceRoot, ".agentcompany", "files", `${sha256}${extname(source)}`);
diff --git a/tests/integration/chat-context.test.ts b/tests/integration/chat-context.test.ts
index 0c60849d..63fe89c3 100644
--- a/tests/integration/chat-context.test.ts
+++ b/tests/integration/chat-context.test.ts
@@ -1,4 +1,4 @@
-import { randomUUID } from "node:crypto";
+import { createHash, randomUUID } from "node:crypto";
 import { writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -87,8 +87,8 @@ async function founderLine() {
     const fileId = `fil_${randomUUID()}`;
     connection.db.prepare(`
       insert into file_objects(id, org_id, name, kind, size_bytes, sha256, storage_path, owner_type, owner_id, created_at)
-      values (?, ?, ?, 'md', ?, 'x', ?, 'employee', ?, ?)
-    `).run(fileId, orgId, name, content.length, path, founderId, new Date().toISOString());
+      values (?, ?, ?, 'md', ?, ?, ?, 'employee', ?, ?)
+    `).run(fileId, orgId, name, content.length, createHash("sha256").update(content).digest("hex"), path, founderId, new Date().toISOString());
     return fileId;
   };
   const sayAs = (senderType: "employee" | "board", senderId: string, bodyMd: string, threadId: string) =>
diff --git a/tests/unit/agent-attachment-tools.test.ts b/tests/unit/agent-attachment-tools.test.ts
index 52b4932e..59fe0442 100644
--- a/tests/unit/agent-attachment-tools.test.ts
+++ b/tests/unit/agent-attachment-tools.test.ts
@@ -1,4 +1,4 @@
-import { randomUUID } from "node:crypto";
+import { createHash, randomUUID } from "node:crypto";
 import { writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -23,8 +23,8 @@ function harness() {
     const fileId = `fil_${randomUUID()}`;
     connection.db.prepare(`
       insert into file_objects(id, org_id, name, kind, size_bytes, sha256, storage_path, owner_type, owner_id, created_at, scan_state)
-      values (?, ?, ?, 'md', 1, 'x', ?, 'employee', null, ?, ?)
-    `).run(fileId, owner, name, path, new Date().toISOString(), scanState);
+      values (?, ?, ?, 'md', 1, ?, ?, 'employee', null, ?, ?)
+    `).run(fileId, owner, name, createHash("sha256").update(content).digest("hex"), path, new Date().toISOString(), scanState);
     return fileId;
   };
   const ctx: AgentToolContext = {
diff --git a/tests/unit/agent-task-handoff.test.ts b/tests/unit/agent-task-handoff.test.ts
index 11e6b31b..7e4803de 100644
--- a/tests/unit/agent-task-handoff.test.ts
+++ b/tests/unit/agent-task-handoff.test.ts
@@ -86,6 +86,15 @@ describe("structured task handoff", () => {
     h.connection.close();
   });
 
+  test("an input whose bytes no longer match its record is refused, not pinned", () => {
+    const h = harness();
+    writeFileSync(h.specPath, "tampered");
+    expect(dispatchAgentTool(h.deps, h.ctx, call("create_task", { title: "Y", description: "d", input_file_ids: ["fil_spec"] })))
+      .toBe("ERROR: spec.md changed on disk since it was recorded; it is not the content that was handed over.");
+    expect(h.connection.db.prepare("select count(*) as n from tasks where title = 'Y'").get()).toEqual({ n: 0 });
+    h.connection.close();
+  });
+
   test("an unknown input file id fails the call and creates nothing", () => {
     const h = harness();
     expect(dispatchAgentTool(h.deps, h.ctx, call("create_task", { title: "X", description: "d", input_file_ids: ["fil_missing"] })))
diff --git a/tests/unit/chat-governance.test.ts b/tests/unit/chat-governance.test.ts
index c64f9114..e1240c8b 100644
--- a/tests/unit/chat-governance.test.ts
+++ b/tests/unit/chat-governance.test.ts
@@ -1,4 +1,5 @@
-import { randomUUID } from "node:crypto";
+import { writeFileSync } from "node:fs";
+import { createHash, randomUUID } from "node:crypto";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { describe, expect, test } from "vitest";
@@ -11,6 +12,14 @@ import { providerAdapters } from "../../src/main/services/provider-adapters";
 import { SnapshotService } from "../../src/main/services/snapshot-service";
 import { RISK_LEVEL } from "../../src/shared/domain";
 
+/** A real file and its hash, as a recorded file needs: bytes are checked against the hash when read. */
+function realFile(content: string): [string, string] {
+  const path = join(tmpdir(), `ac-file-${randomUUID()}.md`);
+  writeFileSync(path, content);
+  return [createHash("sha256").update(content).digest("hex"), path];
+}
+
+
 function createConnection(): DatabaseConnection {
   return new DatabaseConnection(join(tmpdir(), `agentcompany-new-chat-${randomUUID()}.db`));
 }
@@ -273,7 +282,7 @@ describe("chat governance", () => {
       connection.db.prepare(`
         insert into file_objects(id, org_id, name, kind, size_bytes, sha256, storage_path, owner_type, owner_id, created_at)
         values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
-      `).run(fileId, orgId, "release-notes.md", "md", 24, "8f3a", "artifacts/release-notes.md", "employee", employeeId, now);
+      `).run(fileId, orgId, "release-notes.md", "md", 24, ...realFile("Release notes."), "employee", employeeId, now);
       agentMessageId = chat.sendMessage({
         orgId,
         spaceId,
diff --git a/tests/unit/domain-services.test.ts b/tests/unit/domain-services.test.ts
index d10c9688..75295f31 100644
--- a/tests/unit/domain-services.test.ts
+++ b/tests/unit/domain-services.test.ts
@@ -15,6 +15,17 @@ import { OnboardingService } from "../../src/main/services/onboarding-service";
 import { RISK_LEVEL, TASK_STATUS } from "../../src/shared/domain";
 import { createId } from "../../src/shared/ids";
 
+/** A real file and its hash, as a recorded file needs: bytes are checked against the hash when read. */
+const runbook = realFile("Runbook steps.");
+const releaseNotes = realFile("Release notes.");
+
+function realFile(content: string): [string, string] {
+  const path = join(tmpdir(), `ac-file-${randomUUID()}.md`);
+  writeFileSync(path, content);
+  return [createHash("sha256").update(content).digest("hex"), path];
+}
+
+
 function createConnection(): DatabaseConnection {
   return new DatabaseConnection(join(tmpdir(), `agentcompany-new-${randomUUID()}.db`));
 }
@@ -287,7 +298,7 @@ describe("domain services", () => {
       connection.db.prepare(`
         insert into file_objects(id, org_id, name, kind, size_bytes, sha256, storage_path, owner_type, owner_id, created_at)
         values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
-      `).run(fileId, orgId, "runbook.md", "text/markdown", 16, "8f3a", "artifacts/runbook.md", "employee", employee.id, new Date().toISOString());
+      `).run(fileId, orgId, "runbook.md", "text/markdown", 16, ...runbook, "employee", employee.id, new Date().toISOString());
 
       chat.sendMessage({
         orgId,
@@ -311,10 +322,10 @@ describe("domain services", () => {
       fileId,
       targetPrincipalType: "board",
       targetPrincipalId: orgId,
-      checksum: "8f3a",
+      checksum: runbook[0],
       scanState: "approved",
       policyDecision: "approved_for_handoff",
-      objectRef: "artifacts/runbook.md",
+      objectRef: runbook[1],
     });
     expect(state.chatMentions[0]).toMatchObject({ targetType: "board", targetId: orgId });
     expect(state.chatSlashInvocations[0]).toMatchObject({ command: "handoff" });
@@ -376,7 +387,7 @@ describe("domain services", () => {
           owner_type, owner_id, space_id, pinned, scan_state, scan_labels_json, scanned_at, created_at, updated_at
         )
         values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
-      `).run(approvedFileId, orgId, "release-notes.md", "md", 24, "sha-approved", "artifacts/release-notes.md", "agent", adaId, null, 0, "approved", "[]", now, now, now);
+      `).run(approvedFileId, orgId, "release-notes.md", "md", 24, ...releaseNotes, "agent", adaId, null, 0, "approved", "[]", now, now, now);
       connection.db.prepare(`
         insert into file_objects(
           id, org_id, name, kind, size_bytes, sha256, storage_path,
@@ -438,8 +449,8 @@ describe("domain services", () => {
       sourcePrincipalType: "agent",
       sourcePrincipalId: adaId,
       targetPrincipalType: "agent",
-      checksum: "sha-approved",
-      objectRef: "artifacts/release-notes.md",
+      checksum: releaseNotes[0],
+      objectRef: releaseNotes[1],
       scanState: "approved",
       policyDecision: "approved_for_handoff",
     });
diff --git a/tests/unit/workspace-file-versions.test.ts b/tests/unit/workspace-file-versions.test.ts
index bec13905..122620b2 100644
--- a/tests/unit/workspace-file-versions.test.ts
+++ b/tests/unit/workspace-file-versions.test.ts
@@ -6,6 +6,8 @@ import { describe, expect, test } from "vitest";
 import { DatabaseConnection } from "../../src/main/database/connection";
 import { OrganizationService } from "../../src/main/services/organization-service";
 import { preserveRecordedFiles, recordWorkspaceFile } from "../../src/main/services/files/file-object-store";
+import { ChatService } from "../../src/main/services/chat-service";
+import { readAttachmentTool } from "../../src/main/services/agent-attachment-tools";
 
 describe("recorded workspace files", () => {
   test("a file id keeps the bytes it was recorded with after the workspace file is rewritten", () => {
@@ -58,3 +60,24 @@ describe("recorded workspace files", () => {
     connection.close();
   });
 });
+
+describe("opening a recorded file", () => {
+  test("bytes that no longer match the recorded hash are refused, not shown as the recorded file", () => {
+    const connection = new DatabaseConnection(join(tmpdir(), `ac-verify-${randomUUID()}.db`));
+    const root = join(tmpdir(), `ac-verify-ws-${randomUUID()}`);
+    mkdirSync(root, { recursive: true });
+    let orgId = "";
+    connection.transaction(() => {
+      orgId = new OrganizationService(connection.db).createOrganization({ organizationName: "Acme", employeeName: "Dana", mission: "m", goal: "g" });
+    });
+    writeFileSync(join(root, "plan.md"), "recorded");
+    const fileId = recordWorkspaceFile(connection.db, { orgId, ownerType: "agent", ownerId: null, workspaceRoot: root, relativePath: "plan.md" });
+    const deps = { db: connection.db, organization: new OrganizationService(connection.db), chat: new ChatService(connection.db) };
+    const ctx = { orgId, agent: { id: "agt_x", modelProviderId: null, modelId: null }, taskProjectId: null, taskGoalId: null, taskId: null, sourceThreadId: null, workspace: null };
+    expect(readAttachmentTool(deps, ctx, { file_id: fileId })).toContain("recorded");
+    const stored = (connection.db.prepare("select storage_path from file_objects where id = ?").get(fileId) as { storage_path: string }).storage_path;
+    writeFileSync(stored, "tampered");
+    expect(() => readAttachmentTool(deps, ctx, { file_id: fileId })).toThrow("plan.md changed on disk since it was recorded; it is not the content that was handed over.");
+    connection.close();
+  });
+});