diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index dff68dbd..a7cf01e6 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -18,6 +18,36 @@ entry. See `CONTRIBUTING.md` § Releases & changelog.
 
 ## [Unreleased]
 
+### Added — the audience floor now guards attachment-handle resolution (#575)
+
+- **The floor's third and last guard**, completing the trio the spec names. A
+  storage key is a handle: it is minted in one turn and can be redeemed later,
+  potentially in a different room.
+- **The check rides with the handle, not with the call sites.** Enforcement
+  lives in a wrapper around `AttachmentReader` applied at its single
+  construction site, so `read_attachment`, the orchestrator's own
+  `ingestAttachments` and any future resolution path are covered by
+  construction rather than by remembering to add a call.
+- **This closes a path the egress guard did not.** `read_attachment` is a tool
+  and so already passed the first guard, but `ingestAttachments` resolves
+  storage keys straight off the inbound turn with no tool call involved — the
+  path a caller actually controls.
+- **A refusal is indistinguishable from "unknown key" to the caller, on
+  purpose.** Confirming that a key exists but is off-limits would leak the
+  document's existence to a room that may not know it. The real reason goes to
+  the operator log, where it is actionable and not a side channel. The inner
+  reader is never reached, so a refused redemption does not even touch the
+  store.
+- Redeeming a handle and invoking the read tool are separate capabilities;
+  neither grants the other.
+
+> **Remaining gap, named rather than assumed closed.** This checks the floor at
+> *redemption*, not at *minting*. It stops a room from redeeming a handle that
+> room may not read, but cannot yet stop a handle minted in a narrow room from
+> being redeemed in a wider one that happens to hold the capability. Binding the
+> minting audience to the handle needs the attachment store to persist it, and
+> that store lives in the channel plugins.
+
 ### Added — the audience floor now guards context recall (#575)
 
 - **The floor's second guard**, at the single context-assembly call site. In a
diff --git a/middleware/packages/harness-orchestrator/src/attachmentReaderFactory.ts b/middleware/packages/harness-orchestrator/src/attachmentReaderFactory.ts
index f15e77c2..b72ccd2f 100644
--- a/middleware/packages/harness-orchestrator/src/attachmentReaderFactory.ts
+++ b/middleware/packages/harness-orchestrator/src/attachmentReaderFactory.ts
@@ -16,6 +16,7 @@
 import type { Readable } from 'node:stream';
 
 import type { AttachmentReader } from './tools/readAttachmentTool.js';
+import { guardAttachmentRead } from './audienceFloorGuard.js';
 
 /** Minimal structural view of the kernel's `tigrisStore` service. */
 export interface AttachmentByteStore {
@@ -40,10 +41,53 @@ function fileNameFromKey(key: string): string | undefined {
   return seg && seg.length > 0 ? seg : undefined;
 }
 
+/**
+ * #575 — wrap a reader so every handle redemption passes the audience floor.
+ *
+ * The check rides with the handle rather than sitting at each call site, which
+ * is the point spec §5.2 makes: a storage key outlives the turn that minted it,
+ * and a resolution site added later would otherwise silently escape the guard.
+ * Wrapping the reader covers `read_attachment`, the orchestrator's own
+ * `ingestAttachments`, and anything added next, by construction.
+ *
+ * A refusal returns `undefined` — the reader's existing "unavailable" contract —
+ * rather than throwing, so no caller needs new error handling. That does mean a
+ * denial is indistinguishable from "unknown key" *to the model*, and that is
+ * deliberate rather than sloppy: a message confirming that the key exists but
+ * is off-limits would leak the document's existence to a room that may not know
+ * it. The real reason goes to the operator log, where it is actionable and not
+ * a side channel.
+ *
+ * Inert when no audience source is installed, like every other guard here.
+ */
+export function audienceGuardedAttachmentReader(inner: AttachmentReader): AttachmentReader {
+  return {
+    async readByStorageKey(storageKey) {
+      const refusal = await guardAttachmentRead();
+      if (refusal !== undefined) {
+        console.warn(`[harness-orchestrator] attachment read refused by audience floor: ${refusal}`);
+        return undefined;
+      }
+      return inner.readByStorageKey(storageKey);
+    },
+    async readByUrl(url) {
+      const refusal = await guardAttachmentRead();
+      if (refusal !== undefined) {
+        console.warn(`[harness-orchestrator] attachment fetch refused by audience floor: ${refusal}`);
+        return undefined;
+      }
+      return inner.readByUrl(url);
+    },
+  };
+}
+
 /**
  * Construct an {@link AttachmentReader}. When `store` is `undefined`
  * (bucket env not set), `readByStorageKey` always resolves to `undefined`
  * and the feature is inert; `readByUrl` still works via `fetch`.
+ *
+ * Unguarded on its own — `plugin.ts` wraps it in
+ * {@link audienceGuardedAttachmentReader} at the single construction site.
  */
 export function createAttachmentReader(
   store: AttachmentByteStore | undefined,
diff --git a/middleware/packages/harness-orchestrator/src/audienceFloorGuard.ts b/middleware/packages/harness-orchestrator/src/audienceFloorGuard.ts
index a9a184c4..556e4723 100644
--- a/middleware/packages/harness-orchestrator/src/audienceFloorGuard.ts
+++ b/middleware/packages/harness-orchestrator/src/audienceFloorGuard.ts
@@ -76,6 +76,65 @@ export function toolCapability(name: string): Capability {
  */
 export const MEMORY_RECALL_CAPABILITY: Capability = 'memory:recall';
 
+/**
+ * The capability resolving a stored attachment handle requires.
+ *
+ * Separate from `tool:read_attachment` on purpose. That one asks "may this room
+ * invoke the read tool"; this one asks "may this room redeem a storage handle",
+ * and the handle is redeemable from paths that are not tool calls at all — see
+ * {@link guardAttachmentRead}.
+ */
+export const ATTACHMENT_READ_CAPABILITY: Capability = 'attachment:read';
+
+/**
+ * #575 — the third guard: file / credential handle resolution.
+ *
+ * ## Why this is not already covered by the egress guard
+ *
+ * `read_attachment` is a tool, so it passes `dispatchTool` and Guard 1 does
+ * bound it. But that is not the only way a handle gets redeemed: the
+ * orchestrator's own `ingestAttachments` resolves storage keys straight off the
+ * inbound turn, with no tool call in sight. Guarding only the tool would leave
+ * the path a caller actually controls wide open.
+ *
+ * ## Why the check rides with the handle rather than sitting at call sites
+ *
+ * Spec §5.2 says the check "must ride with" the handle, because a handle
+ * outlives the turn that minted it. A storage key issued in a private chat is
+ * just a string, and a string can be pasted into a group chat. Adding a call to
+ * every resolution site would work exactly until somebody adds the next site
+ * and forgets — so the enforcement lives in a wrapper around `AttachmentReader`
+ * itself (`attachmentReaderFactory.ts`). Every consumer, present and future, is
+ * covered by construction.
+ *
+ * ## What this version does NOT do
+ *
+ * It checks the floor **at redemption**, not the floor **at minting**. So it
+ * stops a room from redeeming a handle that room may not read — but it cannot
+ * yet stop a handle minted in a narrow room from being redeemed in a room that
+ * happens to hold the capability. Binding the minting audience to the handle
+ * needs the attachment store to persist it, and that store lives in the channel
+ * plugins rather than here. Stated so the remaining gap is visible rather than
+ * assumed closed.
+ */
+export async function guardAttachmentRead(): Promise<string | undefined> {
+  const provider = turnContext.current()?.audienceFloor;
+  if (!provider) return undefined;
+
+  let floor: AudienceFloor;
+  try {
+    floor = await provider();
+  } catch (err) {
+    return `audience unresolvable (${err instanceof Error ? err.message : String(err)})`;
+  }
+
+  if (floorPermits(floor, ATTACHMENT_READ_CAPABILITY)) return undefined;
+
+  return floor.outcome === 'closed'
+    ? floor.reason
+    : 'not every participant in this conversation may read stored attachments';
+}
+
 /**
  * #575 — the second guard: context / memory recall.
  *
diff --git a/middleware/packages/harness-orchestrator/src/plugin.ts b/middleware/packages/harness-orchestrator/src/plugin.ts
index 218d981a..f4080791 100644
--- a/middleware/packages/harness-orchestrator/src/plugin.ts
+++ b/middleware/packages/harness-orchestrator/src/plugin.ts
@@ -63,6 +63,7 @@ import {
   type OrchestratorDeps,
 } from './buildOrchestrator.js';
 import {
+  audienceGuardedAttachmentReader,
   createAttachmentReader,
   type AttachmentByteStore,
 } from './attachmentReaderFactory.js';
@@ -554,7 +555,13 @@ export async function activate(
   // tool; harness-orchestrator stays free of any @aws-sdk dependency.
   const attachmentByteStore =
     ctx.services.get<AttachmentByteStore>('tigrisStore');
-  const attachmentReader = createAttachmentReader(attachmentByteStore);
+  // #575 — every attachment-handle redemption passes the audience floor. Wrapped
+  // here, at the ONE construction site, so the check rides with the handle rather
+  // than depending on each resolution site remembering to ask. Inert unless an
+  // audience source is installed.
+  const attachmentReader = audienceGuardedAttachmentReader(
+    createAttachmentReader(attachmentByteStore),
+  );
   // Phase-1 of the Kemia integration. Late-bound `responseGuard@1` getter —
   // the orchestrator generally activates BEFORE its tool plugins, so a
   // bind-at-activate lookup would always miss the responseGuard provider
diff --git a/middleware/test/audienceFloorGuard.test.ts b/middleware/test/audienceFloorGuard.test.ts
index 9287aff0..a4c3fc7f 100644
--- a/middleware/test/audienceFloorGuard.test.ts
+++ b/middleware/test/audienceFloorGuard.test.ts
@@ -13,11 +13,14 @@ import { describe, it } from 'node:test';
 import assert from 'node:assert/strict';
 
 import {
+  ATTACHMENT_READ_CAPABILITY,
   MEMORY_RECALL_CAPABILITY,
+  guardAttachmentRead,
   guardContextRecall,
   guardToolEgress,
   toolCapability,
 } from '../packages/harness-orchestrator/src/audienceFloorGuard.js';
+import { audienceGuardedAttachmentReader } from '../packages/harness-orchestrator/src/attachmentReaderFactory.js';
 import { turnContext } from '../packages/harness-orchestrator/src/turnContext.js';
 import type { AudienceFloor } from '../packages/harness-channel-sdk/src/audienceFloor.js';
 
@@ -180,6 +183,90 @@ describe('the context guard', () => {
   });
 });
 
+// ─── guard 3: file / credential handle resolution ──────────────────────────
+
+describe('the handle guard', () => {
+  it('an unconfigured deployment resolves handles exactly as before', async () => {
+    assert.equal(await withFloor(undefined, () => guardAttachmentRead()), undefined);
+  });
+
+  it('permits redemption when the whole room may read stored attachments', async () => {
+    assert.equal(
+      await withFloor(async () => open(ATTACHMENT_READ_CAPABILITY), () => guardAttachmentRead()),
+      undefined,
+    );
+  });
+
+  it('refuses redemption when someone present may not', async () => {
+    const refusal = await withFloor(async () => open('tool:t'), () => guardAttachmentRead());
+    assert.match(refusal ?? '', /not every participant/);
+  });
+
+  it('the read-attachment TOOL capability does not grant handle redemption', async () => {
+    // Being allowed to invoke the tool is a different question from being
+    // allowed to redeem a storage handle — the handle is redeemable from paths
+    // that are not tool calls at all.
+    const refusal = await withFloor(
+      async () => open(toolCapability('read_attachment')),
+      () => guardAttachmentRead(),
+    );
+    assert.ok(refusal);
+  });
+});
+
+describe('the check rides with the handle, not with the call site', () => {
+  const inner = {
+    readByStorageKey: async () => ({ bytes: Buffer.from('secret'), contentType: 'text/plain' }),
+    readByUrl: async () => ({ bytes: Buffer.from('secret'), contentType: 'text/plain' }),
+  };
+
+  it('a wrapped reader serves bytes when the floor permits', async () => {
+    const reader = audienceGuardedAttachmentReader(inner);
+    const got = await withFloor(
+      async () => open(ATTACHMENT_READ_CAPABILITY),
+      () => reader.readByStorageKey('k'),
+    );
+    assert.equal(got?.bytes.toString(), 'secret');
+  });
+
+  it('BOTH resolution methods are guarded, not just the storage-key one', async () => {
+    // `ingestAttachments` prefers `readByStorageKey` but falls back to
+    // `readByUrl`; guarding only the first would leave the fallback open.
+    const reader = audienceGuardedAttachmentReader(inner);
+    await withFloor(async () => closed('nope'), async () => {
+      assert.equal(await reader.readByStorageKey('k'), undefined);
+      assert.equal(await reader.readByUrl('https://x/y'), undefined);
+    });
+  });
+
+  it('a refusal is indistinguishable from "unknown key" TO THE CALLER', async () => {
+    // Deliberate: confirming the key exists but is off-limits would leak the
+    // document's existence. The reason goes to the operator log instead.
+    const reader = audienceGuardedAttachmentReader(inner);
+    const got = await withFloor(async () => closed('nope'), () => reader.readByStorageKey('k'));
+    assert.equal(got, undefined);
+  });
+
+  it('the inner reader is never even reached on a refusal', async () => {
+    // The bytes must not be fetched and then discarded — that would still hit
+    // the store, and a store hit is itself observable.
+    let touched = false;
+    const spy = {
+      readByStorageKey: async () => {
+        touched = true;
+        return undefined;
+      },
+      readByUrl: async () => {
+        touched = true;
+        return undefined;
+      },
+    };
+    const reader = audienceGuardedAttachmentReader(spy);
+    await withFloor(async () => closed('nope'), () => reader.readByStorageKey('k'));
+    assert.equal(touched, false);
+  });
+});
+
 describe('re-evaluation, not a snapshot', () => {
   it('the provider is consulted on EVERY dispatch', async () => {
     // Spec §5.2: a turn-start snapshot is a TOCTOU hole — somebody can join