diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..2909c10
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,69 @@
+# AGENTS.md
+
+Instructions for AI agents (Claude Code, Copilot, Cursor, etc.) working with libenclaveapp.
+
+## Protection Class Safety Rules
+
+**READ THIS BEFORE TOUCHING `bridge.swift`.**
+
+The macOS Apple backend has two distinct protection class use sites that
+**MUST use different values**. Getting this wrong forces every downstream
+user to regenerate all Secure Enclave keys — there is no migration path
+because the SE key access control is baked into the SEP at creation time.
+
+### SE key access control (`makeAccessControl`)
+
+```swift
+// MUST be WhenUnlockedThisDeviceOnly
+SecAccessControlCreateWithFlags(nil, kSecAttrAccessibleWhenUnlockedThisDeviceOnly, flags, &error)
+```
+
+This is passed to `SecureEnclave.P256.Signing.PrivateKey(accessControl:)` at
+key generation time. The protection class is **immutable** — once set, it
+cannot be changed without deleting and regenerating the key.
+
+`WhenUnlockedThisDeviceOnly` is required because CryptoKit's
+`touchIDAuthenticationAllowableReuseDuration` (biometric caching) only works
+with this protection class. With `AfterFirstUnlockThisDeviceOnly`, the SEP
+ignores the LAContext's cached authentication and prompts Touch ID on every
+single sign operation. This was discovered in PR #158 — the blanket protection
+class change broke biometric caching entirely, requiring full key regeneration
+for all affected users.
+
+### Keychain wrapping key (`keychain_store`)
+
+```swift
+// MUST be AfterFirstUnlockThisDeviceOnly
+SecAccessControlCreateWithFlags(nil, kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly, ...)
+```
+
+The wrapping key is stored as a `kSecClassGenericPassword` keychain item.
+`AfterFirstUnlockThisDeviceOnly` keeps the keybag class key in memory from
+first unlock until reboot, so background agents (like sshenc-agent) can
+access wrapping keys after sleep/wake without requiring the screen to be
+unlocked. `WhenUnlockedThisDeviceOnly` purges the class key on device
+lock/sleep, causing `-25308 (errSecInteractionNotAllowed)` failures.
+
+### Rules
+
+1. **NEVER do a blanket find-and-replace of protection class constants.** These
+   two sites have different requirements for different reasons.
+2. **NEVER change `makeAccessControl`'s protection class** unless you have
+   verified with Apple documentation that biometric caching
+   (`touchIDAuthenticationAllowableReuseDuration`) works with the new class.
+   The cost of getting this wrong is catastrophic — every user must regenerate
+   all keys.
+3. **Test biometric caching after any change to `bridge.swift`**: first sign
+   should trigger Touch ID (~2-3s), second sign within the cache window should
+   complete in <50ms. If every sign takes 2-3s, biometric caching is broken.
+4. When changing keychain-related code, consider both sites independently and
+   document which one you are changing and why.
+
+## Build Safety (macOS)
+
+See the consuming app's AGENTS.md (e.g., sshenc/AGENTS.md) for rules about
+never running unsigned development builds as agents on macOS.
+
+## Commits
+
+Do not add Co-Authored-By lines for Claude Code in commit messages.
diff --git a/CLAUDE.md b/CLAUDE.md
index e92f911..6ad3882 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -53,6 +53,29 @@ Rust workspace under `crates/`:
 - **enclaveapp-build-support** — Shared build.rs helper for Windows PE resource compilation.
 - **enclaveapp-test-support** — `MockKeyBackend` implementing all three traits with deterministic in-memory operations. XOR-based mock crypto for testing control flow without hardware.
 
+### macOS Keychain Protection Classes — CRITICAL
+
+There are **two separate protection class decisions** in the Apple backend, and
+they **MUST use different values**. Mixing them up causes catastrophic user
+impact (broken biometric caching → Touch ID on every sign → key regeneration
+required, because the SE key's access control is immutable after creation).
+
+| What | Function | Protection class | Why |
+|------|----------|-----------------|-----|
+| **SE key access control** | `makeAccessControl()` in `bridge.swift` | `kSecAttrAccessibleWhenUnlockedThisDeviceOnly` | Required for `touchIDAuthenticationAllowableReuseDuration` biometric caching in CryptoKit. Using `AfterFirstUnlock` silently breaks LAContext reuse — every sign requires Touch ID. |
+| **Keychain wrapping key** | `keychain_store()` in `bridge.swift` | `kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly` | Must survive sleep/wake. `WhenUnlocked` purges the keybag class key on device lock, making the wrapping key inaccessible to background agents. |
+
+**NEVER do a blanket find-and-replace of protection class constants.** These two
+use sites have different requirements for different reasons. If you change one,
+verify you are not changing the other.
+
+**The SE key access control is immutable.** Once a key is generated with a given
+`SecAccessControl`, that access control is baked into the Secure Enclave
+Processor. There is no migration path — the key must be deleted and
+regenerated. This means getting `makeAccessControl()` wrong forces every user
+to regenerate all their keys. This has already happened once (PR #158) and
+must never happen again.
+
 ### Key Patterns
 
 - `EnclaveKeyManager` trait is the base — `EnclaveSigner` and `EnclaveEncryptor` extend it.
diff --git a/crates/enclaveapp-apple/swift/bridge.swift b/crates/enclaveapp-apple/swift/bridge.swift
index 3be65b0..a3e9976 100644
--- a/crates/enclaveapp-apple/swift/bridge.swift
+++ b/crates/enclaveapp-apple/swift/bridge.swift
@@ -276,9 +276,14 @@ func makeAccessControl(_ authPolicy: Int32) -> SecAccessControl? {
         setLastError("unsupported auth_policy value \(authPolicy)")
         return nil
     }
+    // SE key access control uses WhenUnlockedThisDeviceOnly — this is
+    // required for touchIDAuthenticationAllowableReuseDuration biometric
+    // caching to work in CryptoKit. The keychain wrapping key uses
+    // AfterFirstUnlockThisDeviceOnly separately (in keychain_store) so
+    // it survives sleep/wake.
     var error: Unmanaged<CFError>?
     let ac = SecAccessControlCreateWithFlags(
-        nil, kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly, flags, &error
+        nil, kSecAttrAccessibleWhenUnlockedThisDeviceOnly, flags, &error
     )
     if ac == nil {
         let desc = error.map { $0.takeRetainedValue().localizedDescription } ?? "unknown"