diff --git a/doc/api-reference.md b/doc/api-reference.md
index 363c865..1069082 100644
--- a/doc/api-reference.md
+++ b/doc/api-reference.md
@@ -75,10 +75,30 @@ A dictionary containing the chat completion response with the following structur
         "prompt_tokens": int,
         "completion_tokens": int,
         "total_tokens": int
+    },
+    "_metadata": {
+        "payload_id": str,
+        "processed_at": int,          # Unix timestamp
+        "is_encrypted": bool,
+        "response_status": str,
+        "security_tier": str,         # "standard", "high", or "maximum"
+        "memory_protection": dict,    # server-side memory protection info
+        "cuda_device": dict,          # privacy-safe GPU info (hashed identifiers)
+        "tpm_attestation": {          # TPM 2.0 hardware attestation (see Security Guide)
+            "is_available": bool,
+            # Present only when is_available is True:
+            "pcr_banks": str,         # e.g. "sha256:0,7,10"
+            "pcr_values": dict,       # {bank: {pcr_index: hex_digest}}
+            "quote_b64": str,         # base64-encoded TPMS_ATTEST (signed by AIK)
+            "signature_b64": str,     # base64-encoded TPMT_SIGNATURE
+            "aik_pubkey_b64": str,    # base64-encoded TPM2B_PUBLIC (ephemeral AIK)
+        }
     }
 }
 ```
 
+The `_metadata` field is added by the client library and is not part of the OpenAI API response format. See the [Security Guide](security-guide.md) for how to interpret and verify `tpm_attestation`.
+
 #### acreate(model, messages, **kwargs)
 
 Async alias for create() method.
diff --git a/doc/security-guide.md b/doc/security-guide.md
index 6c34f71..6e4abdc 100644
--- a/doc/security-guide.md
+++ b/doc/security-guide.md
@@ -162,6 +162,81 @@ Secure memory features:
 - Guarantees zeroing of sensitive memory
 - Prevents memory dumps from containing sensitive data
 
+## Hardware Attestation (TPM 2.0)
+
+### What it is
+
+When the server has a TPM 2.0 chip, every response includes a `tpm_attestation` block in `_metadata`. This is a cryptographically signed hardware quote proving:
+
+- Which firmware and Secure Boot state the server is running (PCR 0, 7)
+- Which application binary is running, when IMA is active (PCR 10)
+
+The quote is signed by an ephemeral AIK (Attestation Identity Key) generated fresh for each request and tied to the `payload_id` nonce, so it cannot be replayed for a different request.
+
+### Reading the attestation
+
+```python
+response = await client.create(
+    model="Qwen/Qwen3-0.6B",
+    messages=[{"role": "user", "content": "..."}],
+    security_tier="maximum"
+)
+
+tpm = response["_metadata"].get("tpm_attestation", {})
+
+if tpm.get("is_available"):
+    print("PCR banks:", tpm["pcr_banks"])         # e.g. "sha256:0,7,10"
+    print("PCR values:", tpm["pcr_values"])        # {bank: {index: hex}}
+    print("AIK key:", tpm["aik_pubkey_b64"][:32], "...")
+else:
+    print("TPM not available on this server")
+```
+
+### Verifying the quote
+
+The response is self-contained: `aik_pubkey_b64` is the full public key of the AIK that signed the quote, so no separate key-fetch round-trip is needed.
+
+Verification steps using `tpm2-pytss`:
+
+```python
+import base64
+from tpm2_pytss.types import TPM2B_PUBLIC, TPMT_SIGNATURE, TPM2B_ATTEST
+
+# 1. Decode the quote components
+aik_pub = TPM2B_PUBLIC.unmarshal(base64.b64decode(tpm["aik_pubkey_b64"]))[0]
+quote   = TPM2B_ATTEST.unmarshal(base64.b64decode(tpm["quote_b64"]))[0]
+sig     = TPMT_SIGNATURE.unmarshal(base64.b64decode(tpm["signature_b64"]))[0]
+
+# 2. Verify the signature over the quote using the AIK public key
+#    (use a TPM ESAPI verify_signature call or an offline RSA verify)
+
+# 3. Inspect the qualifying_data inside the quote — it must match
+#    SHA-256(payload_id.encode())[:16] to confirm this quote is for this request
+
+# 4. Check pcr_values against your known-good baseline
+```
+
+> Full verification requires `tpm2-pytss` on the client side (`pip install tpm2-pytss` + `sudo apt install libtss2-dev`). It is optional — the attestation is informational unless your deployment policy requires verification.
+
+### Behaviour per security tier
+
+| Tier | TPM unavailable |
+|------|----------------|
+| `standard` | `tpm_attestation: {"is_available": false}` — request proceeds |
+| `high` | same as standard |
+| `maximum` | `ServiceUnavailableError` (HTTP 503) — request rejected |
+
+For `maximum` tier, the server enforces TPM availability as a hard requirement. If your server has no TPM and you request `maximum`, catch the error explicitly:
+
+```python
+from nomyo import ServiceUnavailableError
+
+try:
+    response = await client.create(..., security_tier="maximum")
+except ServiceUnavailableError as e:
+    print("Server does not meet TPM requirements for maximum tier:", e)
+```
+
 ## Compliance Considerations
 
 ### HIPAA Compliance
@@ -207,9 +282,11 @@ response = await client.create(
     messages=[{"role": "user", "content": "Hello"}]
 )
 
-print(response["_metadata"])  # Contains security-related information
+print(response["_metadata"])  # Contains security_tier, memory_protection, tpm_attestation, etc.
 ```
 
+See [Hardware Attestation](#hardware-attestation-tpm-20) for details on the `tpm_attestation` field.
+
 ### Logging
 
 Enable logging to see security operations:
diff --git a/nomyo/SecureCompletionClient.py b/nomyo/SecureCompletionClient.py
index 4c1d96d..6aa5379 100644
--- a/nomyo/SecureCompletionClient.py
+++ b/nomyo/SecureCompletionClient.py
@@ -1,5 +1,5 @@
 import asyncio, ctypes, json, base64, urllib.parse, httpx, os, secrets, sys, warnings, logging
-from typing import Dict, Any, Optional
+from typing import Dict, Any, Optional, Union
 from cryptography.hazmat.primitives import serialization, hashes
 from cryptography.hazmat.primitives.asymmetric import rsa, padding
 from cryptography.hazmat.backends import default_backend