diff --git a/AGENTS.md b/AGENTS.md
index 9230927..1f15c38 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -41,6 +41,9 @@ This repository owns the published `elydora` package, its CLI, synchronous and a
 - Propagate unexpected errors to the CLI boundary.
 - Use documented defaults only for genuinely optional configuration.
 - Avoid compatibility shims without a named public or user configuration contract.
+- Interpret `max_retries` as retries after the initial attempt and reject negative or non-integer values.
+- Automatically retry RFC-idempotent methods; retry non-idempotent methods only when the transport proves the request was never sent.
+- Honor valid `Retry-After` delay-seconds and HTTP-date values, and release retryable responses before sleeping.
 
 ## Verification
 
diff --git a/README.md b/README.md
index aa87ae0..9f0414c 100644
--- a/README.md
+++ b/README.md
@@ -129,11 +129,15 @@ client = ElydoraClient(
     private_key="<seed>",       # Base64url-encoded Ed25519 seed
     base_url="https://...",     # API base URL (default: https://api.elydora.com)
     ttl_ms=30000,               # Operation TTL in ms (default: 30000)
-    max_retries=3,              # Max retries on transient failures (default: 3)
+    max_retries=3,              # Retries after the initial attempt (default: 3)
     token="<api-token>",         # Optional API token
 )
 ```
 
+Automatic retries apply to RFC-idempotent methods.
+Connection failures that prove a request was never sent can also retry non-idempotent methods.
+Retryable responses honor both `Retry-After` seconds and HTTP-date values.
+
 ### Authentication
 
 ```python
diff --git a/elydora/_async_http.py b/elydora/_async_http.py
new file mode 100644
index 0000000..12d7ebf
--- /dev/null
+++ b/elydora/_async_http.py
@@ -0,0 +1,89 @@
+"""Asynchronous HTTP execution with the Elydora retry contract."""
+
+import asyncio
+import logging
+from typing import Any, Awaitable, Callable, Dict, Optional
+
+import aiohttp
+
+from ._retry import can_retry, log_retry, retry_delay_seconds, should_retry_response
+
+logger = logging.getLogger(__name__)
+
+
+async def request_with_retries(
+    session: aiohttp.ClientSession,
+    method: str,
+    url: str,
+    *,
+    path: str,
+    max_retries: int,
+    response_handler: Callable[[aiohttp.ClientResponse], Awaitable[Any]],
+    json_body: Any = None,
+    params: Optional[Dict[str, str]] = None,
+    headers: Optional[Dict[str, str]] = None,
+) -> Any:
+    retry_count = 0
+    while True:
+        try:
+            delay: Optional[float] = None
+            reason = ""
+            async with session.request(
+                method,
+                url,
+                json=json_body,
+                params=params,
+                headers=headers,
+                timeout=aiohttp.ClientTimeout(total=30),
+            ) as response:
+                if should_retry_response(
+                    method, response.status, retry_count, max_retries
+                ):
+                    delay = retry_delay_seconds(
+                        retry_count, response.headers.get("Retry-After")
+                    )
+                    reason = f"http_{response.status}"
+                else:
+                    return await response_handler(response)
+            if delay is not None:
+                log_retry(logger, method, path, retry_count, max_retries, delay, reason)
+                await asyncio.sleep(delay)
+                retry_count += 1
+                continue
+        except aiohttp.ClientConnectorError as error:
+            if can_retry(
+                method,
+                retry_count,
+                max_retries,
+                request_known_unprocessed=True,
+            ):
+                delay = retry_delay_seconds(retry_count)
+                log_retry(
+                    logger,
+                    method,
+                    path,
+                    retry_count,
+                    max_retries,
+                    delay,
+                    type(error).__name__,
+                )
+                await asyncio.sleep(delay)
+                retry_count += 1
+                continue
+            raise
+        except (aiohttp.ClientConnectionError, asyncio.TimeoutError) as error:
+            if can_retry(method, retry_count, max_retries):
+                delay = retry_delay_seconds(retry_count)
+                log_retry(
+                    logger,
+                    method,
+                    path,
+                    retry_count,
+                    max_retries,
+                    delay,
+                    type(error).__name__,
+                )
+                await asyncio.sleep(delay)
+                retry_count += 1
+                continue
+            raise
diff --git a/elydora/_retry.py b/elydora/_retry.py
new file mode 100644
index 0000000..6ff81e6
--- /dev/null
+++ b/elydora/_retry.py
@@ -0,0 +1,95 @@
+"""Shared retry policy for the synchronous and asynchronous clients."""
+
+from datetime import datetime, timezone
+from email.utils import parsedate_to_datetime
+from logging import Logger
+from typing import Optional
+
+IDEMPOTENT_METHODS = frozenset({"DELETE", "GET", "HEAD", "OPTIONS", "PUT"})
+RETRYABLE_STATUS_CODES = frozenset({408, 429, 500, 502, 503, 504})
+MAX_BACKOFF_SECONDS = 8
+
+
+def require_max_retries(value: object) -> int:
+    if isinstance(value, bool) or not isinstance(value, int) or value < 0:
+        raise ValueError("max_retries must be a non-negative integer")
+    return value
+
+
+def can_retry(
+    method: str,
+    retry_count: int,
+    max_retries: int,
+    *,
+    request_known_unprocessed: bool = False,
+) -> bool:
+    if retry_count >= max_retries:
+        return False
+    return request_known_unprocessed or method.upper() in IDEMPOTENT_METHODS
+
+
+def should_retry_response(
+    method: str,
+    status_code: int,
+    retry_count: int,
+    max_retries: int,
+) -> bool:
+    return status_code in RETRYABLE_STATUS_CODES and can_retry(
+        method, retry_count, max_retries
+    )
+
+
+def retry_delay_seconds(
+    retry_count: int,
+    retry_after: Optional[str] = None,
+    *,
+    now: Optional[datetime] = None,
+) -> float:
+    parsed_delay = _parse_retry_after(retry_after, now=now)
+    if parsed_delay is not None:
+        return parsed_delay
+    return float(min(2**retry_count, MAX_BACKOFF_SECONDS))
+
+
+def log_retry(
+    logger: Logger,
+    method: str,
+    path: str,
+    retry_count: int,
+    max_retries: int,
+    delay: float,
+    reason: str,
+) -> None:
+    logger.warning(
+        "Retrying Elydora request method=%s path=%s retry=%d/%d "
+        "delay_seconds=%s reason=%s",
+        method.upper(),
+        path,
+        retry_count + 1,
+        max_retries,
+        delay,
+        reason,
+    )
+
+
+def _parse_retry_after(
+    value: Optional[str],
+    *,
+    now: Optional[datetime],
+) -> Optional[float]:
+    if value is None:
+        return None
+
+    normalized = value.strip()
+    if normalized.isascii() and normalized.isdecimal():
+        return float(int(normalized))
+
+    try:
+        retry_at = parsedate_to_datetime(normalized)
+    except (TypeError, ValueError, OverflowError):
+        return None
+    if retry_at.tzinfo is None:
+        retry_at = retry_at.replace(tzinfo=timezone.utc)
+
+    current_time = now or datetime.now(timezone.utc)
+    return max(0.0, (retry_at - current_time).total_seconds())
diff --git a/elydora/_sync_http.py b/elydora/_sync_http.py
new file mode 100644
index 0000000..6ceaf43
--- /dev/null
+++ b/elydora/_sync_http.py
@@ -0,0 +1,96 @@
+"""Synchronous HTTP execution with the Elydora retry contract."""
+
+import logging
+import time
+from typing import Any, Callable, Dict, Optional
+
+import requests
+
+from ._retry import can_retry, log_retry, retry_delay_seconds, should_retry_response
+
+logger = logging.getLogger(__name__)
+
+
+def request_with_retries(
+    session: requests.Session,
+    method: str,
+    url: str,
+    *,
+    path: str,
+    max_retries: int,
+    response_handler: Callable[[requests.Response], Any],
+    json_body: Any = None,
+    params: Optional[Dict[str, str]] = None,
+    headers: Optional[Dict[str, str]] = None,
+) -> Any:
+    retry_count = 0
+    while True:
+        try:
+            response = session.request(
+                method,
+                url,
+                json=json_body,
+                params=params,
+                headers=headers,
+                timeout=30,
+            )
+            if should_retry_response(
+                method, response.status_code, retry_count, max_retries
+            ):
+                delay = retry_delay_seconds(
+                    retry_count, response.headers.get("Retry-After")
+                )
+                response.close()
+                log_retry(
+                    logger,
+                    method,
+                    path,
+                    retry_count,
+                    max_retries,
+                    delay,
+                    f"http_{response.status_code}",
+                )
+                time.sleep(delay)
+                retry_count += 1
+                continue
+            return response_handler(response)
+        except requests.exceptions.ConnectTimeout as error:
+            if can_retry(
+                method,
+                retry_count,
+                max_retries,
+                request_known_unprocessed=True,
+            ):
+                delay = retry_delay_seconds(retry_count)
+                log_retry(
+                    logger,
+                    method,
+                    path,
+                    retry_count,
+                    max_retries,
+                    delay,
+                    type(error).__name__,
+                )
+                time.sleep(delay)
+                retry_count += 1
+                continue
+            raise
+        except (
+            requests.exceptions.ConnectionError,
+            requests.exceptions.Timeout,
+        ) as error:
+            if can_retry(method, retry_count, max_retries):
+                delay = retry_delay_seconds(retry_count)
+                log_retry(
+                    logger,
+                    method,
+                    path,
+                    retry_count,
+                    max_retries,
+                    delay,
+                    type(error).__name__,
+                )
+                time.sleep(delay)
+                retry_count += 1
+                continue
+            raise
diff --git a/elydora/async_client.py b/elydora/async_client.py
index 4f2ecc7..2dd525c 100644
--- a/elydora/async_client.py
+++ b/elydora/async_client.py
@@ -3,12 +3,13 @@
 from __future__ import annotations
 
 import time
-import asyncio
 import warnings
-from typing import Any, Dict, List, Optional, Union
+from typing import Any, Dict, Optional, Union
 
 import aiohttp
 
+from ._async_http import request_with_retries
+from ._retry import require_max_retries
 from .crypto import compute_chain_hash, compute_payload_hash, sign_eor
 from .errors import ElydoraError
 from .integration_types import require_integration_type
@@ -20,7 +21,6 @@ from .types import (
     DeepHealthResponse,
     DeleteAgentResponse,
     EOR,
-    FreezeAgentResponse,
     GetAgentResponse,
     GetEpochResponse,
     GetExportResponse,
@@ -81,7 +81,7 @@ class AsyncElydoraClient:
         self.private_key = private_key
         self.base_url = base_url.rstrip("/")
         self.ttl_ms = ttl_ms
-        self.max_retries = max_retries
+        self.max_retries = require_max_retries(max_retries)
         self.token = token
 
         self._prev_chain_hash = GENESIS_CHAIN_HASH
@@ -129,23 +129,17 @@ class AsyncElydoraClient:
         hdrs = headers or self._headers()
         session = await self._get_session()
 
-        last_exc: Optional[Exception] = None
-        for attempt in range(0, self.max_retries + 1):
-            try:
-                async with session.request(
-                    method, url, json=json_body, params=params, headers=hdrs, timeout=aiohttp.ClientTimeout(total=30)
-                ) as resp:
-                    return await self._handle_response(resp)
-            except ElydoraError:
-                raise
-            except (aiohttp.ClientConnectionError, asyncio.TimeoutError) as exc:
-                last_exc = exc
-                if attempt < self.max_retries:
-                    await asyncio.sleep(min(2 ** attempt, 8))
-                    continue
-                raise
-
-        raise last_exc  # type: ignore[misc]
+        return await request_with_retries(
+            session,
+            method,
+            url,
+            path=path,
+            max_retries=self.max_retries,
+            response_handler=self._handle_response,
+            json_body=json_body,
+            params=params,
+            headers=hdrs,
+        )
 
     @staticmethod
     async def _handle_response(resp: aiohttp.ClientResponse) -> Any:
diff --git a/elydora/client.py b/elydora/client.py
index a6ca713..1eb6b4b 100644
--- a/elydora/client.py
+++ b/elydora/client.py
@@ -4,10 +4,12 @@ from __future__ import annotations
 
 import time
 import warnings
-from typing import Any, Dict, List, Optional, Union
+from typing import Any, Dict, Optional, Union
 
 import requests
 
+from ._retry import require_max_retries
+from ._sync_http import request_with_retries
 from .crypto import compute_chain_hash, compute_payload_hash, sign_eor
 from .errors import ElydoraError
 from .integration_types import require_integration_type
@@ -19,7 +21,6 @@ from .types import (
     DeepHealthResponse,
     DeleteAgentResponse,
     EOR,
-    FreezeAgentResponse,
     GetAgentResponse,
     GetEpochResponse,
     GetExportResponse,
@@ -80,7 +81,7 @@ class ElydoraClient:
         self.private_key = private_key
         self.base_url = base_url.rstrip("/")
         self.ttl_ms = ttl_ms
-        self.max_retries = max_retries
+        self.max_retries = require_max_retries(max_retries)
         self.token = token
 
         self._prev_chain_hash = GENESIS_CHAIN_HASH
@@ -117,33 +118,17 @@ class ElydoraClient:
         url = f"{self.base_url}{path}"
         hdrs = headers or self._headers()
 
-        last_exc: Optional[Exception] = None
-        for attempt in range(0, self.max_retries + 1):
-            try:
-                resp = self._session.request(
-                    method, url, json=json_body, params=params, headers=hdrs, timeout=30
-                )
-                # Retry on 429 or 5xx
-                if resp.status_code == 429 or resp.status_code >= 500:
-                    if attempt < self.max_retries:
-                        retry_after = resp.headers.get("Retry-After")
-                        if retry_after and retry_after.isdigit():
-                            delay = int(retry_after)
-                        else:
-                            delay = min(2 ** attempt, 8)
-                        time.sleep(delay)
-                        continue
-                return self._handle_response(resp)
-            except ElydoraError:
-                raise
-            except (requests.exceptions.ConnectionError, requests.exceptions.Timeout) as exc:
-                last_exc = exc
-                if attempt < self.max_retries:
-                    time.sleep(min(2 ** attempt, 8))
-                    continue
-                raise
-
-        raise last_exc  # type: ignore[misc]
+        return request_with_retries(
+            self._session,
+            method,
+            url,
+            path=path,
+            max_retries=self.max_retries,
+            response_handler=self._handle_response,
+            json_body=json_body,
+            params=params,
+            headers=hdrs,
+        )
 
     @staticmethod
     def _handle_response(resp: requests.Response) -> Any:
diff --git a/tests/test_retry_contract.py b/tests/test_retry_contract.py
new file mode 100644
index 0000000..2a1c35e
--- /dev/null
+++ b/tests/test_retry_contract.py
@@ -0,0 +1,249 @@
+import json
+import logging
+import threading
+from datetime import datetime, timedelta, timezone
+from email.utils import format_datetime
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+from typing import Dict, List, Tuple, cast
+
+import pytest
+
+from elydora import AsyncElydoraClient, ElydoraClient, ElydoraError
+from elydora._retry import (
+    can_retry,
+    retry_delay_seconds,
+    should_retry_response,
+)
+
+ScriptedResponse = Tuple[int, Dict[str, str], object]
+
+
+class ScriptedServer(ThreadingHTTPServer):
+    responses: List[ScriptedResponse]
+    methods: List[str]
+
+
+class ScriptedHandler(BaseHTTPRequestHandler):
+    def do_GET(self) -> None:
+        self._respond()
+
+    def do_POST(self) -> None:
+        content_length = int(self.headers.get("Content-Length", "0"))
+        if content_length:
+            self.rfile.read(content_length)
+        self._respond()
+
+    def _respond(self) -> None:
+        server = cast(ScriptedServer, self.server)
+        server.methods.append(self.command)
+        status, headers, body = server.responses.pop(0)
+        encoded = json.dumps(body).encode()
+        self.send_response(status)
+        for name, value in headers.items():
+            self.send_header(name, value)
+        self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(encoded)))
+        self.end_headers()
+        self.wfile.write(encoded)
+
+    def log_message(self, _format: str, *args: object) -> None:
+        pass
+
+
+@pytest.fixture
+def scripted_server():
+    server = ScriptedServer(("127.0.0.1", 0), ScriptedHandler)
+    server.responses = []
+    server.methods = []
+    thread = threading.Thread(target=server.serve_forever, daemon=True)
+    thread.start()
+    try:
+        yield server
+    finally:
+        server.shutdown()
+        server.server_close()
+        thread.join()
+
+
+def base_url(server: ScriptedServer) -> str:
+    host, port = server.server_address
+    return f"http://{host}:{port}"
+
+
+def error_response(status: int, retry_after: str = "") -> ScriptedResponse:
+    headers = {"Retry-After": retry_after} if retry_after else {}
+    return (
+        status,
+        headers,
+        {"error": {"code": "INTERNAL_ERROR", "message": "temporary failure"}},
+    )
+
+
+def test_retry_policy_matches_http_semantics() -> None:
+    assert should_retry_response("GET", 503, 0, 1)
+    assert should_retry_response("delete", 429, 0, 1)
+    assert not should_retry_response("POST", 503, 0, 1)
+    assert not should_retry_response("GET", 501, 0, 1)
+    assert not should_retry_response("GET", 503, 1, 1)
+    assert can_retry("POST", 0, 1, request_known_unprocessed=True)
+
+
+def test_retry_after_supports_seconds_and_http_dates() -> None:
+    now = datetime(2026, 7, 19, 12, 0, tzinfo=timezone.utc)
+    retry_at = format_datetime(now + timedelta(seconds=30), usegmt=True)
+
+    assert retry_delay_seconds(0, "15", now=now) == 15
+    assert retry_delay_seconds(0, retry_at, now=now) == 30
+    assert retry_delay_seconds(2, "invalid", now=now) == 4
+    assert retry_delay_seconds(4, None, now=now) == 8
+
+
+@pytest.mark.parametrize("value", [-1, True, 1.5, "2"])
+def test_clients_reject_invalid_retry_configuration(value: object) -> None:
+    with pytest.raises(ValueError, match="non-negative integer"):
+        ElydoraClient("org", "agent", "key", max_retries=value)  # type: ignore[arg-type]
+    with pytest.raises(ValueError, match="non-negative integer"):
+        AsyncElydoraClient("org", "agent", "key", max_retries=value)  # type: ignore[arg-type]
+
+
+def test_zero_retries_still_performs_one_sync_request(
+    scripted_server: ScriptedServer,
+) -> None:
+    scripted_server.responses = [(200, {}, {"user_id": "user-1"})]
+    client = ElydoraClient(
+        "org", "agent", "key", base_url=base_url(scripted_server), max_retries=0
+    )
+
+    assert client.get_me()["user_id"] == "user-1"
+    assert scripted_server.methods == ["GET"]
+
+
+@pytest.mark.asyncio
+async def test_zero_retries_still_performs_one_async_request(
+    scripted_server: ScriptedServer,
+) -> None:
+    scripted_server.responses = [(200, {}, {"user_id": "user-1"})]
+    client = AsyncElydoraClient(
+        "org", "agent", "key", base_url=base_url(scripted_server), max_retries=0
+    )
+    try:
+        response = await client.get_me()
+        assert response["user_id"] == "user-1"
+        assert scripted_server.methods == ["GET"]
+    finally:
+        await client.close()
+
+
+def test_sync_client_retries_idempotent_responses(
+    scripted_server: ScriptedServer,
+    monkeypatch: pytest.MonkeyPatch,
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    scripted_server.responses = [
+        error_response(503, "0"),
+        error_response(500),
+        (200, {}, {"user_id": "user-1"}),
+    ]
+    delays: List[float] = []
+    monkeypatch.setattr("elydora._sync_http.time.sleep", delays.append)
+    caplog.set_level(logging.WARNING, logger="elydora._sync_http")
+    client = ElydoraClient(
+        "org", "agent", "key", base_url=base_url(scripted_server), max_retries=2
+    )
+
+    assert client.get_me()["user_id"] == "user-1"
+    assert scripted_server.methods == ["GET", "GET", "GET"]
+    assert delays == [0.0, 2.0]
+    retry_logs = [
+        record.getMessage()
+        for record in caplog.records
+        if record.name == "elydora._sync_http"
+    ]
+    assert "retry=1/2 delay_seconds=0.0 reason=http_503" in retry_logs[0]
+    assert "retry=2/2 delay_seconds=2.0 reason=http_500" in retry_logs[1]
+
+
+@pytest.mark.asyncio
+async def test_async_client_retries_idempotent_responses(
+    scripted_server: ScriptedServer,
+    monkeypatch: pytest.MonkeyPatch,
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    scripted_server.responses = [
+        error_response(429, "0"),
+        error_response(502),
+        (200, {}, {"user_id": "user-1"}),
+    ]
+    delays: List[float] = []
+
+    async def record_sleep(delay: float) -> None:
+        delays.append(delay)
+
+    monkeypatch.setattr("elydora._async_http.asyncio.sleep", record_sleep)
+    caplog.set_level(logging.WARNING, logger="elydora._async_http")
+    client = AsyncElydoraClient(
+        "org", "agent", "key", base_url=base_url(scripted_server), max_retries=2
+    )
+    try:
+        response = await client.get_me()
+        assert response["user_id"] == "user-1"
+        assert scripted_server.methods == ["GET", "GET", "GET"]
+        assert delays == [0.0, 2.0]
+        retry_logs = [
+            record.getMessage()
+            for record in caplog.records
+            if record.name == "elydora._async_http"
+        ]
+        assert "retry=1/2 delay_seconds=0.0 reason=http_429" in retry_logs[0]
+        assert "retry=2/2 delay_seconds=2.0 reason=http_502" in retry_logs[1]
+    finally:
+        await client.close()
+
+
+def test_sync_client_does_not_replay_non_idempotent_requests(
+    scripted_server: ScriptedServer,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    scripted_server.responses = [
+        error_response(503),
+        (200, {}, {"token": "duplicate"}),
+    ]
+    delays: List[float] = []
+    monkeypatch.setattr("elydora._sync_http.time.sleep", delays.append)
+    client = ElydoraClient(
+        "org", "agent", "key", base_url=base_url(scripted_server), max_retries=3
+    )
+
+    with pytest.raises(ElydoraError) as error:
+        client.issue_token()
+    assert error.value.status_code == 503
+    assert scripted_server.methods == ["POST"]
+    assert delays == []
+
+
+@pytest.mark.asyncio
+async def test_async_client_does_not_replay_non_idempotent_requests(
+    scripted_server: ScriptedServer,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    scripted_server.responses = [
+        error_response(503),
+        (200, {}, {"token": "duplicate"}),
+    ]
+    delays: List[float] = []
+
+    async def record_sleep(delay: float) -> None:
+        delays.append(delay)
+
+    monkeypatch.setattr("elydora._async_http.asyncio.sleep", record_sleep)
+    client = AsyncElydoraClient(
+        "org", "agent", "key", base_url=base_url(scripted_server), max_retries=3
+    )
+    try:
+        with pytest.raises(ElydoraError) as error:
+            await client.issue_token()
+        assert error.value.status_code == 503
+        assert scripted_server.methods == ["POST"]
+        assert delays == []
+    finally:
+        await client.close()