diff --git a/eio/handler.ml b/eio/handler.ml
index 649981b..321fbcc 100644
--- a/eio/handler.ml
+++ b/eio/handler.ml
@@ -56,7 +56,7 @@ type t = {
   prompts: registered_prompt list;
   completion_handler: completion_handler option;
   task_handlers: task_handlers option;
-  mutable subscribed_uris: StringSet.t;
+  subscribed_uris: StringSet.t Atomic.t;
 }
 
 let create ~name ~version ?instructions () =
@@ -64,7 +64,7 @@ let create ~name ~version ?instructions () =
     tools = []; resources = []; prompts = [];
     completion_handler = None;
     task_handlers = None;
-    subscribed_uris = StringSet.empty }
+    subscribed_uris = Atomic.make StringSet.empty }
 
 (** H3 fix: Check for duplicate tool names before adding.
     Previously, registering two tools with the same name would silently
@@ -129,7 +129,7 @@ let instructions s = s.instructions
 let tools s = List.map (fun rt -> rt.tool) s.tools
 let resources s = List.map (fun rr -> rr.resource) s.resources
 let prompts s = List.map (fun rp -> rp.prompt) s.prompts
-let subscribed_uris s = StringSet.elements s.subscribed_uris
+let subscribed_uris s = StringSet.elements (Atomic.get s.subscribed_uris)
 
 (* ── capabilities ─────────────────────────────────────── *)
 
@@ -341,12 +341,23 @@ let handle_prompts_get s ctx id params =
 
 (* ── resources/subscribe + unsubscribe ────────────────── *)
 
+(** Atomically update subscribed_uris via compare-and-set loop.
+    Lock-free: StringSet is immutable, so the CAS swaps a pointer. *)
+let atomic_update_uris s f =
+  let rec loop () =
+    let old = Atomic.get s.subscribed_uris in
+    let updated = f old in
+    if Atomic.compare_and_set s.subscribed_uris old updated then ()
+    else loop ()
+  in
+  loop ()
+
 let handle_resources_subscribe s id params =
   match params with
   | Some (`Assoc fields) ->
     begin match List.assoc_opt "uri" fields with
     | Some (`String uri) ->
-      s.subscribed_uris <- StringSet.add uri s.subscribed_uris;
+      atomic_update_uris s (StringSet.add uri);
       Jsonrpc.make_response ~id ~result:(`Assoc [])
     | _ ->
       Jsonrpc.make_error ~id ~code:Error_codes.invalid_params
@@ -361,7 +372,7 @@ let handle_resources_unsubscribe s id params =
   | Some (`Assoc fields) ->
     begin match List.assoc_opt "uri" fields with
     | Some (`String uri) ->
-      s.subscribed_uris <- StringSet.remove uri s.subscribed_uris;
+      atomic_update_uris s (StringSet.remove uri);
       Jsonrpc.make_response ~id ~result:(`Assoc [])
     | _ ->
       Jsonrpc.make_error ~id ~code:Error_codes.invalid_params
diff --git a/test/test_bug_fixes.ml b/test/test_bug_fixes.ml
index a14ff6a..afc732a 100644
--- a/test/test_bug_fixes.ml
+++ b/test/test_bug_fixes.ml
@@ -236,6 +236,152 @@ let test_l2_validate_state_length_mismatch () =
   Alcotest.(check bool) "different length"
     false (Oauth_client.validate_state ~expected:"short" ~received:"longer_string")
 
+(* ── C4: ensure_rng consolidation ──────────────── *)
+
+let test_c4_ensure_rng_works_via_tls_helpers () =
+  (* C4 fix consolidated ensure_rng to Tls_helpers.ensure_rng only.
+     Oauth_client.generate_state calls ensure_rng_initialized which is
+     Tls_helpers.ensure_rng. If we can generate a state, the consolidated
+     initialization path works correctly. *)
+  let state = Oauth_client.generate_state () in
+  Alcotest.(check bool) "ensure_rng works via Tls_helpers"
+    true (String.length state > 0)
+
+(* ── H1: session state transitions ───────────── *)
+
+let test_h1_initialize_twice_fails () =
+  let session = Http_session.create () in
+  (match Http_session.state session with
+   | Http_session.Uninitialized -> ()
+   | _ -> Alcotest.fail "should start Uninitialized");
+  (match Http_session.initialize session with
+   | Ok _sid -> ()
+   | Error e -> Alcotest.fail e);
+  (match Http_session.state session with
+   | Http_session.Initializing -> ()
+   | _ -> Alcotest.fail "should be Initializing after initialize");
+  (* H1: second initialize must fail — concurrent call protection *)
+  (match Http_session.initialize session with
+   | Error _ -> ()
+   | Ok _ -> Alcotest.fail "H1 regression: double initialize should be rejected")
+
+let test_h1_ready_requires_initializing () =
+  let session = Http_session.create () in
+  (* ready before initialize should fail *)
+  (match Http_session.ready session with
+   | Error _ -> ()
+   | Ok () -> Alcotest.fail "H1: ready on Uninitialized should fail");
+  ignore (Http_session.initialize session);
+  (match Http_session.ready session with
+   | Ok () -> ()
+   | Error e -> Alcotest.fail e);
+  (match Http_session.state session with
+   | Http_session.Ready -> ()
+   | _ -> Alcotest.fail "should be Ready after ready()")
+
+let test_h1_close_from_any_state () =
+  let session = Http_session.create () in
+  Http_session.close session;
+  (match Http_session.state session with
+   | Http_session.Closed -> ()
+   | _ -> Alcotest.fail "should be Closed after close()")
+
+(* ── H2: version negotiation with unknown version ── *)
+
+let test_h2_unsupported_version_returns_latest () =
+  (* When a client requests a version the server doesn't know,
+     the server should fall back to latest (with a warning log). *)
+  let h = Mcp_protocol_eio.Handler.create ~name:"test" ~version:"1.0" () in
+  let params = `Assoc [
+    ("protocolVersion", `String "9999-12-31");
+    ("capabilities", `Assoc []);
+    ("clientInfo", `Assoc [("name", `String "client"); ("version", `String "1")]);
+  ] in
+  let req = Jsonrpc.make_request ~id:(Jsonrpc.Int 1) ~method_:"initialize" ~params () in
+  let log_ref = ref Logging.Warning in
+  let dummy_ctx : Mcp_protocol_eio.Handler.context = {
+    send_notification = (fun ~method_:_ ~params:_ -> Ok ());
+    send_log = (fun _ _ -> Ok ());
+    send_progress = (fun ~token:_ ~progress:_ ~total:_ -> Ok ());
+    request_sampling = (fun _ -> Error "n/a");
+    request_roots_list = (fun () -> Error "n/a");
+    request_elicitation = (fun _ -> Error "n/a");
+  } in
+  match Mcp_protocol_eio.Handler.dispatch h dummy_ctx log_ref req with
+  | Some (Response r) ->
+    (match r.result with
+     | `Assoc fields ->
+       (match List.assoc_opt "protocolVersion" fields with
+        | Some (`String v) ->
+          Alcotest.(check string) "falls back to latest" Version.latest v
+        | _ -> Alcotest.fail "missing protocolVersion in response")
+     | _ -> Alcotest.fail "expected assoc result")
+  | _ -> Alcotest.fail "H2: unsupported version should still produce a Response"
+
+let test_h2_supported_version_negotiated () =
+  (* When a client requests a supported version, negotiate returns it. *)
+  let h = Mcp_protocol_eio.Handler.create ~name:"test" ~version:"1.0" () in
+  let params = `Assoc [
+    ("protocolVersion", `String "2024-11-05");
+    ("capabilities", `Assoc []);
+    ("clientInfo", `Assoc [("name", `String "client"); ("version", `String "1")]);
+  ] in
+  let req = Jsonrpc.make_request ~id:(Jsonrpc.Int 1) ~method_:"initialize" ~params () in
+  let log_ref = ref Logging.Warning in
+  let dummy_ctx : Mcp_protocol_eio.Handler.context = {
+    send_notification = (fun ~method_:_ ~params:_ -> Ok ());
+    send_log = (fun _ _ -> Ok ());
+    send_progress = (fun ~token:_ ~progress:_ ~total:_ -> Ok ());
+    request_sampling = (fun _ -> Error "n/a");
+    request_roots_list = (fun () -> Error "n/a");
+    request_elicitation = (fun _ -> Error "n/a");
+  } in
+  match Mcp_protocol_eio.Handler.dispatch h dummy_ctx log_ref req with
+  | Some (Response r) ->
+    (match r.result with
+     | `Assoc fields ->
+       (match List.assoc_opt "protocolVersion" fields with
+        | Some (`String v) ->
+          Alcotest.(check string) "negotiated version" "2024-11-05" v
+        | _ -> Alcotest.fail "missing protocolVersion")
+     | _ -> Alcotest.fail "expected assoc result")
+  | _ -> Alcotest.fail "expected Response"
+
+(* ── L1: max response size constant ──────────── *)
+
+let test_l1_default_max_response_size () =
+  Alcotest.(check int) "default_max_response_size is 1MB"
+    (1024 * 1024) Oauth_client.default_max_response_size
+
+(* ── M1: pre-init session validation ──────────── *)
+
+let test_m1_pre_init_accepts_any_header () =
+  let session = Http_session.create () in
+  (* Before initialize, validate should accept any header value *)
+  (match Http_session.validate session None with
+   | Ok () -> ()
+   | Error _ -> Alcotest.fail "M1: pre-init should accept no header");
+  (match Http_session.validate session (Some "anything") with
+   | Ok () -> ()
+   | Error _ -> Alcotest.fail "M1: pre-init should accept any header value")
+
+let test_m1_post_init_requires_matching_header () =
+  let session = Http_session.create () in
+  let sid = match Http_session.initialize session with
+    | Ok sid -> sid
+    | Error e -> Alcotest.fail e
+  in
+  (* After initialize, must match session id *)
+  (match Http_session.validate session (Some sid) with
+   | Ok () -> ()
+   | Error _ -> Alcotest.fail "matching header should pass");
+  (match Http_session.validate session (Some "wrong-id") with
+   | Error `Not_found -> ()
+   | _ -> Alcotest.fail "wrong header should return Not_found");
+  (match Http_session.validate session None with
+   | Error (`Bad_request _) -> ()
+   | _ -> Alcotest.fail "missing header should return Bad_request")
+
 (* ── L3: scope checking with StringSet ───────── *)
 
 let test_l3_scope_check_subset () =
@@ -294,6 +440,25 @@ let () =
       Alcotest.test_case "validate mismatch" `Quick test_l2_validate_state_mismatch;
       Alcotest.test_case "validate length mismatch" `Quick test_l2_validate_state_length_mismatch;
     ];
+    "C4_ensure_rng", [
+      Alcotest.test_case "consolidated via Tls_helpers" `Quick test_c4_ensure_rng_works_via_tls_helpers;
+    ];
+    "H1_session_state", [
+      Alcotest.test_case "double initialize rejected" `Quick test_h1_initialize_twice_fails;
+      Alcotest.test_case "ready requires Initializing" `Quick test_h1_ready_requires_initializing;
+      Alcotest.test_case "close from any state" `Quick test_h1_close_from_any_state;
+    ];
+    "H2_version_negotiation", [
+      Alcotest.test_case "unsupported version -> latest" `Quick test_h2_unsupported_version_returns_latest;
+      Alcotest.test_case "supported version negotiated" `Quick test_h2_supported_version_negotiated;
+    ];
+    "L1_max_response_size", [
+      Alcotest.test_case "default is 1MB" `Quick test_l1_default_max_response_size;
+    ];
+    "M1_pre_init_validation", [
+      Alcotest.test_case "pre-init accepts any" `Quick test_m1_pre_init_accepts_any_header;
+      Alcotest.test_case "post-init requires match" `Quick test_m1_post_init_requires_matching_header;
+    ];
     "L3_scope_check", [
       Alcotest.test_case "subset passes" `Quick test_l3_scope_check_subset;
       Alcotest.test_case "missing fails" `Quick test_l3_scope_check_missing;