diff --git a/src/provider/opencode/client.lisp b/src/provider/opencode/client.lisp
index 0d00ad48..0aae48a9 100644
--- a/src/provider/opencode/client.lisp
+++ b/src/provider/opencode/client.lisp
@@ -8,7 +8,9 @@
 ;;; Autolith's private store with an optional OPENCODE_API_KEY environment
 ;;; bootstrap. Tool names ride in the Chat Completions Base64 encoding
 ;;; inherited from OPENAI-COMPATIBLE-PROVIDER, so conversations persist in
-;;; the same namespaced shape regardless of the serving provider.
+;;; the same namespaced shape regardless of the serving provider. Every
+;;; request also carries the provider's stable session identifier in the
+;;; x-opencode-session header so the backend can pin caching.
 
 (defparameter *opencode-model-prefix* "opencode/"
   "The namespace distinguishing OpenCode models from other provider models.")
@@ -56,14 +58,19 @@
 
 (-> opencode-provider-create (configuration) opencode-chat-completions-provider)
 (defun opencode-provider-create (configuration)
-  "Create the OpenCode API key provider for CONFIGURATION."
-  (make-instance 'opencode-chat-completions-provider
-                 :configuration configuration
-                 :credential-manager (opencode-credential-manager-create
-                                      configuration)
-                 :session-id (make-identifier)
-                 :display-name "OpenCode"
-                 :family ':opencode))
+  "Create the OpenCode API key provider for CONFIGURATION.
+
+The provider's stable session identifier also rides as the x-opencode-session
+request header so the backend can pin caching to one session."
+  (let ((session-id (make-identifier)))
+    (make-instance 'opencode-chat-completions-provider
+                   :configuration configuration
+                   :credential-manager (opencode-credential-manager-create
+                                        configuration)
+                   :session-id session-id
+                   :headers (list (cons "x-opencode-session" session-id))
+                   :display-name "OpenCode"
+                   :family ':opencode)))
 
 (defmethod provider-request-object :around
     ((provider opencode-chat-completions-provider)
diff --git a/tests/opencode-provider-tests.lisp b/tests/opencode-provider-tests.lisp
index 2a87e33c..504708ae 100644
--- a/tests/opencode-provider-tests.lisp
+++ b/tests/opencode-provider-tests.lisp
@@ -334,6 +334,76 @@
       (uiop:delete-directory-tree root :validate t :if-does-not-exist ':ignore)))
   nil)
 
+(-> opencode-provider-test--session-header () null)
+(defun opencode-provider-test--session-header ()
+  "Test that OpenCode transport sends its stable session caching header."
+  (let* ((registry-snapshot (provider--registry-snapshot))
+         (configuration (opencode-provider-test--configuration))
+         (root (test-configuration-root configuration))
+         (conversation
+           (conversation-create configuration :identifier "opencode-session"))
+         (provider (opencode-provider-create configuration))
+         (credentials
+           (make-instance 'oauth-credentials
+                          :access-token "synthetic-opencode-key"
+                          :account-id "opencode"
+                          :source-path #P"/tmp/opencode-test-key"))
+         (captured-headers nil))
+    (unwind-protect
+         (progn
+           (register-provider
+            "opencode-session-test"
+            :family ':opencode
+            :models '("opencode/session-test")
+            :endpoint *opencode-chat-completions-endpoint*
+            :factory
+            (lambda (selected &key reasoning-summaries-p)
+              (declare (ignore reasoning-summaries-p))
+              (opencode-provider-create selected))
+            :source ':runtime)
+           (test-call-with-function-replacements
+            (list
+             (list 'dexador:post
+                   (lambda (url &key headers &allow-other-keys)
+                     (declare (ignore url))
+                     (setf captured-headers headers)
+                     (values (make-string-input-stream "") 200 nil nil))))
+            (lambda ()
+              (provider-open-response-stream
+               provider
+               (json-object "model" "gpt-5.6-luna")
+               :credentials credentials
+               :conversation conversation)))
+           (test-assert
+            (string= (rest (assoc "x-opencode-session" captured-headers
+                                  :test #'string-equal))
+                     (provider-session-id provider))
+            "OpenCode transport sends the stable session caching header")
+           (test-assert
+            (string= (rest (assoc "x-opencode-session"
+                                  (openai-compatible-provider-headers provider)
+                                  :test #'string-equal))
+                     (provider-session-id provider))
+            "the OpenCode session header carries the provider session identity")
+           (let ((reconfigured
+                   (provider-with-configuration
+                    provider
+                    (configuration-with-model
+                     configuration "opencode/session-test"))))
+             (test-assert
+              (equal (openai-compatible-provider-headers reconfigured)
+                     (list (cons "x-opencode-session"
+                                 (provider-session-id provider))))
+              "OpenCode reconfiguration preserves the session caching header")
+             (test-assert
+              (string= (provider-session-id reconfigured)
+                       (provider-session-id provider))
+              "OpenCode reconfiguration preserves the session identity")))
+      (provider--registry-restore registry-snapshot)
+      (uiop:delete-directory-tree root :validate t :if-does-not-exist ':ignore)))
+  nil)
+
+
 (-> opencode-provider-test--discovery () null)
 (defun opencode-provider-test--discovery ()
   "Test OpenCode discovery namespacing, collisions, caching, and failures."
@@ -615,6 +685,7 @@
   (opencode-provider-test--login)
   (opencode-provider-test--authentication-bootstrap)
   (opencode-provider-test--request-model)
+  (opencode-provider-test--session-header)
   (opencode-provider-test--discovery)
   (opencode-provider-test--legacy-registry-snapshot)
   (opencode-provider-test--builtin-registration)