Document per-run session and reasoning overrides

docs/formats.md

@@ -91,7 +91,8 @@ of a named input. Missing variables and input references are errors.
 
 The optional `session_id` uses the same template data and input helper. Its
 rendered value is trimmed, omitted when empty, and limited to 256 Unicode code
-points.
+points. A nonblank direct request session ID bypasses this template completely;
+a blank direct value leaves the template behavior unchanged.
 
 ### Cache Control
 
@@ -203,9 +204,12 @@ explicit zero is preserved. In particular, an explicit request
 the caller context and transport timeout intact.
 
 Non-empty profile strings replace backend defaults, and non-empty request
-strings replace both. Backend identity is retained when either layer overrides
-the endpoint. A non-empty `extra_params` map at each layer replaces the entire
-lower-precedence map rather than merging keys.
+strings replace both. Request reasoning is the exception: a nil
+`ReasoningEffort` pointer inherits the profile, a pointer to a nonblank string
+trims and replaces it, and a pointer to a blank string clears it. Backend
+identity is retained when either layer overrides the endpoint. A non-empty
+`extra_params` map at each layer replaces the entire lower-precedence map
+rather than merging keys.
 The [outbound integration contract](integrations/openai-compatible-chat.md)
 defines how the effective settings are serialized.