Model affinity for consistent model selection in agentic loops (#827)

crates/common/src/configuration.rs

@@ -7,6 +7,14 @@ use crate::api::open_ai::{
     ChatCompletionTool, FunctionDefinition, FunctionParameter, FunctionParameters, ParameterType,
 };
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Routing {
+    pub llm_provider: Option<String>,
+    pub model: Option<String>,
+    pub session_ttl_seconds: Option<u64>,
+    pub session_max_entries: Option<usize>,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ModelAlias {
     pub target: String,
@@ -182,6 +190,7 @@ pub struct Configuration {
     pub model_providers: Vec<LlmProvider>,
     pub model_aliases: Option<HashMap<String, ModelAlias>>,
     pub overrides: Option<Overrides>,
+    pub routing: Option<Routing>,
     pub system_prompt: Option<String>,
     pub prompt_guards: Option<PromptGuards>,
     pub prompt_targets: Option<Vec<PromptTarget>>,

crates/common/src/consts.rs

@@ -22,6 +22,7 @@ pub const X_ARCH_TOOL_CALL: &str = "x-arch-tool-call-message";
 pub const X_ARCH_FC_MODEL_RESPONSE: &str = "x-arch-fc-model-response";
 pub const ARCH_FC_MODEL_NAME: &str = "Arch-Function";
 pub const REQUEST_ID_HEADER: &str = "x-request-id";
+pub const MODEL_AFFINITY_HEADER: &str = "x-model-affinity";
 pub const ENVOY_ORIGINAL_PATH_HEADER: &str = "x-envoy-original-path";
 pub const TRACE_PARENT_HEADER: &str = "traceparent";
 pub const ARCH_INTERNAL_CLUSTER_NAME: &str = "arch_internal";