fix(agents): improve middleware type inference with const type parameters (#9238)

Co-authored-by: Claude <[email protected]>

Author: pokey

examples/src/createAgent/dynamicTools/advanced.ts

@@ -80,7 +80,7 @@ const selectToolsMiddleware = createMiddleware({
 const semanticAgent = createAgent({
   model: "openai:gpt-4o",
   tools: fullCatalog, // superset for validation and typing
-  middleware: [selectToolsMiddleware] as const,
+  middleware: [selectToolsMiddleware],
 });
 
 const result = await semanticAgent.invoke({

examples/src/createAgent/middleware/hitl.ts

@@ -72,7 +72,7 @@ const agent = createAgent({
   systemPrompt:
     "You are a helpful assistant. Use the tools provided to help the user.",
   tools: [calculateTool, writeFileTool],
-  middleware: [hitlMiddleware] as const,
+  middleware: [hitlMiddleware],
 });
 const config = {
   configurable: {

examples/src/createAgent/middleware/promptCaching.ts

@@ -5,7 +5,7 @@ import { anthropicPromptCachingMiddleware } from "langchain";
 const agent = createAgent({
   model: "anthropic:claude-sonnet-4-20250514",
   tools: [],
-  middleware: [anthropicPromptCachingMiddleware({ ttl: "5m" })] as const,
+  middleware: [anthropicPromptCachingMiddleware({ ttl: "5m" })],
 });
 
 // Usage example with a long chat history for testing caching

examples/src/createAgent/middleware/simpleHitl.ts

@@ -48,7 +48,7 @@ const agent = createAgent({
   model: "openai:gpt-4o-mini",
   tools: [],
   checkpointer,
-  middleware: [humanInTheLoopMiddleware] as const,
+  middleware: [humanInTheLoopMiddleware],
 });
 
 console.log("🚀 Human in the Loop Example - Missing Information Flow");

examples/src/createAgent/middleware/simpleSummarization.ts

@@ -56,7 +56,7 @@ const agent = createAgent({
     userId: z.string(),
   }),
   tools: [],
-  middleware: [summarizationMiddleware] as const,
+  middleware: [summarizationMiddleware],
 });
 
 const config = {

examples/src/createAgent/middleware/summarization.ts

@@ -33,7 +33,7 @@ const summaryMiddleware = summarizationMiddleware({
 const agent = createAgent({
   model: "openai:gpt-4o-mini",
   tools: [],
-  middleware: [summaryMiddleware] as const,
+  middleware: [summaryMiddleware],
 });
 
 console.log("🚀 Summarization Middleware Example");

libs/langchain/src/agents/index.ts

@@ -146,7 +146,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     T,
@@ -169,7 +169,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     ExtractZodArrayTypes<T> extends Record<string, any>
@@ -200,7 +200,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     Record<string, unknown>,
@@ -222,7 +222,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     Record<string, unknown>,
@@ -244,7 +244,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     Record<string, unknown>,
@@ -267,7 +267,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     T,
@@ -290,7 +290,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<T, StateSchema, ContextSchema, ToolStrategy<T>> & {
     responseFormat: ToolStrategy<T>;
@@ -308,7 +308,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     T,
@@ -330,7 +330,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: Omit<
     CreateAgentParams<
@@ -352,7 +352,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: Omit<
     CreateAgentParams<
@@ -378,7 +378,7 @@ export function createAgent<
   ContextSchema extends
     | AnyAnnotationRoot
     | InteropZodObject = AnyAnnotationRoot,
-  TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
+  const TMiddleware extends readonly AgentMiddleware[] = readonly AgentMiddleware[]
 >(
   params: CreateAgentParams<
     StructuredResponseFormat,

libs/langchain/src/agents/middleware/tests/callLimit.test.ts

@@ -76,7 +76,7 @@ describe("ModelCallLimitMiddleware", () => {
         const agent = createAgent({
           model,
           tools,
-          middleware: [middleware] as const,
+          middleware: [middleware],
         });
 
         const result = await agent.invoke({ messages: ["Hello, world!"] });
@@ -130,7 +130,7 @@ describe("ModelCallLimitMiddleware", () => {
           model,
           tools,
           checkpointer,
-          middleware: [middleware] as const,
+          middleware: [middleware],
         });
         await expect(
           agent.invoke({ messages: ["Hello, world!"] }, config)
@@ -139,7 +139,7 @@ describe("ModelCallLimitMiddleware", () => {
         const agent2 = createAgent({
           model,
           tools,
-          middleware: [middleware] as const,
+          middleware: [middleware],
           checkpointer,
         });
         if (exitBehavior === "throw") {
@@ -165,7 +165,7 @@ describe("ModelCallLimitMiddleware", () => {
           model,
           tools,
           checkpointer,
-          middleware: [middleware] as const,
+          middleware: [middleware],
         });
         if (exitBehavior === "throw") {
           await expect(

libs/langchain/src/agents/middleware/tests/contextEditing.test.ts

@@ -65,7 +65,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       // Create a conversation that doesn't exceed default 100K token threshold
@@ -113,7 +113,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -156,7 +156,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -189,7 +189,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -221,7 +221,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages: BaseMessage[] = [
@@ -289,7 +289,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -342,7 +342,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -376,7 +376,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -442,7 +442,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = createToolCallConversation();
@@ -496,7 +496,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = [new HumanMessage("Hello"), new AIMessage("Hi there!")];
@@ -521,7 +521,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages = [
@@ -549,7 +549,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       const messages: BaseMessage[] = [
@@ -585,7 +585,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       // Create a message that's already marked as cleared
@@ -638,7 +638,7 @@ describe("contextEditingMiddleware", () => {
 
       const agent = createAgent({
         model,
-        middleware: [middleware] as const,
+        middleware: [middleware],
       });
 
       // Tool message without a corresponding AI message (malformed conversation)

libs/langchain/src/agents/middleware/tests/dynamicSystemPrompt.test.ts

@@ -41,7 +41,7 @@ describe("dynamicSystemPrompt", () => {
 
     const agent = createAgent({
       model,
-      middleware: [middleware] as const,
+      middleware: [middleware],
       contextSchema,
     });
 
@@ -81,7 +81,7 @@ describe("dynamicSystemPrompt", () => {
 
     const agent = createAgent({
       model,
-      middleware: [middleware] as const,
+      middleware: [middleware],
       contextSchema,
     });