fix(summary-middleware): use summaryPrefix or fall back to default prefix (#9630)

Co-authored-by: Thomas Pischulski <thomas@blackhorne.de>
Co-authored-by: Christian Bromann <git@bromann.dev>

Author: nephix

.changeset/two-ducks-double.md

@@ -0,0 +1,5 @@
+---
+"langchain": patch
+---
+
+fix(summary-middleware): use summaryPrefix or fall back to default prefix

libs/langchain/src/agents/middleware/summarization.ts

@@ -53,6 +53,7 @@ Messages to summarize:
 {messages}
 </messages>`;
 
+const DEFAULT_SUMMARY_PREFIX = "Here is a summary of the conversation to date:";
 const DEFAULT_MESSAGES_TO_KEEP = 20;
 const DEFAULT_TRIM_TOKEN_LIMIT = 4000;
 const DEFAULT_FALLBACK_MESSAGE_COUNT = 15;
@@ -385,6 +386,10 @@ export function summarizationMiddleware(
           : runtime.context?.summaryPrompt ??
             userOptions.summaryPrompt ??
             DEFAULT_SUMMARY_PROMPT;
+      const summaryPrefix =
+        runtime.context.summaryPrefix ??
+        userOptions.summaryPrefix ??
+        DEFAULT_SUMMARY_PREFIX;
       const trimTokensToSummarize =
         runtime.context?.trimTokensToSummarize !== undefined
           ? runtime.context.trimTokensToSummarize
@@ -440,7 +445,7 @@ export function summarizationMiddleware(
       );
 
       const summaryMessage = new HumanMessage({
-        content: `Here is a summary of the conversation to date:\n\n${summary}`,
+        content: `${summaryPrefix}\n\n${summary}`,
         id: uuid(),
       });
 

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

@@ -830,4 +830,78 @@ describe("summarizationMiddleware", () => {
       !(AIMessage.isInstance(firstPreserved) && hasToolCalls(firstPreserved))
     ).toBe(true);
   });
+
+  it("should use default summaryPrefix when not provided", async () => {
+    const summarizationModel = createMockSummarizationModel();
+    const model = createMockMainModel();
+
+    const middleware = summarizationMiddleware({
+      model: summarizationModel as any,
+      trigger: { tokens: 50 },
+      keep: { messages: 2 },
+    });
+
+    const agent = createAgent({
+      model,
+      middleware: [middleware],
+    });
+
+    const messages = [
+      new HumanMessage(`Message 1: ${"x".repeat(200)}`),
+      new AIMessage(`Response 1: ${"x".repeat(200)}`),
+      new HumanMessage(`Message 2: ${"x".repeat(200)}`),
+      new AIMessage(`Response 2: ${"x".repeat(200)}`),
+      new HumanMessage("Final question"),
+    ];
+
+    const result = await agent.invoke({ messages });
+
+    // Verify summarization was triggered
+    expect(summarizationModel.invoke).toHaveBeenCalled();
+
+    // Verify the default prefix is used
+    const summaryMessage = result.messages[0] as HumanMessage;
+    expect(summaryMessage.content).toContain(
+      "Here is a summary of the conversation to date:"
+    );
+  });
+
+  it("should use custom summaryPrefix when provided", async () => {
+    const summarizationModel = createMockSummarizationModel();
+    const model = createMockMainModel();
+
+    const customPrefix = "Custom summary prefix for testing:";
+
+    const middleware = summarizationMiddleware({
+      model: summarizationModel as any,
+      trigger: { tokens: 50 },
+      keep: { messages: 2 },
+      summaryPrefix: customPrefix,
+    });
+
+    const agent = createAgent({
+      model,
+      middleware: [middleware],
+    });
+
+    const messages = [
+      new HumanMessage(`Message 1: ${"x".repeat(200)}`),
+      new AIMessage(`Response 1: ${"x".repeat(200)}`),
+      new HumanMessage(`Message 2: ${"x".repeat(200)}`),
+      new AIMessage(`Response 2: ${"x".repeat(200)}`),
+      new HumanMessage("Final question"),
+    ];
+
+    const result = await agent.invoke({ messages });
+
+    // Verify summarization was triggered
+    expect(summarizationModel.invoke).toHaveBeenCalled();
+
+    // Verify the custom prefix is used
+    const summaryMessage = result.messages[0] as HumanMessage;
+    expect(summaryMessage.content).toContain(customPrefix);
+    expect(summaryMessage.content).not.toContain(
+      "Here is a summary of the conversation to date:"
+    );
+  });
 });