fix: preserve context window during resume preset refresh (#1353)

Co-authored-by: Letta Code <noreply@letta.com>

src/agent/modify.ts

@@ -182,21 +182,32 @@ function buildModelSettings(
  * @param agentId - The agent ID
  * @param modelHandle - The model handle (e.g., "anthropic/claude-sonnet-4-5-20250929")
  * @param updateArgs - Additional config args (context_window, reasoning_effort, enable_reasoner, etc.)
- * @param preserveParallelToolCalls - If true, preserves the parallel_tool_calls setting when updating the model
+ * @param options - Optional update behavior overrides
  * @returns The updated agent state from the server (includes llm_config and model_settings)
  */
+export interface UpdateAgentLLMConfigOptions {
+  preserveContextWindow?: boolean;
+}
+
 export async function updateAgentLLMConfig(
   agentId: string,
   modelHandle: string,
   updateArgs?: Record<string, unknown>,
+  options?: UpdateAgentLLMConfigOptions,
 ): Promise<AgentState> {
   const client = await getClient();
 
   const modelSettings = buildModelSettings(modelHandle, updateArgs);
-  // First try updateArgs, then fall back to API-cached context window for BYOK models
+  const explicitContextWindow = updateArgs?.context_window as
+    | number
+    | undefined;
+  const shouldPreserveContextWindow = options?.preserveContextWindow === true;
+  // Resume refresh updates should not implicitly reset context window.
   const contextWindow =
-    (updateArgs?.context_window as number | undefined) ??
-    (await getModelContextWindow(modelHandle));
+    explicitContextWindow ??
+    (!shouldPreserveContextWindow
+      ? await getModelContextWindow(modelHandle)
+      : undefined);
   const hasModelSettings = Object.keys(modelSettings).length > 0;
 
   // MiniMax doesn't have a dedicated ModelSettings class, so model_settings

src/headless.ts

@@ -918,6 +918,7 @@ export async function handleHeadlessCommand(
             agent.id,
             presetRefresh.modelHandle,
             resumeRefreshUpdateArgs,
+            { preserveContextWindow: true },
           );
         }
       }

src/index.ts

@@ -1761,6 +1761,7 @@ async function main(): Promise<void> {
                   agent.id,
                   presetRefresh.modelHandle,
                   resumeRefreshUpdateArgs,
+                  { preserveContextWindow: true },
                 );
               }
             }

src/tests/agent/model-preset-refresh.wiring.test.ts

@@ -14,7 +14,7 @@ describe("model preset refresh wiring", () => {
     expect(source).toContain("getModelInfoForLlmConfig(modelHandle");
   });
 
-  test("modify.ts keeps direct updateArgs-driven model update flow", () => {
+  test("modify.ts supports preserving context window during resume refresh", () => {
     const path = fileURLToPath(
       new URL("../../agent/modify.ts", import.meta.url),
     );
@@ -32,7 +32,14 @@ describe("model preset refresh wiring", () => {
     expect(updateSegment).toContain(
       "buildModelSettings(modelHandle, updateArgs)",
     );
+    expect(source).toContain("export interface UpdateAgentLLMConfigOptions");
+    expect(updateSegment).toContain("options?: UpdateAgentLLMConfigOptions");
+    expect(updateSegment).toContain("shouldPreserveContextWindow");
     expect(updateSegment).toContain("getModelContextWindow(modelHandle)");
+    expect(updateSegment).toContain("options?.preserveContextWindow === true");
+    expect(updateSegment).not.toContain(
+      "(updateArgs?.context_window as number | undefined) ??\n    (await getModelContextWindow(modelHandle));",
+    );
     expect(updateSegment).not.toContain(
       "const currentAgent = await client.agents.retrieve(",
     );
@@ -175,6 +182,9 @@ describe("model preset refresh wiring", () => {
     expect(source).toContain("needsUpdate");
     expect(source).toContain("await updateAgentLLMConfig(");
     expect(source).toContain("presetRefresh.modelHandle");
+    expect(source).toMatch(
+      /resumeRefreshUpdateArgs,\s*\{\s*preserveContextWindow:\s*true\s*\},/,
+    );
     expect(source).not.toContain(
       "await updateAgentLLMConfig(\n                agent.id,\n                presetRefresh.modelHandle,\n                presetRefresh.updateArgs,",
     );
@@ -194,6 +204,9 @@ describe("model preset refresh wiring", () => {
     expect(source).toContain("needsUpdate");
     expect(source).toContain("await updateAgentLLMConfig(");
     expect(source).toContain("presetRefresh.modelHandle");
+    expect(source).toMatch(
+      /resumeRefreshUpdateArgs,\s*\{\s*preserveContextWindow:\s*true\s*\},/,
+    );
     expect(source).not.toContain(
       "await updateAgentLLMConfig(\n          agent.id,\n          presetRefresh.modelHandle,\n          presetRefresh.updateArgs,",
     );