fix(harness): preserve empty prompt ranges

src/agents/harness/prompt-compaction-hook-helpers.test.ts

@@ -44,4 +44,23 @@ describe("resolveAgentHarnessBeforePromptBuildResult", () => {
       promptInputRange: { start: 16, end: 21 },
     });
   });
+
+  it("keeps an empty input range between prepended and appended context", async () => {
+    const result = await resolveAgentHarnessBeforePromptBuildResult({
+      prompt: "",
+      developerInstructions: "base instructions",
+      messages: [],
+      ctx: {},
+      beforeAgentStartResult: {
+        appendContext: "appended context",
+        prependContext: "prepended context",
+      },
+    });
+
+    expect(result).toEqual({
+      prompt: "prepended context\n\nappended context",
+      developerInstructions: "base instructions",
+      promptInputRange: { start: 17, end: 17 },
+    });
+  });
 });

src/agents/harness/prompt-compaction-hook-helpers.ts

@@ -88,7 +88,12 @@ export async function resolveAgentHarnessBeforePromptBuildResult(params: {
   ]);
   const prompt =
     joinPresentTextSegments([promptPrefix, params.prompt, promptSuffix]) ?? params.prompt;
-  const promptInputStart = promptPrefix ? promptPrefix.length + 2 : 0;
+  const promptInputStart =
+    params.prompt.length === 0
+      ? (promptPrefix?.length ?? 0)
+      : promptPrefix
+        ? promptPrefix.length + 2
+        : 0;
   return {
     prompt,
     developerInstructions: