test: add eval case for implicit ranged reads

evals/read_file_optimization.eval.ts

@@ -69,4 +69,63 @@ describe('optimization_evals', () => {
       // But for this behavior test, the read pattern is the most important part.
     },
   });
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should use ranged read when specific line is targeted',
+    files: {
+      'linter_mess.ts': (() => {
+        const lines = [];
+        for (let i = 0; i < 4000; i++) {
+          if (i === 3000) {
+            lines.push(`var oldVar${i} = "needs fix";`);
+          } else {
+            lines.push(`const goodVar${i} = "clean";`);
+          }
+        }
+        return lines.join('\n');
+      })(),
+    },
+    prompt:
+      'Fix the linter error on line 3000 of linter_mess.ts. The error is the use of "var".',
+    assert: async (rig, result) => {
+      const logs = rig.readToolLogs();
+
+      // Check if the agent read the whole file
+      const readCalls = logs.filter(
+        (log) => log.toolRequest?.name === READ_FILE_TOOL_NAME,
+      );
+      expect(
+        readCalls.length,
+        'Agent should have used read_file to check context',
+      ).toBeGreaterThan(0);
+
+      for (const call of readCalls) {
+        const args = JSON.parse(call.toolRequest.args);
+        if (args.file_path.includes('linter_mess.ts')) {
+          expect(
+            args.limit,
+            'Agent read the entire file (missing limit) instead of using ranged read',
+          ).toBeDefined();
+
+          expect(args.limit, 'Agent read too many lines at once').toBeLessThan(
+            1000,
+          );
+
+          // Since the error is at line 3000, efficient reading implies using an offset
+          // unless they grep first (which is also fine, but less direct if line is known).
+          // However, strict ranged read usually means offset is set.
+          // We'll allow either grep OR offset usage.
+          const hasOffset = args.offset !== undefined && args.offset > 2000;
+
+          // If they didn't use offset, they better have searched first or read a small chunk?
+          // Actually if they read line 0-1000, they won't find line 3000.
+          // So if they read the file, they MUST have used offset to see line 3000 without reading the whole thing.
+          expect(
+            hasOffset,
+            'Agent should use offset to read around line 3000',
+          ).toBe(true);
+        }
+      }
+    },
+  });
 });