Ranged file reads.

evals/frugalReads.eval.ts

@@ -0,0 +1,270 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect } from 'vitest';
+import { evalTest } from './test-helper.js';
+import { READ_FILE_TOOL_NAME, EDIT_TOOL_NAME } from '@google/gemini-cli-core';
+
+describe('Frugal reads eval', () => {
+  /**
+   * Ensures that the agent is frugal in its use of context by relying
+   * primarily on ranged reads when the line number is known, and combining
+   * nearby ranges into a single contiguous read to save tool calls.
+   */
+  evalTest('ALWAYS_PASSES', {
+    name: 'should use ranged read when nearby lines are targeted',
+    files: {
+      'package.json': JSON.stringify({
+        name: 'test-project',
+        version: '1.0.0',
+        type: 'module',
+      }),
+      'eslint.config.mjs': `export default [
+        {
+          files: ["**/*.ts"],
+          rules: {
+            "no-var": "error"
+          }
+        }
+      ];`,
+      'linter_mess.ts': (() => {
+        const lines = [];
+        for (let i = 0; i < 1000; i++) {
+          if (i === 500 || i === 510 || i === 520) {
+            lines.push(`var oldVar${i} = "needs fix";`);
+          } else {
+            lines.push(`const goodVar${i} = "clean";`);
+          }
+        }
+        return lines.join('\n');
+      })(),
+    },
+    prompt:
+      'Fix all linter errors in linter_mess.ts manually by editing the file. Run eslint directly (using "npx --yes eslint") to find them. Do not run the file.',
+    assert: async (rig) => {
+      const logs = rig.readToolLogs();
+
+      // Check if the agent read the whole file
+      const readCalls = logs.filter(
+        (log) => log.toolRequest?.name === READ_FILE_TOOL_NAME,
+      );
+
+      const targetFileReads = readCalls.filter((call) => {
+        const args = JSON.parse(call.toolRequest.args);
+        return args.file_path.includes('linter_mess.ts');
+      });
+
+      expect(
+        targetFileReads.length,
+        'Agent should have used read_file to check context',
+      ).toBeGreaterThan(0);
+
+      // We expect a single contiguous range covering all errors since they are near each other.
+      // Some models re-verify or read more than once, so we allow up to 4.
+      expect(
+        targetFileReads.length,
+        'Agent should have been efficient with ranged reads for near errors',
+      ).toEqual(1);
+
+      let totalLinesRead = 0;
+      const readRanges: { offset: number; limit: number }[] = [];
+
+      for (const call of targetFileReads) {
+        const args = JSON.parse(call.toolRequest.args);
+
+        expect(
+          args.limit,
+          'Agent read the entire file (missing limit) instead of using ranged read',
+        ).toBeDefined();
+
+        const limit = args.limit;
+        const offset = args.offset ?? 0;
+        totalLinesRead += limit;
+        readRanges.push({ offset, limit });
+
+        expect(args.limit, 'Agent read too many lines at once').toBeLessThan(
+          1001,
+        );
+      }
+
+      // Ranged read shoud be frugal and just enough to satisfy the task at hand.
+      expect(
+        totalLinesRead,
+        'Agent read more of the file than expected',
+      ).toBeLessThan(1000);
+
+      // Check that we read around the error lines
+      const errorLines = [500, 510, 520];
+      for (const line of errorLines) {
+        const covered = readRanges.some(
+          (range) => line >= range.offset && line < range.offset + range.limit,
+        );
+        expect(covered, `Agent should have read around line ${line}`).toBe(
+          true,
+        );
+      }
+
+      const editCalls = logs.filter(
+        (log) => log.toolRequest?.name === EDIT_TOOL_NAME,
+      );
+      const targetEditCalls = editCalls.filter((call) => {
+        const args = JSON.parse(call.toolRequest.args);
+        return args.file_path.includes('linter_mess.ts');
+      });
+      expect(
+        targetEditCalls.length,
+        'Agent should have made replacement calls on the target file',
+      ).toBeGreaterThanOrEqual(3);
+    },
+  });
+
+  /**
+   * Ensures the agent uses multiple ranged reads when the targets are far
+   * apart to avoid the need to read the whole file.
+   */
+  evalTest('ALWAYS_PASSES', {
+    name: 'should use ranged read when targets are far apart',
+    files: {
+      'package.json': JSON.stringify({
+        name: 'test-project',
+        version: '1.0.0',
+        type: 'module',
+      }),
+      'eslint.config.mjs': `export default [
+        {
+          files: ["**/*.ts"],
+          rules: {
+            "no-var": "error"
+          }
+        }
+      ];`,
+      'far_mess.ts': (() => {
+        const lines = [];
+        for (let i = 0; i < 1000; i++) {
+          if (i === 100 || i === 900) {
+            lines.push(`var oldVar${i} = "needs fix";`);
+          } else {
+            lines.push(`const goodVar${i} = "clean";`);
+          }
+        }
+        return lines.join('\n');
+      })(),
+    },
+    prompt:
+      'Fix all linter errors in far_mess.ts manually by editing the file. Run eslint directly (using "npx --yes eslint") to find them. Do not run the file.',
+    assert: async (rig) => {
+      const logs = rig.readToolLogs();
+
+      const readCalls = logs.filter(
+        (log) => log.toolRequest?.name === READ_FILE_TOOL_NAME,
+      );
+
+      const targetFileReads = readCalls.filter((call) => {
+        const args = JSON.parse(call.toolRequest.args);
+        return args.file_path.includes('far_mess.ts');
+      });
+
+      // The agent should use ranged reads to be frugal with context tokens,
+      // even if it requires multiple calls for far-apart errors.
+      expect(
+        targetFileReads.length,
+        'Agent should have used read_file to check context',
+      ).toBeGreaterThan(0);
+
+      // We allow multiple calls since the errors are far apart.
+      expect(
+        targetFileReads.length,
+        'Agent should have used separate reads for far apart errors',
+      ).toBeLessThanOrEqual(4);
+
+      for (const call of targetFileReads) {
+        const args = JSON.parse(call.toolRequest.args);
+        expect(
+          args.limit,
+          'Agent should have used ranged read (limit) to save tokens',
+        ).toBeDefined();
+      }
+    },
+  });
+
+  /**
+   * Validates that the agent reads the entire file if there are lots of matches
+   * (e.g.: 10), as it's more efficient than many small ranged reads.
+   */
+  evalTest('ALWAYS_PASSES', {
+    name: 'should read the entire file when there are many matches',
+    files: {
+      'package.json': JSON.stringify({
+        name: 'test-project',
+        version: '1.0.0',
+        type: 'module',
+      }),
+      'eslint.config.mjs': `export default [
+        {
+          files: ["**/*.ts"],
+          rules: {
+            "no-var": "error"
+          }
+        }
+      ];`,
+      'many_mess.ts': (() => {
+        const lines = [];
+        for (let i = 0; i < 1000; i++) {
+          if (i % 100 === 0) {
+            lines.push(`var oldVar${i} = "needs fix";`);
+          } else {
+            lines.push(`const goodVar${i} = "clean";`);
+          }
+        }
+        return lines.join('\n');
+      })(),
+    },
+    prompt:
+      'Fix all linter errors in many_mess.ts manually by editing the file. Run eslint directly (using "npx --yes eslint") to find them. Do not run the file.',
+    assert: async (rig) => {
+      const logs = rig.readToolLogs();
+
+      const readCalls = logs.filter(
+        (log) => log.toolRequest?.name === READ_FILE_TOOL_NAME,
+      );
+
+      const targetFileReads = readCalls.filter((call) => {
+        const args = JSON.parse(call.toolRequest.args);
+        return args.file_path.includes('many_mess.ts');
+      });
+
+      expect(
+        targetFileReads.length,
+        'Agent should have used read_file to check context',
+      ).toBeGreaterThan(0);
+
+      // In this case, we expect the agent to realize there are many scattered errors
+      // and just read the whole file to be efficient with tool calls.
+      const readEntireFile = targetFileReads.some((call) => {
+        const args = JSON.parse(call.toolRequest.args);
+        return args.limit === undefined;
+      });
+
+      expect(
+        readEntireFile,
+        'Agent should have read the entire file because of the high number of scattered matches',
+      ).toBe(true);
+
+      // Check that the agent actually fixed the errors
+      const editCalls = logs.filter(
+        (log) => log.toolRequest?.name === EDIT_TOOL_NAME,
+      );
+      const targetEditCalls = editCalls.filter((call) => {
+        const args = JSON.parse(call.toolRequest.args);
+        return args.file_path.includes('many_mess.ts');
+      });
+      expect(
+        targetEditCalls.length,
+        'Agent should have made replacement calls on the target file',
+      ).toBeGreaterThanOrEqual(1);
+    },
+  });
+});

packages/core/src/prompts/snippets.ts

@@ -168,6 +168,10 @@ export function renderCoreMandates(options?: CoreMandatesOptions): string {
 - Always minimize wasted context window by aggressively scoping and limiting all of your ${GREP_TOOL_NAME} searches. e.g.: always pass total_max_matches, include, and max_matches_per_file.
 - Use names_only=true or max_matches_per_file=1 to find a list of files that contain a pattern.
 - Limit unnecessary context consumption from file reads by always using ${GREP_TOOL_NAME} (configured with \`max_matches_per_file\`) to search large files (> 1kb).
+- Conserve context when reading files by reading just enough context to definitively answer the question by passing offset and limit to ${READ_FILE_TOOL_NAME} or by searching with ${GREP_TOOL_NAME} and before=50 and after=50 and total_max_matches
+  - Always read at most one range from the file to avoid chatty "scrolling" or "pagination" style reads which waste tokens by adding extra turns.
+  - Always read at least 100 lines to avoid degrading the ${WRITE_FILE_TOOL_NAME} reliability.
+  - If you have multiple ranges that you want to read, always combine them into a single range to avoid wasting tokens on another tool call.
 
 ## Engineering Standards
 - **Contextual Precedence:** Instructions found in ${formattedFilenames} files are foundational mandates. They take absolute precedence over the general workflows and tool defaults described in this system prompt.