feat: add read_file tool

mcp/checkout.ts

@@ -487,9 +487,8 @@ export function CheckoutPrTool(ctx: ToolContext) {
       hookWarning: checkoutResult.hookWarning,
       instructions:
         `the diff file at diffPath contains a table of contents (TOC) listing every changed file with its line range. ` +
-        `to read the diff, use the shell MCP tool — for example: shell({ command: 'sed -n "<start>,<end>p" ${join(tempDir, `pr-${pull_number}-${headShort}.diff`)}', description: 'read diff section' }). ` +
-        `use the TOC line ranges as your checklist and read specific sections. ` +
-        `for example, if the TOC says "src/foo.ts → lines 5-42", run: shell({ command: 'sed -n "5,42p" <diffPath>', description: 'read foo.ts diff' }). ` +
+        `use the read_file MCP tool to read sections of it. ` +
+        `for example, if the TOC says "src/foo.ts → lines 5-42", call: read_file({ path: diffPath, start_line: 5, end_line: 42 }). ` +
         `IMPORTANT: to inspect the PR's changed files, read diffPath directly — ` +
         `do NOT run git diff or git show. The PR base branch is '${pr.baseRef}', NOT necessarily 'main' — ` +
         `if you must use git, use 'origin/${pr.baseRef}' as the base (e.g. git log origin/${pr.baseRef}..HEAD), ` +

mcp/readFile.ts

@@ -0,0 +1,38 @@
+import { readFileSync } from "node:fs";
+import { type } from "arktype";
+import type { ToolContext } from "./server.ts";
+import { execute, tool } from "./shared.ts";
+
+export const ReadFileParams = type({
+  path: type.string.describe("absolute path to the file to read"),
+  "start_line?": type.number.describe("start line, 1-based inclusive (default: 1)"),
+  "end_line?": type.number.describe("end line, 1-based inclusive (default: end of file)"),
+});
+
+export function ReadFileTool(_ctx: ToolContext) {
+  return tool({
+    name: "read_file",
+    description:
+      "Read lines from a file. Use this to read sections of the PR diff returned by checkout_pr. " +
+      "Example: `read_file({ path: diffPath, start_line: 5, end_line: 42 })`.",
+    parameters: ReadFileParams,
+    execute: execute(async ({ path, start_line, end_line }) => {
+      let content: string;
+      try {
+        content = readFileSync(path, "utf-8");
+      } catch (err) {
+        const msg = err instanceof Error ? err.message : String(err);
+        throw new Error(`failed to read ${path}: ${msg}`);
+      }
+
+      if (start_line === undefined && end_line === undefined) {
+        return { content };
+      }
+
+      const lines = content.split("\n");
+      const start = Math.max(0, (start_line ?? 1) - 1);
+      const end = Math.min(lines.length, end_line ?? lines.length);
+      return { content: lines.slice(start, end).join("\n") };
+    }),
+  });
+}

mcp/server.ts

@@ -35,6 +35,7 @@ import {
   GetReviewCommentsTool,
   ListPullRequestReviewsTool,
 } from "./reviewComments.ts";
+import { ReadFileTool } from "./readFile.ts";
 import { SelectModeTool } from "./selectMode.ts";
 import { addTools } from "./shared.ts";
 import { KillBackgroundTool, ShellTool } from "./shell.ts";
@@ -118,6 +119,7 @@ function buildCommonTools(ctx: ToolContext, outputSchema?: JsonSchema): Tool<any
     GitTool(ctx),
     GitFetchTool(ctx),
     UploadFileTool(ctx),
+    ReadFileTool(ctx),
   ];
 
   const isStandalone = ctx.payload.event.trigger === "unknown";