feat: add read_file tool

agents/ollama.ts

@@ -117,6 +117,7 @@ async function runOllamaLoop(ctx: AgentRunContext): Promise<AgentResult> {
         model,
         messages,
         tools,
+        keep_alive: "10m",
         options: {
           think: false,
         } as Record<string, unknown>,
@@ -140,7 +141,9 @@ async function runOllamaLoop(ctx: AgentRunContext): Promise<AgentResult> {
       // of whether the mode nudge is still pending (the model may have stopped
       // right after select_mode before acting on the guidance).
       if (!calledOutputTool && !addedContinueNudge) {
-        log.info("» model stopped before completing task — nudging to continue");
+        log.info(
+          "» model stopped before completing task — nudging to continue",
+        );
         addedContinueNudge = true;
         messages.push({
           role: "user",

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}-${headShort}.diff`)}', description: 'read diff section' }). ` +
+        `use the read_file MCP tool to read sections of it. ` +
-        `use the TOC line ranges as your checklist and read specific sections. ` +
+        `for example, if the TOC says "src/foo.ts → lines 5-42", call: read_file({ path: diffPath, start_line: 5, end_line: 42 }). ` +
-        `for example, if the TOC says "src/foo.ts → lines 5-42", run: shell({ command: 'sed -n "5,42p" <diffPath>', description: 'read foo.ts diff' }). ` +
         `IMPORTANT: to inspect the PR's changed files, read diffPath directly — ` +
         `do NOT run git diff or git show. The PR base branch is '${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";

utils/instructions.ts

@@ -99,7 +99,7 @@ function getShellInstructions(
     case "restricted":
       return `### Shell commands\n\nUse the \`${t("shell")}\` MCP tool for all shell command execution. This tool provides a secure environment with filtered credentials. Do NOT use any native shell tool — it is disabled for security. For long-running processes, use \`shell({ command, background: true })\`. Use \`${t("kill_background")}\` to stop background processes.`;
     case "enabled":
-      return `### Shell commands\n\nUse the \`${t("shell")}\` MCP tool for all shell command execution (unrestricted environment, credentials available).`;
+      return `### Shell commands\n\nUse your native shell tool for shell command execution.`;
     default: {
       const _exhaustive: never = shell;
       return _exhaustive satisfies never;
@@ -107,11 +107,8 @@ function getShellInstructions(
   }
 }
 
-function getFileInstructions(shell: ResolvedPayload["shell"]): string {
+function getFileInstructions(_shell: ResolvedPayload["shell"]): string {
-  if (shell === "disabled") {
+  return `### File operations\n\nUse the \`read_file\` MCP tool to read files. For example: \`read_file({ path: diffPath, start_line: 5, end_line: 42 })\`. This is the primary way to read the PR diff returned by \`checkout_pr\`.`;
-    return `### File operations\n\nShell is disabled — you cannot read arbitrary files. Use the diff content returned by MCP tools directly.`;
-  }
-  return `### File operations\n\nUse the \`shell\` MCP tool for all file operations. Examples:\n- Read a file: \`shell({ command: 'cat /path/to/file', description: 'read file' })\`\n- Read specific lines: \`shell({ command: 'sed -n "<start>,<end>p" /path/to/file', description: 'read lines' })\`\n- Search in a file: \`shell({ command: 'grep -n "pattern" /path/to/file', description: 'search' })\``;
 }
 
 function getStandaloneModeInstructions(