fix(core): preserve trailing blank lines in read pages

Author: kitlangton

packages/core/src/tool/plugin/read.ts

@@ -192,7 +192,9 @@ export const toModelContent = (path: string, offset: number | undefined, output:
   }
 
   const start = output.type === "text-page" ? output.offset : 1
-  const lines = output.content === "" ? [] : output.content.replace(/\n$/, "").split("\n")
+  // Pages already join selected lines; a trailing newline represents a selected blank line.
+  const text = output.type === "file" ? output.content.replace(/\n$/, "") : output.content
+  const lines = output.content === "" ? [] : text.split("\n")
   const content = [
     lines.length === 0 ? `Read file ${path}, 0 lines` : `Read file ${path}, lines ${start}-${start + lines.length - 1}`,
   ]

packages/core/test/tool-read-filesystem.test.ts

@@ -3,6 +3,7 @@ import fs from "fs/promises"
 import path from "path"
 import { Environment } from "@opencode/core/environment/index"
 import { AbsolutePath } from "@opencode/core/schema"
+import { ReadTool } from "@opencode/core/tool/plugin/read"
 import { ReadToolFileSystem } from "@opencode/core/tool/read-filesystem"
 import { CrossSpawnSpawner } from "@opencode/util/cross-spawn-spawner"
 import { LayerNodePlatform } from "@opencode/util/effect/app-node-platform"
@@ -20,6 +21,96 @@ const fixture = Effect.gen(function* () {
 })
 const absolute = (value: string) => AbsolutePath.make(value)
 
+describe("ReadTool text serialization", () => {
+  const cases = [
+    {
+      name: "preserves a selected trailing blank line before continuation",
+      content: "alpha\n\nomega\n",
+      page: { offset: 1, limit: 2 },
+      output: { type: "text-page", content: "alpha\n", offset: 1, truncated: true, next: 3 },
+      model: "Read file lines.txt, lines 1-2\n1: alpha\n2: \n[Output truncated. Continue reading with offset: 3]",
+    },
+    {
+      name: "preserves multiple selected trailing blank lines at a noninitial offset",
+      content: "before\nalpha\n\n\nomega\n",
+      page: { offset: 2, limit: 3 },
+      output: { type: "text-page", content: "alpha\n\n", offset: 2, truncated: true, next: 5 },
+      model: "Read file lines.txt, lines 2-4\n2: alpha\n3: \n4: \n[Output truncated. Continue reading with offset: 5]",
+    },
+    {
+      name: "preserves a selected trailing blank line at EOF",
+      content: "alpha\n\n",
+      page: { limit: 2 },
+      output: { type: "text-page", content: "alpha\n", offset: 1, truncated: false },
+      model: "Read file lines.txt, lines 1-2\n1: alpha\n2: ",
+    },
+    {
+      name: "preserves internal blank lines in a page",
+      content: "alpha\n\nomega\n",
+      page: { limit: 3 },
+      output: { type: "text-page", content: "alpha\n\nomega", offset: 1, truncated: false },
+      model: "Read file lines.txt, lines 1-3\n1: alpha\n2: \n3: omega",
+    },
+    {
+      name: "preserves continuation for a nonblank page",
+      content: "alpha\n\nomega\n",
+      page: { limit: 1 },
+      output: { type: "text-page", content: "alpha", offset: 1, truncated: true, next: 2 },
+      model: "Read file lines.txt, lines 1-1\n1: alpha\n[Output truncated. Continue reading with offset: 2]",
+    },
+    {
+      name: "strips only the terminal file newline in a whole-file read",
+      content: "alpha\n\n",
+      page: {},
+      output: { type: "file", content: "alpha\n\n", encoding: "utf8" },
+      model: "Read file lines.txt, lines 1-2\n1: alpha\n2: ",
+    },
+    {
+      name: "does not add a line for a whole-file terminal newline",
+      content: "alpha\n",
+      page: {},
+      output: { type: "file", content: "alpha\n", encoding: "utf8" },
+      model: "Read file lines.txt, lines 1-1\n1: alpha",
+    },
+    {
+      name: "preserves a whole-file read without a terminal newline",
+      content: "alpha",
+      page: {},
+      output: { type: "file", content: "alpha", encoding: "utf8" },
+      model: "Read file lines.txt, lines 1-1\n1: alpha",
+    },
+    {
+      name: "preserves empty whole-file output",
+      content: "",
+      page: {},
+      output: { type: "file", content: "", encoding: "utf8" },
+      model: "Read file lines.txt, 0 lines",
+    },
+    {
+      name: "preserves empty-file page output",
+      content: "",
+      page: { limit: 2 },
+      output: { type: "text-page", content: "", offset: 1, truncated: false },
+      model: "Read file lines.txt, 0 lines",
+    },
+  ]
+
+  cases.forEach((input) => {
+    it.live(input.name, () =>
+      Effect.gen(function* () {
+        const current = yield* fixture
+        const file = absolute(path.join(current.directory, "lines.txt"))
+        yield* current.files.writeFileString(file, input.content)
+
+        const result = yield* ReadToolFileSystem.read(current.environment, file, "lines.txt", input.page)
+
+        expect(result).toMatchObject(input.output)
+        expect(ReadTool.toModelContent("lines.txt", undefined, result)).toBe(input.model)
+      }),
+    )
+  })
+})
+
 describe("ReadToolFileSystem", () => {
   it.effect("preserves the environment not-found error", () =>
     Effect.gen(function* () {