diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index f49084d9c..4ef72139b 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1022,9 +1022,9 @@ export namespace SessionPrompt {
                       }
                     }
                   }
-                  offset = Math.max(start - 1, 0)
+                  offset = Math.max(start, 1)
                   if (end) {
-                    limit = end - offset
+                    limit = end - (offset - 1)
                   }
                 }
                 const args = { filePath: filepath, offset, limit }
diff --git a/packages/opencode/src/tool/read.ts b/packages/opencode/src/tool/read.ts
index d3bff27d6..7f5a9a9bd 100644
--- a/packages/opencode/src/tool/read.ts
+++ b/packages/opencode/src/tool/read.ts
@@ -18,10 +18,13 @@ export const ReadTool = Tool.define("read", {
   description: DESCRIPTION,
   parameters: z.object({
     filePath: z.string().describe("The absolute path to the file or directory to read"),
-    offset: z.coerce.number().describe("The 0-based line offset to start reading from").optional(),
+    offset: z.coerce.number().describe("The line number to start reading from (1-indexed)").optional(),
     limit: z.coerce.number().describe("The maximum number of lines to read (defaults to 2000)").optional(),
   }),
   async execute(params, ctx) {
+    if (params.offset !== undefined && params.offset < 1) {
+      throw new Error("offset must be greater than or equal to 1")
+    }
     let filepath = params.filePath
     if (!path.isAbsolute(filepath)) {
       filepath = path.resolve(Instance.directory, filepath)
@@ -78,9 +81,10 @@ export const ReadTool = Tool.define("read", {
       entries.sort((a, b) => a.localeCompare(b))
 
       const limit = params.limit ?? DEFAULT_READ_LIMIT
-      const offset = params.offset || 0
-      const sliced = entries.slice(offset, offset + limit)
-      const truncated = offset + sliced.length < entries.length
+      const offset = params.offset ?? 1
+      const start = offset - 1
+      const sliced = entries.slice(start, start + limit)
+      const truncated = start + sliced.length < entries.length
 
       const output = [
         `<path>${filepath}</path>`,
@@ -138,13 +142,15 @@ export const ReadTool = Tool.define("read", {
     if (isBinary) throw new Error(`Cannot read binary file: ${filepath}`)
 
     const limit = params.limit ?? DEFAULT_READ_LIMIT
-    const offset = params.offset || 0
+    const offset = params.offset ?? 1
+    const start = offset - 1
     const lines = await file.text().then((text) => text.split("\n"))
+    if (start >= lines.length) throw new Error(`Offset ${offset} is out of range for this file (${lines.length} lines)`)
 
     const raw: string[] = []
     let bytes = 0
     let truncatedByBytes = false
-    for (let i = offset; i < Math.min(lines.length, offset + limit); i++) {
+    for (let i = start; i < Math.min(lines.length, start + limit); i++) {
       const line = lines[i].length > MAX_LINE_LENGTH ? lines[i].substring(0, MAX_LINE_LENGTH) + "..." : lines[i]
       const size = Buffer.byteLength(line, "utf-8") + (raw.length > 0 ? 1 : 0)
       if (bytes + size > MAX_BYTES) {
@@ -156,7 +162,7 @@ export const ReadTool = Tool.define("read", {
     }
 
     const content = raw.map((line, index) => {
-      return `${index + offset + 1}: ${line}`
+      return `${index + offset}: ${line}`
     })
     const preview = raw.slice(0, 20).join("\n")
 
@@ -164,7 +170,7 @@ export const ReadTool = Tool.define("read", {
     output += content.join("\n")
 
     const totalLines = lines.length
-    const lastReadLine = offset + raw.length
+    const lastReadLine = offset + raw.length - 1
     const hasMoreLines = totalLines > lastReadLine
     const truncated = hasMoreLines || truncatedByBytes
 
diff --git a/packages/opencode/src/tool/read.txt b/packages/opencode/src/tool/read.txt
index 308aaea54..368174cc8 100644
--- a/packages/opencode/src/tool/read.txt
+++ b/packages/opencode/src/tool/read.txt
@@ -3,6 +3,7 @@ Read a file or directory from the local filesystem. If the path does not exist,
 Usage:
 - The filePath parameter should be an absolute path.
 - By default, this tool returns up to 2000 lines from the start of the file.
+- The offset parameter is the line number to start from (1-indexed).
 - To read later sections, call this tool again with a larger offset.
 - Use the grep tool to find specific content in large files or files with long lines.
 - If you are unsure of the correct file path, use the glob tool to look up filenames by glob pattern.
diff --git a/packages/opencode/test/tool/read.test.ts b/packages/opencode/test/tool/read.test.ts
index d6c5cf439..095c7bce2 100644
--- a/packages/opencode/test/tool/read.test.ts
+++ b/packages/opencode/test/tool/read.test.ts
@@ -258,7 +258,7 @@ describe("tool.read truncation", () => {
   test("respects offset parameter", async () => {
     await using tmp = await tmpdir({
       init: async (dir) => {
-        const lines = Array.from({ length: 20 }, (_, i) => `line${i}`).join("\n")
+        const lines = Array.from({ length: 20 }, (_, i) => `line${i + 1}`).join("\n")
         await Bun.write(path.join(dir, "offset.txt"), lines)
       },
     })
@@ -275,11 +275,29 @@ describe("tool.read truncation", () => {
     })
   })
 
+  test("throws when offset is beyond end of file", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        const lines = Array.from({ length: 3 }, (_, i) => `line${i + 1}`).join("\n")
+        await Bun.write(path.join(dir, "short.txt"), lines)
+      },
+    })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const read = await ReadTool.init()
+        await expect(
+          read.execute({ filePath: path.join(tmp.path, "short.txt"), offset: 4, limit: 5 }, ctx),
+        ).rejects.toThrow("Offset 4 is out of range for this file (3 lines)")
+      },
+    })
+  })
+
   test("does not mark final directory page as truncated", async () => {
     await using tmp = await tmpdir({
       init: async (dir) => {
         await Promise.all(
-          Array.from({ length: 10 }, (_, i) => Bun.write(path.join(dir, "dir", `file-${i}.txt`), `line${i}`)),
+          Array.from({ length: 10 }, (_, i) => Bun.write(path.join(dir, "dir", `file-${i + 1}.txt`), `line${i}`)),
         )
       },
     })
@@ -287,7 +305,7 @@ describe("tool.read truncation", () => {
       directory: tmp.path,
       fn: async () => {
         const read = await ReadTool.init()
-        const result = await read.execute({ filePath: path.join(tmp.path, "dir"), offset: 5, limit: 5 }, ctx)
+        const result = await read.execute({ filePath: path.join(tmp.path, "dir"), offset: 6, limit: 5 }, ctx)
         expect(result.metadata.truncated).toBe(false)
         expect(result.output).not.toContain("Showing 5 of 10 entries")
       },