diff --git a/packages/core/src/tool/plugin/read.ts b/packages/core/src/tool/plugin/read.ts
index b18ce6b5fad0..44da6ed002b9 100644
--- a/packages/core/src/tool/plugin/read.ts
+++ b/packages/core/src/tool/plugin/read.ts
@@ -19,6 +19,8 @@ const LocationInput = Schema.Struct({
   path: Schema.String.annotate({ description: "File or directory to read" }),
   offset: ReadToolFileSystem.PageInput.fields.offset.annotate({
     description: "The line or directory entry to start reading from (1-based)",
+    message:
+      "The offset must be a non-negative integer. Line and entry numbers are 1-based, so prefer omitting the offset or using 1.",
   }),
   limit: ReadToolFileSystem.PageInput.fields.limit.annotate({
     description: "The maximum number of lines or directory entries to read (defaults to 2000)",
diff --git a/packages/core/test/tool-read.test.ts b/packages/core/test/tool-read.test.ts
index 776235c610b7..e0353941af78 100644
--- a/packages/core/test/tool-read.test.ts
+++ b/packages/core/test/tool-read.test.ts
@@ -213,7 +213,25 @@ describe("ReadTool", () => {
           page: { offset: undefined, limit: undefined },
         },
       ])
-      expect(listCalls).toEqual([])
+    }),
+  )
+
+  it.effect("surfaces 1-based guidance when the model hallucinates a negative offset", () =>
+    Effect.gen(function* () {
+      const registry = yield* Tool.Service
+      const execution = yield* executeTool(registry, {
+        sessionID,
+        ...toolIdentity,
+        call: {
+          type: "tool-call",
+          id: "call-read-negative-offset",
+          name: "read",
+          input: { path: "README.md", offset: -1 },
+        },
+      })
+      expect(execution.status).toBe("error")
+      if (execution.status !== "error") return
+      expect(JSON.stringify(execution.error)).toContain("1-based")
     }),
   )