tell() fails fast when the budget is exhausted

No message is sent to the LLM; the error points at add_budget().
Previously the agent received the message, hit tool refusals, and
spent tokens explaining it could not work.

Author: mattyoreilly

R/session.R

@@ -249,6 +249,12 @@ atlas_session <- R6::R6Class("atlas_session",
     #' @return The agent's reply (invisibly).
     tell = function(text, verbose = TRUE) {
       stopifnot(is.character(text), length(text) == 1)
+      reason <- private$budget_reason()
+      if (!is.null(reason)) {
+        stop("session budget exhausted (", reason, ") - nothing was sent ",
+             "to the LLM. Grant more with $add_budget(steps = , seconds = ) ",
+             "and retry.", call. = FALSE)
+      }
       private$verbose <- verbose
       if (private$context_tokens() >= private$compact_at) {
         self$compact()

tests/testthat/test-session.R

@@ -232,6 +232,22 @@ test_that("add_budget() unblocks an exhausted session", {
   expect_equal(readRDS(file.path(dir, "meta.rds"))$max_steps, 3)
 })
 
+test_that("tell() on an exhausted session fails fast, without an LLM call", {
+  chat <- FakeChat$new()   # empty script: any message sent would error
+  s <- atlas_session$new(mtcars, "mpg", chat = chat, dir = temp_dir(),
+                         max_steps = 1)
+  s$chat$get_tools()$run_r_code("1 + 1")   # spend the budget
+
+  expect_error(s$tell("remove the worst predictor"), "budget exhausted")
+  expect_error(s$tell("remove the worst predictor"), "add_budget")
+  expect_length(chat$log, 0)               # nothing reached the model
+
+  s$add_budget(steps = 5)
+  # now the message goes through (FakeChat script exhausted = it was sent)
+  expect_error(s$tell("try again", verbose = FALSE), "script exhausted")
+  expect_length(chat$log, 1)
+})
+
 test_that("max_runtime blocks execution after the deadline", {
   s <- new_session(max_runtime = 0.01)     # expires almost immediately
   Sys.sleep(0.05)