diff --git a/lib/prompt.mjs b/lib/prompt.mjs
index 2b0af31..12f9dae 100644
--- a/lib/prompt.mjs
+++ b/lib/prompt.mjs
@@ -41,3 +41,14 @@ export function derivePromptCharBudget(models, { charsPerToken = 3, floor = 1500
   if (windows.length === 0) return floor;
   return Math.max(floor, Math.max(...windows) * charsPerToken);
 }
+
+// Resolve the effective budget from the env var + SPOT. TRUTHINESS (not != null) on the env
+// value deliberately: an EMPTY value ("CLAUDE_MAX_PROMPT_CHARS=" in a systemd EnvironmentFile
+// or .env) must mean "use the default" — exactly the old `parseInt(env || "150000")` contract.
+// Treating "" as explicit gives parseInt("") = NaN, and a NaN cap silently DISABLES the
+// runaway-context guard while injecting a false "[System] Note: 0 older messages were
+// truncated" line into every prompt (caught in PR #179 review). Non-empty garbage still
+// parses to NaN — the pre-existing class, slated for parseIntEnv routing in PR #154.
+export function resolvePromptCharBudget(rawEnv, models, opts) {
+  return rawEnv ? parseInt(rawEnv, 10) : derivePromptCharBudget(models, opts);
+}
diff --git a/server.mjs b/server.mjs
index 1367482..112e512 100644
--- a/server.mjs
+++ b/server.mjs
@@ -49,7 +49,7 @@ import { TuiSemaphore, SemaphoreAbortError, recordTuiEntrypoint, buildTuiHealthB
 import { TuiPanePool, resolvePoolSize, POOL_MAX_SIZE } from "./lib/tui/pool.mjs";
 import { TuiDeltaAssembler, DEFAULT_HOLDBACK_CHARS, resolveStreamHoldback } from "./lib/tui/stream.mjs";
 import { createSerialMutex, createTtlCache, isTokenExpiring, orderLabelsLastGoodFirst } from "./lib/spawn-auth.mjs";
-import { appendOperatorPrompt, derivePromptCharBudget } from "./lib/prompt.mjs";
+import { appendOperatorPrompt, resolvePromptCharBudget } from "./lib/prompt.mjs";
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const _pkg = JSON.parse(readFileSync(join(__dirname, "package.json"), "utf8"));
@@ -1154,9 +1154,7 @@ function buildCliArgs(cliModel, systemPrompt) {
 // English tokens), which silently under-delivered the advertised window by ~5×. The env
 // var (and the runtime settings API below) remain absolute operator overrides. If
 // models.json ever advertises a bigger window, this budget follows automatically.
-let MAX_PROMPT_CHARS = process.env.CLAUDE_MAX_PROMPT_CHARS != null
-  ? parseInt(process.env.CLAUDE_MAX_PROMPT_CHARS, 10)
-  : derivePromptCharBudget(modelsConfig.models);
+let MAX_PROMPT_CHARS = resolvePromptCharBudget(process.env.CLAUDE_MAX_PROMPT_CHARS, modelsConfig.models);
 
 // Flatten OpenAI content (string | array of parts) to plain text for the prompt.
 // Array content: concatenate text parts; replace non-text parts (e.g. image_url)
diff --git a/test-features.mjs b/test-features.mjs
index 8633cdc..1ad17c1 100644
--- a/test-features.mjs
+++ b/test-features.mjs
@@ -844,7 +844,7 @@ test("doctor falls back to currentVersion when origin/main unreachable (no stale
 // contract lives in lib/prompt.mjs. Mutation-proof: make appendOperatorPrompt
 // return `base` unconditionally and the first test fails; make it stop trimming
 // and the whitespace test fails.
-import { appendOperatorPrompt, derivePromptCharBudget } from "./lib/prompt.mjs";
+import { appendOperatorPrompt, derivePromptCharBudget, resolvePromptCharBudget } from "./lib/prompt.mjs";
 
 console.log("\nPrompt-char budget (ADR 0009 — SPOT-derived):");
 
@@ -873,6 +873,21 @@ test("derivePromptCharBudget: charsPerToken and floor are tunable parameters", (
   assert.equal(derivePromptCharBudget([], { floor: 42 }), 42);
 });
 
+// PR #179 review regression: EMPTY env value must mean "use the default" (the old
+// `parseInt(env || "150000")` contract). Mutation-proof: switch the resolver's
+// truthiness check to `!= null` and the empty-string test fails (NaN ≠ 600000).
+test("resolvePromptCharBudget: empty/unset env → SPOT-derived default, never NaN", () => {
+  const models = [{ contextWindow: 200000 }];
+  assert.equal(resolvePromptCharBudget("", models), 600000, "CLAUDE_MAX_PROMPT_CHARS= (empty) must fall back to derived");
+  assert.equal(resolvePromptCharBudget(undefined, models), 600000);
+});
+
+test("resolvePromptCharBudget: a set env value overrides the derivation absolutely", () => {
+  const models = [{ contextWindow: 200000 }];
+  assert.equal(resolvePromptCharBudget("300000", models), 300000);
+  assert.equal(resolvePromptCharBudget("150000", models), 150000, "explicit legacy value wins over the bigger derived default");
+});
+
 console.log("\nSystem-prompt operator append:");
 
 test("appendOperatorPrompt: appends the operator prompt LAST, blank-line separated", () => {