diff --git a/shared/learn/scenario.ts b/shared/learn/scenario.ts
index 750628a..18edb3b 100644
--- a/shared/learn/scenario.ts
+++ b/shared/learn/scenario.ts
@@ -1,4 +1,5 @@
 import type { AirlineData, AirportData, Frequency, FrequencyType, Scenario } from './types'
+import { generateSquawk } from '../utils/transponder'
 
 const natoMap: Record<string, string> = {
   A: 'Alpha',
@@ -214,14 +215,6 @@ function codeToPhonetic(value: string): string {
     .trim()
 }
 
-function generateSquawk(): string {
-  let code = ''
-  for (let i = 0; i < 4; i++) {
-    code += String(randInt(0, 7))
-  }
-  return code
-}
-
 export function digitsToWords(value: string): string {
   return value
     .split('')
diff --git a/shared/utils/communicationsEngine.ts b/shared/utils/communicationsEngine.ts
index 3db5401..efdc387 100644
--- a/shared/utils/communicationsEngine.ts
+++ b/shared/utils/communicationsEngine.ts
@@ -9,6 +9,7 @@ import type {
 } from '../types/decision'
 import type { FlowActivationInstruction, FlowActivationMode, LLMDecisionTrace } from '../types/llm'
 import { normalizeRadioPhrase, DEFAULT_AIRLINE_TELEPHONY } from './radioSpeech'
+import { generateSquawk } from './transponder'
 
 // --- DecisionTree runtime types ---
 type Role = 'pilot' | 'atc' | 'system'
@@ -783,7 +784,7 @@ export default function useCommunicationsEngine() {
             dest: fpl.arr || fpl.arrival || 'EDDM',
             stand: genStand(),
             runway: genRunway(),
-            squawk: fpl.assignedsquawk || genSquawk(),
+            squawk: fpl.assignedsquawk || generateSquawk(),
             atis_code: genATIS(),
             sid: genSID(fpl.route || ''),
             transition: 'DCT',
@@ -1509,10 +1510,6 @@ export default function useCommunicationsEngine() {
         return arr[Math.floor(Math.random() * arr.length)]
     }
 
-    function genSquawk() {
-        return String(Math.floor(Math.random() * 8000 + 1000)).padStart(4, '0')
-    }
-
     function genATIS() {
         const letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
         return letters[Math.floor(Math.random() * letters.length)]
diff --git a/shared/utils/transponder.ts b/shared/utils/transponder.ts
new file mode 100644
index 0000000..8628f9d
--- /dev/null
+++ b/shared/utils/transponder.ts
@@ -0,0 +1,39 @@
+/**
+ * Transponder (SSR) code generation.
+ *
+ * A squawk is an *octal* four-digit code — every digit is 0–7, because each
+ * digit is encoded in three pulse positions. A code containing 8 or 9 cannot be
+ * dialled into a real transponder, so it must never reach a clearance.
+ */
+
+/**
+ * Codes ATC will not assign as a discrete code.
+ *
+ * 7500/7600/7700 are the ICAO emergency codes (unlawful interference, radio
+ * failure, general emergency); 7000 is the European VFR conspicuity code and
+ * 1200 its US equivalent; 2000 is the code for entering from a non-SSR area;
+ * 0000 is a non-code used to indicate a transponder fault.
+ */
+export const RESERVED_SQUAWKS = new Set([
+    '7500', '7600', '7700', '7000', '2000', '1200', '0000',
+])
+
+/** True when `code` is a well-formed, assignable discrete squawk. */
+export function isValidSquawk(code: string): boolean {
+    if (!/^[0-7]{4}$/.test(code)) return false
+    return !RESERVED_SQUAWKS.has(code)
+}
+
+/** A random assignable discrete squawk: four octal digits, no reserved code. */
+export function generateSquawk(): string {
+    // The reserved set is tiny next to the 4096-code space, so re-rolling
+    // terminates immediately in practice; the bound just makes that guaranteed.
+    for (let attempt = 0; attempt < 20; attempt++) {
+        let code = ''
+        for (let digit = 0; digit < 4; digit++) {
+            code += String(Math.floor(Math.random() * 8))
+        }
+        if (isValidSquawk(code)) return code
+    }
+    return '1000'
+}
diff --git a/tests/shared/transponder.test.ts b/tests/shared/transponder.test.ts
new file mode 100644
index 0000000..0760bdf
--- /dev/null
+++ b/tests/shared/transponder.test.ts
@@ -0,0 +1,49 @@
+import { test } from 'node:test'
+import assert from 'node:assert/strict'
+import { RESERVED_SQUAWKS, generateSquawk, isValidSquawk } from '../../shared/utils/transponder.ts'
+
+// Enough draws that a decimal generator (which would emit 8/9 roughly a quarter
+// of the time per digit) could not pass by chance.
+const DRAWS = 5000
+
+test('generateSquawk only ever emits octal digits', () => {
+  for (let i = 0; i < DRAWS; i++) {
+    const code = generateSquawk()
+    assert.match(code, /^[0-7]{4}$/, `non-octal squawk generated: ${code}`)
+  }
+})
+
+test('generateSquawk never emits a reserved code', () => {
+  for (let i = 0; i < DRAWS; i++) {
+    const code = generateSquawk()
+    assert.ok(!RESERVED_SQUAWKS.has(code), `reserved squawk generated: ${code}`)
+  }
+})
+
+test('generateSquawk covers the code space rather than returning a constant', () => {
+  const seen = new Set<string>()
+  for (let i = 0; i < DRAWS; i++) seen.add(generateSquawk())
+  assert.ok(seen.size > 1000, `expected a spread of codes, got ${seen.size} distinct`)
+})
+
+test('isValidSquawk rejects the digits a transponder cannot display', () => {
+  assert.equal(isValidSquawk('2891'), false)
+  assert.equal(isValidSquawk('4592'), false)
+  assert.equal(isValidSquawk('1234'), true)
+})
+
+test('isValidSquawk rejects reserved codes', () => {
+  assert.equal(isValidSquawk('7500'), false)
+  assert.equal(isValidSquawk('7600'), false)
+  assert.equal(isValidSquawk('7700'), false)
+  assert.equal(isValidSquawk('7000'), false)
+  assert.equal(isValidSquawk('2000'), false)
+  assert.equal(isValidSquawk('0000'), false)
+})
+
+test('isValidSquawk rejects malformed input', () => {
+  assert.equal(isValidSquawk(''), false)
+  assert.equal(isValidSquawk('123'), false)
+  assert.equal(isValidSquawk('12345'), false)
+  assert.equal(isValidSquawk('12A4'), false)
+})