diff --git a/newLang/docs/architecture.md b/newLang/docs/architecture.md
index f00a3e8..fd9eb83 100644
--- a/newLang/docs/architecture.md
+++ b/newLang/docs/architecture.md
@@ -344,8 +344,17 @@ fresh image per run.
 ## 11. Diagnostics and Faults
 
 Compile-time diagnostics use stable IDs such as `E014`. Runtime faults use a
-separate stable namespace such as `R003`. Numeric assignments live in
-`src/common/errors.inc` once story E02-S03 freezes the catalog.
+separate stable namespace such as `R003`. The compile-time catalog is
+`src/common/diagcat.inc` (one line per diagnostic, numbered by category
+range); `errors.inc` turns it into `E_*` constants and `diagnostics.s` into
+messages. `docs/diagnostics.md` is generated from it (E02-S04).
+
+`diagnostics.s` prints a two-line screen form (header, message clipped to 39
+characters) and appends one full line per diagnostic to `NAME.ERR`. The ERR
+file is scratched when compilation starts and created only on the first
+diagnostic. After 20 errors the next is replaced by E221 and the caller must
+stop. If `NAME.ERR` cannot be written, E201 is shown once and reporting
+continues on screen.
 
 Compiler errors include source name, line, column, and compact message. Runtime
 faults always include a stable fault code and the best available program address
diff --git a/newLang/docs/diagnostics.md b/newLang/docs/diagnostics.md
new file mode 100644
index 0000000..452df76
--- /dev/null
+++ b/newLang/docs/diagnostics.md
@@ -0,0 +1,94 @@
+# Nova128 Diagnostics
+
+Generated from `src/common/diagcat.inc` by `scripts/build-diag-test.ps1 -UpdateDocs`.
+Do not edit by hand. Numbers are stable: never renumber or reuse one.
+
+Each `%` is replaced by an argument such as a name or type. On screen a
+diagnostic is two lines, the header and the message clipped to 39 characters:
+
+```text
+E010 TEST.N8 3 11
+CHARACTER LITERAL NEEDS ONE CHARACTER
+```
+
+`NAME.ERR` holds one full line per diagnostic: `E010 TEST.N8 3 11 CHARACTER ...`.
+It is scratched when compilation starts and created only if a diagnostic occurs.
+After 20 errors the next one is replaced by E221 and compilation stops.
+
+## Source and lexical (E001-E019)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E001 | `SRC_LINE_TOO_LONG` | LINE LONGER THAN 80 CHARACTERS |
+| E002 | `SRC_FILE_TOO_LARGE` | SOURCE LARGER THAN 18999 BYTES |
+| E003 | `SRC_TOO_MANY_LINES` | MORE THAN 599 LINES |
+| E004 | `LEX_INVALID_CHAR` | INVALID CHARACTER |
+| E005 | `LEX_IDENT_TOO_LONG` | NAME LONGER THAN 16 CHARACTERS |
+| E006 | `LEX_NUMBER_RANGE` | NUMBER LARGER THAN 65535 |
+| E007 | `LEX_BAD_NUMBER` | MALFORMED NUMBER |
+| E008 | `LEX_UNTERMINATED_STRING` | STRING NOT CLOSED ON THIS LINE |
+| E009 | `LEX_BAD_ESCAPE` | UNKNOWN ESCAPE SEQUENCE |
+| E010 | `LEX_BAD_CHAR_LITERAL` | CHARACTER LITERAL NEEDS ONE CHARACTER |
+
+## Syntax (E020-E059)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E020 | `SYN_EXPECTED_IDENTIFIER` | NAME EXPECTED |
+| E021 | `SYN_EXPECTED_EOL` | END OF LINE EXPECTED |
+| E022 | `SYN_UNEXPECTED_EOL` | LINE ENDS TOO EARLY |
+| E023 | `SYN_EXPECTED_RPAREN` | ')' EXPECTED |
+| E024 | `SYN_EXPECTED_END_PROGRAM` | END PROGRAM EXPECTED |
+| E025 | `SYN_EXPECTED_END_METHOD` | END METHOD EXPECTED |
+| E026 | `SYN_EXPECTED_END_IF` | END IF EXPECTED |
+| E027 | `SYN_EXPECTED_END_CLASS` | END CLASS EXPECTED |
+| E028 | `SYN_EXPECTED_INTERFACE_NAME` | INTERFACE NAME EXPECTED |
+| E029 | `SYN_INTERFACE_METHOD_BODY` | INTERFACE METHODS HAVE NO BODY |
+| E030 | `SYN_UNEXPECTED_TOKEN` | % IS NOT ALLOWED HERE |
+
+## Names and declarations (E060-E079)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E060 | `SEM_DUPLICATE_NAME` | % IS ALREADY DECLARED |
+| E061 | `SEM_UNKNOWN_NAME` | % IS NOT DECLARED |
+
+## Types (E080-E119)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E080 | `TYP_MISMATCH` | % EXPECTED; % FOUND |
+| E081 | `TYP_CONDITION_NOT_BOOLEAN` | CONDITION MUST BE BOOLEAN |
+
+## Access (E120-E129)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E120 | `ACC_PRIVATE_MEMBER` | % IS PRIVATE |
+
+## Interfaces (E130-E149)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E130 | `IFC_METHOD_MISSING` | % IS NOT IMPLEMENTED |
+| E131 | `IFC_SIGNATURE_MISMATCH` | % DIFFERS FROM INTERFACE |
+
+## Methods and returns (E150-E169)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E150 | `SEM_MISSING_RETURN` | MISSING RETURN VALUE |
+
+## Disk (E200-E219)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E200 | `DSK_SOURCE_UNREADABLE` | CANNOT READ SOURCE; DOS % |
+| E201 | `DSK_ERR_FILE_UNWRITABLE` | CANNOT WRITE ERROR FILE; DOS % |
+
+## Resource limits (E220-E239)
+
+| Code | Symbol | Message |
+|---|---|---|
+| E220 | `RES_TOO_MANY_NAMES` | MORE THAN 255 NAMES |
+| E221 | `RES_ERROR_LIMIT` | TOO MANY ERRORS; COMPILATION STOPPED |
diff --git a/newLang/planning/claims/E02-S04.md b/newLang/planning/claims/E02-S04.md
new file mode 100644
index 0000000..4f4e886
--- /dev/null
+++ b/newLang/planning/claims/E02-S04.md
@@ -0,0 +1,77 @@
+---
+story: E02-S04
+owner: e02_diagnostics
+status: done
+started: 2026-11-07
+depends-on: [E01-S03, E00-S02]
+---
+
+# E02-S04 Claim
+
+## Intended change
+
+Add the stable diagnostic catalog (`src/common/diagcat.inc`, one line per
+diagnostic, included as constants and as messages), `src/common/errors.inc`,
+and `src/compiler/diagnostics.s`, which prints the 40-column screen form and
+writes `NAME.ERR`, with a default limit of 20 errors. Extend `diskio.s` with
+output-file and DOS-command wrappers.
+
+## File area
+
+- `src/common/diagcat.inc`, `src/common/errors.inc`, `src/common/diskio.inc`
+- `src/compiler/diagnostics.s`, `src/compiler/diskio.s`
+- `tests/diag/`
+- `scripts/build-diag-test.ps1`, `scripts/verify-diag-test.ps1`,
+  `scripts/lib/Nova128Test.psm1`
+- `docs/diagnostics.md` (generated from the catalog), `docs/architecture.md`
+- `planning/claims/E02-S04.md`
+- generated files under `build/`
+
+## Acceptance evidence
+
+- [x] Lexical, syntax, type, access, interface, disk, and resource categories
+  have stable IDs: 33 diagnostics in `src/common/diagcat.inc` across nine
+  category ranges; the build rejects out-of-range, duplicate, or reused
+  numbers and any grammar `.expect` symbol missing from the catalog.
+- [x] Screen form fits 40 columns for representative diagnostics: x128 screen
+  RAM shows each diagnostic as a header row and a message row, all at most
+  39 characters, including the longest message, a clipped two-argument
+  message, and the widest header (`E023 DIAG.N8 65535 255`).
+- [x] ERR output contains source, line, column, code, and message:
+  `diag.err` read back from the D81 equals the expected six lines exactly;
+  a stale `diag.err` was replaced and a stale `clean.err` was scratched when
+  no diagnostics occurred.
+- [x] Error limiting defaults to 20 and terminates cleanly: 25 reports give
+  carry 0 x20 then 1 x5; `limit.err` holds 20 errors plus E221; no files are
+  left open after any phase. An unwritable ERR name (DOS 33) shows E201 once
+  and leaves no file. A mutation that stops counting is caught.
+
+## Coordination notes
+
+The grammar fixtures' symbolic `.expect` names become catalog symbols
+unchanged. Later stories append diagnostics within their category range.
+
+## Work log
+
+- The first x128 run showed garbage before every message: `find_message`
+  clobbered X, the formatter's buffer index. Fixed; found by dumping screen
+  RAM and the format buffer.
+- `c1541 -read` needs `name,s` for SEQ files.
+- The ERR width check uses spec limits (15-character source name, line <= 599,
+  column <= 81), so the longest ERR line is 77 characters.
+
+## Completion
+
+- Tests run: `scripts/verify-diag-test.ps1` (pass plus counting mutation).
+- Results: result block, 19 screen rows, `diag.err`, `limit.err`, and the
+  directory all match expectations.
+- Files changed: `src/common/diagcat.inc`, `src/common/errors.inc`,
+  `src/common/diskio.inc`, `src/common/c128.inc` (CHKOUT),
+  `src/compiler/diagnostics.s`, `src/compiler/diskio.s`, `tests/diag/`,
+  `scripts/build-diag-test.ps1`, `scripts/verify-diag-test.ps1`,
+  `docs/diagnostics.md`, `docs/architecture.md`,
+  `tests/grammar/README.md`, this claim.
+- Remaining risks: later stories will append diagnostics, and their wording
+  should be reviewed by the user; the 80-column display path was not
+  exercised here (E01-S04 covers KERNAL output in both modes); a disk that
+  fills up while NAME.ERR is being written is not tested.
diff --git a/newLang/scripts/build-diag-test.ps1 b/newLang/scripts/build-diag-test.ps1
new file mode 100644
index 0000000..4728073
--- /dev/null
+++ b/newLang/scripts/build-diag-test.ps1
@@ -0,0 +1,145 @@
+# E02-S04: check the diagnostic catalog, keep docs/diagnostics.md in sync with
+# it, build the diagnostics test PRG, and prepare a D81 with stale ERR files.
+#   -UpdateDocs rewrites docs/diagnostics.md from src/common/diagcat.inc.
+[CmdletBinding()]
+param([switch]$UpdateDocs)
+
+$ErrorActionPreference = "Stop"
+$project = Split-Path $PSScriptRoot -Parent
+Import-Module (Join-Path $PSScriptRoot "lib\Nova128Test.psm1") -Force
+$out = Join-Path $project "build\diag-test"
+New-Item -ItemType Directory -Force -Path $out | Out-Null
+
+# ------------------------------------------------------------ the catalog
+$catalogPath = Join-Path $project "src\common\diagcat.inc"
+$catalog = @()
+foreach ($line in Get-Content -LiteralPath $catalogPath) {
+    $m = [regex]::Match($line, '^\s*DIAGNOSTIC\s+(\w+),\s*(\d+),\s*"(.*)"\s*$')
+    if ($m.Success) {
+        $catalog += [pscustomobject]@{ Symbol = $m.Groups[1].Value; Code = [int]$m.Groups[2].Value
+            Text = $m.Groups[3].Value }
+    }
+}
+$ranges = @{
+    SRC = @(, @(1, 19)); LEX = @(, @(1, 19)); SYN = @(, @(20, 59)); SEM = @(@(60, 79), @(150, 169))
+    TYP = @(, @(80, 119)); ACC = @(, @(120, 129)); IFC = @(, @(130, 149)); DSK = @(, @(200, 219))
+    RES = @(, @(220, 239))
+}
+$categories = [ordered]@{
+    "Source and lexical" = @(1, 19); "Syntax" = @(20, 59); "Names and declarations" = @(60, 79)
+    "Types" = @(80, 119); "Access" = @(120, 129); "Interfaces" = @(130, 149)
+    "Methods and returns" = @(150, 169); "Disk" = @(200, 219); "Resource limits" = @(220, 239)
+}
+$problems = @()
+foreach ($d in $catalog) {
+    $prefix = $d.Symbol.Split('_')[0]
+    if (-not $ranges.ContainsKey($prefix)) { $problems += "$($d.Symbol): unknown category prefix" }
+    elseif (-not ($ranges[$prefix] | Where-Object { $d.Code -ge $_[0] -and $d.Code -le $_[1] })) {
+        $problems += "$($d.Symbol) = $($d.Code) is outside the $prefix range"
+    }
+    if ($d.Code -lt 1 -or $d.Code -gt 255) { $problems += "$($d.Symbol): code must be 1..255" }
+    $argCount = ([regex]::Matches($d.Text, '%')).Count
+    if ($argCount -gt 2) { $problems += "$($d.Symbol): more than two arguments" }
+    if ($argCount -eq 0 -and $d.Text.Length -gt 39) { $problems += "$($d.Symbol): message longer than 39" }
+    if ($d.Text -cmatch '[a-z]') { $problems += "$($d.Symbol): use uppercase ASCII (PETSCII unshifted)" }
+    # ERR line <= 80 (Vim's width). Spec limits: name <= 15 ("12345678901
+    # 2.N8"), line <= 599, column <= 81, so "Ennn NAME LLL CC " <= 28.
+    if ((28 + $d.Text.Length - $argCount + 16 * $argCount) -gt 80) { $problems += "$($d.Symbol): ERR line can exceed 80" }
+}
+$dupes = $catalog | Group-Object Code | Where-Object Count -gt 1
+foreach ($g in $dupes) { $problems += "code $($g.Name) used by $(($g.Group.Symbol) -join ', ')" }
+$dupes = $catalog | Group-Object Symbol | Where-Object Count -gt 1
+foreach ($g in $dupes) { $problems += "symbol $($g.Name) declared twice" }
+foreach ($e in Get-ChildItem (Join-Path $project "tests\grammar\rejected") -Filter *.expect) {
+    $sym = [regex]::Match((Get-Content -Raw $e.FullName), '(?m)^diagnostic: (\w+)').Groups[1].Value
+    if (-not ($catalog | Where-Object Symbol -eq $sym)) { $problems += "$($e.Name): $sym is not in the catalog" }
+}
+if ($problems) { throw ("diagnostic catalog problems:`n  " + ($problems -join "`n  ")) }
+
+# ------------------------------------------------- docs/diagnostics.md
+$doc = New-Object System.Text.StringBuilder
+[void]$doc.AppendLine("# Nova128 Diagnostics")
+[void]$doc.AppendLine("")
+[void]$doc.AppendLine("Generated from ``src/common/diagcat.inc`` by ``scripts/build-diag-test.ps1 -UpdateDocs``.")
+[void]$doc.AppendLine("Do not edit by hand. Numbers are stable: never renumber or reuse one.")
+[void]$doc.AppendLine("")
+[void]$doc.AppendLine("Each ``%`` is replaced by an argument such as a name or type. On screen a")
+[void]$doc.AppendLine("diagnostic is two lines, the header and the message clipped to 39 characters:")
+[void]$doc.AppendLine("")
+[void]$doc.AppendLine('```text')
+[void]$doc.AppendLine("E010 TEST.N8 3 11")
+[void]$doc.AppendLine("CHARACTER LITERAL NEEDS ONE CHARACTER")
+[void]$doc.AppendLine('```')
+[void]$doc.AppendLine("")
+[void]$doc.AppendLine("``NAME.ERR`` holds one full line per diagnostic: ``E010 TEST.N8 3 11 CHARACTER ...``.")
+[void]$doc.AppendLine("It is scratched when compilation starts and created only if a diagnostic occurs.")
+[void]$doc.AppendLine("After 20 errors the next one is replaced by E221 and compilation stops.")
+foreach ($c in $categories.Keys) {
+    $lo = $categories[$c][0]; $hi = $categories[$c][1]
+    [void]$doc.AppendLine("")
+    [void]$doc.AppendLine(("## {0} (E{1:D3}-E{2:D3})" -f $c, $lo, $hi))
+    [void]$doc.AppendLine("")
+    [void]$doc.AppendLine("| Code | Symbol | Message |")
+    [void]$doc.AppendLine("|---|---|---|")
+    foreach ($d in ($catalog | Where-Object { $_.Code -ge $lo -and $_.Code -le $hi } | Sort-Object Code)) {
+        [void]$doc.AppendLine(("| E{0:D3} | ``{1}`` | {2} |" -f $d.Code, $d.Symbol, $d.Text.Replace('|', '\|')))
+    }
+}
+$docPath = Join-Path $project "docs\diagnostics.md"
+$docText = $doc.ToString().Replace("`r`n", "`n")
+if ($UpdateDocs) { [IO.File]::WriteAllText($docPath, $docText) }
+elseif (-not (Test-Path $docPath) -or [IO.File]::ReadAllText($docPath) -ne $docText) {
+    throw "docs/diagnostics.md is out of date; run build-diag-test.ps1 -UpdateDocs and review"
+}
+
+# -------------------------------------------------------- expectations
+function Format-Diag([string]$name, [int]$line, [int]$col, [string]$symbol, [string[]]$arguments = @()) {
+    $d = $catalog | Where-Object Symbol -eq $symbol
+    $msg = $d.Text                                   # each % takes the next argument
+    foreach ($a in $arguments) { $idx = $msg.IndexOf('%'); $msg = $msg.Substring(0, $idx) + $a + $msg.Substring($idx + 1) }
+    $header = "E{0:D3} {1} {2} {3}" -f $d.Code, $name, $line, $col
+    return [pscustomobject]@{ Header = $header; Message = $msg; ErrLine = "$header $msg" }
+}
+$phaseA = @(
+    (Format-Diag "DIAG.N8" 3 11 "LEX_BAD_CHAR_LITERAL"),
+    (Format-Diag "DIAG.N8" 599 80 "SYN_EXPECTED_END_PROGRAM"),
+    (Format-Diag "DIAG.N8" 12 5 "SEM_DUPLICATE_NAME" @("COUNTER")),
+    (Format-Diag "DIAG.N8" 40 17 "TYP_MISMATCH" @("INTEGER", "VERYLONGCLASSNAM")),
+    (Format-Diag "DIAG.N8" 1 1 "DSK_SOURCE_UNREADABLE" @("62")),
+    (Format-Diag "DIAG.N8" 65535 255 "SYN_EXPECTED_RPAREN"))
+$phaseD = @(
+    (Format-Diag "BAD*.N8" 7 3 "LEX_INVALID_CHAR"),
+    (Format-Diag "BAD*.N8" 7 3 "DSK_ERR_FILE_UNWRITABLE" @("##")),   # ## = DOS code, checked by pattern
+    (Format-Diag "BAD*.N8" 8 4 "LEX_INVALID_CHAR"))
+$limit = @(1..20 | ForEach-Object { (Format-Diag "LIMIT.N8" $_ 1 "LEX_INVALID_CHAR").ErrLine })
+$limit += (Format-Diag "LIMIT.N8" 21 1 "RES_ERROR_LIMIT").ErrLine
+$screen = @()
+foreach ($d in ($phaseA + $phaseD)) {
+    $screen += $d.Header
+    $screen += $(if ($d.Message.Length -gt 39) { $d.Message.Substring(0, 39) } else { $d.Message })
+}
+$screen += "NOVA128 DIAG TEST OK"
+[ordered]@{
+    diag = @($phaseA | ForEach-Object ErrLine)
+    limit = $limit
+    screen = $screen
+} | ConvertTo-Json -Depth 3 | Set-Content (Join-Path $out "expected.json")
+
+# ------------------------------------------------------------- program
+$link = Invoke-Nova128Link -Name "diag-test" -BuildDir $out `
+    -Config (Join-Path $project "cfg\compiler.cfg") -Sources @(
+        (Join-Path $project "src\compiler\mmu.s"),
+        (Join-Path $project "src\compiler\diskio.s"),
+        (Join-Path $project "src\compiler\diagnostics.s"),
+        (Join-Path $project "tests\diag\diag-test.s"))
+if ((Get-Nova128Label $link.Labels "test_entry") -ne 0x1C0D) { throw "test_entry is not at `$1C0D" }
+
+$stale = Join-Path $out "stale.pet"
+[IO.File]::WriteAllBytes($stale, [byte[]](0x53, 0x54, 0x41, 0x4C, 0x45, 0x0D))   # "STALE"
+New-Nova128Disk -Path (Join-Path $out "diag-test.d81") -Files @(
+    @{ Name = "diag-test"; Path = $link.Program; Type = "prg" },
+    @{ Name = "diag.err"; Path = $stale; Type = "seq" },
+    @{ Name = "limit.err"; Path = $stale; Type = "seq" },
+    @{ Name = "clean.err"; Path = $stale; Type = "seq" })
+Write-Host ("catalog: {0} diagnostics in range; docs/diagnostics.md in sync" -f $catalog.Count)
+Write-Host ("built {0}" -f $link.Program)
diff --git a/newLang/scripts/verify-diag-test.ps1 b/newLang/scripts/verify-diag-test.ps1
new file mode 100644
index 0000000..2495a40
--- /dev/null
+++ b/newLang/scripts/verify-diag-test.ps1
@@ -0,0 +1,85 @@
+# E02-S04: run the diagnostics test in x128, then check the result block, the
+# 40-column screen, and the ERR files left on the disk image.
+[CmdletBinding()]
+param([int]$Port = 6534, [int]$TimeoutSeconds = 180)
+
+$ErrorActionPreference = "Stop"
+$project = Split-Path $PSScriptRoot -Parent
+Import-Module (Join-Path $PSScriptRoot "lib\Nova128Test.psm1") -Force
+& (Join-Path $PSScriptRoot "build-diag-test.ps1")
+
+$out = Join-Path $project "build\diag-test"
+$disk = Join-Path $out "diag-test.d81"
+$expected = Get-Content -Raw (Join-Path $out "expected.json") | ConvertFrom-Json
+$failures = @()
+
+$session = Start-Nova128X128 -Autostart $disk -Drive8Image $disk -Port $Port `
+    -Screenshot (Join-Path $out "diag-test-vicii.png")
+try {
+    $mb = Wait-Nova128Mailbox $session -TimeoutSeconds $TimeoutSeconds
+    $results = Read-Nova128Memory $session ($mb[8] + 256 * $mb[9]) 34
+    Invoke-Nova128Monitor $session "x" | Out-Null
+    Start-Sleep -Seconds 2                       # let the OK line print
+    $screenRam = Read-Nova128Memory $session 0x0400 1000
+} finally {
+    Stop-Nova128X128 $session -SettleSeconds 0
+}
+
+# ------------------------------------------------------------ results
+$want = [ordered]@{ "phase A count" = @(0, 6); "phase D count" = @(1, 2); "phase D file state (2=failed)" = @(2, 2)
+    "phase B count" = @(3, 20); "phase C count" = @(4, 0) }
+foreach ($k in $want.Keys) {
+    if ($results[$want[$k][0]] -ne $want[$k][1]) { $failures += "${k}: $($results[$want[$k][0]]) expected $($want[$k][1])" }
+}
+for ($i = 5; $i -le 8; $i++) { if ($results[$i] -ne 0) { $failures += "files left open after phase $($i - 4): $($results[$i])" } }
+$carries = $results[9..33]
+$wantCarries = @(0) * 20 + @(1) * 5
+if (($carries -join ',') -ne ($wantCarries -join ',')) {
+    $failures += "phase B carries $($carries -join '') expected $($wantCarries -join '')"
+}
+
+# ------------------------------------------------------------- screen
+$rows = for ($r = 0; $r -lt 25; $r++) {
+    $chars = for ($c = 0; $c -lt 40; $c++) {
+        $code = $screenRam[$r * 40 + $c]
+        if ($code -le 0x1F) { [char]($code + 0x40) } elseif ($code -le 0x3F) { [char]$code } else { '?' }
+    }
+    (-join $chars).TrimEnd()
+}
+for ($r = 0; $r -lt $expected.screen.Count; $r++) {
+    $pattern = '^' + [regex]::Escape($expected.screen[$r]).Replace('\#\#', '\d\d').Replace('##', '\d\d') + '$'
+    if ($rows[$r] -notmatch $pattern) { $failures += "screen row ${r}: '$($rows[$r])' expected '$($expected.screen[$r])'" }
+}
+$wide = @($rows | Where-Object { $_.Length -gt 39 })
+if ($wide) { $failures += "screen rows longer than 39: $($wide -join ' | ')" }
+
+# ---------------------------------------------------------- ERR files
+$c1541 = Get-ViceTool "c1541"
+$listing = (& $c1541 -attach $disk -list 2>&1) | Out-String
+function Read-SeqText([string]$name) {
+    $dest = Join-Path $out ($name.Replace('*', '_') + ".bin")
+    if (Test-Path $dest) { Remove-Item $dest -Force }
+    & $c1541 -attach $disk -read "$name,s" $dest 2>&1 | Out-Null   # SEQ needs ,s
+    if (-not (Test-Path $dest)) { return $null }
+    $bytes = [IO.File]::ReadAllBytes($dest)
+    return @(([Text.Encoding]::ASCII.GetString($bytes).TrimEnd("`r")) -split "`r")
+}
+$diagLines = Read-SeqText "diag.err"
+if ($null -eq $diagLines) { $failures += "diag.err missing" }
+elseif (($diagLines -join "`n") -cne ($expected.diag -join "`n")) {
+    $failures += "diag.err differs:`n    got:  " + ($diagLines -join "`n          ") + "`n    want: " + ($expected.diag -join "`n          ")
+}
+$limitLines = Read-SeqText "limit.err"
+if ($null -eq $limitLines) { $failures += "limit.err missing" }
+elseif (($limitLines -join "`n") -cne ($expected.limit -join "`n")) {
+    $failures += "limit.err differs ($($limitLines.Count) lines): first '$($limitLines[0])', last '$($limitLines[-1])'"
+}
+if ($listing -match '"clean\.err"') { $failures += "stale clean.err was not scratched" }
+if ($listing -match '"bad') { $failures += "a bad*.err file was created" }
+
+if ($failures) { throw ("diagnostics verification failed:`n  " + ($failures -join "`n  ")) }
+Write-Host "screen (rows 0-$($expected.screen.Count - 1)):"
+$rows[0..($expected.screen.Count - 1)] | ForEach-Object { Write-Host "  |$_" }
+Write-Host ("diag.err: {0} lines, limit.err: {1} lines (20 + limit), clean.err scratched, no bad*.err" -f
+    $diagLines.Count, $limitLines.Count)
+Write-Host "verified diagnostics: results, screen, and ERR files match"
diff --git a/newLang/src/common/c128.inc b/newLang/src/common/c128.inc
index 5f1b76f..c7728b3 100644
--- a/newLang/src/common/c128.inc
+++ b/newLang/src/common/c128.inc
@@ -60,6 +60,7 @@ SETNAM          = $FFBD
 OPEN            = $FFC0         ; no-name OPEN sends nothing on the bus
 CLOSE           = $FFC3
 CHKIN           = $FFC6         ; absent device: C=1, A=5, ST=$80
+CHKOUT          = $FFC9
 CLRCHN          = $FFCC
 CHRIN           = $FFCF
 CHROUT          = $FFD2
diff --git a/newLang/src/common/diagcat.inc b/newLang/src/common/diagcat.inc
new file mode 100644
index 0000000..2d242f4
--- /dev/null
+++ b/newLang/src/common/diagcat.inc
@@ -0,0 +1,68 @@
+; diagcat.inc - THE Nova128 diagnostic catalog (E02-S04).
+;
+; One line per diagnostic: DIAGNOSTIC symbol, number, "message"
+; The including file defines the DIAGNOSTIC macro: errors.inc turns each line
+; into a constant E_<symbol>; diagnostics.s turns it into the message table.
+; docs/diagnostics.md is generated from this file by build-diag-test.ps1.
+;
+; Rules (checked by scripts/build-diag-test.ps1):
+; - Numbers are stable. Never renumber or reuse; append within the range.
+; - Ranges:  1-19 source and lexical      20-59 syntax
+;           60-79 names and declarations  80-119 types
+;          120-129 access                130-149 interfaces
+;          150-169 methods and returns   200-219 disk
+;          220-239 resource limits
+; - Messages are uppercase ASCII (PETSCII unshifted letters), at most 39
+;   characters with no arguments. Each % is replaced by an argument (at most
+;   two, each at most 16 characters); the screen clips the result to 39.
+
+; ---------------------------------------------------- source and lexical
+        DIAGNOSTIC SRC_LINE_TOO_LONG,         1, "LINE LONGER THAN 80 CHARACTERS"
+        DIAGNOSTIC SRC_FILE_TOO_LARGE,        2, "SOURCE LARGER THAN 18999 BYTES"
+        DIAGNOSTIC SRC_TOO_MANY_LINES,        3, "MORE THAN 599 LINES"
+        DIAGNOSTIC LEX_INVALID_CHAR,          4, "INVALID CHARACTER"
+        DIAGNOSTIC LEX_IDENT_TOO_LONG,        5, "NAME LONGER THAN 16 CHARACTERS"
+        DIAGNOSTIC LEX_NUMBER_RANGE,          6, "NUMBER LARGER THAN 65535"
+        DIAGNOSTIC LEX_BAD_NUMBER,            7, "MALFORMED NUMBER"
+        DIAGNOSTIC LEX_UNTERMINATED_STRING,   8, "STRING NOT CLOSED ON THIS LINE"
+        DIAGNOSTIC LEX_BAD_ESCAPE,            9, "UNKNOWN ESCAPE SEQUENCE"
+        DIAGNOSTIC LEX_BAD_CHAR_LITERAL,     10, "CHARACTER LITERAL NEEDS ONE CHARACTER"
+
+; ----------------------------------------------------------------- syntax
+        DIAGNOSTIC SYN_EXPECTED_IDENTIFIER,  20, "NAME EXPECTED"
+        DIAGNOSTIC SYN_EXPECTED_EOL,         21, "END OF LINE EXPECTED"
+        DIAGNOSTIC SYN_UNEXPECTED_EOL,       22, "LINE ENDS TOO EARLY"
+        DIAGNOSTIC SYN_EXPECTED_RPAREN,      23, "')' EXPECTED"
+        DIAGNOSTIC SYN_EXPECTED_END_PROGRAM, 24, "END PROGRAM EXPECTED"
+        DIAGNOSTIC SYN_EXPECTED_END_METHOD,  25, "END METHOD EXPECTED"
+        DIAGNOSTIC SYN_EXPECTED_END_IF,      26, "END IF EXPECTED"
+        DIAGNOSTIC SYN_EXPECTED_END_CLASS,   27, "END CLASS EXPECTED"
+        DIAGNOSTIC SYN_EXPECTED_INTERFACE_NAME, 28, "INTERFACE NAME EXPECTED"
+        DIAGNOSTIC SYN_INTERFACE_METHOD_BODY, 29, "INTERFACE METHODS HAVE NO BODY"
+        DIAGNOSTIC SYN_UNEXPECTED_TOKEN,     30, "% IS NOT ALLOWED HERE"
+
+; -------------------------------------------- names and declarations
+        DIAGNOSTIC SEM_DUPLICATE_NAME,       60, "% IS ALREADY DECLARED"
+        DIAGNOSTIC SEM_UNKNOWN_NAME,         61, "% IS NOT DECLARED"
+
+; ------------------------------------------------------------------ types
+        DIAGNOSTIC TYP_MISMATCH,             80, "% EXPECTED; % FOUND"
+        DIAGNOSTIC TYP_CONDITION_NOT_BOOLEAN, 81, "CONDITION MUST BE BOOLEAN"
+
+; ----------------------------------------------------------------- access
+        DIAGNOSTIC ACC_PRIVATE_MEMBER,      120, "% IS PRIVATE"
+
+; ------------------------------------------------------------- interfaces
+        DIAGNOSTIC IFC_METHOD_MISSING,      130, "% IS NOT IMPLEMENTED"
+        DIAGNOSTIC IFC_SIGNATURE_MISMATCH,  131, "% DIFFERS FROM INTERFACE"
+
+; ---------------------------------------------------- methods and returns
+        DIAGNOSTIC SEM_MISSING_RETURN,      150, "MISSING RETURN VALUE"
+
+; ------------------------------------------------------------------- disk
+        DIAGNOSTIC DSK_SOURCE_UNREADABLE,   200, "CANNOT READ SOURCE; DOS %"
+        DIAGNOSTIC DSK_ERR_FILE_UNWRITABLE, 201, "CANNOT WRITE ERROR FILE; DOS %"
+
+; ------------------------------------------------------- resource limits
+        DIAGNOSTIC RES_TOO_MANY_NAMES,      220, "MORE THAN 255 NAMES"
+        DIAGNOSTIC RES_ERROR_LIMIT,         221, "TOO MANY ERRORS; COMPILATION STOPPED"
diff --git a/newLang/src/common/diskio.inc b/newLang/src/common/diskio.inc
index 5b4e647..522f17f 100644
--- a/newLang/src/common/diskio.inc
+++ b/newLang/src/common/diskio.inc
@@ -7,6 +7,8 @@ DIO_DEVICE      = 8
 DIO_LFN_CMD     = 15            ; command/status channel: opened first, closed last
 DIO_LFN_IN      = 2             ; source input
 DIO_SA_IN       = 2
+DIO_LFN_ERR     = 4             ; NAME.ERR output
+DIO_SA_ERR      = 4
 DIO_STATUS_MAX  = 40            ; bytes kept from a drive status line
 
 .endif
diff --git a/newLang/src/common/errors.inc b/newLang/src/common/errors.inc
new file mode 100644
index 0000000..0ea17da
--- /dev/null
+++ b/newLang/src/common/errors.inc
@@ -0,0 +1,16 @@
+; errors.inc - diagnostic numbers as E_<symbol> constants (E02-S04).
+; The catalog itself is src/common/diagcat.inc.
+
+.ifndef ERRORS_INC
+ERRORS_INC = 1
+
+.macro DIAGNOSTIC sym, number, text
+        .ident(.concat("E_", .string(sym))) = number
+.endmacro
+.include "diagcat.inc"
+.delmacro DIAGNOSTIC
+
+DIAG_LIMIT_DEFAULT = 20         ; errors before compilation stops
+DIAG_SCREEN_WIDTH  = 39         ; longest screen line (no editor wrap)
+
+.endif
diff --git a/newLang/src/compiler/diagnostics.s b/newLang/src/compiler/diagnostics.s
new file mode 100644
index 0000000..c8916f4
--- /dev/null
+++ b/newLang/src/compiler/diagnostics.s
@@ -0,0 +1,539 @@
+; diagnostics.s - diagnostic reporting and the ERR writer (E02-S04).
+;
+; Usage (bank-0 compiler state):
+;   diag_init    A/X = source name (bank 0), Y = length. Scratches a stale
+;                NAME.ERR (".N8" replaced by ".ERR"); resets the count.
+;   diag_line/diag_col, optional diag_arg1/diag_arg2 (address + length),
+;   then diag_report with A = E_* number.
+;     C=0: reported, keep going.  C=1: the error limit was reached; the
+;     limit diagnostic has been reported and the caller must stop.
+;   diag_finish  closes NAME.ERR; A = number of errors reported.
+;
+; Screen form (40 columns, no line ever longer than 39 characters):
+;   E010 TEST.N8 3 11
+;   CHARACTER LITERAL NEEDS ONE CHARACTER
+; ERR form, one line per diagnostic, $0D terminated:
+;   E010 TEST.N8 3 11 CHARACTER LITERAL NEEDS ONE CHARACTER
+; NAME.ERR is created only when the first diagnostic is reported. If it cannot
+; be written, DSK_ERR_FILE_UNWRITABLE is shown once on screen and reporting
+; continues on screen only.
+
+.setcpu "6502"
+.include "../common/c128.inc"
+.include "../common/diskio.inc"
+.include "../common/errors.inc"
+
+.import kcall_inline
+.import dio_open_cmd, dio_open_out, dio_read_status, dio_chkout, dio_putc
+.import dio_clrchn, dio_close, dio_command, dio_dos_code
+
+.export diag_init, diag_report, diag_finish
+.export diag_line, diag_col, diag_arg1, diag_arg1_len, diag_arg2, diag_arg2_len
+.export diag_count, diag_limit, diag_screen, diag_file_state, diag_buf
+
+DIAG_BUF_MAX = 96
+NAME_MAX = 16
+
+FILE_NONE   = 0                    ; NAME.ERR not opened yet
+FILE_OPEN   = 1
+FILE_FAILED = 2
+
+; ------------------------------------------------------------------ CODE
+.segment "CODE"
+
+diag_init:
+        sta @src+1
+        stx @src+2
+        cpy #NAME_MAX + 1
+        bcc :+
+        ldy #NAME_MAX
+:       sty name_len
+        ldy #$00
+@copy:  cpy name_len
+        beq @copied
+@src:   lda $FFFF,y                ; patched
+        sta name_text,y
+        iny
+        bne @copy
+@copied:
+        lda #$00
+        sta diag_count
+        sta diag_stopped
+        sta diag_file_state
+        sta diag_arg1_len
+        sta diag_arg2_len
+        lda #DIAG_LIMIT_DEFAULT
+        sta diag_limit
+        lda #$01
+        sta diag_screen
+
+        ; base = name without a trailing ".N8" (either letter range)
+        lda name_len
+        sta base_len
+        cmp #3
+        bcc @names
+        tay
+        lda name_text-1,y
+        cmp #'8'
+        bne @names
+        lda name_text-2,y
+        and #$7F
+        cmp #'N'
+        bne @names
+        lda name_text-3,y
+        cmp #'.'
+        bne @names
+        dey
+        dey
+        dey
+        sty base_len
+@names: ldx #$00                   ; scratch_cmd = "S0:" base ".ERR"
+        ldy #$00
+:       lda scratch_prefix,y
+        sta scratch_cmd,x
+        inx
+        iny
+        cpy #3
+        bne :-
+        jsr append_base
+        stx scratch_len
+        ldx #$00                   ; open_name = base ".ERR,S,W"
+        jsr append_base_to_open
+        stx open_len
+
+        jsr dio_open_cmd           ; remove any stale NAME.ERR
+        bcs @done
+        lda #<scratch_cmd
+        ldx #>scratch_cmd
+        ldy scratch_len
+        jsr dio_command
+        lda #DIO_LFN_CMD
+        jsr dio_close
+@done:  rts
+
+; Append base + ".ERR" to scratch_cmd at X.
+append_base:
+        ldy #$00
+:       cpy base_len
+        beq :+
+        lda name_text,y
+        sta scratch_cmd,x
+        inx
+        iny
+        bne :-
+:       ldy #$00
+:       lda err_suffix,y
+        sta scratch_cmd,x
+        inx
+        iny
+        cpy #4
+        bne :-
+        rts
+
+append_base_to_open:
+        ldy #$00
+:       cpy base_len
+        beq :+
+        lda name_text,y
+        sta open_name,x
+        inx
+        iny
+        bne :-
+:       ldy #$00
+:       lda err_suffix,y
+        sta open_name,x
+        inx
+        iny
+        cpy #8
+        bne :-
+        rts
+
+diag_report:
+        ldx diag_stopped
+        beq :+
+        sec                        ; already stopped: report nothing more
+        rts
+:       ldx diag_count
+        cpx diag_limit
+        bcc @count
+        lda #E_RES_ERROR_LIMIT     ; the (limit+1)th error stops compilation
+        sta diag_code
+        lda #$00
+        sta diag_arg1_len
+        sta diag_arg2_len
+        lda #$01
+        sta diag_stopped
+        jsr emit
+        sec
+        rts
+@count: sta diag_code
+        inc diag_count
+        jsr emit
+        clc
+        rts
+
+; Report to screen and file, then clear the arguments.
+emit:   jsr format
+        lda diag_screen
+        beq :+
+        jsr to_screen
+:       jsr to_file
+        lda #$00
+        sta diag_arg1_len
+        sta diag_arg2_len
+        rts
+
+diag_finish:
+        lda diag_file_state
+        cmp #FILE_OPEN
+        bne @done
+        jsr dio_clrchn
+        lda #DIO_LFN_ERR
+        jsr dio_close
+        lda #DIO_LFN_CMD           ; command channel last
+        jsr dio_close
+        lda #FILE_NONE
+        sta diag_file_state
+@done:  lda diag_count
+        rts
+
+; --------------------------------------------------------------- format
+; diag_buf = "Ennn NAME line col message"; hdr_len, msg_start, buf_len.
+format: ldx #$00
+        lda #'E'
+        jsr put
+        lda diag_code              ; three digits, zero padded
+        ldy #'0' - 1
+:       iny
+        sec
+        sbc #100
+        bcs :-
+        adc #100
+        pha
+        tya
+        jsr put
+        pla
+        ldy #'0' - 1
+:       iny
+        sec
+        sbc #10
+        bcs :-
+        adc #10 + '0'
+        pha
+        tya
+        jsr put
+        pla
+        jsr put
+        lda #' '
+        jsr put
+        ldy #$00
+:       cpy name_len
+        beq :+
+        lda name_text,y
+        jsr put
+        iny
+        bne :-
+:       lda #' '
+        jsr put
+        lda diag_line
+        ldy diag_line+1
+        jsr put_decimal
+        lda #' '
+        jsr put
+        lda diag_col
+        ldy #$00
+        jsr put_decimal
+        stx hdr_len
+        lda #' '
+        jsr put
+        stx msg_start
+
+        jsr find_message           ; template address -> fmt_tpl
+        lda #$00
+        sta arg_index
+        ldy #$00
+fmt_loop:
+        sty tpl_y
+fmt_tpl:
+        lda $FFFF,y                ; patched by find_message
+        beq fmt_done
+        cmp #'%'
+        bne fmt_plain
+        jsr put_arg
+        jmp fmt_next
+fmt_plain:
+        jsr put
+fmt_next:
+        ldy tpl_y
+        iny
+        bne fmt_loop
+fmt_done:
+        stx buf_len
+        rts
+
+; X = index; store A unless the buffer is full.
+put:    cpx #DIAG_BUF_MAX
+        bcs :+
+        sta diag_buf,x
+        inx
+:       rts
+
+; Append argument 1 or 2 (in order of the % markers).
+put_arg:
+        lda arg_index              ; 0 = first %, otherwise the second
+        inc arg_index
+        cmp #$00
+        bne @two
+        lda diag_arg1
+        sta @src+1
+        lda diag_arg1+1
+        sta @src+2
+        lda diag_arg1_len
+        jmp @copy
+@two:   lda diag_arg2
+        sta @src+1
+        lda diag_arg2+1
+        sta @src+2
+        lda diag_arg2_len
+@copy:  sta arg_len
+        ldy #$00
+:       cpy arg_len
+        beq @done
+@src:   lda $FFFF,y                ; patched
+        jsr put
+        iny
+        bne :-
+@done:  rts
+
+; Append the unsigned 16-bit value A (low) / Y (high) in decimal.
+put_decimal:
+        sta num
+        sty num+1
+        lda #$00
+        sta num_started
+        ldy #4                     ; 10000, 1000, 100, 10, 1
+@digit: lda #'0'
+        sta num_digit
+@sub:   lda num
+        sec
+        sbc pow_lo,y
+        sta num_t
+        lda num+1
+        sbc pow_hi,y
+        bcc @emit
+        sta num+1
+        lda num_t
+        sta num
+        inc num_digit
+        bne @sub
+@emit:  lda num_digit
+        cpy #$00                   ; always print the units digit
+        beq @out
+        cmp #'0'
+        bne @out
+        lda num_started
+        beq @skip
+        lda num_digit
+@out:   sta num_started
+        jsr put
+@skip:  dey
+        bpl @digit
+        rts
+
+; Point the fmt_tpl load in format at the template for diag_code.
+; Preserves X: it is format's buffer index.
+find_message:
+        ldy #$00
+@scan:  lda catalog,y
+        beq @unknown
+        cmp diag_code
+        beq @found
+        iny
+        iny
+        iny
+        bne @scan
+@unknown:
+        lda #<unknown_text
+        sta fmt_tpl+1
+        lda #>unknown_text
+        sta fmt_tpl+2
+        rts
+@found: lda catalog+1,y
+        sta fmt_tpl+1
+        lda catalog+2,y
+        sta fmt_tpl+2
+        rts
+
+; ------------------------------------------------------------- outputs
+to_screen:
+        ldy #$00
+:       cpy hdr_len
+        beq :+
+        lda diag_buf,y
+        jsr screen_putc
+        iny
+        bne :-
+:       lda #$0D
+        jsr screen_putc
+        lda msg_start              ; message clipped to DIAG_SCREEN_WIDTH
+        clc
+        adc #DIAG_SCREEN_WIDTH
+        sta clip_end
+        ldy msg_start
+:       cpy buf_len
+        beq :+
+        cpy clip_end
+        beq :+
+        lda diag_buf,y
+        jsr screen_putc
+        iny
+        bne :-
+:       lda #$0D
+        jmp screen_putc
+
+screen_putc:
+        sty out_y
+        jsr kcall_inline
+        .word CHROUT
+        ldy out_y
+        rts
+
+to_file:
+        lda diag_file_state
+        cmp #FILE_FAILED
+        beq @done
+        cmp #FILE_OPEN
+        beq @write
+        jsr open_err_file
+        bcs @done
+@write: ldx #DIO_LFN_ERR
+        jsr dio_chkout
+        bcs @done
+        ldy #$00
+:       cpy buf_len
+        beq :+
+        lda diag_buf,y
+        sty out_y
+        jsr dio_putc
+        ldy out_y
+        iny
+        bne :-
+:       lda #$0D
+        jsr dio_putc
+        jsr dio_clrchn
+@done:  rts
+
+; Open the command channel, then NAME.ERR. On failure close both, mark the
+; file failed, and show DSK_ERR_FILE_UNWRITABLE once on screen. C=1 on failure.
+open_err_file:
+        jsr dio_open_cmd
+        bcs @fail_cmd
+        lda #<open_name
+        ldx #>open_name
+        ldy open_len
+        jsr dio_open_out
+        bcs @fail
+        jsr dio_read_status
+        lda dio_dos_code
+        cmp #20
+        bcs @fail
+        lda #FILE_OPEN
+        sta diag_file_state
+        clc
+        rts
+@fail:  lda #DIO_LFN_ERR
+        jsr dio_close
+@fail_cmd:
+        lda #DIO_LFN_CMD
+        jsr dio_close
+        lda #FILE_FAILED
+        sta diag_file_state
+        lda dio_dos_code           ; screen-only notice; the report that
+                                   ; triggered it is already on screen           ; argument: DOS code as two digits
+        ldy #$00
+        jsr dos_digits
+        lda #E_DSK_ERR_FILE_UNWRITABLE
+        sta diag_code
+        lda #<dos_text
+        sta diag_arg1
+        lda #>dos_text
+        sta diag_arg1+1
+        lda #2
+        sta diag_arg1_len
+        lda #$00
+        sta diag_arg2_len
+        jsr format
+        jsr to_screen
+        sec
+        rts
+
+dos_digits:
+        ldy #'0' - 1
+:       iny
+        sec
+        sbc #10
+        bcs :-
+        adc #10 + '0'
+        sty dos_text
+        sta dos_text+1
+        rts
+
+.segment "RODATA"
+pow_lo: .byte <1, <10, <100, <1000, <10000
+pow_hi: .byte >1, >10, >100, >1000, >10000
+scratch_prefix: .byte "S0:"
+err_suffix:     .byte ".ERR,S,W"
+unknown_text:   .byte "INTERNAL ERROR", 0
+
+; code, template address; 0 ends the table.
+.macro DIAGNOSTIC sym, number, text
+        .local t
+        .byte number
+        .word t
+        .pushseg
+        .segment "RODATA"
+t:      .byte text, 0
+        .popseg
+.endmacro
+.segment "DATA"
+catalog:
+.include "../common/diagcat.inc"
+        .byte 0
+.delmacro DIAGNOSTIC
+.assert * - catalog < 256, error, "catalog table must stay below 256 bytes"
+
+; ------------------------------------------------------------------- BSS
+.segment "BSS"
+diag_line:      .res 2
+diag_col:       .res 1
+diag_arg1:      .res 2
+diag_arg1_len:  .res 1
+diag_arg2:      .res 2
+diag_arg2_len:  .res 1
+diag_count:     .res 1
+diag_limit:     .res 1
+diag_screen:    .res 1
+diag_file_state: .res 1
+diag_stopped:   .res 1
+name_len:       .res 1
+name_text:      .res NAME_MAX
+base_len:       .res 1
+scratch_len:    .res 1
+scratch_cmd:    .res 3 + NAME_MAX + 4
+open_len:       .res 1
+open_name:      .res NAME_MAX + 8
+tpl_y:          .res 1
+arg_index:      .res 1
+arg_len:        .res 1
+num:            .res 2
+num_t:          .res 1
+num_digit:      .res 1
+num_started:    .res 1
+clip_end:       .res 1
+out_y:          .res 1
+dos_text:       .res 2
+diag_code:      .res 1
+hdr_len:        .res 1
+msg_start:      .res 1
+buf_len:        .res 1
+diag_buf:       .res DIAG_BUF_MAX
diff --git a/newLang/src/compiler/diskio.s b/newLang/src/compiler/diskio.s
index 0a418c8..d49f583 100644
--- a/newLang/src/compiler/diskio.s
+++ b/newLang/src/compiler/diskio.s
@@ -14,6 +14,7 @@
 .export dio_open_cmd, dio_open_in, dio_chkin_in, dio_read_status
 .export dio_read_chunk, dio_clrchn, dio_close
 .export dio_count, dio_eof, dio_st, dio_dos_code, dio_status
+.export dio_open_out, dio_chkout, dio_putc, dio_command
 
 .import __DISKBUF_START__
 
@@ -124,6 +125,61 @@ dio_close:
         .word CLOSE
         rts
 
+; OPEN DIO_LFN_ERR,8,DIO_SA_ERR,name for writing. In: A/X = name (bank 0,
+; including ",S,W"), Y = length. Check dio_read_status afterwards.
+dio_open_out:
+        sta dio_tmp
+        stx dio_tmp+1
+        sty dio_tmp+2
+        jsr dio_setbank
+        lda dio_tmp+2
+        ldx dio_tmp
+        ldy dio_tmp+1
+        jsr kcall_inline
+        .word SETNAM
+        lda #DIO_LFN_ERR
+        ldx #DIO_DEVICE
+        ldy #DIO_SA_ERR
+        jsr kcall_inline
+        .word SETLFS
+        jsr kcall_inline
+        .word OPEN
+        rts
+
+; Select logical file X for output. C=1 with A = KERNAL error on failure.
+dio_chkout:
+        jsr kcall_inline
+        .word CHKOUT
+        rts
+
+; Write A to the current output channel.
+dio_putc:
+        jsr kcall_inline
+        .word CHROUT
+        rts
+
+; Send a DOS command (A/X = text, Y = length) on the open command channel,
+; then read the reply into dio_status / dio_dos_code.
+dio_command:
+        sta @src+1
+        stx @src+2
+        sty dio_tmp+2
+        ldx #DIO_LFN_CMD
+        jsr dio_chkout
+        bcs @out
+        ldy #$00
+@byte:  cpy dio_tmp+2
+        beq @sent
+@src:   lda $FFFF,y                ; patched
+        sty dio_tmp
+        jsr dio_putc
+        ldy dio_tmp
+        iny
+        bne @byte
+@sent:  jsr dio_clrchn             ; UNLISTEN executes the command
+        jmp dio_read_status
+@out:   rts
+
 dio_setbank:
         lda #$00                   ; data and filename in bank 0
         tax
diff --git a/newLang/tests/diag/diag-test.s b/newLang/tests/diag/diag-test.s
new file mode 100644
index 0000000..7bf85dc
--- /dev/null
+++ b/newLang/tests/diag/diag-test.s
@@ -0,0 +1,199 @@
+; E02-S04: diagnostics and ERR-writer test in x128.
+;
+; Phase A  DIAG.N8   six representative diagnostics on a cleared screen
+; Phase D  BAD*.N8   ERR name the drive refuses: report, screen-only notice,
+;                    second report on screen only
+; Phase B  LIMIT.N8  25 errors, screen off: 20 reported, then the limit
+; Phase C  CLEAN.N8  no errors: the stale CLEAN.ERR is scratched, none made
+; Results (address in MB_DATA+0/1):
+;   +0 A count  +1 D count  +2 D file state  +3 B count  +4 C count
+;   +5..+8  LDTND after A, D, B, C
+;   +9..+33 carry returned by each of B's 25 diag_report calls
+; Mailbox PASS means the program ran to the end; the host checks the rest.
+
+.setcpu "6502"
+.include "../../src/common/c128.inc"
+.include "../../src/common/mailbox.inc"
+.include "../../src/common/errors.inc"
+
+.import mmu_init, mmu_bank0, mmu_shutdown, kcall_inline
+.import diag_init, diag_report, diag_finish, diag_line, diag_col
+.import diag_arg1, diag_arg1_len, diag_arg2, diag_arg2_len
+.import diag_screen, diag_file_state
+.import __COMPILER_ZP_PAGE__
+
+.segment "LOADADDR"
+        .word $1C01
+
+.segment "EXEHDR"
+        .word basic_end
+        .word 10
+        .byte $9E
+        .byte "7181"
+        .byte $00
+basic_end:
+        .word $0000
+
+.segment "MAILBOX"
+        .res MB_SIZE
+
+.macro REPORT line, col, code
+        lda #<(line)
+        sta diag_line
+        lda #>(line)
+        sta diag_line+1
+        lda #col
+        sta diag_col
+        lda #code
+        jsr diag_report
+.endmacro
+
+.macro ARG1 text, len
+        lda #<text
+        sta diag_arg1
+        lda #>text
+        sta diag_arg1+1
+        lda #len
+        sta diag_arg1_len
+.endmacro
+
+.macro ARG2 text, len
+        lda #<text
+        sta diag_arg2
+        lda #>text
+        sta diag_arg2+1
+        lda #len
+        sta diag_arg2_len
+.endmacro
+
+.macro INIT name, len
+        lda #<name
+        ldx #>name
+        ldy #len
+        jsr diag_init
+.endmacro
+
+.segment "STARTUP"
+.export test_entry, results
+test_entry:
+        php
+        sei
+        ldx #MB_SIZE - 1
+        lda #$00
+@clr:   sta MB_BASE,x
+        dex
+        bpl @clr
+        lda #MB_MAGIC0_VALUE
+        sta MB_MAGIC0
+        lda #MB_MAGIC1_VALUE
+        sta MB_MAGIC1
+        lda #MB_PROD_PLATFORM
+        sta MB_PRODUCER
+        lda #MB_ST_RUNNING
+        sta MB_STATUS
+        jsr mmu_init
+        jsr mmu_bank0
+
+        lda #$93                   ; clear screen: the host reads it back
+        jsr kcall_inline
+        .word CHROUT
+
+        ; ------------------------------------------------------ phase A
+        INIT n_diag, 7
+        REPORT 3, 11, E_LEX_BAD_CHAR_LITERAL
+        REPORT 599, 80, E_SYN_EXPECTED_END_PROGRAM
+        ARG1 a_counter, 7
+        REPORT 12, 5, E_SEM_DUPLICATE_NAME
+        ARG1 a_integer, 7
+        ARG2 a_long, 16
+        REPORT 40, 17, E_TYP_MISMATCH
+        ARG1 a_dos, 2
+        REPORT 1, 1, E_DSK_SOURCE_UNREADABLE
+        REPORT 65535, 255, E_SYN_EXPECTED_RPAREN
+        jsr diag_finish
+        sta results+0
+        jsr open_files
+        sta results+5
+
+        ; ------------------------------------------------------ phase D
+        INIT n_bad, 7
+        REPORT 7, 3, E_LEX_INVALID_CHAR
+        REPORT 8, 4, E_LEX_INVALID_CHAR
+        lda diag_file_state
+        sta results+2
+        jsr diag_finish
+        sta results+1
+        jsr open_files
+        sta results+6
+
+        ; ------------------------------------------------------ phase B
+        INIT n_limit, 8
+        lda #$00
+        sta diag_screen
+        ldx #$00
+@limit: stx b_index
+        inx                        ; line = 1..25
+        stx diag_line
+        lda #$00
+        sta diag_line+1
+        lda #1
+        sta diag_col
+        lda #E_LEX_INVALID_CHAR
+        jsr diag_report
+        lda #$00
+        rol a                      ; carry -> 0/1
+        ldx b_index
+        sta results+9,x
+        inx
+        cpx #25
+        bne @limit
+        jsr diag_finish
+        sta results+3
+        jsr open_files
+        sta results+7
+
+        ; ------------------------------------------------------ phase C
+        INIT n_clean, 8
+        jsr diag_finish
+        sta results+4
+        jsr open_files
+        sta results+8
+
+        jsr mmu_shutdown
+        lda #<results
+        sta MB_DATA+0
+        lda #>results
+        sta MB_DATA+1
+        lda #MB_ST_PASS
+        sta MB_STATUS
+        plp
+        ldy #$00
+@print: lda ok_msg,y
+        beq @end
+        sty b_index
+        jsr CHROUT
+        ldy b_index
+        iny
+        bne @print
+@end:   rts
+
+.segment "CODE"
+; A = number of open logical files (system zero page via the P0 swap).
+open_files:
+        lda __COMPILER_ZP_PAGE__ * 256 + LDTND
+        rts
+
+.segment "RODATA"
+n_diag:    .byte "DIAG.N8"
+n_bad:     .byte "BAD*.N8"
+n_limit:   .byte "LIMIT.N8"
+n_clean:   .byte "CLEAN.N8"
+a_counter: .byte "COUNTER"
+a_integer: .byte "INTEGER"
+a_long:    .byte "VERYLONGCLASSNAM"
+a_dos:     .byte "62"
+ok_msg:    .byte "NOVA128 DIAG TEST OK", $0D, $00
+
+.segment "BSS"
+results:   .res 34
+b_index:   .res 1
diff --git a/newLang/tests/grammar/README.md b/newLang/tests/grammar/README.md
index 47a313f..125e31b 100644
--- a/newLang/tests/grammar/README.md
+++ b/newLang/tests/grammar/README.md
@@ -20,8 +20,10 @@ The validator enforces the Vim editor profile for every `.n8` fixture:
 - one symbolic diagnostic expectation for every rejected fixture;
 - no diagnostic sidecars for accepted fixtures.
 
-`.expect` files deliberately use symbolic diagnostic names. Story E02-S04 will
-assign stable numeric codes without requiring grammar fixtures to guess them.
+`.expect` files deliberately use symbolic diagnostic names. E02-S04 assigned
+their stable numbers in `src/common/diagcat.inc` (listed in
+`docs/diagnostics.md`); `scripts/build-diag-test.ps1` fails if an `.expect`
+name is missing from the catalog.
 
 ## Accepted Coverage