fix: distinguish compiler errors and report compilation termination

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

2 files changed, 12 insertions, 4 deletions

diff --git a/AGENTS/ARCH.md b/AGENTS/ARCH.md
index 181c207..8245027 100644
--- a/AGENTS/ARCH.md
+++ b/AGENTS/ARCH.md
@@ -57,12 +57,17 @@ release because MSYS2 has no Pandoc package. Only the Linux build is verified.
 Driver behavior (`tools/driver/main.cc`):
 
 - One source file, with optional `-o path` and `-r`; options may precede or follow the source.
-  No stdin source input. No arguments exits silently with status 0 (help TODO).
+  No stdin source input. Missing input, including no arguments, prints
+  `cpsc: fatal error: no input files` and `compilation terminated.`, then returns 1.
 - IR is written only after successful parsing, code generation and module
   verification. Diagnostics stay on stderr; failures do not overwrite existing IR.
 - Lexer, Parser and CodeGen track error flags (`hadError()`); any error, including
   invalid IR from `verifyModule`, makes the driver return 1. File/argument errors
-  also return 1. Check the exit code rather than grepping stderr.
+  also return 1. Ordinary compiler errors use `error:`; failures that prevent
+  compilation, such as file I/O errors, use `fatal error:`. The driver prints
+  `compilation terminated.` once on compilation failure, including argument errors.
+  Execution-stage failures do not print that line. Check the exit code rather
+  than grepping stderr.
 - Successful compilation overwrites the output, but paths aliasing the source
   (including symlinks/hard links) are rejected.
 - In PATH mode, `-r` executes PATH's `lli` using LLVM Support process APIs, with
diff --git a/AGENTS/CONVENTIONS.md b/AGENTS/CONVENTIONS.md
index a49b381..638013a 100644
--- a/AGENTS/CONVENTIONS.md
+++ b/AGENTS/CONVENTIONS.md
@@ -92,12 +92,15 @@ scope stack. `FunctionGen` saves/clears/restores the table around a function bod
 `emitExpr` walk a linear if-chain of casts. **A statement type missing from the chain is
 silently ignored** — no error. Adding an AST node always means touching these chains.
 
-**Error style everywhere: print an `Error:` line, set the component's error flag, and
+**Compiler error style: print an `error:` line, set the component's error flag, and
 keep compiling.** No exceptions, no exit mid-compile. `Lexer`/`Parser`/`CodeGen` each
 expose `hadError()`; `CodeGen::reportError(fmt, ...)` is the helper for codegen-side
 diagnostics (`ArrayHandler` reaches it through its `CodeGen &` parameter,
 `FunctionGen`/`ArithmeticHandler` hold a `bool &HadError` reference). The driver exits
-non-zero when any flag is set. The parser drops a failed statement and resyncs at the
+non-zero when any flag is set, printing `compilation terminated.` once. Errors that
+prevent compilation (missing input, file I/O or invalid IR) use `fatal error:`.
+Execution-stage errors do not print the compilation termination line.
+The parser drops a failed statement and resyncs at the
 next token that can start a statement or end a block (`syncToStatementStart`), then
 continues; the failure is always diagnosed and reflected in the exit code.