fix: don't abort when stderr or stdout is closed early (#40)

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 All notable changes to this project will be documented in this file.
 
+## Unreleased
+
+- **A closed stderr or stdout pipe no longer aborts the run (#40).** `context-builder … 2>&1 | head -3` used to panic with exit code 101 and could leave the output file missing. Progress and warning lines now ignore write errors, and `-o - | head` ends with exit code 0.
+
 ## v0.11.0 (2026-10-10) — "Quiet Defaults"
 
 v0.11.0 trims what a default run puts in the document and removes the surprises around it. **Breaking changes** (details below):

src/cache.rs

@@ -117,7 +117,11 @@ impl CacheManager {
         for pattern in &old_cache_patterns {
             let old_cache_path = self.cache_dir.join(pattern);
             if old_cache_path.exists() {
-                eprintln!("Migrating old cache format: removing {}", pattern);
+                let _ = writeln!(
+                    std::io::stderr(),
+                    "Migrating old cache format: removing {}",
+                    pattern
+                );
                 let _ = fs::remove_file(&old_cache_path);
             }
         }
@@ -128,7 +132,11 @@ impl CacheManager {
                 let file_name = entry.file_name();
                 let name = file_name.to_string_lossy();
                 if name.ends_with(".md") && (name.contains("_20") || name.starts_with("output_")) {
-                    eprintln!("Migrating old cache format: removing {}", name);
+                    let _ = writeln!(
+                        std::io::stderr(),
+                        "Migrating old cache format: removing {}",
+                        name
+                    );
                     let _ = fs::remove_file(entry.path());
                 }
             }

src/config.rs

@@ -1,5 +1,6 @@
 use serde::Deserialize;
 use std::fs;
+use std::io::Write as _;
 use std::path::Path;
 
 use crate::content_filter::{self, DEFAULT_MAX_FILE_SIZE_BYTES};
@@ -222,7 +223,8 @@ pub fn load_config() -> Option<Config> {
         match toml::from_str(&content) {
             Ok(config) => Some(config),
             Err(e) => {
-                eprintln!(
+                let _ = writeln!(
+                    std::io::stderr(),
                     "⚠️  Failed to parse context-builder.toml: {}. Config will be ignored.",
                     e
                 );
@@ -243,7 +245,8 @@ pub fn load_config_from_path(project_root: &Path) -> Option<Config> {
         match toml::from_str(&content) {
             Ok(config) => Some(config),
             Err(e) => {
-                eprintln!(
+                let _ = writeln!(
+                    std::io::stderr(),
                     "⚠️  Failed to parse {}: {}. Config will be ignored.",
                     config_path.display(),
                     e

src/content_filter.rs

@@ -423,7 +423,7 @@ pub fn report_skips(skipped: &[SkippedFile], silent: bool) {
         return;
     }
     for line in report_lines(skipped) {
-        eprintln!("{line}");
+        let _ = writeln!(std::io::stderr(), "{line}");
     }
 }
 

src/file_utils.rs

@@ -753,7 +753,7 @@ fn can_prompt(stdin_tty: bool, stderr_tty: bool) -> bool {
 /// Prompts must not go to stdout: `-o -` and any caller capturing stdout would
 /// otherwise treat the question as document content.
 fn prompt_yes(prompt: &str) -> io::Result<bool> {
-    eprint!("{prompt}");
+    let _ = write!(std::io::stderr(), "{prompt}");
     io::stderr().flush()?;
     let mut input = String::new();
     io::stdin().read_line(&mut input)?;