feat(tools): salience-aware shell output compression — retain error lines through HeadTail (#67)

HeadTail now accepts an optional salience regex + cap: middle lines matching
it (error|fail|exception|panic|traceback|✗, case-insensitive) survive
truncation in place instead of being silently dropped, so a decisive error
buried in the middle of a long build/test log still reaches the model-facing
context entry. ShellTool's outputCompressor spec wires this in; the raw
build-gate receipt path is untouched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

core/tools/src/main/kotlin/com/correx/core/tools/compression/DeclarativeCompressor.kt

@@ -41,10 +41,38 @@ class DeclarativeCompressor(private val spec: OutputCompressionSpec) : ToolOutpu
 
     private fun applyHeadTail(rule: HeadTail, lines: List<String>): List<String> {
         if (lines.size <= rule.head + rule.tail) return lines
-        val elided = lines.size - rule.head - rule.tail
-        return lines.take(rule.head) +
-            "… $elided lines elided …" +
-            lines.takeLast(rule.tail)
+        val middle = lines.subList(rule.head, lines.size - rule.tail)
+        val salienceRegex = rule.salience?.let { Regex(it, RegexOption.IGNORE_CASE) }
+        val middleOut = if (salienceRegex == null) {
+            listOf("… ${middle.size} lines elided …")
+        } else {
+            renderSalientMiddle(middle, salienceRegex, rule.salienceCap)
+        }
+        return lines.take(rule.head) + middleOut + lines.takeLast(rule.tail)
+    }
+
+    // Walks the elided middle, keeping order and retaining up to [cap] salient lines in place;
+    // runs of dropped lines around them collapse into a single elision marker each, so a decisive
+    // error at line 250 of 500 survives even though the bulk of the log is still bounded.
+    private fun renderSalientMiddle(middle: List<String>, salienceRegex: Regex, cap: Int): List<String> {
+        val out = ArrayList<String>()
+        var elidedCount = 0
+        var keptSalient = 0
+        for (line in middle) {
+            val isSalient = keptSalient < cap && salienceRegex.containsMatchIn(line)
+            if (isSalient) {
+                if (elidedCount > 0) {
+                    out += "… $elidedCount lines elided …"
+                    elidedCount = 0
+                }
+                out += line
+                keptSalient++
+            } else {
+                elidedCount++
+            }
+        }
+        if (elidedCount > 0) out += "… $elidedCount lines elided …"
+        return out
     }
 
     private fun collapseConsecutive(lines: List<String>): List<String> {

core/tools/src/main/kotlin/com/correx/core/tools/compression/OutputCompressionSpec.kt

@@ -38,11 +38,20 @@ sealed interface CompressionRule {
 
     /**
      * When line count exceeds [head] + [tail], keep the first [head] and last
-     * [tail] lines, replacing the middle with a single elision marker line.
+     * [tail] lines. If [salience] is set, middle lines matching it (case-insensitive)
+     * are also retained in place — up to [salienceCap] of them — so a decisive error
+     * buried in the middle of a long log survives truncation; remaining elided runs
+     * collapse to a single "… N lines elided …" marker. Without [salience] the whole
+     * middle collapses to one marker, as before.
      */
     @Serializable
     @SerialName("head_tail")
-    data class HeadTail(val head: Int, val tail: Int) : CompressionRule
+    data class HeadTail(
+        val head: Int,
+        val tail: Int,
+        val salience: String? = null,
+        val salienceCap: Int = 30,
+    ) : CompressionRule
 
     /**
      * Collapse runs of identical adjacent lines into a single line, appending an

core/tools/src/test/kotlin/com/correx/core/tools/compression/DeclarativeCompressorTest.kt

@@ -44,6 +44,30 @@ class DeclarativeCompressorTest {
         assertEquals("line1\nline2\n… 6 lines elided …\nline9\nline10", out)
     }
 
+    @Test
+    fun `HeadTail with salience retains a matching middle line otherwise elided`() {
+        val lines = (1..20).map { if (it == 10) "boom: ERROR occurred" else "line$it" }
+        val out = compress(OutputCompressionSpec(listOf(HeadTail(2, 2, salience = "error"))), lines.joinToString("\n"))
+        assertEquals(
+            "line1\nline2\n… 7 lines elided …\nboom: ERROR occurred\n… 8 lines elided …\nline19\nline20",
+            out,
+        )
+    }
+
+    @Test
+    fun `HeadTail with salience caps the number of retained salient lines`() {
+        val lines = (1..20).map { "error$it" }
+        val out = compress(OutputCompressionSpec(listOf(HeadTail(0, 0, salience = "error", salienceCap = 3))), lines.joinToString("\n"))
+        assertEquals("error1\nerror2\nerror3\n… 17 lines elided …", out)
+    }
+
+    @Test
+    fun `HeadTail with salience preserves order and behaves like plain HeadTail when nothing matches`() {
+        val raw = (1..10).joinToString("\n") { "line$it" }
+        val out = compress(OutputCompressionSpec(listOf(HeadTail(2, 2, salience = "error"))), raw)
+        assertEquals("line1\nline2\n… 6 lines elided …\nline9\nline10", out)
+    }
+
     @Test
     fun `HeadTail is a no-op at or under threshold`() {
         val raw = (1..4).joinToString("\n") { "line$it" }

infrastructure/tools/src/main/kotlin/com/correx/infrastructure/tools/shell/ShellTool.kt

@@ -62,7 +62,11 @@ class ShellTool(
             // head/tail window holds distinct content rather than duplicates. Dedup is lossless
             // (count is preserved) and order-preserving, so it is safe as a default — unlike a
             // progress-dropping regex, which can silently eat real output.
-            listOf(StripBlankLines, CollapseConsecutiveDuplicates, HeadTail(HEAD_LINES, TAIL_LINES)),
+            listOf(
+                StripBlankLines,
+                CollapseConsecutiveDuplicates,
+                HeadTail(HEAD_LINES, TAIL_LINES, salience = SALIENCE_PATTERN),
+            ),
             bypassOnError = true,
         ),
     )
@@ -213,6 +217,9 @@ class ShellTool(
     private companion object {
         const val HEAD_LINES = 40
         const val TAIL_LINES = 40
+        // Case-insensitive: a decisive error line buried in the middle of a long build/test log
+        // (line 250 of 500) survives the head/tail window instead of being silently dropped.
+        const val SALIENCE_PATTERN = "error|fail|exception|panic|traceback|✗"
         const val EMPTY_MSG = "Missing or empty 'argv' parameter. Expected a JSON array of strings."
         val SHELL_BUILTINS = setOf("cd", "export", "source", ".", "pushd", "popd", "umask", "ulimit")
         val SHELL_OPERATORS = setOf("&&", "||", "|", ">", ">>", "<", ";", "&", "2>", "2>&1")