perf(judge): earlier check outputs shrink to a reminder before the next round

Measured on prod: 7 of 9 verdicts ran to the 12-check cap. Every round
resends the whole history, and each check's output is bounded at 12 KB — so
by the last round the judge was paying for ~144 KB of outputs it had already
read, on top of up to 120 KB of evidence, and it paid that on every round.
That is the quadratic term in a verdict's cost, and the reason a single
blocked phase could empty a weekly plan.

Before this round's results go in, every earlier tool result compacts to an
800-byte head plus a marker saying the rest was shown when the check ran.
The round that just ran stays whole; a result already carrying the marker is
left alone. The budget of checks is unchanged — each one is cheaper to
remember, not fewer to run.

Also: docs/NEXT-SESSION.md rewritten for the state as of today.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WZb5A2kfVfjpdwSochkuHz

crates/cm-api/src/evaluator.rs

@@ -767,6 +767,9 @@ async fn judge_with_tools(
                 content: Value::String(evidence),
             });
         }
+        // Everything the judge has already read shrinks to a reminder before
+        // this round's results go in full. See `compact_earlier_results`.
+        compact_earlier_results(&mut messages);
         messages.push(ChatMessage {
             role: ChatRole::User,
             parts: results,
@@ -775,6 +778,44 @@ async fn judge_with_tools(
     Err("evaluator exceeded its verification budget without reaching a verdict".into())
 }
 
+/// How much of an earlier check's output stays in the history.
+///
+/// Enough to recognise the command and its outcome — a test summary line, a
+/// grep hit, an error — not enough to re-read the whole thing, which the judge
+/// already did in the round it arrived.
+const KEPT_OF_EARLIER_RESULT: usize = 800;
+
+/// Shrink every tool result from EARLIER rounds to a short head.
+///
+/// The judge's history is resent whole on every round, and each check's
+/// output is bounded at `evaluator_tools::MAX_OUTPUT_BYTES` (12 KB). Measured
+/// on prod, 7 of 9 verdicts ran to the 12-check cap, so by the last round the
+/// history carried ~144 KB of outputs the judge had already read, on top of
+/// up to 120 KB of evidence — and every round paid for all of it again. That
+/// is the quadratic term in a verdict's cost, and it is why one blocked phase
+/// could empty a weekly plan.
+///
+/// The round that just ran keeps its results in full; only what came before
+/// is compacted, and it is compacted once — a result already carrying the
+/// marker is left alone. The judge's budget of checks is unchanged: this
+/// makes each check cheaper to remember, not fewer to run.
+fn compact_earlier_results(messages: &mut [cm_llm::ChatMessage]) {
+    use cm_llm::ContentPart;
+    const MARKER: &str = "\n[… output elided here — it was shown in full when this check ran]";
+    for m in messages.iter_mut() {
+        for part in m.parts.iter_mut() {
+            if let ContentPart::ToolResult { content, .. } = part {
+                if let Some(text) = content.as_str() {
+                    if text.len() > KEPT_OF_EARLIER_RESULT && !text.ends_with(MARKER) {
+                        let kept = head(text, KEPT_OF_EARLIER_RESULT);
+                        *content = Value::String(format!("{kept}{MARKER}"));
+                    }
+                }
+            }
+        }
+    }
+}
+
 /// Parse the model's reply into a verdict, failing closed.
 fn parse_verdict(model: &str, text: &str) -> Verdict {
     let trimmed = text.trim();
@@ -925,6 +966,41 @@ pub async fn latest(
 
 #[cfg(test)]
 mod cross_provider_tests {
+    /// The quadratic term: earlier outputs resent whole every round.
+    #[test]
+    fn earlier_tool_results_shrink_and_the_latest_stays_whole() {
+        use cm_llm::{ChatMessage, ChatRole, ContentPart};
+        let big = "line of output\n".repeat(900); // ~13 KB
+        let mut messages = vec![
+            ChatMessage {
+                role: ChatRole::User,
+                parts: vec![ContentPart::text("judge this")],
+            },
+            ChatMessage {
+                role: ChatRole::User,
+                parts: vec![
+                    ContentPart::ToolResult { tool_use_id: "a".into(), content: Value::String(big.clone()) },
+                    ContentPart::ToolResult { tool_use_id: "b".into(), content: Value::String(big.clone()) },
+                ],
+            },
+        ];
+        compact_earlier_results(&mut messages);
+        for part in &messages[1].parts {
+            let ContentPart::ToolResult { content, .. } = part else { panic!() };
+            let s = content.as_str().unwrap();
+            assert!(s.len() < KEPT_OF_EARLIER_RESULT + 120, "not compacted: {} bytes", s.len());
+            assert!(s.starts_with("line of output"), "the head survives");
+            assert!(s.contains("elided"), "and says so");
+        }
+        // Idempotent: a second pass must not shrink the reminder further.
+        let once: Vec<String> = messages[1].parts.iter().map(|p| serde_json::to_string(p).unwrap()).collect();
+        compact_earlier_results(&mut messages);
+        let twice: Vec<String> = messages[1].parts.iter().map(|p| serde_json::to_string(p).unwrap()).collect();
+        assert_eq!(once, twice);
+        // Plain text parts are untouched.
+        assert!(matches!(&messages[0].parts[0], ContentPart::Text { text } if text == "judge this"));
+    }
+
     /// `LlmEvent::Usage` arrives on every provider call. It was matched by
     /// `Ok(_) => {}` and dropped, which is how two plan exhaustions happened
     /// with no row anywhere saying a judge token was spent.