fix: better validate for dev_brain and 50k reminder not triggering rebuild

src/engine.rs

@@ -568,14 +568,15 @@ impl Generator {
         mut progress: impl FnMut(u32, u32, Option<f32>),
         mut phase: impl FnMut(&'static str),
     ) -> Result<GenerationOutput, String> {
-        let history = messages
-            .split_last()
-            .map_or(messages, |(_, history)| history);
         let checkpoint_present = checkpoint.is_file();
-        let selected = self.select_checkpoint(
-            checkpoint,
-            conversation_tag(&settings.system_prompt, settings.reasoning_mode, history),
-        )?;
+        let selected = self.select_checkpoint(checkpoint, |tag| {
+            checkpoint_matches_prefix(
+                tag,
+                &settings.system_prompt,
+                settings.reasoning_mode,
+                messages,
+            )
+        })?;
         if checkpoint_present && !selected.found {
             phase(checkpoint_rebuild_activity(selected.incompatible));
         }
@@ -788,7 +789,10 @@ impl Generator {
             return Ok(self.checkpoint.clone());
         }
         if let Some(entry) = store.find(key, self.executor.context()) {
-            if self.select_checkpoint(&entry.checkpoint, entry.tag)?.found {
+            if self
+                .select_checkpoint(&entry.checkpoint, |tag| tag == entry.tag)?
+                .found
+            {
                 store.touch(&entry)?;
                 self.last_store_tokens = entry.tokens;
                 return Ok(Some(entry.checkpoint));
@@ -955,10 +959,10 @@ impl Generator {
     fn select_checkpoint(
         &mut self,
         checkpoint: &Path,
-        expected_tag: [u8; 32],
+        matches: impl Fn([u8; 32]) -> bool,
     ) -> Result<CheckpointSelection, String> {
         let resident_hit = self.activate_resident(checkpoint.to_owned())?;
-        if resident_hit && self.executor.checkpoint_tag() == expected_tag {
+        if resident_hit && matches(self.executor.checkpoint_tag()) {
             self.checkpoint = Some(checkpoint.to_owned());
             self.metrics.kv_lookup(KvLookup::MemoryHit);
             return Ok(CheckpointSelection {
@@ -976,7 +980,7 @@ impl Generator {
                     incompatible: false,
                 });
             }
-            let found = self.executor.checkpoint_tag() == expected_tag;
+            let found = matches(self.executor.checkpoint_tag());
             self.metrics.kv_lookup(if found {
                 KvLookup::MemoryHit
             } else {
@@ -995,7 +999,7 @@ impl Generator {
             .load_checkpoint(checkpoint, &mut |bytes| self.metrics.kv_read_bytes(bytes));
         self.metrics
             .kv_read_finished(started.elapsed(), loaded.is_err());
-        let found = matches!(loaded, Ok(true)) && self.executor.checkpoint_tag() == expected_tag;
+        let found = matches!(loaded, Ok(true)) && matches(self.executor.checkpoint_tag());
         let incompatible = loaded.is_err();
         let lookup = match loaded {
             Ok(true) if found => KvLookup::DiskHit,
@@ -1527,6 +1531,20 @@ fn conversation_tag(system: &str, reasoning: ReasoningMode, messages: &[ChatTurn
     Sha256::digest(conversation_key(system, reasoning, messages)).into()
 }
 
+#[cfg(target_os = "macos")]
+fn checkpoint_matches_prefix(
+    checkpoint: [u8; 32],
+    system: &str,
+    reasoning: ReasoningMode,
+    messages: &[ChatTurn],
+) -> bool {
+    // ponytail: appended control messages are few; carry incremental hashes if
+    // scanning a genuinely changed, very long history becomes measurable.
+    (0..messages.len())
+        .rev()
+        .any(|end| checkpoint == conversation_tag(system, reasoning, &messages[..end]))
+}
+
 #[cfg(target_os = "macos")]
 fn resident_key(directory: &Path, tag: [u8; 32]) -> PathBuf {
     let mut name = String::with_capacity(64);
@@ -1735,6 +1753,53 @@ mod sampling_tests {
         assert!(conversation_key("System", ReasoningMode::High, &messages).starts_with(&prefix));
     }
 
+    #[test]
+    fn checkpoint_tag_accepts_an_unchanged_prefix_before_reminders() {
+        let mut messages = vec![ChatTurn {
+            user: true,
+            tool: false,
+            system: false,
+            skip_previous_eos: false,
+            reasoning: None,
+            reasoning_complete: true,
+            content: "Question".into(),
+        }];
+        let checkpoint = conversation_tag("System", ReasoningMode::High, &messages);
+        messages.extend([
+            ChatTurn {
+                user: false,
+                tool: true,
+                system: false,
+                skip_previous_eos: false,
+                reasoning: None,
+                reasoning_complete: true,
+                content: "Tool result".into(),
+            },
+            ChatTurn {
+                user: false,
+                tool: false,
+                system: true,
+                skip_previous_eos: false,
+                reasoning: None,
+                reasoning_complete: true,
+                content: "System prompt reminder".into(),
+            },
+        ]);
+
+        assert!(checkpoint_matches_prefix(
+            checkpoint,
+            "System",
+            ReasoningMode::High,
+            &messages,
+        ));
+        assert!(!checkpoint_matches_prefix(
+            checkpoint,
+            "Changed",
+            ReasoningMode::High,
+            &messages,
+        ));
+    }
+
     #[test]
     fn bootstrap_key_is_the_prefix_before_dynamic_session_context() {
         let system = "System\n\nProject instructions from AGENTS.md:\n\nkeep this";