diff --git a/nexus/reconfigurator/planning/src/blueprint_builder/builder.rs b/nexus/reconfigurator/planning/src/blueprint_builder/builder.rs
index df486ac5bf..bb20b38df7 100644
--- a/nexus/reconfigurator/planning/src/blueprint_builder/builder.rs
+++ b/nexus/reconfigurator/planning/src/blueprint_builder/builder.rs
@@ -343,13 +343,28 @@ impl<'a> BlueprintBuilder<'a> {
         // Prefer the sled state from our parent blueprint for sleds
         // that were in it; there may be new sleds in `input`, in which
         // case we'll use their current state as our starting point.
-        //
-        // TODO-correctness When can we drop decommissioned sleds from this map?
         let mut sled_state = parent_blueprint.sled_state.clone();
+        let mut commissioned_sled_ids = BTreeSet::new();
         for (sled_id, details) in input.all_sleds(SledFilter::Commissioned) {
+            commissioned_sled_ids.insert(sled_id);
             sled_state.entry(sled_id).or_insert(details.state);
         }
 
+        // Make a garbage collection pass through `sled_state`. We want to keep
+        // any sleds which either:
+        //
+        // 1. do not have a desired state of `Decommissioned`
+        // 2. do have a desired state of `Decommissioned` and are still included
+        //    in our input's list of commissioned sleds
+        //
+        // Sleds that don't fall into either of these cases have reached the
+        // actual `Decommissioned` state, which means we no longer need to carry
+        // forward that desired state.
+        sled_state.retain(|sled_id, state| {
+            *state != SledState::Decommissioned
+                || commissioned_sled_ids.contains(sled_id)
+        });
+
         Ok(BlueprintBuilder {
             log,
             parent_blueprint,
@@ -1197,6 +1212,7 @@ pub mod test {
     use crate::system::SledBuilder;
     use expectorate::assert_contents;
     use nexus_types::deployment::BlueprintZoneFilter;
+    use nexus_types::external_api::views::SledPolicy;
     use omicron_common::address::IpRange;
     use omicron_test_utils::dev::test_setup_log;
     use std::collections::BTreeSet;
@@ -1403,6 +1419,83 @@ pub mod test {
         logctx.cleanup_successful();
     }
 
+    #[test]
+    fn test_prune_decommissioned_sleds() {
+        static TEST_NAME: &str =
+            "blueprint_builder_test_prune_decommissioned_sleds";
+        let logctx = test_setup_log(TEST_NAME);
+        let (_, input, mut blueprint1) =
+            example(&logctx.log, TEST_NAME, DEFAULT_N_SLEDS);
+        verify_blueprint(&blueprint1);
+
+        // Mark one sled as having a desired state of decommissioned.
+        let decommision_sled_id = blueprint1
+            .sled_state
+            .keys()
+            .copied()
+            .next()
+            .expect("at least one sled");
+        *blueprint1.sled_state.get_mut(&decommision_sled_id).unwrap() =
+            SledState::Decommissioned;
+
+        // Change the input to note that the sled is expunged, but still active.
+        let mut builder = input.into_builder();
+        builder.sleds_mut().get_mut(&decommision_sled_id).unwrap().policy =
+            SledPolicy::Expunged;
+        builder.sleds_mut().get_mut(&decommision_sled_id).unwrap().state =
+            SledState::Active;
+        let input = builder.build();
+
+        // Generate a new blueprint. This sled should still be included: even
+        // though the desired state is decommissioned, the current state is
+        // still active, so we should carry it forward.
+        let blueprint2 = BlueprintBuilder::new_based_on(
+            &logctx.log,
+            &blueprint1,
+            &input,
+            "test_prune_decommissioned_sleds",
+        )
+        .expect("created builder")
+        .build();
+        verify_blueprint(&blueprint2);
+
+        // We carried forward the desired state.
+        assert_eq!(
+            blueprint2.sled_state.get(&decommision_sled_id).copied(),
+            Some(SledState::Decommissioned)
+        );
+
+        // Change the input to mark the sled decommissioned. (Normally realizing
+        // blueprint2 would make this change.)
+        let mut builder = input.into_builder();
+        builder.sleds_mut().get_mut(&decommision_sled_id).unwrap().state =
+            SledState::Decommissioned;
+        let input = builder.build();
+
+        // Generate a new blueprint. This desired sled state should no longer be
+        // present: it has reached the terminal decommissioned state, so there's
+        // no more work to be done.
+        let blueprint3 = BlueprintBuilder::new_based_on(
+            &logctx.log,
+            &blueprint2,
+            &input,
+            "test_prune_decommissioned_sleds",
+        )
+        .expect("created builder")
+        .build();
+        verify_blueprint(&blueprint3);
+
+        // Ensure we've dropped the decommissioned sled. (We may still have
+        // _zones_ for it that need cleanup work, but all state transitions for
+        // it are complete.)
+        assert_eq!(
+            blueprint3.sled_state.get(&decommision_sled_id).copied(),
+            None,
+        );
+
+        logctx.cleanup_successful();
+    }
+
     #[test]
     fn test_add_physical_disks() {
         static TEST_NAME: &str = "blueprint_builder_test_add_physical_disks";