diff --git a/docs/p1-continuation-diagnostics.md b/docs/p1-continuation-diagnostics.md
new file mode 100644
index 0000000..eeac1f5
--- /dev/null
+++ b/docs/p1-continuation-diagnostics.md
@@ -0,0 +1,37 @@
+# P1 continuation diagnostics
+
+## Scope
+
+This document records the diagnostic events added around Codex tool-result continuation and state replay.
+
+The change is logging-only. It does not change protocol conversion, state lookup rules, fallback behavior, or Responses output shape.
+
+## Events
+
+| Event | Level | Location | Meaning |
+|---|---:|---|---|
+| `stored_response_not_found` | warn | `StateStore::get` | A requested `previous_response_id` was not found in non-expired state. The log includes `response_id` and TTL cutoff. |
+| `tool_history_unique_fallback_hit` | debug | `StateStore::find_by_call_ids` | No `previous_response_id` was available, but the adapter restored continuation via a unique pending `call_id` match. The log includes the restored `response_id`, requested call IDs, and stored pending call IDs. |
+| `tool_history_call_id_ambiguous` | warn | `StateStore::find_by_call_ids` | The same requested `call_id` matched multiple stored responses. The log includes both candidate response IDs and `candidate_count = 2`. |
+| `tool_history_response_ambiguous` | warn | `StateStore::find_by_call_ids` | The requested call-id set matched multiple stored responses. The log includes both candidate response IDs and `candidate_count = 2`. |
+| `tool_history_call_id_not_found` | warn | `StateStore::find_by_call_ids` | The adapter could not restore continuation via pending `call_id` fallback. The log includes requested and matched call-id counts. |
+| `stateless_tool_history_bypass_state_lookup` | debug | `conversion::function_output_call_ids` | Responses input already contains matching tool calls for every tool output, so the server bypasses stored-state lookup and allows stateless history repair. |
+
+## Operational use
+
+When a real Codex subagent continuation fails, check logs in this order:
+
+1. `stored_response_not_found` means Codex supplied a `previous_response_id`, but the adapter no longer has the stored response, commonly because the state TTL expired or a different state DB is being used.
+2. `tool_history_unique_fallback_hit` means Codex omitted or rewrote `previous_response_id`, but the adapter recovered through unique `call_id` fallback.
+3. `tool_history_call_id_ambiguous` or `tool_history_response_ambiguous` means the adapter refused recovery because multiple stored responses could match the requested tool output.
+4. `stateless_tool_history_bypass_state_lookup` means Codex sent a self-contained stateless history; recovery should continue through `build_chat_payload()` and `repair_stateless_history()`.
+
+## Validation
+
+Run locally:
+
+```bash
+cargo fmt --check
+cargo test --lib
+cargo test
+```
diff --git a/src/conversion/mod.rs b/src/conversion/mod.rs
index f79b66e..c5e3546 100644
--- a/src/conversion/mod.rs
+++ b/src/conversion/mod.rs
@@ -25,6 +25,11 @@ pub fn function_output_call_ids(body: &Value) -> Result<Vec<String>, HistoryErro
     }
 
     if input_contains_all_tool_calls(body, &ids) {
+        tracing::debug!(
+            event = "stateless_tool_history_bypass_state_lookup",
+            call_ids = ?ids,
+            "Responses input contains matching tool calls for every tool output; using stateless history repair instead of stored-state lookup"
+        );
         Ok(Vec::new())
     } else {
         Ok(ids)
diff --git a/src/state.rs b/src/state.rs
index d32fb3d..afafaab 100644
--- a/src/state.rs
+++ b/src/state.rs
@@ -79,6 +79,12 @@ impl StateStore {
             let payload: String = row.get(0)?;
             Ok(Some(serde_json::from_str(&payload)?))
         } else {
+            tracing::warn!(
+                event = "stored_response_not_found",
+                response_id,
+                cutoff,
+                "requested previous_response_id was not found in non-expired state"
+            );
             Ok(None)
         }
     }
@@ -113,6 +119,15 @@ impl StateStore {
                 intersects_wanted = true;
                 match matched_by_call_id.get(call_id) {
                     Some(existing_response_id) if existing_response_id != &item.response_id => {
+                        tracing::warn!(
+                            event = "tool_history_call_id_ambiguous",
+                            call_id,
+                            candidate_count = 2,
+                            existing_response_id,
+                            candidate_response_id = %item.response_id,
+                            requested_call_ids = ?call_ids,
+                            "call_id matched multiple stored responses; unique fallback disabled"
+                        );
                         return Ok(None);
                     }
                     Some(_) => {}
@@ -125,6 +140,14 @@ impl StateStore {
             if intersects_wanted && wanted.is_subset(&pending) {
                 match &unique_response_id {
                     Some(existing_response_id) if existing_response_id != &item.response_id => {
+                        tracing::warn!(
+                            event = "tool_history_response_ambiguous",
+                            candidate_count = 2,
+                            existing_response_id,
+                            candidate_response_id = %item.response_id,
+                            requested_call_ids = ?call_ids,
+                            "requested call ids matched multiple stored responses; unique fallback disabled"
+                        );
                         return Ok(None);
                     }
                     Some(_) => {}
@@ -137,8 +160,24 @@ impl StateStore {
         }
 
         if matched_by_call_id.len() == wanted.len() {
+            if let Some(response) = unique_response.as_ref() {
+                tracing::debug!(
+                    event = "tool_history_unique_fallback_hit",
+                    response_id = %response.response_id,
+                    requested_call_ids = ?call_ids,
+                    pending_call_ids = ?response.pending_call_ids,
+                    "restored tool continuation by unique pending call_id fallback"
+                );
+            }
             Ok(unique_response)
         } else {
+            tracing::warn!(
+                event = "tool_history_call_id_not_found",
+                requested_call_ids = ?call_ids,
+                matched_call_id_count = matched_by_call_id.len(),
+                requested_call_id_count = wanted.len(),
+                "could not restore tool continuation by pending call_id fallback"
+            );
             Ok(None)
         }
     }