fix: mark truncated assistant replies as incomplete instead of complete

pr-body-1980.md

@@ -0,0 +1,42 @@
+## Fix: Mark truncated assistant replies as incomplete instead of complete
+
+Closes #1980
+
+### Problem
+
+When the model output stream ends prematurely (provider-side interruption, timeout, connection drop, or `max_tokens` cut), the execution engine marks the dialog turn as `complete` / `hasFinalResponse=true` instead of `interrupted`/`incomplete`. This causes the frontend to persist and display an incomplete assistant reply as if it were a complete, final response.
+
+### Root Cause
+
+In [`execution_engine.rs`](src/crates/assembly/core/src/agentic/execution/execution_engine.rs), three code paths contribute to the bug:
+
+1. **`partial_truncated` marks as final (line ~4464):** When continuation attempts are exhausted after a partial recovery, `finalization_reason` is set to `"partial_truncated"`, but the code then sets `has_final_response = true` — treating the truncated text as a complete final response.
+
+2. **Success calculation excludes `partial_truncated` (line ~4480):** The `success` variable is `has_final_response || matches!(effective_finish_reason, "max_rounds" | "repeated_tool_failures")`. Since `"partial_truncated"` is not in the matches, and after fix #1 `has_final_response` is `false`, the turn would be marked as failed (`success=false`), which is too harsh — the turn did produce partial output.
+
+3. **Cancellation path leaves `finalization_reason = None` (line ~4194):** When `should_continue_after_partial_response(reason)` returns `false` (reason contains "cancelled"), the code breaks immediately without setting `finalization_reason`, so it defaults to `None` → `effective_finish_reason = "complete"` → `has_final_response = true`. A cancelled stream with partial text is reported as complete.
+
+### Fix
+
+Three surgical changes:
+
+1. **Line ~4465:** `has_final_response = true` → `has_final_response = false` for the `partial_truncated` branch. A truncated response is not a complete final response.
+
+2. **Line ~4483:** Add `"partial_truncated"` to the `matches!` list for `success`. The turn produced partial output, so `success=true` with `has_final_response=false` tells the UI: "the turn produced output, but it is incomplete."
+
+3. **Line ~4199:** Set `finalization_reason = Some("partial_truncated")` before `break` in the cancellation path. This ensures a cancelled stream with partial text is reported as `partial_truncated` (incomplete) rather than `complete`.
+
+### Event Flow After Fix
+
+The `DialogTurnCompleted` event now carries for truncated turns:
+- `finish_reason: "partial_truncated"` (was `"complete"`)
+- `has_final_response: false` (was `true`)
+- `success: true` (was `true` for path 1, `false` would have been too harsh)
+- `partial_recovery_reason: <actual reason>` (unchanged — e.g., "idle_timeout", "cancelled")
+
+The frontend projection (`frontend_projection.rs`) forwards these fields as `finishReason`, `hasFinalResponse`, `success`, and `partialRecoveryReason`, allowing the UI to distinguish incomplete turns from complete ones.
+
+### Validation
+
+- `cargo build -p bitfun-core --lib` — compiles clean (1 pre-existing warning, unrelated)
+- `cargo test -p bitfun-core --lib execution::execution_engine` — all 38 existing tests pass

src/crates/assembly/core/src/agentic/execution/execution_engine.rs

@@ -4196,6 +4196,10 @@ impl ExecutionEngine {
                                 "Model round {} ended with partial answer after cancellation, reason: {:?}",
                                 round_index, round_result.finish_reason
                             );
+                            // Mark as truncated so the turn is not reported as
+                            // complete — a cancelled stream with partial text is
+                            // still incomplete.
+                            finalization_reason = Some("partial_truncated");
                             break;
                         }
                     } else {
@@ -4462,7 +4466,9 @@ impl ExecutionEngine {
                     }
                 }
             } else if reason == "partial_truncated" {
-                has_final_response = true;
+                // A truncated response is not a complete final response — the
+                // model output stream ended prematurely.
+                has_final_response = false;
             }
         }
 
@@ -4480,7 +4486,7 @@ impl ExecutionEngine {
         let success = has_final_response
             || matches!(
                 effective_finish_reason,
-                "max_rounds" | "repeated_tool_failures"
+                "max_rounds" | "repeated_tool_failures" | "partial_truncated"
             );
 
         // Post-processing hook: when a DeepResearch dialog turn finishes