[hylin (linhongyu510)]

Title

fix(python): preserve non-adjacent tool pairs during history truncation

Motivation and Context

• Problem: ChatHistoryTruncationReducer could retain a FunctionResultContent while dropping its matching FunctionCallContent when the two were separated by non-function messages.
• Existing contract: adjacent tool blocks were already protected, and matched ID pairs are used elsewhere by the reducer utilities. A retained tool result should not become an orphan after truncation.
• Why this is minimal: the fix chooses the existing user-threshold candidate boundary first, then applies a final matched-ID pair closure.
• Scope: offline chat-history reduction only; no provider, network, E2E, public API, dependency, lockfile, or documentation-only refactor.
• Issue: self-discovered during audit; independent dedup found no equivalent open issue/PR.

Description

locate_safe_reduction_index now builds an O(n) suffix minimum-call map from get_call_result_pairs(history). After the existing adjacency and threshold-window candidate selection, it moves the cut back only when a retained result requires an earlier matched call. Chained non-adjacent pairs are closed transitively. If the required call is before offset_count, reduction returns None rather than crossing offset/summary territory.

Offset, threshold, system/developer preservation, unknown/unpaired content, and existing user-boundary search semantics are otherwise unchanged.

Local verification

• Base: 483ffb487a33a1644886992d56599feae3b69b76.
• Pre-fix formal regression: 4 failed, 4 passed.
• Post-fix formal regression: 8 passed.
• Production-only revert cycle: red exit 1, restore green exit 0.
• Related chat-history suite: 105 passed.
• Ruff: 0.9.10; check and format check pass.
• Mypy: targeted changed production file passes.
• Multi-Python matrix and wheel build were not run in this preparation step.
• AI assistance was used; an independent local review was rerun before publication. No external CLI review is claimed.

Contribution Checklist

• The code builds clean without any errors or warnings in the focused local checks.
• The PR follows the Semantic Kernel Contribution Guide and local Ruff/Mypy checks.
• All unit tests pass, and I added focused tests for the non-adjacent call/result regression.
• I did not break anyone.
• AI assistance is disclosed.

[hylin (linhongyu510)]

Rebased onto upstream main snapshot b5b526d9b8bd39da41db4a5c0f1aff20d26f7ec8 (2026-10-27).

Local verification on this rebased HEAD (4c489f2d44a9d41164d40c2b27aaf59333421b70), frozen lock env (uv 0.13.1, uv sync --frozen --all-extras --dev):

• Formal regression suite: 8 passed.
• Production-only revert (base bytes restored via try/finally): exactly 4 failed / 4 passed, exit 1; restored HEAD green 8 passed.
• Focused reducer suite: 43 passed.
• Related chat-history suite: 212 passed.
• Official type gate mypy -p semantic_kernel --config-file mypy.ini: no issues found in 556 source files (HEAD and base).
• Ruff check: clean.
• Official pre-commit (--all-files): 13 hooks passed (exit 0).
• Warnings: pytest-timeout config warning resolved by installing the locked plugin; Pydantic deprecation warnings are pre-existing on base (40 in focused, 52 in related suite).
• AI assistance was used and reviewed; this is an independent local verification, not a maintainer review. Multi-Python matrix / wheel build not run in this step.

[Copilot]

🟡 Changes recommended

History truncation can still orphan tool results because only the first matched result per message is collected and backward cut adjustments can split adjacent tool blocks.

2 open issues

• Collect all batched function results before calculating the cut · New
• Reapply tool-block adjacency after backward cut adjustments · New

What changed in this PR

This PR updates Python chat-history truncation to retain matching function calls when their results appear later, separated by other messages.

Changes:

• Adds matched-ID boundary adjustment after existing adjacency and user-threshold selection.
• Handles chained pair dependencies and rejects cuts that require crossing the offset boundary.
• Adds regression tests for pair preservation, thresholds, offsets, and system-message retention.

File	Description
python/​tests/​unit/​contents/​test_chat_history_truncation_pair_invariant.py	Adds focused truncation and pair-preservation regression tests.
python/​semantic_kernel/​contents/​history_reducer/​chat_history_reducer_utils.py	Adjusts truncation boundaries using suffix minima of matched call indices.

_{Review effort: Balanced}