fix(context-assembly): kill 123 pre-existing mutation survivors in condensers.py (#228)

[cdeust]

What

Issue #228 asked for the 125 pre-existing mutation survivors in
core/context_assembly/condensers.py — every function outside #196's
condense_assembled_context — to be killed by contract tests or
documented as equivalent (§12.1: no third disposition).

The file was also 391 lines (over the 300-line §4.1 cap), with two
functions over the 40-line §4.2 cap (condense_assistant_message 63,
condense_assembled_context 66) — so the behaviour-preserving split
came first, per the issue's own framing: "if the file also breaches §4
sizes, the behaviour-preserving split comes first (existing suite
passing unchanged is the proof), then the tests."

Split (behaviour-preserving)

condensers.py (391 lines) → one file per condenser family, plus a thin
re-export facade so every existing import path (including the
mock.patch("...condensers.condense_assembled_context") target in
tests_py/core/test_pg_recall_assemble_context_condensation.py) keeps
resolving unchanged:

File	Contents	Lines
condense_text.py	condense_user_message, condense_timeline_event + sentence helpers	100
condense_code.py	condense_code_block, condense_assistant_message + fence helpers	189
condense_structured.py	condense_entity_triples	46
condense_dispatch.py	condense_memory_content (shape/tag dispatch)	76
condense_stage.py	condense_assembled_context (#196 wiring)	113
condensers.py	re-export facade (docstring + imports only)	70

Proof of zero behaviour change: the pre-existing 36-test
test_condensers.py + the 3-test pg_recall wiring suite (39 tests, 216
across the whole context_assembly test directory) pass unmodified
against the split, verified before a single new test was written.

One dead-code branch surfaced by the mutation run is deleted, not kept
(§9/§12.1 — "every mutant of an unreachable branch survives... remove
it, don't test it"): condense_assistant_message's trailing
if prose_parts and prose_budget > 0: ...; return join(code_parts) # pragma: no cover fallback, every mutant of which survived. The
reachability proof moves to the use site in condense_code.py and is
pinned by two tests (see ledger). A stale docstring claim about a "late
import" of assemble_prompt (the import was already module-scope, not
late) is corrected in the same pass — a seen defect in touched material
(§14).

Mutation run

Full methodology, per-mutant verification (mutmut show, not asserted
from memory), and a reproducible 2nd run confirming the same 6 survivor
IDs: docs/audits/condensers-mutation-run-2026-07-30.md.

	mutants	killed	survived
This PR	383	377	6 (all equivalent, all documented at the use site)

condense_assembled_context (#196's own function, already triaged to 0
non-equivalent survivors) re-confirms at 0 after the split — no
regression.

Completion Ledger

Diff is git diff origin/main...HEAD. No new production branches are
introduced (this is a behaviour-preserving split + one dead-code
deletion); every pre-existing branch that had a test in test_condensers.py
now additionally has an exact-equality mutation-hardening test in the file
mirroring its new module.

Path / behaviour	Test(s)	Evidence
condense_user_message boundary (<= fast path, mid-question keep, tighter-budget rebuild)	test_condense_text.py::test_user_message_at_exact_budget_is_returned_verbatim, ::test_user_message_keeps_every_middle_question_in_order, ::test_user_message_result_at_exact_budget_is_not_truncated, ::test_user_message_truncates_the_rejoined_result_when_over_budget	exact-equality, ran green
condense_user_message 2-sentence fallback (len(sentences) <= 2)	test_condense_text.py::test_two_sentence_message_truncates_the_original_not_the_rejoin	ran green
condense_timeline_event 3 date-format arms + no-date + boundary + over-budget	test_condense_text.py::test_timeline_event_reads_the_bracketed_date_marker, ::test_timeline_event_reads_an_iso_date, ::test_timeline_event_reads_a_month_day_year_date, ::test_timeline_event_at_exact_budget_is_returned_verbatim, ::test_timeline_event_compressed_at_exact_budget_is_not_truncated, ::test_timeline_event_truncates_the_compressed_form_not_the_raw_text	ran green
_split_sentences/_first_sentence helpers	test_condense_text.py::test_first_sentence_returns_the_first_sentence_not_the_whole_text, ::test_split_sentences_strips_and_drops_empty_parts	ran green
condense_assistant_message code-only branch (code_tokens >= token_budget), boundary, oversized-block degrade	test_condense_code.py::test_assistant_code_only_path_stops_at_the_budget_boundary, ::test_assistant_code_equal_to_budget_takes_the_code_only_path, ::test_assistant_message_at_exact_budget_is_returned_verbatim	ran green
condense_assistant_message prose-interleave branch + floor + blank-fallback	test_condense_code.py::test_assistant_interleaves_prose_and_code_in_original_order, ::test_assistant_prose_share_scales_with_the_leftover_budget, ::test_assistant_blank_only_content_falls_back_to_truncation	ran green
Deleted dead-code branch's unreachability proof	test_condense_code.py::test_assistant_unclosed_fence_is_a_single_code_segment, ::test_split_by_code_blocks_segments_preserve_original_newlines	ran green
condense_code_block signature scan, boundary, prefix roster	test_condense_code.py::test_code_block_at_exact_budget_is_returned_verbatim, ::test_code_block_keeps_every_declared_signature_prefix, ::test_code_block_signature_accounting_stops_at_the_budget_boundary	ran green
_has_code_blocks/_split_by_code_blocks helpers	test_condense_code.py::test_has_code_blocks_indent_run_threshold_is_three, ::test_has_code_blocks_detects_a_fence	ran green
condense_entity_triples boundary + accounting	test_condense_structured.py::test_entity_triples_at_exact_budget_is_returned_verbatim, ::test_entity_triples_stop_at_the_budget_boundary	ran green
condense_memory_content tag-dispatch (code/file/timeline/event arms)	test_condense_dispatch.py::test_code_tag_routes_to_the_code_block_condenser, ::test_file_tag_routes_to_the_code_block_condenser, ::test_timeline_tag_routes_to_the_timeline_condenser, ::test_event_tag_routes_to_the_timeline_condenser	ran green
condense_memory_content shape-dispatch (arrows, speaker prefix, indent)	test_condense_dispatch.py::test_two_unicode_arrows_reach_the_triple_threshold, ::test_two_ascii_arrows_reach_the_triple_threshold, ::test_assistant_prefixed_transcript_routes_to_the_assistant_condenser, ::test_user_prefixed_transcript_naming_the_assistant_routes_to_assistant, ::test_indent_runs_alone_route_to_the_assistant_condenser	ran green
condense_memory_content fast-path boundary	test_condense_dispatch.py::test_memory_content_at_exact_budget_is_returned_verbatim	ran green
condense_assembled_context under/over-budget split (unchanged)	pre-existing test_condensers.py (36 tests, unmodified)	ran green
Facade re-exports every public name unchanged	test_condensers.py + test_pg_recall_assemble_context_condensation.py (3 tests) import through the old path	ran green
§4 size caps on all 6 source + 4 test files	AST script (below)	0 findings
Mutation adequacy	mutmut scoped run	383/377/6, 6 documented equivalent

Pre-push gate output (quoted)

§4 sizes (AST, verbatim script from the task):

$ python3 - <files> <<'PY' ... PY
scan complete: no output above this line means all clear

(0 findings across all 6 source files + 4 new test files.)

ruff:

$ .venv/bin/python -m ruff check .
All checks passed!
$ .venv/bin/python -m ruff format --check mcp_server/core/context_assembly/ tests_py/core/context_assembly/
30 files already formatted

doc claims + badges:

$ .venv/bin/python scripts/check_doc_claims.py
doc claims OK (1 declared not-a-claim exemption(s))
  CONTRIBUTING.md:36: exempt from the tests claim
$ .venv/bin/python scripts/generate_repo_badges.py --check
badges OK (4 checked)

Targeted tests:

$ .venv/bin/python -m pytest tests_py/core/context_assembly/ tests_py/core/test_pg_recall_assemble_context_condensation.py -q
........................................................................ [ 33%]
........................................................................ [ 66%]
........................................................................ [100%]
216 passed in 2.34s

Full suite (post-rebase onto origin/main@2590922):

$ .venv/bin/python -m pytest tests_py/ -q
6725 passed, 5 skipped, 116 subtests passed in 190.29s

Conflict markers / JSON: none; .bestpractices.json parses.

Mutation (scoped, pyproject.toml [tool.mutmut] patched then
restored — no diff on pyproject.toml in this PR):

383 mutants, 377 killed, 6 survived (all documented equivalent)

Full per-file breakdown, per-mutant mutmut show diffs, and the
independent second-run reproduction: docs/audits/condensers-mutation-run-2026-07-30.md.

Boy-scout items fixed in this PR

• Dead code deleted (condense_assistant_message's unreachable
  code-only fallback), not kept as future-proofing.
• Stale "late import" docstring claim about assemble_prompt corrected
  (the import was already module-scope).
• docs/module-inventory.md's core/context_assembly/ file count,
  already stale by one file before this PR (said "9", measured "10"),
  corrected to the current measured 15.

No unfixed, un-issued seen defects remain in touched material.

Closes #228

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_012Yu6EnWspTfqHoGkExyS6u