Lay the prompt out so the endpoint can cache most of it

Prompt caching bills on a shared prefix: the endpoint reuses the request up
to the first byte that differs from last time and no further. The live
world-state block sat third from the top of the system message, so every turn
re-priced the instructions, the plot essentials and the whole story history
underneath it. The retrieved memories and the rewritten summary did it again.

Everything fixed is emitted first now, and everything that moves goes after
the history, ordered least-volatile first — which is also where recency serves
it best, the reasoning that already put the emit reminder last. The three tail
sections that are last for their own reasons stay last. The moved sections are
still charged to the token budget; only their position changed.

Two smaller halves of the same problem. OpenRouter serves a model from
whichever upstream is free and each upstream holds its own cache, so a
deepseek model now names deepseek as its preferred upstream — a preference,
not a restriction, so a turn still runs if that upstream is down. And the
endpoint's usage block is read back off the response and kept per attempt, so
the hit rate shows up in Insights and the debug log instead of being assumed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DfMCsN1KBLsTqMkj5hSgrY

backend/tests/test_prompt_caching.py

@@ -0,0 +1,241 @@
+"""Prompt caching: the prompt has to start with the same bytes every turn.
+
+Every endpoint that caches prompts caches a *prefix* — it reuses the request up
+to the first byte that differs from last time and no further. So the cost of a
+turn is decided by layout: one section that changes each turn, placed near the
+top, re-prices everything underneath it, and underneath it is the story
+history, which is most of the prompt.
+
+Three things have to hold, and each is easy to undo by accident:
+
+* the static block is byte-identical across turns — adding a section that moves
+  (live stats, retrieved memories, a rewritten summary) to `system_sections` is
+  the mistake this file exists to catch;
+* the sections that move sit *after* the history, but still *before* the tail
+  that is last for its own reasons (front memory, the length hint, and
+  EMIT_REMINDER, which is what keeps the state block emitted at all);
+* moving a section out of the system block does not drop it from the token
+  budget — it is still in the prompt.
+
+Plus the two request-level halves: preferring one OpenRouter upstream, since
+each upstream holds its own cache, and reading back the usage the endpoint
+reports so the hit rate is measurable rather than assumed.
+
+    python -m pytest tests/test_prompt_caching.py -v
+"""
+import os
+import tempfile
+
+_tmp = tempfile.NamedTemporaryFile(suffix=".db", delete=False)
+_tmp.close()
+os.environ["AIDND_DB_PATH"] = _tmp.name
+os.environ.pop("AIDND_DATABASE_URL", None)
+os.environ.pop("DATABASE_URL", None)
+
+import pytest
+
+from app import models, worldstate
+from app.context import builder
+from app.database import Base, SessionLocal, engine
+from app.providers.openai_compatible import OpenAICompatibleProvider
+
+SCHEMA = {
+    "player": {"hp": {"min": 0, "max": 100, "initial": 100, "desc": "Health"}},
+    "world": {"alarm": {"min": 0, "max": 10, "initial": 0, "desc": "Alarm level"}},
+}
+
+
+# ------------------------------------------------- preferring one upstream
+
+def _routed(endpoint, model):
+    provider = OpenAICompatibleProvider(endpoint, "k", model, "chat", 0)
+    body = {"max_tokens": 100}
+    provider._apply_provider_routing(body)
+    return body
+
+
+def test_openrouter_deepseek_pins_the_upstream():
+    """Each upstream has its own cache, so routing has to be deterministic."""
+    body = _routed("https://openrouter.ai/api/v1", "deepseek/deepseek-v4-flash-0731")
+    assert body["provider"] == {"order": ["deepseek"]}
+
+
+def test_fallbacks_stay_on():
+    """A preference, not a restriction: if deepseek is down the turn still runs
+    somewhere else and merely misses the cache."""
+    body = _routed("https://openrouter.ai/api/v1", "deepseek/deepseek-v4-flash-0731")
+    assert "allow_fallbacks" not in body["provider"]
+
+
+def test_non_openrouter_endpoints_get_no_provider_field():
+    """Ollama and friends reject fields they do not know — the same trap the
+    `reasoning` param is written around."""
+    body = _routed("http://localhost:11434/v1", "deepseek/deepseek-v4-flash-0731")
+    assert "provider" not in body
+
+
+def test_unknown_vendors_are_left_alone():
+    """The vendor half of a slug is not reliably a provider slug: Google's
+    models are served by "google-ai-studio", and there is no "google". A guess
+    would be a routing preference naming an upstream that does not exist."""
+    body = _routed("https://openrouter.ai/api/v1", "google/gemma-4-26b-a4b-it:free")
+    assert "provider" not in body
+
+
+# ------------------------------------------------------ reading usage back
+
+def test_usage_is_recorded_from_a_final_chunk():
+    """In a stream the usage block rides on a last chunk carrying no choices,
+    which is why it is read separately from the text extraction."""
+    provider = OpenAICompatibleProvider("https://openrouter.ai/api/v1", "k", "m")
+    assert provider.last_usage is None
+    provider._record_usage({"choices": [{"delta": {"content": "hi"}}]})
+    assert provider.last_usage is None
+    provider._record_usage(
+        {"choices": [], "usage": {"prompt_tokens": 900,
+                                  "prompt_tokens_details": {"cached_tokens": 768}}}
+    )
+    assert provider.last_usage["prompt_tokens_details"]["cached_tokens"] == 768
+
+
+def test_a_later_chunk_without_usage_does_not_erase_it():
+    provider = OpenAICompatibleProvider("https://openrouter.ai/api/v1", "k", "m")
+    provider._record_usage({"usage": {"prompt_tokens": 5}})
+    provider._record_usage({"choices": [{"delta": {"content": "x"}}]})
+    provider._record_usage({"usage": {}})
+    assert provider.last_usage == {"prompt_tokens": 5}
+
+
+# ------------------------------------------------------------ prompt layout
+
+def _with_hp(world_state, hp):
+    """`world_state` is nested by group, and the JSON column only notices a
+    whole new object — so build one rather than mutating in place."""
+    return {**world_state, "player": {**world_state["player"], "hp": hp}}
+
+
+@pytest.fixture()
+def story():
+    Base.metadata.create_all(bind=engine)
+    db = SessionLocal()
+    user = models.User(is_guest=False, email="cache@example.com")
+    db.add(user)
+    db.flush()
+    settings = models.Settings(user_id=user.id, api_key="enc:dummy", model="m")
+    db.add(settings)
+    scenario = models.Scenario(
+        user_id=user.id, title="S", prompt="A road.", stat_schema=SCHEMA
+    )
+    db.add(scenario)
+    db.flush()
+    adventure = models.Adventure(
+        user_id=user.id, title="A", scenario_id=scenario.id, script_state={},
+        memory="The hero hunts bandits.",
+        ai_instructions="Write in second person.",
+        story_summary="The hero left the village.",
+        world_state=worldstate.instantiate(SCHEMA),
+    )
+    db.add(adventure)
+    db.flush()
+    for i in range(6):
+        db.add(models.Action(adventure_id=adventure.id, index=i,
+                             type="ai" if i % 2 else "do",
+                             text=f"[{i}] The road bends onward past the treeline."))
+    db.commit()
+    db.expire_all()
+    adventure = db.get(models.Adventure, adventure.id)
+    settings = db.get(models.Settings, settings.id)
+    try:
+        yield db, adventure, settings
+    finally:
+        db.close()
+        Base.metadata.drop_all(bind=engine)
+
+
+def test_changing_a_stat_leaves_the_static_block_untouched(story):
+    """The whole point. Live values used to sit third from the top, so a single
+    point of damage re-priced the instructions, the plot and the history."""
+    db, adventure, settings = story
+    before, _, _ = builder.build_context(adventure, settings)
+    adventure.world_state = _with_hp(adventure.world_state, 40)
+    db.commit()
+    after, story_text, _ = builder.build_context(adventure, settings)
+    assert before == after
+    assert "hp 40/100" in story_text, "the new value still has to reach the model"
+
+
+def test_the_static_block_holds_the_things_that_do_not_move(story):
+    db, adventure, settings = story
+    system_text, story_text, _ = builder.build_context(adventure, settings)
+    for fixed in ("Write in second person.", "The hero hunts bandits."):
+        assert fixed in system_text
+    # The stat *guide* is derived from the schema and so is fixed; the live
+    # values it describes are not, and belong to the story text.
+    assert "Stat guide" in system_text
+    for moves in ("The hero left the village.", "hp 100/100"):
+        assert moves not in system_text
+        assert moves in story_text
+
+
+def test_volatile_sections_sit_after_the_history(story):
+    db, adventure, settings = story
+    _, story_text, _ = builder.build_context(adventure, settings)
+    history_at = story_text.index("[5] The road bends")
+    for label in ("Story summary:", "World state"):
+        assert story_text.index(label) > history_at, label
+
+
+def test_the_tail_stays_the_tail(story):
+    """front memory, the length hint and EMIT_REMINDER are last for reasons of
+    their own, and the live sections must not have displaced them."""
+    db, adventure, settings = story
+    _, story_text, report = builder.build_context(adventure, settings)
+    labels = [s["label"] for s in report["sections"]]
+    assert labels[-1] == "world_state_reminder"
+    assert labels[-2] == "length_hint"
+    assert labels.index("world_state") < labels.index("length_hint")
+    assert story_text.rstrip().endswith(worldstate.EMIT_REMINDER.rstrip())
+
+
+def test_live_sections_are_still_charged_to_the_budget(story):
+    """They moved out of `system_sections`, so it would be easy to stop
+    counting them in `reserved` — and then the history, which is budgeted with
+    what is left over, would quietly overrun."""
+    db, adventure, settings = story
+    for i in range(6, 90):
+        db.add(models.Action(
+            adventure_id=adventure.id, index=i, type="do",
+            text=f"[{i}] " + "The road bends onward past the treeline. " * 6,
+        ))
+    settings.context_token_budget = 4000
+    db.commit()
+    db.expire_all()
+    adventure = db.get(models.Adventure, adventure.id)
+    settings = db.get(models.Settings, settings.id)
+
+    _, _, lean = builder.build_context(adventure, settings)
+    adventure.story_summary = "The hero left the village. " * 150
+    db.commit()
+    _, _, fat = builder.build_context(adventure, settings)
+
+    assert fat["history"]["included"] < lean["history"]["included"], (
+        "a bigger summary has to leave less room for history"
+    )
+
+
+def test_a_new_turn_only_appends_to_the_cached_prefix(story):
+    """Playing on must extend the previous prompt, not rewrite it: the shared
+    prefix has to still contain the whole static block and the older history."""
+    db, adventure, settings = story
+    system_a, story_a, _ = builder.build_context(adventure, settings)
+    db.add(models.Action(adventure_id=adventure.id, index=6, type="do",
+                         text="[6] You step into the clearing."))
+    db.commit()
+    db.expire_all()
+    adventure = db.get(models.Adventure, adventure.id)
+    system_b, story_b, _ = builder.build_context(adventure, settings)
+
+    assert system_a == system_b
+    shared = os.path.commonprefix([story_a, story_b])
+    assert "[0] The road bends" in shared
+    assert "[5] The road bends" in shared