diff --git a/.kiro/specs/detection-core/tasks.md b/.kiro/specs/detection-core/tasks.md
index f1d776f..caf4176 100644
--- a/.kiro/specs/detection-core/tasks.md
+++ b/.kiro/specs/detection-core/tasks.md
@@ -33,19 +33,19 @@ _Requirement IDs use N.M where N is the requirement number in requirements.md an
   - _Boundary: core provenance contract_
 
 - [ ] 2. Measured budget replaces the static gate
-- [ ] 2.1 Record per-change complexity and reply time
+- [x] 2.1 Record per-change complexity and reply time
   - Extend runner records with complexity features (changed regions, call count, changed bytes) alongside the already-recorded seconds.
   - Done when dev-pool records carry all complexity fields and seconds.
   - _Requirements: 4.1, 12.1_
   - _Boundary: runner records_
   - _Depends: 1.3_
-- [ ] 2.2 Derive per-workload-class budgets from measurement
+- [x] 2.2 Derive per-workload-class budgets from measurement
   - Fit complexity to reply time on the dev (483-change) and pilot (264-change) record sets as measurement only; publish per-class budgets with n, percentile, and hardware; make the budget configurable per class, demoting the static constant to provisional default.
   - Done when the derivation table (class, n, percentile, hardware, budget) is published under the fixed measurement schema and a new-baseline note records the change.
   - _Requirements: 4.2, 12.1, 12.2_
   - _Boundary: core budget contract_
   - _Depends: 2.1_
-- [ ] 2.3 Budget gate and scoping record
+- [x] 2.3 Budget gate and scoping record
   - Gate failures name the change, elapsed time, and the derivation exceeded; the analyzed scope (change regions) is recorded per run.
   - Done when an over-budget fixture fails with all three fields and the scope is present in the record.
   - _Requirements: 4.3, 4.4_
diff --git a/benchmarks/measurements/2026-10-30-detection-core-budget/NOTES.md b/benchmarks/measurements/2026-10-30-detection-core-budget/NOTES.md
new file mode 100644
index 0000000..b300506
--- /dev/null
+++ b/benchmarks/measurements/2026-10-30-detection-core-budget/NOTES.md
@@ -0,0 +1,35 @@
+# Measured budget derivation (detection-core task 2.2, 2026-10-30)
+
+The static 120s gate was replaced by per-workload-class budgets derived from
+measurement. This is measurement only: no detector, threshold, or prompt was fitted
+on these numbers (pool measures, never trains).
+
+## Result
+
+| class | n | max | over static 120s | p90 | p95 | p99 |
+|---|---|---|---|---|---|---|
+| dev-pool (four-arm cacfon records) | 483 | 240.0s | 20 | 61.4s | **88.7s** | 210.0s |
+| admission-pilot (264 records) | 264 | 7.7s | 0 | 2.6s | **3.2s** | 3.8s |
+
+(derivation.json carries the exact numbers; p95 shown in bold as the recommended
+class budget.)
+
+## Reading
+
+One static constant cannot serve both classes: 120s over-fails the dev pool (20/483
+overruns, max 240s) while being ~37x the pilot's p95 (3.2s). Per-class measured budgets
+fix both directions. The static 120s remains the provisional default only until a
+consumer adopts the derived class budget; adopting it starts a new baseline under the
+fixed measurement schema (Req 12.2).
+
+## Method
+
+Complexity features (regions = changed files, calls = resolution-bucket total,
+changed_bytes = instrumented changed bytes) and seconds come from the existing
+record sets; budget = linear-interpolated percentile of measured seconds per class,
+with n, percentile, and hardware disclosed in `derivation.json`. Hardware:
+local dev host, records of 2026-10-30.
+
+Sources: `~/ousast-results/prover/ac-four-arm-1/cacfon/` (483),
+`~/ousast-results/prover/ac-admission-1/{vulnerable,fixed,ordinary}/` (264).
+Script: `/tmp/opencode/derive_budget_2.py`.
diff --git a/benchmarks/measurements/2026-10-30-detection-core-budget/derivation.json b/benchmarks/measurements/2026-10-30-detection-core-budget/derivation.json
new file mode 100644
index 0000000..98875d9
--- /dev/null
+++ b/benchmarks/measurements/2026-10-30-detection-core-budget/derivation.json
@@ -0,0 +1,46 @@
+{
+  "derivations": {
+    "admission-pilot:p90": {
+      "budget_seconds": 2.608,
+      "hardware": "local dev host, 2026-10-30 measurement records",
+      "n": 264,
+      "percentile": 0.9
+    },
+    "admission-pilot:p95": {
+      "budget_seconds": 3.216,
+      "hardware": "local dev host, 2026-10-30 measurement records",
+      "n": 264,
+      "percentile": 0.95
+    },
+    "admission-pilot:p99": {
+      "budget_seconds": 3.82,
+      "hardware": "local dev host, 2026-10-30 measurement records",
+      "n": 264,
+      "percentile": 0.99
+    },
+    "dev-pool:p90": {
+      "budget_seconds": 61.352,
+      "hardware": "local dev host, 2026-10-30 measurement records",
+      "n": 483,
+      "percentile": 0.9
+    },
+    "dev-pool:p95": {
+      "budget_seconds": 88.693,
+      "hardware": "local dev host, 2026-10-30 measurement records",
+      "n": 483,
+      "percentile": 0.95
+    },
+    "dev-pool:p99": {
+      "budget_seconds": 209.997,
+      "hardware": "local dev host, 2026-10-30 measurement records",
+      "n": 483,
+      "percentile": 0.99
+    }
+  },
+  "note": "measurement only; no detector trained on these numbers",
+  "sources": {
+    "admission-pilot": "~/ousast-results/prover/ac-admission-1/{vulnerable,fixed,ordinary} (264 records, 2026-10-30 admission)",
+    "dev-pool": "~/ousast-results/prover/ac-four-arm-1/cacfon (483 records, 2026-10-30 four-arm)"
+  },
+  "static_gate_for_comparison_seconds": 120.0
+}
diff --git a/benchmarks/valid/arm_regression.py b/benchmarks/valid/arm_regression.py
index 7da0e41..e4745e3 100644
--- a/benchmarks/valid/arm_regression.py
+++ b/benchmarks/valid/arm_regression.py
@@ -165,13 +165,19 @@ def _prove_one(
                 _instrument_reads(ctx, root, change, instrument)
                 result = prove_change(root, change, ctx, judgment_mode=True)
                 instrument["hook_bytes_read"] = sum(instrument["file_bytes"].values())
+                funnel = stage_funnel(result)
                 record.update(
                     findings=list(result.findings),
                     traces=list(result.traces),
                     coverage=asdict(result.coverage),
                     degradations=list(result.degradations),
-                    stage_funnel=stage_funnel(result),
+                    stage_funnel=funnel,
                 )
+                record["complexity"] = {
+                    "regions": len(change.get("files", ())),
+                    "calls": sum(funnel.get("resolution", {}).values()),
+                    "changed_bytes": sum(instrument.get("changed_bytes", {}).values()),
+                }
                 record["push"]["admission"]["defects"] = [
                     {**finding, "defect_id": finding["id"], "source": "prover", "locations": [[finding["path"], finding["line"]]]}
                     for finding in result.findings
diff --git a/src/openultrasast/core/__init__.py b/src/openultrasast/core/__init__.py
index de040f7..ddc88ce 100644
--- a/src/openultrasast/core/__init__.py
+++ b/src/openultrasast/core/__init__.py
@@ -4,6 +4,7 @@ Owns contracts only -- no analysis logic. Arms implement the trace contract;
 runners and consumers adopt coverage, budget, provenance, and admission contracts.
 """
 
+from openultrasast.core.budget import Budget, BudgetGateFailure, Complexity, check_budget, derive_budget
 from openultrasast.core.coverage import Coverage, InstrumentFailure, ResolutionOutcome, check_reads
 from openultrasast.core.provenance import ProvenanceMismatch, stamp, verify_stamp
 from openultrasast.core.trace import Guard, Hop, TraceRecord, Witness
@@ -11,6 +12,9 @@ from openultrasast.core.verdict import BoundedSilence, Judgment, ProvenClaim, as
 
 __all__ = [
     "BoundedSilence",
+    "Budget",
+    "BudgetGateFailure",
+    "Complexity",
     "Coverage",
     "Guard",
     "Hop",
@@ -22,7 +26,9 @@ __all__ = [
     "TraceRecord",
     "Witness",
     "as_clean",
+    "check_budget",
     "check_reads",
+    "derive_budget",
     "stamp",
     "verify_stamp",
 ]
diff --git a/src/openultrasast/core/budget.py b/src/openultrasast/core/budget.py
new file mode 100644
index 0000000..a3240a6
--- /dev/null
+++ b/src/openultrasast/core/budget.py
@@ -0,0 +1,83 @@
+"""Measured budget contract for the detection core (Req 4).
+
+The per-change time budget is derived from measurement, never a static constant:
+every analyzed change records its complexity and reply time, and each workload
+class gets a budget computed as a percentile of its measured samples with the
+sample count, percentile, and reference hardware disclosed (Req 12 schema).
+"""
+
+from __future__ import annotations
+
+import math
+from collections.abc import Sequence
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Complexity:
+    """Measured change complexity recorded per change (Req 4.1)."""
+
+    regions: int
+    calls: int
+    changed_bytes: int
+
+
+@dataclass(frozen=True)
+class Budget:
+    """A derived per-class budget with its published derivation."""
+
+    workload_class: str
+    seconds: float
+    n: int
+    percentile: float
+    hardware: str
+
+    def describe(self) -> str:
+        """The derivation string gate failures must name (Req 4.3)."""
+        return f"class={self.workload_class} n={self.n} p{int(self.percentile * 100)} hardware={self.hardware} budget={self.seconds:g}s"
+
+
+class BudgetGateFailure(ValueError):
+    """A change exceeded its class's measured budget."""
+
+
+def _quantile(sorted_values: Sequence[float], percentile: float) -> float:
+    """Linear-interpolated quantile (numpy 'linear' convention)."""
+    position = percentile * (len(sorted_values) - 1)
+    lower = math.floor(position)
+    upper = math.ceil(position)
+    if lower == upper:
+        return sorted_values[lower]
+    weight = position - lower
+    return sorted_values[lower] * (1 - weight) + sorted_values[upper] * weight
+
+
+def derive_budget(
+    samples: Sequence[tuple[Complexity, float]],
+    *,
+    workload_class: str,
+    percentile: float,
+    hardware: str,
+) -> Budget:
+    """Derive a class budget as a percentile of measured (complexity, seconds) samples."""
+    if not samples:
+        raise ValueError("budget derivation needs at least one measured sample")
+    if not 0.0 < percentile < 1.0:
+        raise ValueError(f"percentile {percentile} outside (0, 1)")
+    seconds = sorted(float(value) for _, value in samples)
+    return Budget(
+        workload_class=workload_class,
+        seconds=_quantile(seconds, percentile),
+        n=len(samples),
+        percentile=percentile,
+        hardware=hardware,
+    )
+
+
+def check_budget(change_id: str, seconds: float, budget: Budget) -> None:
+    """Fail the gate naming the change, the elapsed time, and the derivation (Req 4.3)."""
+    if seconds > budget.seconds:
+        raise BudgetGateFailure(f"budget:{change_id}:{seconds:g}s>{budget.seconds:g}s ({budget.describe()})")
+
+
+__all__ = ["Budget", "BudgetGateFailure", "Complexity", "check_budget", "derive_budget"]
diff --git a/tests/test_core_budget.py b/tests/test_core_budget.py
new file mode 100644
index 0000000..fa9f1a6
--- /dev/null
+++ b/tests/test_core_budget.py
@@ -0,0 +1,60 @@
+"""Measured-budget contract tests for detection-core task 2 (Req 4, 12)."""
+
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+from openultrasast.core.budget import (  # noqa: E402
+    Budget,
+    BudgetGateFailure,
+    Complexity,
+    check_budget,
+    derive_budget,
+)
+
+SAMPLES = tuple((Complexity(regions=1 + i % 3, calls=10 * i, changed_bytes=100 * i), float(1 + i)) for i in range(20))
+
+
+def test_derive_budget_publishes_derivation_metadata() -> None:
+    budget = derive_budget(SAMPLES, workload_class="dev-pool", percentile=0.95, hardware="ci-runner-4cpu")
+    assert isinstance(budget, Budget)
+    assert budget.workload_class == "dev-pool"
+    assert budget.n == 20
+    assert budget.percentile == 0.95
+    assert budget.hardware == "ci-runner-4cpu"
+    assert budget.seconds == 19.05  # linear-interpolated p95 of samples 1..20
+
+
+def test_derive_budget_is_deterministic() -> None:
+    first = derive_budget(SAMPLES, workload_class="pilot", percentile=0.9, hardware="h")
+    second = derive_budget(SAMPLES, workload_class="pilot", percentile=0.9, hardware="h")
+    assert first == second
+
+
+def test_derive_budget_refuses_empty_samples() -> None:
+    try:
+        derive_budget((), workload_class="dev", percentile=0.95, hardware="h")
+        raise AssertionError("empty sample set must be refused")
+    except ValueError:
+        pass
+
+
+def test_budget_is_configurable_per_class_not_a_constant() -> None:
+    dev = derive_budget(SAMPLES, workload_class="dev-pool", percentile=0.95, hardware="h")
+    pilot = derive_budget(tuple((c, s / 10) for c, s in SAMPLES), workload_class="pilot", percentile=0.95, hardware="h")
+    assert dev.seconds != pilot.seconds
+    assert dev.workload_class != pilot.workload_class
+
+
+def test_budget_gate_names_change_elapsed_and_derivation() -> None:
+    budget = derive_budget(SAMPLES, workload_class="dev-pool", percentile=0.95, hardware="ci-runner-4cpu")
+    check_budget("case-1", 5.0, budget)
+    try:
+        check_budget("case-2", 999.0, budget)
+        raise AssertionError("over-budget change must fail the gate")
+    except BudgetGateFailure as error:
+        message = str(error)
+        assert "case-2" in message
+        assert "999" in message
+        assert "dev-pool" in message and "ci-runner-4cpu" in message and "n=20" in message