diff --git a/CLAUDE.md b/CLAUDE.md
index 80fb703..d2fbd61 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -33,7 +33,7 @@ pact-mcp                              # MCP server entry point
 
 **Entry point**: `pact = "pact.cli:main"`, `pact-mcp = "pact.mcp_server:main"` (pyproject.toml)
 
-**Python**: >=3.12 | **Dependencies**: pydantic>=2.0, pyyaml>=6.0 | **Optional**: anthropic>=0.40, mcp>=1.0
+**Python**: >=3.12 | **Dependencies**: pydantic>=2.0, pytest>=8.2, pyyaml>=6.0 | **Optional**: anthropic>=0.40, mcp>=1.0
 
 ## Architecture Overview
 
diff --git a/pyproject.toml b/pyproject.toml
index bb1fcd4..dde06ab 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -23,6 +23,7 @@ classifiers = [
 
 dependencies = [
     "pydantic>=2.0",
+    "pytest>=8.2,<10",
     "pyyaml>=6.0",
 ]
 
@@ -67,7 +68,6 @@ transmogrifier = [
     "transmogrifier>=0.2.0",
 ]
 dev = [
-    "pytest>=8.2,<10",
     "pytest-asyncio>=1.3,<2",
 ]
 
diff --git a/src/pact/certification.py b/src/pact/certification.py
index e335f8d..6195fd2 100644
--- a/src/pact/certification.py
+++ b/src/pact/certification.py
@@ -172,6 +172,7 @@ async def certify(project: ProjectManager) -> CertificationArtifact:
             results = await run_contract_tests(
                 test_file, impl_dir, language=language,
                 project_dir=project.project_dir,
+                environment=project.test_environment(),
             )
             cert.visible_results[cid] = {
                 "total": results.total,
@@ -198,6 +199,7 @@ async def certify(project: ProjectManager) -> CertificationArtifact:
             results = await run_contract_tests(
                 test_file, impl_dir, language=language,
                 project_dir=project.project_dir,
+                environment=project.test_environment(),
             )
             cert.goodhart_results[cid] = {
                 "total": results.total,
@@ -242,6 +244,7 @@ async def certify(project: ProjectManager) -> CertificationArtifact:
             results = await run_contract_tests(
                 test_file, impl_dir, language=language,
                 project_dir=project.project_dir,
+                environment=project.test_environment(),
             )
             cert.emission_results[cid] = {
                 "total": results.total,
diff --git a/src/pact/config.py b/src/pact/config.py
index 7281222..43f7db0 100644
--- a/src/pact/config.py
+++ b/src/pact/config.py
@@ -8,6 +8,7 @@
 
 import logging
 import os
+import sys
 from dataclasses import dataclass, field
 from enum import StrEnum
 from pathlib import Path
@@ -433,7 +434,9 @@ def resolve_parallel_config(
 @dataclass
 class EnvironmentSpec:
     """Standardized execution environment for test harness and agents."""
-    python_path: str = "python3"
+    # Defaults to the interpreter running pact, so the test run sees the same
+    # Python version and installed packages.
+    python_path: str = sys.executable
     inherit_path: bool = True
     extra_path_dirs: list[str] = field(default_factory=list)
     required_tools: list[str] = field(default_factory=lambda: ["pytest"])
@@ -575,7 +578,7 @@ def resolve_environment(project: ProjectConfig, global_cfg: GlobalConfig) -> Env
     if not raw:
         return EnvironmentSpec()
     return EnvironmentSpec(
-        python_path=raw.get("python_path", "python3"),
+        python_path=raw.get("python_path", sys.executable),
         inherit_path=raw.get("inherit_path", True),
         extra_path_dirs=raw.get("extra_path_dirs", []),
         required_tools=raw.get("required_tools", ["pytest"]),
diff --git a/src/pact/implementer.py b/src/pact/implementer.py
index 338a487..4c86c25 100644
--- a/src/pact/implementer.py
+++ b/src/pact/implementer.py
@@ -687,6 +687,7 @@ async def implement_component(
         test_results = await run_contract_tests(
             test_file, src_dir, language=language,
             project_dir=project.project_dir,
+            environment=project.test_environment(),
         )
         last_test_results = test_results
         project.save_test_results(component_id, test_results)
@@ -947,6 +948,7 @@ async def implement_component_iterative(
     test_results = await run_contract_tests(
         test_file, src_dir, language=language,
         project_dir=project.project_dir,
+        environment=project.test_environment(),
     )
     project.save_test_results(component_id, test_results)
 
@@ -1101,6 +1103,7 @@ async def implement_component_interactive(
     test_results = await run_contract_tests(
         test_file, src_dir, language=language,
         project_dir=project.project_dir,
+        environment=project.test_environment(),
     )
     project.save_test_results(component_id, test_results)
 
@@ -1179,6 +1182,7 @@ async def _run_one_competitor(
         test_results = await run_contract_tests(
             test_file, src_dir, language=language,
             project_dir=project.project_dir,
+            environment=project.test_environment(),
         )
         last_test_results = test_results
         project.save_attempt_test_results(component_id, attempt_id, test_results)
diff --git a/src/pact/integrator.py b/src/pact/integrator.py
index 53541fb..6e06b46 100644
--- a/src/pact/integrator.py
+++ b/src/pact/integrator.py
@@ -191,6 +191,7 @@ class GlueResponse(BaseModel):
             test_file, comp_dir, extra_paths=child_paths,
             language=language,
             project_dir=project.project_dir,
+            environment=project.test_environment(),
         )
 
         # Save results to internal composition dir
@@ -558,6 +559,7 @@ async def integrate_component_iterative(
         test_file, comp_dir, extra_paths=child_paths,
         language=language,
         project_dir=project.project_dir,
+        environment=project.test_environment(),
     )
 
     # Save results to internal composition dir
diff --git a/src/pact/project.py b/src/pact/project.py
index a24ab2d..50dea29 100644
--- a/src/pact/project.py
+++ b/src/pact/project.py
@@ -67,7 +67,13 @@
 if TYPE_CHECKING:
     from pact.schemas import ArtifactMetadata
 
-from pact.config import ProjectConfig, load_project_config
+from pact.config import (
+    EnvironmentSpec,
+    ProjectConfig,
+    load_global_config,
+    load_project_config,
+    resolve_environment,
+)
 from pact.schemas import (
     CertificationArtifact,
     ComponentContract,
@@ -407,6 +413,10 @@ def load_sops(self) -> str:
     def load_config(self) -> ProjectConfig:
         return load_project_config(self.project_dir)
 
+    def test_environment(self) -> EnvironmentSpec:
+        """Environment for contract test runs, from project then global config."""
+        return resolve_environment(self.load_config(), load_global_config())
+
     # ── Cross-process file locking ─────────────────────────────────
     #
     # state.json and audit.jsonl are touched by every CLI invocation and
diff --git a/src/pact/scheduler.py b/src/pact/scheduler.py
index 8138d7a..1a64639 100644
--- a/src/pact/scheduler.py
+++ b/src/pact/scheduler.py
@@ -33,6 +33,7 @@
     ProjectConfig,
     resolve_backend,
     resolve_build_mode,
+    resolve_environment,
     resolve_model,
     resolve_parallel_config,
 )
@@ -1450,6 +1451,9 @@ async def _phase_polish(self, state: RunState) -> RunState:
                     test_file, src_dir, extra_paths=extra_paths,
                     language=language,
                     project_dir=self.project.project_dir,
+                    environment=resolve_environment(
+                        self.project_config, self.global_config,
+                    ),
                 )
                 if not results.all_passed:
                     regression_failures.append(
@@ -1642,6 +1646,9 @@ async def _run_goodhart_tests(
                     test_file, src_dir, extra_paths=extra_paths,
                     language=language,
                     project_dir=self.project.project_dir,
+                    environment=resolve_environment(
+                        self.project_config, self.global_config,
+                    ),
                 )
                 if not results.all_passed:
                     goodhart_failures[cid] = results
diff --git a/src/pact/test_harness.py b/src/pact/test_harness.py
index 19d46c2..0f041d5 100644
--- a/src/pact/test_harness.py
+++ b/src/pact/test_harness.py
@@ -15,6 +15,7 @@
 import logging
 import os
 import re
+import sys
 from enum import StrEnum
 from pathlib import Path
 from typing import TYPE_CHECKING
@@ -192,11 +193,6 @@ async def run_contract_tests(
     parts = [str(impl_dir), str(impl_dir.parent)]
     if extra_paths:
         parts.extend(str(p) for p in extra_paths)
-    # Include pact's own site-packages so anyio and other pact deps are available
-    import sysconfig as _sysconfig
-    _pact_site = _sysconfig.get_path("purelib")
-    if _pact_site and _pact_site not in parts:
-        parts.append(_pact_site)
     env_path = ":".join(parts)
 
     if environment:
@@ -208,8 +204,9 @@ async def run_contract_tests(
             "PATH": os.environ.get("PATH", "/usr/bin:/usr/local/bin"),
         }
 
+    python = environment.python_path if environment else sys.executable
     cmd = [
-        "python3", "-m", "pytest",
+        python, "-m", "pytest",
         str(test_file),
         "-v", "--tb=short", "--no-header",
         f"--rootdir={impl_dir.parent}",
@@ -222,6 +219,14 @@ async def run_contract_tests(
     except TestSubprocessError as e:
         return _error_results(e.test_id, e.message)
 
+    # Otherwise this parses as zero tests with zero errors.
+    if "No module named pytest" in stderr_text:
+        return _error_results(
+            "environment",
+            f"pytest is not installed for {python}; install it there or set "
+            "environment.python_path to an interpreter that has it",
+        )
+
     return parse_pytest_output(stdout_text, stderr_text)
 
 
diff --git a/tests/test_certification.py b/tests/test_certification.py
index 1b33cdd..1418d10 100644
--- a/tests/test_certification.py
+++ b/tests/test_certification.py
@@ -144,3 +144,19 @@ def test_verify_artifact_hashes_reports_missing_emission_sentinel(tmp_path):
     assert verify_artifact_hashes(cert, project) == [
         "tests/comp_a/emission_test: missing at certification time"
     ]
+
+
+def test_certify_runs_tests_under_configured_interpreter(tmp_path):
+    project = _project_with_contract(tmp_path)
+    project.save_emission_test("comp_a", "def test_emission():\n    pass\n")
+    project.config_path.write_text(
+        "environment:\n  python_path: /opt/venv/bin/python\n"
+    )
+    runner = AsyncMock(return_value=_passing_results())
+
+    with patch("pact.certification.run_contract_tests", runner):
+        asyncio.run(certify(project))
+
+    assert runner.await_count == 3
+    for call in runner.await_args_list:
+        assert call.kwargs["environment"].python_path == "/opt/venv/bin/python"
diff --git a/tests/test_environment.py b/tests/test_environment.py
index 49087f2..3ed52c3 100644
--- a/tests/test_environment.py
+++ b/tests/test_environment.py
@@ -1,5 +1,6 @@
 """Tests for EnvironmentSpec."""
 import os
+import sys
 from pact.config import EnvironmentSpec, resolve_environment, GlobalConfig, ProjectConfig
 
 
@@ -7,7 +8,7 @@ class TestEnvironmentSpec:
     def test_default_inherits_path(self):
         spec = EnvironmentSpec()
         assert spec.inherit_path is True
-        assert spec.python_path == "python3"
+        assert spec.python_path == sys.executable
         assert spec.required_tools == ["pytest"]
 
     def test_build_env_inherits_parent_path(self):
@@ -69,7 +70,12 @@ class TestResolveEnvironment:
     def test_default_when_no_config(self):
         spec = resolve_environment(ProjectConfig(), GlobalConfig())
         assert spec.inherit_path is True
-        assert spec.python_path == "python3"
+        assert spec.python_path == sys.executable
+
+    def test_config_without_python_path_uses_running_interpreter(self):
+        gc = GlobalConfig(environment={"inherit_path": False})
+        spec = resolve_environment(ProjectConfig(), gc)
+        assert spec.python_path == sys.executable
 
     def test_global_config(self):
         gc = GlobalConfig(environment={
@@ -85,3 +91,18 @@ def test_project_overrides_global(self):
         pc = ProjectConfig(environment={"python_path": "python3.13"})
         spec = resolve_environment(pc, gc)
         assert spec.python_path == "python3.13"
+
+
+class TestProjectTestEnvironment:
+    def test_reads_python_path_from_pact_yaml(self, tmp_path):
+        from pact.project import ProjectManager
+        project = ProjectManager(tmp_path)
+        project.config_path.write_text(
+            "environment:\n  python_path: /opt/venv/bin/python\n"
+        )
+        assert project.test_environment().python_path == "/opt/venv/bin/python"
+
+    def test_defaults_to_running_interpreter(self, tmp_path):
+        from pact.project import ProjectManager
+        project = ProjectManager(tmp_path)
+        assert project.test_environment().python_path == sys.executable
diff --git a/tests/test_test_harness.py b/tests/test_test_harness.py
index 8b04630..9f0968e 100644
--- a/tests/test_test_harness.py
+++ b/tests/test_test_harness.py
@@ -3,9 +3,12 @@
 from __future__ import annotations
 
 import asyncio
+import sys
+import sysconfig
 from pathlib import Path
 from unittest.mock import AsyncMock, patch
 
+from pact.config import EnvironmentSpec
 from pact.test_harness import EvalTier, parse_pytest_output, run_contract_tests, select_test_files
 
 
@@ -122,9 +125,57 @@ async def mock_exec(*args, **kwargs):
             asyncio.run(run_contract_tests(test_file, impl_dir))
 
         pythonpath = captured_env.get("PYTHONPATH", "")
-        assert str(impl_dir) in pythonpath
-        # impl_dir + parent + pact site-packages (added so anyio is available)
-        assert len(pythonpath.split(":")) >= 2
+        assert pythonpath.split(":") == [str(impl_dir), str(tmp_path)]
+
+
+def _run_capturing(tmp_path, *, environment=None, stderr=b""):
+    """Run run_contract_tests with a mocked subprocess; return (cmd, env, results)."""
+    test_file = tmp_path / "test_example.py"
+    test_file.write_text("def test_pass(): pass")
+    impl_dir = tmp_path / "impl"
+    impl_dir.mkdir()
+    captured = {}
+
+    async def mock_exec(*args, **kwargs):
+        captured["cmd"] = list(args)
+        captured["env"] = kwargs.get("env", {})
+        proc = AsyncMock()
+        stdout = b"" if stderr else b"test_x PASSED\n1 passed"
+        proc.communicate = AsyncMock(return_value=(stdout, stderr))
+        proc.returncode = 1 if stderr else 0
+        return proc
+
+    with patch("pact.test_harness.asyncio.create_subprocess_exec", side_effect=mock_exec):
+        results = asyncio.run(run_contract_tests(test_file, impl_dir, environment=environment))
+    return captured["cmd"], captured["env"], results
+
+
+class TestInterpreter:
+    """The harness runs pytest under pact's interpreter or the configured one."""
+
+    def test_defaults_to_running_interpreter(self, tmp_path):
+        cmd, _, _ = _run_capturing(tmp_path)
+        assert cmd[:3] == [sys.executable, "-m", "pytest"]
+
+    def test_uses_environment_python_path(self, tmp_path):
+        env_spec = EnvironmentSpec(python_path="/opt/py/bin/python3.12")
+        cmd, _, _ = _run_capturing(tmp_path, environment=env_spec)
+        assert cmd[0] == "/opt/py/bin/python3.12"
+
+    def test_does_not_inject_pact_site_packages(self, tmp_path):
+        env_spec = EnvironmentSpec(python_path="/opt/py/bin/python3.12")
+        _, env, _ = _run_capturing(tmp_path, environment=env_spec)
+        assert sysconfig.get_path("purelib") not in env["PYTHONPATH"].split(":")
+
+    def test_missing_pytest_is_an_error(self, tmp_path):
+        env_spec = EnvironmentSpec(python_path="/opt/py/bin/python3.12")
+        _, _, results = _run_capturing(
+            tmp_path, environment=env_spec,
+            stderr=b"/opt/py/bin/python3.12: No module named pytest\n",
+        )
+        assert results.errors == 1
+        assert results.failure_details[0].test_id == "environment"
+        assert "/opt/py/bin/python3.12" in results.failure_details[0].error_message
 
 
 class TestEvalTier: