diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c694525c..162ee866f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -107,6 +107,11 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ### Security
 
+- **PyYAML upgraded to >=6.0.3 to address known security vulnerability** (#9055):
+  Added an explicit `pyyaml>=6.0.3` dependency constraint to `pyproject.toml` to
+  prevent installation of vulnerable older versions with known YAML parsing security
+  issues.
+
 - **aiohttp upgraded to >=3.13.4 to remediate CVE-2026-34513 and CVE-2026-34515** (#1549, #1544):
   Added an explicit `aiohttp>=3.13.4` dependency constraint to `pyproject.toml` to remediate
   two high-severity open redirect vulnerabilities. Both CVEs affect the CleverAgents platform's
diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 358d21b58..3a60bf737 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -35,4 +35,5 @@ Below are some of the specific details of various contributions.
 * HAL 9000 has contributed the ACMS Index Data Model and File Traversal Engine (PR #9664 / issue #9579): foundational data structures for indexed context entries with hot/warm/cold/archive storage tier classification, tag system, and a timeout-safe chunked file traversal engine for large projects with 10,000+ files.
 * HAL 9000 has contributed the error-suppression removal fix (PR #9247 / issue #9060): removed both `try...except Exception:` blocks in `register_registry_agents()` that silently suppressed errors from `actor_registry.list_actors()` and the route bridge refresh, enabling exceptions to propagate per CONTRIBUTING.md fail-fast policy. Added three Behave scenarios verifying RuntimeError, AttributeError, and TypeError propagation.
 * HAL 9000 has contributed the Strategize phase full context snapshot fix (issue #9056): added `_build_strategize_context_snapshot()` helper to `PlanLifecycleService`, updated `_try_record_decision()` to accept and forward a `ContextSnapshot` parameter, and added BDD test coverage verifying all four `ContextSnapshot` fields (`hot_context_hash`, `hot_context_ref`, `actor_state_ref`, `relevant_resources`) are populated during the Strategize phase.
-* HAL 9000 has contributed the ACMS context path matching fix (PR #10975 / issue #10972): corrects `_path_matches()` and `_matches_pattern()` to properly match absolute fragment paths against relative glob patterns by auto-prefixing with `**/` before calling `PurePath.full_match()`, preventing silent inefficacy of include/exclude filters for absolute paths in fragment metadata.
+* HAL 9000 has contributed the PyYAML security upgrade (PR #9244 / issue #9055): added `pyyaml>=6.0.3` dependency constraint to address known YAML parsing vulnerabilities.
+ * HAL 9000 has contributed the ACMS context path matching fix (PR #10975 / issue #10972): corrects `_path_matches()` and `_matches_pattern()` to properly match absolute fragment paths against relative glob patterns by auto-prefixing with `**/` before calling `PurePath.full_match()`, preventing silent inefficacy of include/exclude filters for absolute paths in fragment metadata.
diff --git a/features/security_pyyaml_dependency.feature b/features/security_pyyaml_dependency.feature
new file mode 100644
index 000000000..985f286b3
--- /dev/null
+++ b/features/security_pyyaml_dependency.feature
@@ -0,0 +1,23 @@
+@tdd_issue @tdd_issue_9055
+Feature: PyYAML is a declared project dependency with minimum secure version
+  As a CleverAgents developer
+  I want the PyYAML dependency to be listed in pyproject.toml with a minimum secure version
+  So that vulnerable older versions of PyYAML cannot be installed
+
+  Background:
+    Given the pyproject.toml file exists at "pyproject.toml"
+
+  @tdd_issue @tdd_issue_9055
+  Scenario: PyYAML is listed in project dependencies with minimum version constraint
+    When I read the project dependencies from pyproject.toml
+     Then the dependency list should include a package matching "pyyaml>="
+
+  @tdd_issue @tdd_issue_9055
+  Scenario: PyYAML minimum version is >=6.0.3
+    When I read the PyYAML dependency specification from pyproject.toml
+     Then the minimum version should be at least "6.0.3"
+
+  @tdd_issue @tdd_issue_9055
+  Scenario: yaml module is importable as a project dependency
+    When I attempt to import the "yaml" module
+     Then the import should succeed without errors
diff --git a/features/steps/security_pyyaml_dependency_steps.py b/features/steps/security_pyyaml_dependency_steps.py
new file mode 100644
index 000000000..ef9e98579
--- /dev/null
+++ b/features/steps/security_pyyaml_dependency_steps.py
@@ -0,0 +1,132 @@
+"""Step definitions for PyYAML security dependency verification (#9055)."""
+
+from __future__ import annotations
+
+import importlib
+import re
+from pathlib import Path
+from typing import Any
+
+from behave import given, then, when
+from behave.runner import Context
+
+
+@given('the pyproject.toml file exists at "{path}"')
+def step_pyproject_exists(context: Context, path: str) -> None:
+    """Verify pyproject.toml exists at the given path."""
+    pyproject_path = Path(path)
+    assert pyproject_path.exists(), f"pyproject.toml not found at {path}"
+    context.pyproject_path = pyproject_path
+
+
+@when("I read the project dependencies from pyproject.toml")
+def step_read_project_dependencies(context: Context) -> None:
+    """Read the [project.dependencies] list from pyproject.toml."""
+    content = context.pyproject_path.read_bytes()
+    data: dict[str, Any] = _load_toml(content)
+    context.project_dependencies = data.get("project", {}).get("dependencies", [])
+
+
+@then('the dependency list should include a package matching "{pattern}"')
+def step_dependency_matches_pattern(context: Context, pattern: str) -> None:
+    """Assert that a dependency in the project dependencies matches the given regex pattern."""
+    deps: list[str] = context.project_dependencies
+    found = any(re.search(pattern, dep) for dep in deps)
+    assert found, (
+        f"No dependency matching '{pattern}' found in "
+        f"[project.dependencies]. Current deps: {deps}"
+    )
+
+
+@when("I read the PyYAML dependency specification from pyproject.toml")
+def step_read_pyyaml_dependency(context: Context) -> None:
+    """Read and parse the PyYAML dependency from pyproject.toml."""
+    content = context.pyproject_path.read_bytes()
+    data: dict[str, Any] = _load_toml(content)
+    deps: list[str] = data.get("project", {}).get("dependencies", [])
+
+    # Find the pyyaml dependency entry
+    pyyaml_dep = None
+    for dep in deps:
+        if dep.strip().startswith("pyyaml"):
+            pyyaml_dep = dep.strip()
+            break
+
+    assert pyyaml_dep is not None, (
+        f"PyYAML dependency not found in [project.dependencies]. Current deps: {deps}"
+    )
+
+    # Parse the minimum version from the constraint
+    # Examples: "pyyaml>=6.0.3", "pyyaml>=6.0", "pyyaml==6.0.2"
+    match = re.search(r"(?:>=|==|~>)([\d]+(?:\.[\d]+)*(?:\.\w+)*)", pyyaml_dep)
+    assert match is not None, f"Could not parse version from PyYAML dependency: {pyyaml_dep}"
+
+    context.pyyaml_version_spec = pyyaml_dep
+    context.pyyaml_min_version = match.group(1)
+
+
+@then("the minimum version should be at least \"{expected}\"")
+def step_pyyaml_minimum_version(context: Context, expected: str) -> None:
+    """Assert that the PyYAML minimum version is >= expected version."""
+    min_version = context.pyyaml_min_version
+    assert min_version == expected, (
+        f"PyYAML minimum version {min_version} does not match expected {expected}. "
+        f"Dependency spec: {context.pyyaml_version_spec}"
+    )
+
+
+@when('I attempt to import the "{module_name}" module')
+def step_attempt_import(context: Context, module_name: str) -> None:
+    """Attempt to import the given module and record the result."""
+    try:
+        context.imported_module = importlib.import_module(module_name)
+        context.import_error = None
+    except ImportError as exc:
+        context.imported_module = None
+        context.import_error = exc
+
+
+@then("the import should succeed without errors")
+def step_import_succeeded(context: Context) -> None:
+    """Assert that the previous import attempt succeeded."""
+    assert context.import_error is None, f"Import failed with: {context.import_error}"
+    assert context.imported_module is not None, "Module was not imported"
+
+
+@then('the version constraint in uv.lock should include specifier "{specifier}"')
+def step_uv_lock_specifier(context: Context, specifier: str) -> None:
+    """Assert that pyyaml appears in both package.dependencies and requires-dist in uv.lock."""
+    lock_path = context.pyproject_path.parent / "uv.lock"
+    content = lock_path.read_bytes().decode("utf-8")
+
+    # Check that pyyaml is in the cleveragents package dependencies section
+    assert (
+        '[[package]]\nname = "cleveragents"' in content
+    ), "cleveragents package section not found in uv.lock"
+
+    # Verify pyyaml entry exists in file after cleveragents section
+    cleveragents_match = re.search(
+        r'\[\[package\]\]\s*name = "cleveragents"(.*?)^\[\[package\]\]',
+        content,
+        re.DOTALL | re.MULTILINE,
+    )
+    assert cleveragents_match, "Could not find cleveragents package section"
+
+    cleveragents_block = cleveragents_match.group(1)
+
+    assert '{ name = "pyyaml" }' in cleveragents_block, (
+        f"pyyaml not found in cleveragents package dependencies in uv.lock"
+    )
+
+
+def _load_toml(content: bytes) -> dict[str, Any]:
+    """Load a TOML file from bytes. Uses tomllib if available, else fallback."""
+    try:
+        import tomllib
+
+        return tomllib.loads(content.decode("utf-8"))
+    except ImportError:
+        # Fallback for older Python versions without tomllib
+        import tomlkit
+
+        return tomlkit.parse(content.decode("utf-8")).unwrap()  # type: ignore[no-any-return]
diff --git a/pyproject.toml b/pyproject.toml
index c1fdf4a71..f711440c7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -47,6 +47,7 @@ dependencies = [
     "tomlkit>=0.13.0",  # TOML writing with comment preservation for config CLI
     "tenacity>=8.2.0",  # Retry framework for service layer resilience
     "aiohttp>=3.13.4",  # CVE-2026-34515 mitigation: open redirect vulnerability
+    "pyyaml>=6.0.3",  # Security: address known YAML parsing vulnerabilities
     "a2a-sdk>=0.3.0,<1.0.0",  # A2A Python SDK — required transport for local (stdio) and server (HTTP) modes (ADR-047); pinned <1.0.0 (removed legacy A2AClient)
 ]
 
diff --git a/uv.lock b/uv.lock
index 31d88e524..c0ed72e3f 100644
--- a/uv.lock
+++ b/uv.lock
@@ -446,6 +446,7 @@ dependencies = [
     { name = "pydantic" },
     { name = "pydantic-settings" },
     { name = "python-ulid" },
+    { name = "pyyaml" },
     { name = "restrictedpython" },
     { name = "rx" },
     { name = "structlog" },
@@ -528,6 +529,7 @@ requires-dist = [
     { name = "pytest", marker = "extra == 'dev'", specifier = ">=8.0.0" },
     { name = "pytest-asyncio", marker = "extra == 'dev'", specifier = ">=0.23.0" },
     { name = "pytest-cov", marker = "extra == 'dev'", specifier = ">=4.1.0" },
+    { name = "pyyaml", specifier = ">=6.0.3" },
     { name = "python-ulid", specifier = ">=2.7.0" },
     { name = "radon", marker = "extra == 'dev'", specifier = ">=6.0.1" },
     { name = "restrictedpython", specifier = ">=7.0" },