diff --git a/src/agentex/lib/core/tracing/code_revision.py b/src/agentex/lib/core/tracing/code_revision.py
index 7b08dd45f..2c6f96292 100644
--- a/src/agentex/lib/core/tracing/code_revision.py
+++ b/src/agentex/lib/core/tracing/code_revision.py
@@ -1,10 +1,11 @@
-"""Opt-in stamping of the agent's source commit onto its spans.
+"""Stamping of the agent's source commit onto its spans.
 
-Nothing is stamped until the agent calls :func:`enable`, mirroring the
-``lineage`` registry next door: a process-wide switch the agent sets once at
-import, rather than automatic behaviour every agent inherits. When enabled the
-resolved commit lands in span data under ``__commit_sha__`` and is searchable in
-the SGP Traces UI as ``__commit_sha__:<sha>``.
+Stamping turns on when the process starts with ``AGENT_COMMIT_SHA`` set, which
+the SGP cloud deploy does from the build record's attested commit, or when the
+agent calls :func:`enable` itself. Nothing is stamped otherwise: upgrading the
+SDK alone never starts emitting the field. When on, the resolved commit lands in
+span data under ``__commit_sha__`` and is searchable in the SGP Traces UI as
+``__commit_sha__:<sha>``.
 
 This is deliberately separate from ``__agent_version__``, which is automatic and
 carries the deployed image tag verbatim ("image tag or git sha"). That tag is a
@@ -42,13 +43,16 @@
 
 
 def enable(commit_sha: str | None = None) -> None:
-    """Opt this process in to stamping ``__commit_sha__`` onto every span.
+    """Turn on stamping ``__commit_sha__`` onto every span from this process.
 
     Value precedence: the explicit ``commit_sha`` argument, else
     ``AGENT_COMMIT_SHA``, else ``AGENT_VERSION`` when the deployment happened to
     set it to a bare commit SHA. A value that is not a git object name is
     refused with a warning and leaves stamping off -- better an absent field
     than one named for a commit that holds an image tag.
+
+    Called once at import when ``AGENT_COMMIT_SHA`` is set, so a deployment that
+    supplies the commit needs no code change in the agent.
     """
     global _commit_sha
 
@@ -103,3 +107,12 @@ def is_enabled() -> bool:
 def commit_sha() -> str | None:
     """The resolved commit SHA, or ``None`` when stamping is not enabled."""
     return _commit_sha
+
+
+def _enable_from_environment() -> None:
+    """Auto-enable on ``AGENT_COMMIT_SHA`` only; ``AGENT_VERSION`` stays an explicit fallback."""
+    if os.environ.get(_COMMIT_SHA_ENV, "").strip():
+        enable()
+
+
+_enable_from_environment()
diff --git a/src/agentex/lib/environment_variables.py b/src/agentex/lib/environment_variables.py
index 00dbbaada..7492037ce 100644
--- a/src/agentex/lib/environment_variables.py
+++ b/src/agentex/lib/environment_variables.py
@@ -68,11 +68,8 @@ class EnvironmentVariables(BaseModel):
     AGENT_ID: str | None = None
     # Build/version discriminator (image tag or git sha), set by the deployment
     AGENT_VERSION: str | None = None
-    # The agent's source commit, baked into the image or set by the deployment.
-    # Unlike AGENT_VERSION this is expected to be a git SHA and nothing else, and
-    # it is OPT-IN: nothing is stamped unless the agent calls
-    # `adk.code_revision.enable()`, which also refuses a value that is not a git
-    # object name. See agentex.lib.core.tracing.code_revision.
+    # The agent's source commit, set by the deployment or baked into the image; a git
+    # SHA and nothing else. Stamped as __commit_sha__ when set (see tracing.code_revision).
     AGENT_COMMIT_SHA: str | None = None
     AGENT_API_KEY: str | None = None
     ACP_TYPE: str | None = "async"
diff --git a/tests/lib/core/tracing/processors/test_sgp_tracing_processor.py b/tests/lib/core/tracing/processors/test_sgp_tracing_processor.py
index 6cd324f01..7b5c129d6 100644
--- a/tests/lib/core/tracing/processors/test_sgp_tracing_processor.py
+++ b/tests/lib/core/tracing/processors/test_sgp_tracing_processor.py
@@ -56,13 +56,13 @@ def test_agent_identity_and_version_stamped_into_span_data(self):
 
     SHA = "b362b171a9c4e1f09d8e7a6b5c4d3e2f1a0b9c8d"
 
-    def test_commit_sha_is_not_stamped_without_opt_in(self, monkeypatch):
-        """Upgrading the SDK must not start emitting __commit_sha__ on its own,
-        even when the environment carries a perfectly good SHA."""
+    def test_commit_sha_is_not_stamped_when_env_absent(self, monkeypatch):
+        """Upgrading the SDK must not start emitting __commit_sha__ on its own;
+        only AGENT_COMMIT_SHA or an enable() call turns it on."""
         from agentex.lib.core.tracing import code_revision
         from agentex.lib.core.tracing.processors.sgp_tracing_processor import _sgp_metadata
 
-        monkeypatch.setenv("AGENT_COMMIT_SHA", self.SHA)
+        monkeypatch.delenv("AGENT_COMMIT_SHA", raising=False)
         code_revision.disable()
 
         span = _make_span(); span.data = {}
diff --git a/tests/lib/core/tracing/test_code_revision.py b/tests/lib/core/tracing/test_code_revision.py
index 0b89b88f2..a696129d7 100644
--- a/tests/lib/core/tracing/test_code_revision.py
+++ b/tests/lib/core/tracing/test_code_revision.py
@@ -1,7 +1,8 @@
-"""Opt-in commit-SHA stamping.
+"""Commit-SHA stamping.
 
-The contract that matters: an agent that does not call ``enable()`` gets nothing,
-so upgrading the SDK never starts emitting this field on its own.
+The contract that matters: with ``AGENT_COMMIT_SHA`` absent and no ``enable()``
+call, nothing is stamped, so upgrading the SDK never starts emitting this field
+on its own. A deployment that sets the env var turns it on without agent code.
 """
 
 from __future__ import annotations
@@ -21,14 +22,33 @@ def _reset():
     code_revision.disable()
 
 
-class TestOptIn:
-    def test_disabled_by_default(self, monkeypatch):
-        """Even with the env fully populated, nothing resolves until enable()."""
-        monkeypatch.setenv("AGENT_COMMIT_SHA", SHA)
+class TestEnablement:
+    def test_off_when_env_absent(self, monkeypatch):
+        """The import-time hook ignores AGENT_VERSION; that fallback needs enable()."""
+        monkeypatch.delenv("AGENT_COMMIT_SHA", raising=False)
         monkeypatch.setenv("AGENT_VERSION", SHA)
+        code_revision._enable_from_environment()
         assert code_revision.commit_sha() is None
         assert code_revision.is_enabled() is False
 
+    def test_env_set_at_startup_enables_without_a_call(self, monkeypatch):
+        """The cloud deploy sets AGENT_COMMIT_SHA from the build record; the agent
+        should not need to know."""
+        monkeypatch.setenv("AGENT_COMMIT_SHA", SHA)
+        code_revision._enable_from_environment()
+        assert code_revision.commit_sha() == SHA
+
+    def test_env_set_after_import_needs_enable(self, monkeypatch):
+        monkeypatch.setenv("AGENT_COMMIT_SHA", SHA)
+        assert code_revision.commit_sha() is None
+        code_revision.enable()
+        assert code_revision.commit_sha() == SHA
+
+    def test_bad_env_at_startup_leaves_it_off(self, monkeypatch):
+        monkeypatch.setenv("AGENT_COMMIT_SHA", "latest")
+        code_revision._enable_from_environment()
+        assert code_revision.commit_sha() is None
+
     def test_enable_reads_agent_commit_sha(self, monkeypatch):
         monkeypatch.setenv("AGENT_COMMIT_SHA", SHA)
         code_revision.enable()