diff --git a/docs/BUNDLE_GUIDE.md b/docs/BUNDLE_GUIDE.md
index be01701..df88b82 100644
--- a/docs/BUNDLE_GUIDE.md
+++ b/docs/BUNDLE_GUIDE.md
@@ -1266,10 +1266,13 @@ Bundles support multiple source formats for modules:
 | Local path | `./modules/my-module` | Modules within the bundle |
 | Relative path | `../shared-module` | Sibling directories |
 | Git URL | `git+https://github.com/org/repo@main` | External modules |
+| Git pinned to commit | `git+https://github.com/org/repo@32d4052dad46016f91ce698646580473e4121344` | Reproducible installs (eval runs, CI) |
 | Git with subpath | `git+https://github.com/org/repo@main#subdirectory=modules/foo` | Module within larger repo |
 
 **Local paths are resolved relative to the bundle's location.**
 
+**Commit SHA pinning**: A ref that is a full 40-character hex commit SHA pins the source to that exact commit. Pinned sources are automatically skipped by `amplifier update` (a pinned ref can never have updates). Short/abbreviated SHAs are rejected by design — an ambiguous ref defeats reproducibility. Note that a typo'd but valid-looking SHA fails only after a full-history clone fallback: the fast single-commit fetch fails, git falls back to a full clone, and checkout then fails with a clear error (a failed pinned resolve can leave a cache entry behind, and a subsequent resolve of the same URI may serve it — clear the cache entry if a pin fails).
+
 ---
 
 ## Composition with includes:
diff --git a/docs/URI_FORMATS.md b/docs/URI_FORMATS.md
index aea97bb..d689fe7 100644
--- a/docs/URI_FORMATS.md
+++ b/docs/URI_FORMATS.md
@@ -10,8 +10,17 @@ Quick reference for source URIs. For complete details, see `parse_uri()` docstri
 | **Local directory** | `/path/to/bundle/` (finds `bundle.md` inside) |
 | **Git HTTPS** | `git+https://github.com/org/repo@main` |
 | **Git SSH** | `git+ssh://git@github.com/org/repo@main` |
+| **Git pinned to commit** | `git+https://github.com/org/repo@32d4052dad46016f91ce698646580473e4121344` |
 | **Subdirectory** | `git+https://github.com/org/repo@main#subdirectory=path/to/bundle` |
 
+### Pinning to a Commit SHA
+
+A ref that is a full 40-character hex commit SHA pins the source to that exact commit. Use this for reproducible installs — evaluation runs, CI, anywhere "same URI, same code" must hold.
+
+- Pinned sources are automatically **skipped by `amplifier update`** — a pinned ref can never have updates.
+- Short/abbreviated SHAs are **rejected by design**: an ambiguous ref defeats reproducibility. Use the full 40 characters.
+- A typo'd but valid-looking SHA is expensive: the fast single-commit fetch fails, git falls back to a **full-history clone**, and checkout then fails with a clear error (a failed pinned resolve can leave a cache entry behind, and a subsequent resolve of the same URI may serve it — clear the cache entry if a pin fails). A wrong SHA costs a full-history download before it errors.
+
 ## Common Examples
 
 ```python
diff --git a/tests/test_sources.py b/tests/test_sources.py
index 5576a5b..148b60a 100644
--- a/tests/test_sources.py
+++ b/tests/test_sources.py
@@ -12,6 +12,7 @@
 from amplifier_foundation.sources.git import GitSourceHandler
 from amplifier_foundation.sources.git import _is_full_commit_sha
 from amplifier_foundation.sources.http import HttpSourceHandler
+from amplifier_foundation.sources.protocol import SourceStatus
 from amplifier_foundation.sources.zip import ZipSourceHandler
 
 
@@ -375,6 +376,50 @@ def test_rejects_branch_names(self) -> None:
         assert _is_full_commit_sha("v1.0.0") is False
 
 
+class TestShaPredicatesAgree:
+    """Invariant: the full-SHA rule is encoded twice and must not drift.
+
+    `_is_full_commit_sha` (regex, sources/git.py) and `SourceStatus.is_pinned`
+    (length + charset, sources/protocol.py) independently encode "40-char hex
+    commit SHA". A code comment claims they agree; this test checks it.
+
+    Note: `is_pinned` additionally treats v-prefixed version tags as pinned.
+    That branch is deliberate divergence outside the shared SHA rule, so
+    tag-shaped refs are excluded from the equality set and covered separately.
+    """
+
+    @pytest.mark.parametrize(
+        "ref",
+        [
+            "32d4052dad46016f91ce698646580473e4121344",  # 40-hex lowercase
+            "32D4052DAD46016F91CE698646580473E4121344",  # 40-hex uppercase
+            "32D4052dad46016F91ce698646580473E4121344",  # 40-hex mixed case
+            "3" * 39,  # one char too short
+            "3" * 41,  # one char too long
+            "g" + "3" * 39,  # 40 chars, non-hex
+            "main",  # branch name
+            "feat/some-branch",  # branch name with slash
+        ],
+    )
+    def test_predicates_agree_on_sha_classification(self, ref: str) -> None:
+        status = SourceStatus(
+            source_uri=f"git+https://example.com/org/repo@{ref}",
+            is_cached=True,
+            cached_ref=ref,
+        )
+        assert _is_full_commit_sha(ref) == status.is_pinned
+
+    def test_version_tag_is_pinned_but_not_a_sha(self) -> None:
+        """Documents the intentional divergence: tags pin without being SHAs."""
+        status = SourceStatus(
+            source_uri="git+https://example.com/org/repo@v1.0.0",
+            is_cached=True,
+            cached_ref="v1.0.0",
+        )
+        assert _is_full_commit_sha("v1.0.0") is False
+        assert status.is_pinned is True
+
+
 class TestGitSourceHandlerShaRefs:
     """Tests for resolving git refs pinned to a commit SHA."""