diff --git a/CHANGELOG.md b/CHANGELOG.md
index f747adeae..05ed972bf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,7 +32,7 @@ All notable changes to OriginWeave are documented in this file. The format follo
 - Credential-free sensitive-handle lifecycle evidence binds issuance, exclusive expiry, bounded uses, observed resolution count, and revocation to the exact credential-free `OpaqueHandleOnly` sensitive-access receipt, preserving tenant, actor, task, field set, purpose, destination, classification, policy version, and decision time without storing opaque handle tokens or protected values.
 - Credential-free connection and redirect evidence containing canonical addresses, destination classes, target digests, hop numbers, and approved-address counts.
 - Credential-free verified TCP evidence containing the logical origin, requested socket, observed peer, destination class, successful attempt number, and per-attempt timeout.
-- Standard `Display` and `std::error::Error` contracts for destination, redirect, digest, direct-network, TLS, and resource-budget failures, including preserved destination-policy, rustls, and operating-system sources where applicable.
+- Standard `Display` and `std::error::Error` contracts for destination, redirect, evidence-digest, action-intent-digest, direct-network, TLS, and resource-budget failures, including preserved destination-policy, rustls, and operating-system sources where applicable.
 - Real loopback TCP integration proof plus deterministic timeout, refusal, retry, peer-inspection, peer-mismatch, canonicalization, IPv6 metadata, and single-use replay tests.
 - Real loopback rustls integration covering trusted DNS SAN, Common-Name fallback rejection, wrong-name and untrusted-root rejection, fixed-time expiry and not-yet-valid failures, exact IPv4 and IPv6 SANs, TLS 1.2/TLS 1.3, required and optional ALPN, and transport-origin binding.
 - Cumulative interactive-first RAM, VRAM, batch, local-model, admission, pause, and compositor-pressure mitigation plans, including active-consumer reduction at exact hard limits.
@@ -102,4 +102,4 @@ All notable changes to OriginWeave are documented in this file. The format follo
 - The hourly product agent has no Git metadata or repository authority. A separate post-verification publisher opens one PR and cannot approve or merge it.
 - The unprivileged OpenCode user is restricted to loopback egress during model execution, preventing runner-wide allow-listed endpoints from becoming direct source-exfiltration channels.
 
-[Unreleased]: https://github.com/ContextualWisdomLab/OriginWeave/compare/main...HEAD
\ No newline at end of file
+[Unreleased]: https://github.com/ContextualWisdomLab/OriginWeave/compare/main...HEAD
diff --git a/crates/originweave-core/src/lib.rs b/crates/originweave-core/src/lib.rs
index e33a7e7e5..c7e3d5b13 100644
--- a/crates/originweave-core/src/lib.rs
+++ b/crates/originweave-core/src/lib.rs
@@ -503,6 +503,18 @@ pub enum ActionIntentDigestError {
     InvalidFormat,
 }
 
+impl fmt::Display for ActionIntentDigestError {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::InvalidFormat => formatter.write_str(
+                "action intent digest must be sha256: followed by 64 lowercase hexadecimal digits",
+            ),
+        }
+    }
+}
+
+impl std::error::Error for ActionIntentDigestError {}
+
 /// The browser execution mode that owns an action.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum SessionMode {
diff --git a/crates/originweave-core/tests/action_intent_digest_error_contract.rs b/crates/originweave-core/tests/action_intent_digest_error_contract.rs
new file mode 100644
index 000000000..ed5d731cd
--- /dev/null
+++ b/crates/originweave-core/tests/action_intent_digest_error_contract.rs
@@ -0,0 +1,13 @@
+use std::error::Error as _;
+
+use originweave_core::ActionIntentDigestError;
+
+#[test]
+fn action_intent_digest_error_exposes_standard_error_contract() {
+    let error = ActionIntentDigestError::InvalidFormat;
+    assert_eq!(
+        error.to_string(),
+        "action intent digest must be sha256: followed by 64 lowercase hexadecimal digits"
+    );
+    assert!(error.source().is_none());
+}