diff --git a/crates/worker-runtime/src/config_bundle.rs b/crates/worker-runtime/src/config_bundle.rs
index 91837878..18ea771a 100644
--- a/crates/worker-runtime/src/config_bundle.rs
+++ b/crates/worker-runtime/src/config_bundle.rs
@@ -1,8 +1,7 @@
-use crate::catalog::ProfileSelector;
+use crate::catalog::{ConfigBundleRef, ProfileSelector};
 use crate::error::RuntimeError;
 use serde::{Deserialize, Serialize};
 use sha2::{Digest, Sha256};
-use std::path::Path;
 
 pub const CONFIG_BUNDLE_DIGEST_ALGORITHM: &str = "sha256";
 
@@ -169,13 +168,14 @@ pub struct ConfigBundleSummary {
 
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub struct ConfigBundleAvailability {
-    pub reference: crate::catalog::ConfigBundleRef,
+    pub reference: ConfigBundleRef,
     pub summary: ConfigBundleSummary,
 }
 
 pub(crate) fn validate_config_bundle(bundle: &ConfigBundle) -> Result<(), RuntimeError> {
-    validate_non_empty("config bundle id", &bundle.metadata.id)?;
+    validate_config_bundle_id(&bundle.metadata.id)?;
     validate_non_empty("config bundle digest", &bundle.metadata.digest)?;
+    validate_digest("config bundle digest", &bundle.metadata.digest)?;
     validate_non_empty("config bundle revision", &bundle.metadata.revision)?;
     validate_non_empty("config bundle workspace id", &bundle.metadata.workspace_id)?;
     validate_non_empty("config bundle created_at", &bundle.metadata.created_at)?;
@@ -212,9 +212,7 @@ pub(crate) fn validate_config_bundle(bundle: &ConfigBundle) -> Result<(), Runtim
 
     for declaration in &bundle.declarations {
         validate_non_empty("config declaration name", &declaration.name)?;
-        validate_non_empty("config declaration reference", &declaration.reference)?;
         validate_boundary_text("config declaration name", &declaration.name)?;
-        validate_boundary_text("config declaration reference", &declaration.reference)?;
         if declaration.kind == ConfigDeclarationKind::Unsupported {
             return Err(RuntimeError::UnsupportedConfigDeclaration {
                 bundle_id: bundle.metadata.id.clone(),
@@ -222,10 +220,18 @@ pub(crate) fn validate_config_bundle(bundle: &ConfigBundle) -> Result<(), Runtim
                 name: declaration.name.clone(),
             });
         }
+        validate_declaration_reference(&bundle.metadata.id, declaration)?;
     }
     Ok(())
 }
 
+pub(crate) fn validate_config_bundle_ref(reference: &ConfigBundleRef) -> Result<(), RuntimeError> {
+    validate_config_bundle_id(&reference.id)?;
+    validate_non_empty("config bundle reference digest", &reference.digest)?;
+    validate_digest("config bundle reference digest", &reference.digest)?;
+    Ok(())
+}
+
 pub(crate) fn validate_profile_selector(
     selector: ProfileSelector,
     bundle_id: Option<&str>,
@@ -263,6 +269,117 @@ fn validate_non_empty(label: &'static str, value: &str) -> Result<(), RuntimeErr
     }
 }
 
+fn validate_config_bundle_id(value: &str) -> Result<(), RuntimeError> {
+    validate_non_empty("config bundle id", value)?;
+    let trimmed = value.trim();
+    if trimmed.len() > 128 {
+        return Err(RuntimeError::InvalidRequest(
+            "config bundle id is too large".to_string(),
+        ));
+    }
+    if trimmed != value {
+        return Err(RuntimeError::InvalidRequest(
+            "config bundle id must not contain surrounding whitespace".to_string(),
+        ));
+    }
+    if !trimmed
+        .bytes()
+        .next()
+        .is_some_and(|byte| byte.is_ascii_alphanumeric())
+        || !trimmed
+            .bytes()
+            .all(|byte| byte.is_ascii_alphanumeric() || matches!(byte, b'-' | b'_' | b'.' | b':'))
+    {
+        return Err(RuntimeError::InvalidRequest(
+            "config bundle id must be a path-safe stable identifier".to_string(),
+        ));
+    }
+    Ok(())
+}
+
+fn validate_digest(label: &'static str, value: &str) -> Result<(), RuntimeError> {
+    let trimmed = value.trim();
+    if trimmed != value
+        || trimmed.len() != 64
+        || !trimmed.bytes().all(|byte| byte.is_ascii_hexdigit())
+    {
+        return Err(RuntimeError::InvalidRequest(format!(
+            "{label} must be a 64-character lowercase sha256 hex digest"
+        )));
+    }
+    if !trimmed
+        .bytes()
+        .all(|byte| byte.is_ascii_digit() || matches!(byte, b'a'..=b'f'))
+    {
+        return Err(RuntimeError::InvalidRequest(format!(
+            "{label} must be a 64-character lowercase sha256 hex digest"
+        )));
+    }
+    Ok(())
+}
+
+fn validate_declaration_reference(
+    bundle_id: &str,
+    declaration: &ConfigDeclaration,
+) -> Result<(), RuntimeError> {
+    validate_non_empty("config declaration reference", &declaration.reference)?;
+    validate_ref_boundary_text("config declaration reference", &declaration.reference)?;
+    let allowed_prefixes: &[&str] = match declaration.kind {
+        ConfigDeclarationKind::SecretRef => &["secret:", "secret-ref:", "vault:", "keyring:"],
+        ConfigDeclarationKind::MountGrant => &["mount:", "mount-grant:"],
+        ConfigDeclarationKind::NetworkPolicy => &["network:", "network-policy:"],
+        ConfigDeclarationKind::ShellPolicy => &["shell:", "shell-policy:"],
+        ConfigDeclarationKind::GitPolicy => &["git:", "git-policy:"],
+        ConfigDeclarationKind::CapabilityGrant => &["capability:", "capability-grant:"],
+        ConfigDeclarationKind::Unsupported => &[],
+    };
+    if !allowed_prefixes.iter().any(|prefix| {
+        declaration.reference.starts_with(prefix) && declaration.reference.len() > prefix.len()
+    }) {
+        return Err(RuntimeError::UnsupportedConfigDeclaration {
+            bundle_id: bundle_id.to_string(),
+            declaration_kind: declaration.kind.canonical_name().to_string(),
+            name: declaration.name.clone(),
+        });
+    }
+    Ok(())
+}
+
+fn validate_ref_boundary_text(label: &'static str, value: &str) -> Result<(), RuntimeError> {
+    let trimmed = value.trim();
+    validate_boundary_text(label, trimmed)?;
+    if trimmed != value
+        || trimmed.contains('/')
+        || trimmed.contains('\\')
+        || trimmed.contains('?')
+        || trimmed.contains('&')
+        || trimmed.contains('#')
+        || trimmed.contains('%')
+        || trimmed.contains('=')
+        || trimmed.chars().any(char::is_whitespace)
+        || !trimmed.bytes().all(|byte| {
+            byte.is_ascii_alphanumeric() || matches!(byte, b':' | b'-' | b'_' | b'.' | b'@' | b'+')
+        })
+    {
+        return Err(RuntimeError::InvalidRequest(format!(
+            "{label} must be a typed ref/grant/policy token, not a secret value or path"
+        )));
+    }
+    let lower = trimmed.to_ascii_lowercase();
+    if lower.contains(".cache")
+        || lower.contains(".yoi")
+        || lower.contains(".sock")
+        || lower.contains("socket=")
+        || lower.contains("session_path")
+        || lower.contains("cache_path")
+    {
+        return Err(RuntimeError::InvalidRequest(format!(
+            "{label} must not contain host-local cache/session/socket material"
+        )));
+    }
+    Ok(())
+}
+
 fn validate_boundary_text(label: &'static str, value: &str) -> Result<(), RuntimeError> {
     let trimmed = value.trim();
     if trimmed.len() > 2048 {
@@ -275,16 +392,23 @@ fn validate_boundary_text(label: &'static str, value: &str) -> Result<(), Runtim
             "{label} must not contain control characters"
         )));
     }
-    if Path::new(trimmed).is_absolute()
+    let lower = trimmed.to_ascii_lowercase();
+    if trimmed.starts_with('/')
         || trimmed.starts_with('~')
-        || trimmed.contains("/.cache")
-        || trimmed.contains("\\.cache")
-        || trimmed.contains("/run/")
-        || trimmed.contains("\\run\\")
-        || trimmed.contains(".sock")
-        || trimmed.contains("socket=")
-        || trimmed.contains("session_path")
-        || trimmed.contains("cache_path")
+        || trimmed.contains(":\\")
+        || lower.contains(".cache")
+        || lower.contains(".yoi/sessions")
+        || lower.contains(".yoi\\sessions")
+        || lower.contains("/sessions/")
+        || lower.contains("\\sessions\\")
+        || lower.contains("/run/")
+        || lower.contains("\\run\\")
+        || lower.contains(".sock")
+        || lower.contains("/sock")
+        || lower.contains("\\sock")
+        || lower.contains("socket=")
+        || lower.contains("session_path")
+        || lower.contains("cache_path")
     {
         return Err(RuntimeError::InvalidRequest(format!(
             "{label} must be a stable ref/grant/policy declaration, not a host-local path"
@@ -317,3 +441,79 @@ fn hex_digest(bytes: &[u8]) -> String {
     }
     out
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn bundle_with_declaration(reference: &str) -> ConfigBundle {
+        ConfigBundle {
+            metadata: ConfigBundleMetadata {
+                id: "bundle-1".to_string(),
+                digest: String::new(),
+                revision: "rev-1".to_string(),
+                workspace_id: "workspace-1".to_string(),
+                created_at: "2026-06-26T00:00:00Z".to_string(),
+                provenance: ConfigBundleProvenance {
+                    source: "test".to_string(),
+                    detail: None,
+                },
+            },
+            profiles: vec![ConfigProfileDescriptor {
+                selector: ProfileSelector::Builtin("builtin:coder".to_string()),
+                label: None,
+            }],
+            declarations: vec![ConfigDeclaration {
+                kind: ConfigDeclarationKind::SecretRef,
+                name: "credential".to_string(),
+                reference: reference.to_string(),
+            }],
+        }
+        .with_computed_digest()
+    }
+
+    #[test]
+    fn rejects_host_local_cache_session_socket_and_plaintext_secret_refs() {
+        for reference in [
+            ".cache/yoi",
+            ".yoi/sessions/foo.jsonl",
+            "pods/foo/sock",
+            "password=hunter2",
+            "hunter2-secret-value",
+        ] {
+            let error = validate_config_bundle(&bundle_with_declaration(reference)).unwrap_err();
+            assert!(
+                matches!(
+                    error,
+                    RuntimeError::InvalidRequest(_)
+                        | RuntimeError::UnsupportedConfigDeclaration { .. }
+                ),
+                "unexpected error for {reference}: {error:?}"
+            );
+        }
+    }
+
+    #[test]
+    fn accepts_typed_secret_refs() {
+        validate_config_bundle(&bundle_with_declaration("secret:github-token")).unwrap();
+        validate_config_bundle(&bundle_with_declaration("vault:team.api-key")).unwrap();
+    }
+
+    #[test]
+    fn rejects_unsafe_bundle_ids_and_refs() {
+        for id in ["bundle/1", "bundle?x", "bundle&x", "bundle#x", " bundle"] {
+            let mut bundle = bundle_with_declaration("secret:github-token");
+            bundle.metadata.id = id.to_string();
+            bundle = bundle.with_computed_digest();
+            assert!(validate_config_bundle(&bundle).is_err(), "accepted id {id}");
+        }
+
+        assert!(
+            validate_config_bundle_ref(&ConfigBundleRef {
+                id: "bundle/1".to_string(),
+                digest: "0".repeat(64),
+            })
+            .is_err()
+        );
+    }
+}
diff --git a/crates/worker-runtime/src/runtime.rs b/crates/worker-runtime/src/runtime.rs
index 7fa559ea..1cec1f33 100644
--- a/crates/worker-runtime/src/runtime.rs
+++ b/crates/worker-runtime/src/runtime.rs
@@ -4,7 +4,7 @@ use crate::catalog::{
 };
 use crate::config_bundle::{
     ConfigBundle, ConfigBundleAvailability, ConfigBundleSummary, validate_config_bundle,
-    validate_profile_selector,
+    validate_config_bundle_ref, validate_profile_selector,
 };
 use crate::diagnostics::{DiagnosticSeverity, RuntimeDiagnostic};
 use crate::error::RuntimeError;
@@ -864,11 +864,7 @@ impl RuntimeState {
         &self,
         reference: &ConfigBundleRef,
     ) -> Result<ConfigBundleAvailability, RuntimeError> {
-        if reference.id.trim().is_empty() || reference.digest.trim().is_empty() {
-            return Err(RuntimeError::InvalidRequest(
-                "config bundle reference id and digest must not be empty".to_string(),
-            ));
-        }
+        validate_config_bundle_ref(reference)?;
         let bundle = self.config_bundles.get(&reference.id).ok_or_else(|| {
             RuntimeError::ConfigBundleMissing {
                 bundle_id: reference.id.clone(),
@@ -1274,7 +1270,7 @@ mod tests {
         let mismatch = runtime
             .check_config_bundle(&ConfigBundleRef {
                 id: bundle.metadata.id.clone(),
-                digest: "bad-digest".to_string(),
+                digest: "0".repeat(64),
             })
             .unwrap_err();
         assert!(matches!(
diff --git a/crates/workspace-server/src/hosts.rs b/crates/workspace-server/src/hosts.rs
index 4094be3a..bfa68575 100644
--- a/crates/workspace-server/src/hosts.rs
+++ b/crates/workspace-server/src/hosts.rs
@@ -1451,6 +1451,14 @@ impl RemoteWorkerRuntime {
         format!("{}{}", self.base_url, path)
     }
 
+    fn bundle_availability_path(reference: &ConfigBundleRef) -> String {
+        format!(
+            "/v1/config-bundles/{}/availability?digest={}",
+            url_path_segment_encode(&reference.id),
+            url_query_value_encode(&reference.digest)
+        )
+    }
+
     fn ws_endpoint(&self, worker_id: &str) -> String {
         let mut base = self.base_url.clone();
         if let Some(rest) = base.strip_prefix("https://") {
@@ -1782,10 +1790,7 @@ impl WorkspaceWorkerRuntime for RemoteWorkerRuntime {
     }
 
     fn check_config_bundle(&self, reference: ConfigBundleRef) -> ConfigBundleCheckResult {
-        let path = format!(
-            "/v1/config-bundles/{}/availability?digest={}",
-            reference.id, reference.digest
-        );
+        let path = Self::bundle_availability_path(&reference);
         match self.get_json::<RuntimeHttpConfigBundleAvailabilityResponse>(&path) {
             Ok(response) => ConfigBundleCheckResult {
                 state: WorkerOperationState::Accepted,
@@ -2428,6 +2433,30 @@ fn host_id_for_remote_runtime(runtime_id: &str) -> String {
     bounded_backend_identifier("remote-", runtime_id)
 }
 
+fn url_path_segment_encode(input: &str) -> String {
+    percent_encode(input, |byte| {
+        byte.is_ascii_alphanumeric() || matches!(byte, b'-' | b'_' | b'.' | b'~' | b':')
+    })
+}
+
+fn url_query_value_encode(input: &str) -> String {
+    percent_encode(input, |byte| {
+        byte.is_ascii_alphanumeric() || matches!(byte, b'-' | b'_' | b'.' | b'~')
+    })
+}
+
+fn percent_encode(input: &str, keep: impl Fn(u8) -> bool) -> String {
+    let mut encoded = String::with_capacity(input.len());
+    for byte in input.bytes() {
+        if keep(byte) {
+            encoded.push(byte as char);
+        } else {
+            encoded.push_str(&format!("%{byte:02X}"));
+        }
+    }
+    encoded
+}
+
 fn remote_runtime_capabilities(limit: usize, available: bool) -> RuntimeCapabilitySummary {
     RuntimeCapabilitySummary {
         can_list_hosts: true,
@@ -2483,10 +2512,6 @@ fn remote_http_status_diagnostic(
         .as_ref()
         .map(|error| error.error.code.as_str())
         .unwrap_or("remote_http_error");
-    let remote_message = error
-        .as_ref()
-        .map(|error| error.error.message.clone())
-        .unwrap_or_else(|| format!("remote Runtime returned HTTP {status}"));
     let (code, severity) = match status {
         StatusCode::UNAUTHORIZED | StatusCode::FORBIDDEN => {
             ("remote_runtime_auth_failed", DiagnosticSeverity::Error)
@@ -2501,7 +2526,9 @@ fn remote_http_status_diagnostic(
     diagnostic(
         code,
         severity,
-        format!("Remote Runtime '{runtime_id}' rejected request ({remote_code}): {remote_message}"),
+        format!(
+            "Remote Runtime '{runtime_id}' rejected request ({remote_code}, HTTP {status}); internal details were sanitized"
+        ),
     )
 }
 
@@ -3415,6 +3442,76 @@ mod tests {
         assert!(browser_payload.contains("worker_id"));
     }
 
+    #[test]
+    fn remote_config_bundle_sync_and_check_diagnostics_are_sanitized_and_path_safe() {
+        let leaked_store_path = "/var/lib/yoi/runtime/bundles/bundle-1.json";
+        let leaked_session_path = ".yoi/sessions/session.jsonl";
+        let digest = "0".repeat(64);
+        let (base_url, server) = serve_mock_http(vec![
+            mock_response(
+                "POST",
+                "/v1/config-bundles",
+                true,
+                500,
+                json!({
+                    "error": {
+                        "code": "store_io",
+                        "message": format!("failed to write {leaked_store_path}")
+                    }
+                })
+                .to_string(),
+            ),
+            mock_response(
+                "GET",
+                "/v1/config-bundles/bundle%2F1%3Fx/availability?digest=0000000000000000000000000000000000000000000000000000000000000000",
+                true,
+                400,
+                json!({
+                    "error": {
+                        "code": "invalid_request",
+                        "message": format!("invalid path {leaked_session_path}")
+                    }
+                })
+                .to_string(),
+            ),
+        ]);
+        let mut registry = RuntimeRegistry::new(Vec::new());
+        registry.register(
+            RemoteWorkerRuntime::new(RemoteRuntimeConfig::new(
+                "remote:primary",
+                "Remote Primary",
+                base_url,
+                Some("secret-token".to_string()),
+            ))
+            .unwrap(),
+        );
+
+        let sync = registry
+            .sync_config_bundle("remote:primary", test_config_bundle())
+            .unwrap();
+        assert_eq!(sync.state, WorkerOperationState::Rejected);
+        let sync_payload = serde_json::to_string(&sync).unwrap();
+        assert!(!sync_payload.contains(leaked_store_path), "{sync_payload}");
+
+        let check = registry
+            .check_config_bundle(
+                "remote:primary",
+                ConfigBundleRef {
+                    id: "bundle/1?x".to_string(),
+                    digest,
+                },
+            )
+            .unwrap();
+        assert_eq!(check.state, WorkerOperationState::Rejected);
+        let check_payload = serde_json::to_string(&check).unwrap();
+        assert!(
+            !check_payload.contains(leaked_session_path),
+            "{check_payload}"
+        );
+        assert!(!check_payload.contains(".yoi/sessions"), "{check_payload}");
+        server.join().expect("mock remote server finished");
+    }
+
     #[test]
     fn remote_runtime_auth_errors_map_to_typed_backend_error() {
         let (base_url, server) = serve_mock_http(vec![mock_response(