diff --git a/GEMINI.md b/GEMINI.md
index 77b8839..f5dc233 100644
--- a/GEMINI.md
+++ b/GEMINI.md
@@ -184,6 +184,24 @@ It evaluates as an **Independent Declarative Rules Engine**. Every `Case` block
     *   **`array`**: Homogeneous collection of items matching the `items` schema. Validation is homogeneous, so strictness checking does not apply. `"extensible"` is not applicable at the `array` level (tuple-like `prefixItems` are removed).
 *   **Inheritance Boundaries**: Strictness resets when crossing non-primitive `type` boundaries. A schema extending a strict parent remains strict unless it explicitly overrides with `"extensible": true`.
 
+### Immutable Properties (`"immutable": true`)
+To distinguish read-only hydrated endpoint references, computed properties, system-managed timestamps (`created_at`, `modified_at`), or audit fields from writable properties, JSPG introduces the universal `"immutable": true` schema keyword.
+
+*   **Developer Perspective**: Annotate properties in database schemas or trait definitions with `"immutable": true` when the property should be visible on reads (`jspg_query` / `.response`), but rejected or ignored on writes (`jspg_merge` / `.request`).
+    ```json
+    "properties": {
+      "source": {
+        "family": "lite.organization",
+        "immutable": true,
+        "description": "Read-only hydrated member entity summary on a membership edge."
+      }
+    }
+    ```
+*   **Behavior Across Pillars**:
+    *   **Queryer (`jspg_query`)**: Hydrates and includes `immutable` properties in output read responses without restriction.
+    *   **Validator (`jspg_validate`)**: Context-aware. Returns `IMMUTABLE_PROPERTY_VIOLATION` if an `immutable` property is supplied in a write/request payload (schema IDs not ending in `.response`). Response schemas (`.response`) permit `immutable` fields.
+    *   **Merger (`jspg_merge`)**: Automatically skips `immutable` properties during object graph merging so client payloads can never mutate database columns or relationship edges.
+
 ### Format Leniency for Empty Strings
 To simplify frontend form validation, format validators specifically for `uuid`, `date-time`, and `email` explicitly allow empty strings (`""`), treating them as "present but unset".
 
@@ -302,6 +320,7 @@ JSPG implements specific extensions to the Draft 2020-12 standard to support the
 *   **Missing Type Ultimatum**: If an entity logically requires a discriminator and the JSON payload omits it, JSPG short-circuits branch execution entirely, bubbling a single, perfectly-pathed `MISSING_TYPE` error back to the UI natively to prevent confusing cascading failures.
 *   **Golden Match Context**: When exactly one structural candidate perfectly maps a discriminator, the Validator exclusively cascades that specific structural error context directly to the user, stripping away all noise generated by other parallel schemas.
 *   **Topological Array Pathing**: Instead of relying on explicit `$id` references or injected properties, array iteration paths are dynamically typed based on their compiler boundary constraints. If the array's `items` schema resolves to a topological table-backed entity (e.g., inheriting via a `family` macro tracked in the global DB catalog), the array locks paths and derives element indexes from their actual UUID paths (`array/widget-1/name`), natively enforcing database continuity. If evaluating isolated ad-hoc JSONB elements, strict numeric indexing is enforced natively (`array/1/name`) preventing synthetic payload manipulation.
+*   **Context-Aware Immutability Validation**: `jspg_validate` evaluates the target schema ID context. If the target schema ID does not end with `.response` (inbound write/request context), any property present in the payload marked with `"immutable": true` raises an `IMMUTABLE_PROPERTY_VIOLATION` error. Response schemas (`.response`) permit `immutable` fields for read hydration.
 
 ---
 
@@ -316,6 +335,9 @@ The Merger provides an automated, high-performance graph synchronization engine.
 
 *   **Caching Strategy**: The Merger leverages the native `compiled_edges` permanently cached onto the Schema AST via `OnceLock` to instantly resolve Foreign Key mapping graphs natively in absolute `O(1)` time. It additionally utilizes the concurrent `GLOBAL_JSPG` application memory (`DashMap`) to cache statically constructed SQL `SELECT` strings used during deduplication (`lookup_fields`) and difference tracking calculations.
 *   **Deep Graph Merging**: The Merger walks arbitrary levels of deeply nested JSON schemas (e.g. tracking an `order`, its `customer`, and an array of its `lines`). It intelligently discovers the correct parent-to-child or child-to-parent Foreign Keys stored in the registry and automatically maps the UUIDs across the relationships during UPSERT.
+*   **Immutable Property Filtering**: Properties declaring `"immutable": true` are filtered out during schema property traversal prior to edge classification or column assembly, guaranteeing client payloads cannot write or mutate read-only/computed properties or hydrated endpoint references.
+*   **Factual Creation Dependency Validation**: When `stage_entity` confirms `kind == "create"` via database lookup, `jspg_merge` evaluates `schema.obj.dependencies.get("created")`. If required creation fields are missing from `entity_fields`, `entity_objects`, or `entity_arrays`, `jspg_merge` halts and returns a structured `REQUIRED_FIELD_MISSING` `Drop`.
+*   **Structured Fail-Fast Error Propagation**: Internal merger operations return typed `Result<T, crate::drop::Error>` instances, allowing clean fail-fast `?` traversal and returning `Drop::with_errors(vec![err])` directly at the top-level API boundary on failure.
 *   **Prefix Foreign Key Matching**: Handles scenario where multiple relations point to the same table by using database Foreign Key constraint prefixes (`fk_`). For example, if a schema has `shipping_address` and `billing_address`, the merger resolves against `fk_shipping_address_entity` vs `fk_billing_address_entity` automatically to correctly route object properties.
 *   **Dynamic Deduplication & Lookups**: If a nested object is provided without an `id`, the Merger utilizes custom `lookup_fields` declared directly in the schema registry JSON comments. It validates at setup compile-time that a corresponding unique index exists in PostgreSQL for these fields. When merging, it dynamically builds query predicates for any satisfied `lookup_fields` sets in the entity's type hierarchy (checking child-to-parent hierarchies order-independently and combining satisfied keys with `UNION` queries) to discover the correct UUID to perform an UPDATE, preventing data duplication.
 *   **Hierarchical Table Inheritance**: The Punc system uses distributed table inheritance (e.g. `person` inherits `user` inherits `organization` inherits `entity`). The Merger splits the incoming JSON payload and performs atomic row updates across *all* relevant tables in the lineage map.