@@ -7,6 +7,12 @@ this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Added
+- **`@convert` on `origin.passthrough` — acknowledge a deliberate passthrough type change (#185).** A new optional boolean attr. Absent/false (the default), a passthrough is type-preserving (see Changed, below); `@convert: true` opts a field out of the type-equality check when its type intentionally differs from its `@from` source. It is an **acknowledgement only — it does NOT generate a cast**; the value flows through unchanged and the consumer owns any coercion. Real type-converting projections remain `origin.expression`'s job (#159). Registered on `origin.passthrough` in all five ports (cross-port registry-conformance gated).
+
+### Changed
+- **BREAKING — `origin.passthrough` is now type-preserving: a passthrough field must match its `@from` source's type (#185).** A field carrying `origin.passthrough @from: "Entity.field"` forwards another field's value unchanged, so its declared `field.<subType>` and array-ness must be identical to the resolved source field. A divergence now fails load with `ERR_PASSTHROUGH_TYPE_MISMATCH` (e.g. a `field.uuid` source surfaced by a projection as `field.string` — the exact mismodeling that forces hand-written `String↔UUID` bridging and defeats a UUID migration). The check compares **subType and array-ness only — nullability is deliberately not judged** (a view over an outer join legitimately widens `NOT NULL` → nullable). Opt out of a deliberate type change with `@convert: true` (see Added). This **generalizes and retires** the narrow, stored-proc-parameter-ref-only `ERR_PARAMETER_REF_PASSTHROUGH_TYPE_MISMATCH` (FR-015) into one host-agnostic invariant covering projections, entities, values, and parameter refs alike. Enforced in the loader/verify in all five ports (TS / Python / C# / Java / Kotlin), cross-port conformance-gated. **Migration:** if load newly fails, the error names both the declared type and the source type — declare the source type (usually the fix — the projection was wrong), or add `@convert: true` if the divergence is intentional.
+
 ## [0.15.15] — 2026-07-07
 
 _npm `@metaobjectsdev/sdk` + `@metaobjectsdev/cli` `0.15.15` (isolated patch; the other 12 packages stay `0.15.14`). Ships updated agent-context skills — a docs/content change bundled into the SDK and delivered through the CLI (`meta agent-docs` / `meta init`). No runtime or generated-code change; PyPI / NuGet / Maven are unaffected._
 
@@ -601,6 +601,18 @@ child (`source.rdb: { kind: view, table: v_author }`) — codegen keys projectio
 detection + view DDL off that read-only source, so without it `meta gen` emits
 nothing for the projection.
 
+**A `passthrough` field must match its `@from` source's type.** A passthrough
+forwards the source value unchanged, so the projection field's `field.<subType>`
+and array-ness must be identical to the source field's — a `field.uuid` source
+declared as `field.string` on the projection fails load with
+`ERR_PASSTHROUGH_TYPE_MISMATCH` (this is exactly the mismodeling that leaves a
+view `String`-typed over a `uuid` column and forces hand-written coercion).
+Declare the source's type. If the type genuinely must differ on purpose, set
+`@convert: true` on the `origin.passthrough` to acknowledge it — an
+acknowledgement only, it does **not** generate a cast (you own any coercion).
+Nullability may differ (an outer-join view legitimately widens `NOT NULL` →
+nullable) — only subType + array-ness are checked.
+
 ## Abstracts + `extends` (deferred resolution) + `overlay`
 
 An **abstract** node (`abstract: true`) describes a shape but is never emitted as
 
@@ -601,6 +601,18 @@ child (`source.rdb: { kind: view, table: v_author }`) — codegen keys projectio
 detection + view DDL off that read-only source, so without it `meta gen` emits
 nothing for the projection.
 
+**A `passthrough` field must match its `@from` source's type.** A passthrough
+forwards the source value unchanged, so the projection field's `field.<subType>`
+and array-ness must be identical to the source field's — a `field.uuid` source
+declared as `field.string` on the projection fails load with
+`ERR_PASSTHROUGH_TYPE_MISMATCH` (this is exactly the mismodeling that leaves a
+view `String`-typed over a `uuid` column and forces hand-written coercion).
+Declare the source's type. If the type genuinely must differ on purpose, set
+`@convert: true` on the `origin.passthrough` to acknowledge it — an
+acknowledgement only, it does **not** generate a cast (you own any coercion).
+Nullability may differ (an outer-join view legitimately widens `NOT NULL` →
+nullable) — only subType + array-ness are checked.
+
 ## Abstracts + `extends` (deferred resolution) + `overlay`
 
 An **abstract** node (`abstract: true`) describes a shape but is never emitted as
 
@@ -601,6 +601,18 @@ child (`source.rdb: { kind: view, table: v_author }`) — codegen keys projectio
 detection + view DDL off that read-only source, so without it `meta gen` emits
 nothing for the projection.
 
+**A `passthrough` field must match its `@from` source's type.** A passthrough
+forwards the source value unchanged, so the projection field's `field.<subType>`
+and array-ness must be identical to the source field's — a `field.uuid` source
+declared as `field.string` on the projection fails load with
+`ERR_PASSTHROUGH_TYPE_MISMATCH` (this is exactly the mismodeling that leaves a
+view `String`-typed over a `uuid` column and forces hand-written coercion).
+Declare the source's type. If the type genuinely must differ on purpose, set
+`@convert: true` on the `origin.passthrough` to acknowledge it — an
+acknowledgement only, it does **not** generate a cast (you own any coercion).
+Nullability may differ (an outer-join view legitimately widens `NOT NULL` →
+nullable) — only subType + array-ness are checked.
+
 ## Abstracts + `extends` (deferred resolution) + `overlay`
 
 An **abstract** node (`abstract: true`) describes a shape but is never emitted as
 
@@ -601,6 +601,18 @@ child (`source.rdb: { kind: view, table: v_author }`) — codegen keys projectio
 detection + view DDL off that read-only source, so without it `meta gen` emits
 nothing for the projection.
 
+**A `passthrough` field must match its `@from` source's type.** A passthrough
+forwards the source value unchanged, so the projection field's `field.<subType>`
+and array-ness must be identical to the source field's — a `field.uuid` source
+declared as `field.string` on the projection fails load with
+`ERR_PASSTHROUGH_TYPE_MISMATCH` (this is exactly the mismodeling that leaves a
+view `String`-typed over a `uuid` column and forces hand-written coercion).
+Declare the source's type. If the type genuinely must differ on purpose, set
+`@convert: true` on the `origin.passthrough` to acknowledge it — an
+acknowledgement only, it does **not** generate a cast (you own any coercion).
+Nullability may differ (an outer-join view legitimately widens `NOT NULL` →
+nullable) — only subType + array-ness are checked.
+
 ## Abstracts + `extends` (deferred resolution) + `overlay`
 
 An **abstract** node (`abstract: true`) describes a shape but is never emitted as
 
@@ -601,6 +601,18 @@ child (`source.rdb: { kind: view, table: v_author }`) — codegen keys projectio
 detection + view DDL off that read-only source, so without it `meta gen` emits
 nothing for the projection.
 
+**A `passthrough` field must match its `@from` source's type.** A passthrough
+forwards the source value unchanged, so the projection field's `field.<subType>`
+and array-ness must be identical to the source field's — a `field.uuid` source
+declared as `field.string` on the projection fails load with
+`ERR_PASSTHROUGH_TYPE_MISMATCH` (this is exactly the mismodeling that leaves a
+view `String`-typed over a `uuid` column and forces hand-written coercion).
+Declare the source's type. If the type genuinely must differ on purpose, set
+`@convert: true` on the `origin.passthrough` to acknowledge it — an
+acknowledgement only, it does **not** generate a cast (you own any coercion).
+Nullability may differ (an outer-join view legitimately widens `NOT NULL` →
+nullable) — only subType + array-ness are checked.
+
 ## Abstracts + `extends` (deferred resolution) + `overlay`
 
 An **abstract** node (`abstract: true`) describes a shape but is never emitted as
 
@@ -32,6 +32,7 @@
     "ERR_AMBIGUOUS_REF": "Cross-package reference contract — a BARE reference (no `::`) names an object that exists in more than one package and none is in the referrer's own package. The reference is ambiguous; qualify it with the package (FQN). Applies to every object-ref-bearing attribute (@objectRef, @references, @from/@of/@via heads, extends, @payloadRef/@responseRef, @through).",
     "ERR_ORIGIN_CARDINALITY": "FR-024 (ADR-0029): origin cardinality contract broken — a passthrough @via path crosses a to-many hop (row-multiplying passthrough — you meant aggregate), or an aggregate @via path is to-one at every hop (aggregating over a to-one path — you meant passthrough). Checked on explicit AND inferred paths.",
     "ERR_EXTENDS_ORIGIN_MISMATCH": "FR-024 (ADR-0029 decision 7): a field declares both an entity-nested extends (shape lineage) and an origin.passthrough @from (data lineage) and they disagree — the resolved @from target is not the field's resolved extends target (nor anywhere on its extends chain). Host-agnostic (projections, entities, values). origin.aggregate is never judged (it computes something new); a top-level abstract extends target is never judged (shape-only reuse makes no lineage claim).",
+    "ERR_PASSTHROUGH_TYPE_MISMATCH": "#185: a field carrying origin.passthrough @from: \"Entity.field\" declares a field.<subType> or array-ness that differs from its resolved source field — a passthrough forwards the value unchanged, so its type must be identical. Host-agnostic (projections, entities, values, stored-proc parameter refs). Nullability is NOT judged (a view over an outer join legitimately widens NOT NULL → nullable). Opt out of a deliberate type change with @convert: true on the origin.passthrough (an acknowledgement — it does not generate a cast); real type-converting projections are origin.expression's job (#159).",
     "ERR_DERIVED_FIELD_NO_READ_SOURCE": "FR-024 (spec §7): an object.entity field carrying an origin.* child is derived (read-only) and must be providable — the entity must declare at least one source with a read-only @kind (view/materializedView/storedProc/tableFunction, e.g. a @role replica view beside the table primary). Table-only or source-less entities with origin-bearing fields error on the field. Projections and object.value hosts are exempt. Until the Phase-E B4b cutover removes view-primary entities, a read-only-kind PRIMARY source also counts as providable (legacy spelling).",
     "ERR_INVALID_TEMPLATE": "A template (prompt/output) declares a @payloadRef that does not resolve, or @requiredSlots that are not fields on its payload.",
     "ERR_INVALID_RELATIONSHIP": "FR-017: a M:N relationship's slim vocabulary is invalid — @through does not name a junction declaring two identity.reference children, @sourceRefField does not match one of them, or a M:N-only attr (@through/@sourceRefField/@symmetric) is set on a non-M:N (1:N / @cardinality:one) relationship; OR a relationship's @objectRef does not resolve to any object in the loaded tree (a dangling target).",
@@ -54,7 +55,6 @@
     "ERR_PARAMETER_REF_UNRESOLVED": "FR-015: source.rdb @parameterRef names an object that does not exist in the loaded model.",
     "ERR_PARAMETER_REF_NOT_VALUE_OBJECT": "FR-015: source.rdb @parameterRef references an object.entity instead of object.value. Input shapes are value-objects by definition (no identity).",
     "ERR_PARAMETER_REF_ON_NON_CALLABLE_KIND": "FR-015: source.rdb @parameterRef is set with @kind: \"table\" / \"view\" / \"materializedView\". These kinds do not accept parameters; only \"storedProc\" / \"tableFunction\" do.",
-    "ERR_PARAMETER_REF_PASSTHROUGH_TYPE_MISMATCH": "FR-015: a parameter field uses origin.passthrough @from: \"Entity.field\" but the parameter's subtype does not match the referenced field's subtype (drift gate).",
     "ERR_DISCRIMINATOR_FIELD_NOT_FOUND": "FR-014: object.entity @discriminator names a field that does not exist on the entity (including inherited fields).",
     "ERR_DISCRIMINATOR_VALUE_DUPLICATE": "FR-014: two subtypes of the same @discriminator-bearing root declare the same @discriminatorValue.",
     "ERR_DISCRIMINATOR_VALUE_MISSING": "FR-014: a concrete entity extends a chain whose root declares @discriminator but the subtype lacks @discriminatorValue.",
 
@@ -0,0 +1,17 @@
+{
+  "errors": [
+    {
+      "code": "ERR_PASSTHROUGH_TYPE_MISMATCH",
+      "source": {
+        "format": "resolved",
+        "files": [
+          "meta.demo.json"
+        ],
+        "jsonPath": "$['metadata.root'].children[1]['object.projection'].children[2]['field.string'].children[0]['origin.passthrough']",
+        "referrer": "ProductView::tags",
+        "target": "demo::Product.tags"
+      }
+    }
+  ],
+  "warnings": []
+}
@@ -0,0 +1,36 @@
+{
+  "metadata.root": {
+    "package": "demo",
+    "children": [
+      {
+        "object.entity": {
+          "name": "Product",
+          "children": [
+            { "source.rdb": { "@table": "products" } },
+            { "field.uuid": { "name": "id" } },
+            { "field.string": { "name": "tags", "isArray": true } },
+            { "identity.primary": { "name": "id", "@fields": ["id"] } }
+          ]
+        }
+      },
+      {
+        "object.projection": {
+          "name": "ProductView",
+          "children": [
+            { "source.rdb": { "@kind": "view", "@view": "v_product" } },
+            { "field.uuid": { "name": "id", "extends": "demo::Product.id" } },
+            {
+              "field.string": {
+                "name": "tags",
+                "children": [
+                  { "origin.passthrough": { "@from": "demo::Product.tags" } }
+                ]
+              }
+            },
+            { "identity.primary": { "name": "id", "extends": "demo::Product.id" } }
+          ]
+        }
+      }
+    ]
+  }
+}
@@ -0,0 +1 @@
+["metaobjects-core-types", "metaobjects-db"]
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+["metaobjects-core-types", "metaobjects-db"]`