diff --git a/crates/config-source-wasm/src/lib.rs b/crates/config-source-wasm/src/lib.rs
index e19501ad..042312bb 100644
--- a/crates/config-source-wasm/src/lib.rs
+++ b/crates/config-source-wasm/src/lib.rs
@@ -1,7 +1,7 @@
 use config_source::{
     ConfigTreeSnapshot, EvaluationResult, SnapshotEnvironment, ToolchainContract, VirtualPath,
 };
-use serde_wasm_bindgen::{from_value, to_value};
+use serde_wasm_bindgen::{Serializer, from_value};
 use wasm_bindgen::prelude::*;
 
 #[wasm_bindgen]
@@ -12,7 +12,7 @@ pub fn evaluate_snapshot(snapshot: JsValue, contract: JsValue) -> Result<JsValue
         SnapshotEnvironment::new(snapshot)
             .evaluate_contract(&contract)
             .map_err(|diagnostics| {
-                to_value(&diagnostics).unwrap_or_else(|_| JsValue::from_str("evaluation failed"))
+                encode(&diagnostics).unwrap_or_else(|_| JsValue::from_str("evaluation failed"))
             })?,
     )
 }
@@ -40,7 +40,9 @@ fn decode<T: serde::de::DeserializeOwned>(value: JsValue) -> Result<T, JsValue>
 }
 
 fn encode<T: serde::Serialize>(value: T) -> Result<JsValue, JsValue> {
-    to_value(&value).map_err(|error| JsValue::from_str(&error.to_string()))
+    value
+        .serialize(&Serializer::json_compatible())
+        .map_err(|error| JsValue::from_str(&error.to_string()))
 }
 
 fn js_error(error: impl std::fmt::Display) -> JsValue {
diff --git a/web/workspace/src/lib/workspace/config-source/generated/config_source_wasm_bg.wasm b/web/workspace/src/lib/workspace/config-source/generated/config_source_wasm_bg.wasm
index 5e6dcadd..dbbca662 100644
Binary files a/web/workspace/src/lib/workspace/config-source/generated/config_source_wasm_bg.wasm and b/web/workspace/src/lib/workspace/config-source/generated/config_source_wasm_bg.wasm differ
diff --git a/web/workspace/src/lib/workspace/config-source/api.test.ts b/web/workspace/test/config-source/api.test.ts
similarity index 97%
rename from web/workspace/src/lib/workspace/config-source/api.test.ts
rename to web/workspace/test/config-source/api.test.ts
index e072e44d..4b460fae 100644
--- a/web/workspace/src/lib/workspace/config-source/api.test.ts
+++ b/web/workspace/test/config-source/api.test.ts
@@ -6,7 +6,7 @@ import {
   fetchConfigEntry,
   fetchConfigTree,
   previewConfigTree,
-} from "./api.ts";
+} from "../../src/lib/workspace/config-source/api.ts";
 
 function response(body: unknown, status = 200): Response {
   return new Response(JSON.stringify(body), {
diff --git a/web/workspace/test/config-source/wasm-parity.test.ts b/web/workspace/test/config-source/wasm-parity.test.ts
new file mode 100644
index 00000000..a2f30077
--- /dev/null
+++ b/web/workspace/test/config-source/wasm-parity.test.ts
@@ -0,0 +1,67 @@
+/// <reference lib="deno.ns" />
+
+import { assertEquals } from "jsr:@std/assert";
+// The generated wasm-bindgen loader is JavaScript with an adjacent declaration file.
+// @ts-expect-error Deno checks the generated JS implementation rather than its .d.ts.
+import init, {
+  analyze_snapshot,
+  evaluate_snapshot,
+} from "../../src/lib/workspace/config-source/generated/config_source_wasm.js";
+import type { ConfigTreeSnapshot, ToolchainContract } from "../../src/lib/workspace/config-source/types.ts";
+
+const bytes = await Deno.readFile(
+  new URL("../../src/lib/workspace/config-source/generated/config_source_wasm_bg.wasm", import.meta.url),
+);
+await init({ module_or_path: bytes });
+
+const snapshot: ConfigTreeSnapshot = {
+  revision: 4,
+  digest: "sha256:test-tree",
+  entries: {
+    "workspace.dcdl": {
+      path: "workspace.dcdl",
+      content_type: "decodal",
+      content: 'import "./lib/value.dcdl"',
+      content_digest: "sha256:root",
+    },
+    "lib/value.dcdl": {
+      path: "lib/value.dcdl",
+      content_type: "decodal",
+      content: "{ answer = 42; }",
+      content_digest: "sha256:value",
+    },
+  },
+};
+
+const contract: ToolchainContract = {
+  contract_version: 1,
+  decodal_version: "0.2.0",
+  schema_version: 1,
+  entrypoints: ["workspace.dcdl"],
+  import_policy_version: 1,
+  fingerprint: "sha256:test-contract",
+};
+
+Deno.test("generated WASM evaluates the same virtual import contract", () => {
+  const result = evaluate_snapshot(snapshot, contract) as {
+    projections: Array<{ data_json: { answer: number } }>;
+  };
+  assertEquals(result.projections[0].data_json, { answer: 42 });
+});
+
+Deno.test("generated WASM diagnostics carry snapshot provenance", () => {
+  const diagnostics = analyze_snapshot(
+    snapshot,
+    "workspace.dcdl",
+    "{ broken = ; }",
+  ) as Array<{
+    path: string;
+    revision: number;
+    tree_digest: string;
+    kind: string;
+  }>;
+  assertEquals(diagnostics[0].path, "workspace.dcdl");
+  assertEquals(diagnostics[0].revision, 4);
+  assertEquals(diagnostics[0].tree_digest, "sha256:test-tree");
+  assertEquals(diagnostics[0].kind, "syntax");
+});