fix(Formatter): include Error message in formatJson

.changeset/fix-formatjson-error-message.md

@@ -0,0 +1,11 @@
+---
+"effect": patch
+---
+
+Fix `Formatter.formatJson` to include the message when stringifying plain `Error` values, matching the output of `Logger.formatStructured`.
+
+```ts
+import { Formatter } from "effect"
+
+Formatter.formatJson(new Error("boom")) // now `"Error: boom"`, previously `{}`
+```

packages/effect/src/Formatter.ts

@@ -316,6 +316,9 @@ export function formatJson(input: unknown, options?: {
       if (typeof redacted !== "object" || redacted === null) {
         return redacted
       }
+      if (redacted instanceof Error && !Predicate.hasProperty(redacted, "toJSON")) {
+        return safeToString(redacted)
+      }
       while (ancestors.length > 0 && ancestors[ancestors.length - 1] !== this) {
         ancestors.pop()
       }

packages/effect/test/Formatter.test.ts

@@ -344,6 +344,20 @@ describe("Formatter", () => {
       strictEqual(formatJson([1n, 2n]), `["1n","2n"]`)
     })
 
+    it("should stringify Error messages", () => {
+      strictEqual(formatJson(new Error("boom")), `"Error: boom"`)
+      strictEqual(formatJson({ error: new Error("boom") }), `{"error":"Error: boom"}`)
+    })
+
+    it("should keep structured serialization for Errors that define toJSON", () => {
+      class Tagged extends Error {
+        toJSON() {
+          return { _tag: "Tagged", message: "boom" }
+        }
+      }
+      strictEqual(formatJson(new Tagged("boom")), `{"_tag":"Tagged","message":"boom"}`)
+    })
+
     it("should redact sensitive data", () => {
       const date = Object.assign(new Date(0), {
         [Redactable.symbolRedactable]: () => "[REDACTED]"

packages/effect/test/Logger.test.ts

@@ -115,6 +115,24 @@
       assert.strictEqual(json[0].level, "INFO")
     }))
 
+  it.effect("formatJson includes the message of plain Errors", () =>
+    Effect.gen(function*() {
+      const json: Array<{ readonly message: unknown; readonly level: string }> = []
+      const logger = Logger.formatJson.pipe(Logger.map((output) => void json.push(JSON.parse(output))))
+
+      yield* Effect.gen(function*() {
+        yield* Effect.fail(new Error("boom"))
+      }).pipe(
+        Effect.tapError(Effect.logError),
+        Effect.ignore,
+        Effect.provide(Logger.layer([logger]))
+      )
+
+      assert.strictEqual(json.length, 1)
+      assert.strictEqual(json[0].message, "Error: boom")
+      assert.strictEqual(json[0].level, "ERROR")
+    }))
+
   it.effect("annotateLogsScoped applies annotations only while scoped", () =>
     Effect.gen(function*() {
       const annotations: Array<Record<string, unknown>> = []