diff --git a/.changeset/brown-glasses-thank.md b/.changeset/brown-glasses-thank.md
new file mode 100644
index 00000000000..269c6616347
--- /dev/null
+++ b/.changeset/brown-glasses-thank.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Handle BigInt values safely when formatting JSON diagnostics.
diff --git a/packages/effect/src/Formatter.ts b/packages/effect/src/Formatter.ts
index 426131f10ba..d9a6a0a8029 100644
--- a/packages/effect/src/Formatter.ts
+++ b/packages/effect/src/Formatter.ts
@@ -253,9 +253,10 @@ function safeToString(input: any): string {
  * Uses `JSON.stringify` internally with a replacer that tracks the current
  * object ancestry. Circular references are replaced with `undefined`, which
  * omits them from object output. `Redactable` values are automatically redacted
- * before serialization. Values not supported by JSON, such as `BigInt`,
- * `Symbol`, `undefined`, and functions, follow standard `JSON.stringify`
- * behavior. The `space` parameter controls indentation and defaults to `0`.
+ * before serialization. `BigInt` values are stringified with an `n` suffix.
+ * Other values not supported by JSON, such as `Symbol`, `undefined`, and
+ * functions, follow standard `JSON.stringify` behavior. The `space` parameter
+ * controls indentation and defaults to `0`.
  *
  * **Example** (Formatting compact JSON)
  *
@@ -302,6 +303,9 @@ export function formatJson(input: unknown, options?: {
     input,
     function(this: unknown, _key: string, value: unknown) {
       const redacted = redact(value)
+      if (typeof redacted === "bigint") {
+        return `${redacted}n`
+      }
       if (typeof redacted !== "object" || redacted === null) {
         return redacted
       }
diff --git a/packages/effect/test/Formatter.test.ts b/packages/effect/test/Formatter.test.ts
index 782dcedb2dc..aea0f4d2e83 100644
--- a/packages/effect/test/Formatter.test.ts
+++ b/packages/effect/test/Formatter.test.ts
@@ -1,4 +1,4 @@
-import { Context, Option, Redactable, Redacted, Schema } from "effect"
+import { Context, Inspectable, Option, Redactable, Redacted, Schema } from "effect"
 import { format, formatJson } from "effect/Formatter"
 import { describe, it } from "vitest"
 import { strictEqual } from "./utils/assert.ts"
@@ -239,9 +239,25 @@ describe("Formatter", () => {
       strictEqual(formatJson({ left: shared, right: shared }), `{"left":{"a":1},"right":{"a":1}}`)
     })
 
+    it("should stringify BigInt values", () => {
+      strictEqual(formatJson({ value: 123n }), `{"value":"123n"}`)
+      strictEqual(formatJson([1n, 2n]), `["1n","2n"]`)
+    })
+
     it("should redact sensitive data", () => {
       strictEqual(formatJson(data), `{"secret":"[REDACTED]"}`)
       strictEqual(formatJson({ a: data }), `{"a":{"secret":"[REDACTED]"}}`)
     })
   })
+
+  describe("Inspectable.toStringUnknown", () => {
+    it("should stringify BigInt values", () => {
+      strictEqual(
+        Inspectable.toStringUnknown({ value: 123n }),
+        `{
+  "value": "123n"
+}`
+      )
+    })
+  })
 })