diff --git a/.changeset/brown-glasses-thank.md b/.changeset/brown-glasses-thank.md
new file mode 100644
index 00000000000..0445b02de0b
--- /dev/null
+++ b/.changeset/brown-glasses-thank.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Handle BigInt values safely and consistently across JSON diagnostics and logger formats.
diff --git a/packages/effect/src/Formatter.ts b/packages/effect/src/Formatter.ts
index 843b5b88bc8..fb73d19c79c 100644
--- a/packages/effect/src/Formatter.ts
+++ b/packages/effect/src/Formatter.ts
@@ -250,9 +250,9 @@ 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 otherwise follow standard
- * `JSON.stringify` behavior. The `space` parameter controls indentation and
- * defaults to `0`.
+ * before serialization. `BigInt` values are stringified with an `n` suffix.
+ * Values not supported by JSON otherwise follow standard `JSON.stringify`
+ * behavior. The `space` parameter controls indentation and defaults to `0`.
  *
  * **Gotchas**
  *
@@ -300,6 +300,9 @@ export function formatJson(input: unknown, options?: {
     input,
     function(this: unknown, _key: string, value: unknown) {
       const redacted = redact(value)
+      if (typeof redacted === "bigint") {
+        return format(redacted)
+      }
       if (typeof redacted !== "object" || redacted === null) {
         return redacted
       }
diff --git a/packages/effect/src/Inspectable.ts b/packages/effect/src/Inspectable.ts
index e2c06932a2c..ddf3e270108 100644
--- a/packages/effect/src/Inspectable.ts
+++ b/packages/effect/src/Inspectable.ts
@@ -187,7 +187,7 @@ export const toStringUnknown = (u: unknown, whitespace: number | string | undefi
     return u
   }
   try {
-    return typeof u === "object" ? formatJson(u, { space: whitespace }) : String(u)
+    return typeof u === "object" ? formatJson(u, { space: whitespace }) : format(u, { space: whitespace })
   } catch {
     return String(u)
   }
diff --git a/packages/effect/src/internal/effect.ts b/packages/effect/src/internal/effect.ts
index 370b36ec222..99c3632ff8a 100644
--- a/packages/effect/src/internal/effect.ts
+++ b/packages/effect/src/internal/effect.ts
@@ -6302,7 +6302,6 @@ export const formatLogSpan = (self: [label: string, timestamp: number], now: num
 /** @internal */
 export const structuredMessage = (u: unknown): unknown => {
   switch (typeof u) {
-    case "bigint":
     case "function":
     case "symbol": {
       return String(u)
diff --git a/packages/effect/test/Formatter.test.ts b/packages/effect/test/Formatter.test.ts
index a8d586153ea..a862bff56d6 100644
--- a/packages/effect/test/Formatter.test.ts
+++ b/packages/effect/test/Formatter.test.ts
@@ -2,6 +2,7 @@ import { describe, it } from "@effect/vitest"
 import {
   Context,
   Effect,
+  Inspectable,
   Option,
   Redactable,
   Redacted,
@@ -256,12 +257,30 @@ describe("Formatter", () => {
       strictEqual(formatJson({ left: shared, right: shared }), `{"left":{"a":1},"right":{"a":1}}`)
     })
 
+    it("should stringify BigInt values", () => {
+      strictEqual(formatJson(123n), `"123n"`)
+      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(123n), `123n`)
+      strictEqual(
+        Inspectable.toStringUnknown({ value: 123n }),
+        `{
+  "value": "123n"
+}`
+      )
+    })
+  })
+
   describe("SchemaIssue reportInput", () => {
     const formatIssue = SchemaIssue.makeFormatterDefault()
 
diff --git a/packages/effect/test/Logger.test.ts b/packages/effect/test/Logger.test.ts
index 5871e875217..3d54139c300 100644
--- a/packages/effect/test/Logger.test.ts
+++ b/packages/effect/test/Logger.test.ts
@@ -90,6 +90,27 @@ describe("Logger", () => {
       assert.ok(!output.includes("annotation=\"\\\"value with spaces\\\"\""))
     }))
 
+  it.effect("formats BigInt messages consistently", () =>
+    Effect.gen(function*() {
+      const simple: Array<string> = []
+      const logFmt: Array<string> = []
+      const structured: Array<unknown> = []
+      const json: Array<unknown> = []
+      const loggers = [
+        Logger.formatSimple.pipe(Logger.map((output) => void simple.push(output))),
+        Logger.formatLogFmt.pipe(Logger.map((output) => void logFmt.push(output))),
+        Logger.formatStructured.pipe(Logger.map((output) => void structured.push(output.message))),
+        Logger.formatJson.pipe(Logger.map((output) => void json.push(JSON.parse(output).message)))
+      ]
+
+      yield* Effect.logInfo(123n, { value: 123n }).pipe(Effect.provide(Logger.layer(loggers)))
+
+      assert.include(simple[0], `message=123n message="{\\"value\\":123n}"`)
+      assert.include(logFmt[0], `message=123n message="{\\"value\\":123n}"`)
+      assert.deepStrictEqual(structured, [[123n, { value: 123n }]])
+      assert.deepStrictEqual(json, [["123n", { value: "123n" }]])
+    }))
+
   it.effect("annotateLogsScoped applies annotations only while scoped", () =>
     Effect.gen(function*() {
       const annotations: Array<Record<string, unknown>> = []