diff --git a/packages/codemode/interpreter-support.md b/packages/codemode/interpreter-support.md
index 4b8b9001107b..056d60fc082d 100644
--- a/packages/codemode/interpreter-support.md
+++ b/packages/codemode/interpreter-support.md
@@ -36,7 +36,8 @@ ultimate source of truth.
 - [x] Template literals with interpolation.
 - [x] Regular-expression literals.
 - [x] `NaN` and `Infinity` globals.
-- [ ] BigInt literals and in-interpreter BigInt arithmetic; BigInt remains invalid at JSON-like host boundaries.
+- [x] `BigInt(...)`, bigint literals, exact arithmetic, updates, `toString(radix)`, and `valueOf()`; bigint values stay
+      native across tool calls and become precise decimal strings only in final program results.
 - [ ] Arbitrary Symbol primitive values and symbol-keyed properties. The confined `Symbol.iterator` and
       `Symbol.asyncIterator` keys are available only for custom iterator protocols.
 - [ ] Tagged-template calls.
diff --git a/packages/codemode/src/interpreter/errors.ts b/packages/codemode/src/interpreter/errors.ts
index 14962ce38483..f7ddfbb362cf 100644
--- a/packages/codemode/src/interpreter/errors.ts
+++ b/packages/codemode/src/interpreter/errors.ts
@@ -45,9 +45,9 @@ export const normalizeError = (error: unknown): Diagnostic => {
       message = (value as { message: string }).message
     } else {
       try {
-        message = JSON.stringify(copyOut(value, "json")) ?? String(value)
+        message = JSON.stringify(copyOut(value, "nullify")) ?? coerceToString(value)
       } catch {
-        message = String(value)
+        message = coerceToString(value)
       }
     }
     return { kind: "ExecutionFailure", message: `Uncaught: ${message}` }
diff --git a/packages/codemode/src/interpreter/methods.ts b/packages/codemode/src/interpreter/methods.ts
index cae4951d9a87..debadc8185c7 100644
--- a/packages/codemode/src/interpreter/methods.ts
+++ b/packages/codemode/src/interpreter/methods.ts
@@ -97,6 +97,17 @@ export const invokeIntrinsic = <R>(
   if (typeof ref.receiver === "number") {
     return Effect.succeed(invokeNumberMethod(ref.receiver, ref.name, args, node))
   }
+  if (typeof ref.receiver === "bigint") {
+    if (ref.name === "valueOf") return Effect.succeed(ref.receiver)
+    const radix = args[0]
+    if (radix !== undefined && typeof radix !== "number") {
+      throw new InterpreterRuntimeError("BigInt.toString expects a numeric radix.", node).as("TypeError")
+    }
+    if (typeof radix === "number" && (radix < 2 || radix > 36)) {
+      throw new InterpreterRuntimeError("BigInt.toString radix must be between 2 and 36.", node).as("RangeError")
+    }
+    return Effect.succeed(ref.receiver.toString(radix))
+  }
   if (Array.isArray(ref.receiver)) {
     return invokeArrayMethod(runner, ref.receiver, ref.name, args, node)
   }
diff --git a/packages/codemode/src/interpreter/model.ts b/packages/codemode/src/interpreter/model.ts
index b3a82e68867f..847a9fb25b95 100644
--- a/packages/codemode/src/interpreter/model.ts
+++ b/packages/codemode/src/interpreter/model.ts
@@ -137,7 +137,7 @@ export class JsonMethodReference {
 }
 
 export class CoercionFunction {
-  constructor(readonly name: "Number" | "String" | "Boolean" | "parseInt" | "parseFloat" | "isFinite" | "isNaN") {}
+  constructor(readonly name: "Number" | "String" | "Boolean" | "BigInt" | "parseInt" | "parseFloat" | "isFinite" | "isNaN") {}
 }
 
 export class UriFunction {
diff --git a/packages/codemode/src/interpreter/runtime.ts b/packages/codemode/src/interpreter/runtime.ts
index 4457ec6add14..86fa7ad78f5b 100644
--- a/packages/codemode/src/interpreter/runtime.ts
+++ b/packages/codemode/src/interpreter/runtime.ts
@@ -311,6 +311,7 @@ export class Interpreter<R> {
     globalScope.set("Math", { mutable: false, value: new GlobalNamespace("Math") })
     globalScope.set("JSON", { mutable: false, value: new GlobalNamespace("JSON") })
     globalScope.set("Number", { mutable: false, value: new CoercionFunction("Number") })
+    globalScope.set("BigInt", { mutable: false, value: new CoercionFunction("BigInt") })
     globalScope.set("String", { mutable: false, value: new CoercionFunction("String") })
     globalScope.set("Boolean", { mutable: false, value: new CoercionFunction("Boolean") })
     globalScope.set("Array", { mutable: false, value: new GlobalNamespace("Array") })
@@ -1893,18 +1894,18 @@ export class Interpreter<R> {
 
     // CodeMode numeric coercion, not host Number(): null-prototype data objects would make
     // the host throw during ToPrimitive, and opaque runtime references must reject clearly.
-    const operand = (current: unknown): number => {
+    const operand = (current: unknown): number | bigint => {
       if (containsOpaqueReference(current)) {
         throw new InterpreterRuntimeError(`'${operator}' requires a data value.`, argument, "InvalidDataValue")
       }
-      return coerceToNumber(current)
+      return typeof current === "bigint" ? current : coerceToNumber(current)
     }
 
     if (argument.type === "Identifier") {
       return Effect.sync(() => {
         const name = getString(argument, "name")
         const current = operand(this.scopes.get(name, argument))
-        const next = current + increment
+        const next = typeof current === "bigint" ? current + BigInt(increment) : current + increment
         this.scopes.set(name, next, argument)
         return prefix ? next : current
       })
@@ -1913,7 +1914,7 @@ export class Interpreter<R> {
     if (argument.type === "MemberExpression") {
       return this.modifyMember(argument, (current) => {
         const value = operand(current)
-        const next = value + increment
+        const next = typeof value === "bigint" ? value + BigInt(increment) : value + increment
         return Effect.succeed({ write: true, next, result: prefix ? next : value })
       })
     }
@@ -2581,6 +2582,11 @@ export class Interpreter<R> {
         return new ComputedValue(undefined)
       }
 
+      if (typeof objectValue === "bigint") {
+        if (key === "toString" || key === "valueOf") return new IntrinsicReference(objectValue, key)
+        return new ComputedValue(undefined)
+      }
+
       if (objectValue instanceof CoercionFunction) {
         if (typeof key === "string" && isBlockedMember(key)) {
           throw new InterpreterRuntimeError(`${objectValue.name}.${key} is not available.`, propertyNode)
diff --git a/packages/codemode/src/stdlib/value.ts b/packages/codemode/src/stdlib/value.ts
index 68817ea7d73b..620cf5e05788 100644
--- a/packages/codemode/src/stdlib/value.ts
+++ b/packages/codemode/src/stdlib/value.ts
@@ -89,6 +89,12 @@ export const invokeCoercion = (ref: CoercionFunction, args: Array<unknown>, node
   const value = boundedData(raw, `${ref.name} input`)
   if (ref.name === "Number") return coerceToNumber(value)
   if (ref.name === "Boolean") return Boolean(value)
+  if (ref.name === "BigInt") {
+    if (typeof value !== "string" && typeof value !== "number" && typeof value !== "bigint" && typeof value !== "boolean") {
+      throw new InterpreterRuntimeError("BigInt expects a string, integer, bigint, or boolean.", node).as("TypeError")
+    }
+    return BigInt(value)
+  }
   if (ref.name === "isFinite") return Number.isFinite(coerceToNumber(value))
   if (ref.name === "isNaN") return Number.isNaN(coerceToNumber(value))
   if (ref.name === "parseInt") {
diff --git a/packages/codemode/src/tool-runtime.ts b/packages/codemode/src/tool-runtime.ts
index 5200c13b26ad..5e0e42a7a020 100644
--- a/packages/codemode/src/tool-runtime.ts
+++ b/packages/codemode/src/tool-runtime.ts
@@ -149,7 +149,8 @@ const copyBounded = (
     value === undefined ||
     typeof value === "string" ||
     typeof value === "boolean" ||
-    typeof value === "number"
+    typeof value === "number" ||
+    typeof value === "bigint"
   ) {
     return value
   }
@@ -256,11 +257,12 @@ const copyBounded = (
 // "json" mirrors JSON.stringify (undefined object values drop, undefined array elements become
 // null, a bare undefined passes through): use it wherever data leaves as JSON, like tool
 // arguments and stringify-style formatting. "nullify" turns every undefined, including a bare
-// one, into null: use it for program results, where the consumer must never see undefined.
+// one, into null and bigint values into precise decimal strings: use it for final program results.
 export type CopyOutMode = "json" | "nullify"
 
 export const copyOut = (value: unknown, mode: CopyOutMode): unknown => {
   if (value === undefined && mode === "nullify") return null
+  if (typeof value === "bigint" && mode === "nullify") return value.toString()
   if (typeof value === "number" && !Number.isFinite(value)) {
     return null
   }
diff --git a/packages/codemode/src/tool-schema.ts b/packages/codemode/src/tool-schema.ts
index 52e1cd4033d8..98e76d013263 100644
--- a/packages/codemode/src/tool-schema.ts
+++ b/packages/codemode/src/tool-schema.ts
@@ -27,6 +27,7 @@ const MAX_RENDER_DEPTH = 8
 type RenderContext = {
   readonly definitions: Readonly<Record<string, JsonSchema>>
   readonly pretty: boolean
+  readonly bigints?: WeakSet<JsonSchema>
 }
 
 const hasUnresolvedRef = (
@@ -88,6 +89,7 @@ const renderSchema = (
   seen: ReadonlySet<string> = new Set(),
 ): string => {
   if (depth > MAX_RENDER_DEPTH) return "unknown"
+  if (ctx.bigints?.has(schema)) return "bigint"
   const nested =
     schema.definitions === undefined && schema.$defs === undefined
       ? ctx
@@ -165,14 +167,52 @@ const renderSchema = (
   return "unknown"
 }
 
+const markBigInts = (ast: unknown, schema: JsonSchema, bigints: WeakSet<JsonSchema>): void => {
+  if (ast === null || typeof ast !== "object" || !("_tag" in ast)) return
+  if (ast._tag === "BigInt") {
+    bigints.add(schema)
+    return
+  }
+  if (ast._tag === "Objects" && "propertySignatures" in ast && Array.isArray(ast.propertySignatures)) {
+    for (const field of ast.propertySignatures) {
+      if (typeof field !== "object" || field === null || !("name" in field) || !("type" in field)) continue
+      if (typeof field.name !== "string") continue
+      const property = schema.properties?.[field.name]
+      if (property !== undefined) markBigInts(field.type, property, bigints)
+    }
+    if ("indexSignatures" in ast && Array.isArray(ast.indexSignatures) && typeof schema.additionalProperties === "object") {
+      for (const index of ast.indexSignatures) {
+        if (typeof index === "object" && index !== null && "type" in index) {
+          markBigInts(index.type, schema.additionalProperties, bigints)
+        }
+      }
+    }
+    return
+  }
+  if (ast._tag === "Arrays" && "rest" in ast && Array.isArray(ast.rest) && schema.items !== undefined) {
+    for (const item of ast.rest) markBigInts(item, schema.items, bigints)
+    return
+  }
+  if (ast._tag === "Union" && "types" in ast && Array.isArray(ast.types)) {
+    const alternatives = schema.anyOf ?? schema.oneOf
+    if (alternatives === undefined) return
+    for (const [index, item] of ast.types.entries()) {
+      const alternative = alternatives[index]
+      if (alternative !== undefined) markBigInts(item, alternative, bigints)
+    }
+  }
+}
+
 export const toTypeScript = (schema: Schema.Top, decoded = false, pretty = false): string => {
   try {
-    const visible = decoded ? Schema.toType(schema) : schema
+    const visible = decoded ? Schema.toType(schema) : Schema.toEncoded(schema)
     const document = Schema.toJsonSchemaDocument(visible) as {
       readonly schema: JsonSchema
       readonly definitions?: Readonly<Record<string, JsonSchema>>
     }
-    return renderSchema(document.schema, { definitions: document.definitions ?? {}, pretty })
+    const bigints = new WeakSet<JsonSchema>()
+    markBigInts(visible.ast, document.schema, bigints)
+    return renderSchema(document.schema, { definitions: document.definitions ?? {}, pretty, bigints })
   } catch {
     return "unknown"
   }
diff --git a/packages/codemode/test/codemode.test.ts b/packages/codemode/test/codemode.test.ts
index 3124db57e1ec..2305525b643c 100644
--- a/packages/codemode/test/codemode.test.ts
+++ b/packages/codemode/test/codemode.test.ts
@@ -437,6 +437,97 @@ describe("CodeMode console capture", () => {
   })
 })
 
+describe("CodeMode bigint values", () => {
+  test("preserves exact bigint arithmetic and stringifies nested final results", async () => {
+    const result = await Effect.runPromise(
+      CodeMode.execute({
+        code: `
+          const value = BigInt("9007199254740993")
+          return { value, doubled: value * 2n, nested: [value + 1n], hex: value.toString(16) }
+        `,
+      }),
+    )
+
+    expect(result).toStrictEqual({
+      ok: true,
+      value: {
+        value: "9007199254740993",
+        doubled: "18014398509481986",
+        nested: ["9007199254740994"],
+        hex: "20000000000001",
+      },
+      toolCalls: [],
+    })
+    expect(Schema.decodeUnknownSync(CodeMode.Result)(JSON.parse(JSON.stringify(result)))).toStrictEqual(result)
+  })
+
+  test("returns a structured diagnostic when an uncaught object contains bigint values", async () => {
+    const result = await Effect.runPromise(
+      CodeMode.execute({
+        code: `throw { value: 9007199254740993n, nested: [1n] }`,
+      }),
+    )
+
+    expect(result).toStrictEqual({
+      ok: false,
+      error: {
+        kind: "ExecutionFailure",
+        message: 'Uncaught: {"value":"9007199254740993","nested":["1"]}',
+      },
+      toolCalls: [],
+    })
+  })
+
+  test("preserves native bigint values across chained schema-backed tool calls", async () => {
+    const lookup = Tool.make({
+      description: "Look up an exact integer",
+      input: Schema.Struct({}),
+      output: Schema.Struct({ value: Schema.BigIntFromString }),
+      execute: () => Effect.succeed({ value: "9007199254740993" }),
+    })
+    const double = Tool.make({
+      description: "Double an exact integer",
+      input: Schema.Struct({ value: Schema.BigInt }),
+      output: Schema.BigInt,
+      execute: ({ value }) => Effect.succeed(value * 2n),
+    })
+
+    const result = await Effect.runPromise(
+      CodeMode.make({ tools: { lookup, double } }).execute(`
+        const result = await tools.lookup({})
+        return await tools.double({ value: result.value + 1n })
+      `),
+    )
+
+    expect(result).toStrictEqual({
+      ok: true,
+      value: "18014398509481988",
+      toolCalls: [{ name: "lookup" }, { name: "double" }],
+    })
+  })
+
+  test("increments bigint bindings and object properties without losing precision", async () => {
+    const result = await Effect.runPromise(
+      CodeMode.execute({
+        code: `
+          let value = 9007199254740993n
+          const record = { value }
+          value++
+          ++record.value
+          return [value, record.value, typeof value, String(value)]
+        `,
+      }),
+    )
+
+    expect(result.ok ? result.value : result.error).toStrictEqual([
+      "9007199254740994",
+      "9007199254740994",
+      "bigint",
+      "9007199254740994",
+    ])
+  })
+})
+
 describe("CodeMode output budget", () => {
   test("absent maxOutputBytes means no truncation at all", async () => {
     const result = await Effect.runPromise(
diff --git a/packages/codemode/test/parity.test.ts b/packages/codemode/test/parity.test.ts
index 1ea2917d0adb..0cd3fee5b0bd 100644
--- a/packages/codemode/test/parity.test.ts
+++ b/packages/codemode/test/parity.test.ts
@@ -307,6 +307,16 @@ describe("H1: NaN/Infinity flow as intermediates and normalize to null at the bo
   })
 })
 
+describe("copyOut bigint handling per boundary mode", () => {
+  test("preserves native bigint tool arguments but stringifies final result values", () => {
+    expect(ToolRuntime.copyOut(9007199254740993n, "json")).toBe(9007199254740993n)
+    expect(ToolRuntime.copyOut(9007199254740993n, "nullify")).toBe("9007199254740993")
+    expect(ToolRuntime.copyOut({ value: [9007199254740993n] }, "nullify")).toStrictEqual({
+      value: ["9007199254740993"],
+    })
+  })
+})
+
 describe("copyOut undefined handling per boundary mode", () => {
   test("json mode mirrors JSON.stringify for undefined", () => {
     expect(ToolRuntime.copyOut({ q: undefined, keep: 1 }, "json")).toStrictEqual({ keep: 1 })
diff --git a/packages/codemode/test/signature.test.ts b/packages/codemode/test/signature.test.ts
index a4dbd8790a04..c36749beec7a 100644
--- a/packages/codemode/test/signature.test.ts
+++ b/packages/codemode/test/signature.test.ts
@@ -69,6 +69,30 @@ describe("pretty signature rendering", () => {
     expect(outputTypeScript(lookupOrder)).toBe("{ status: string }")
   })
 
+  test("renders native bigint schemas accurately in tool inputs and decoded outputs", () => {
+    const native = Tool.make({
+      description: "Double an exact integer",
+      input: Schema.Struct({ value: Schema.BigInt, values: Schema.Array(Schema.BigInt) }),
+      output: Schema.BigInt,
+      execute: ({ value }) => Effect.succeed(value * 2n),
+    })
+    const decoded = Tool.make({
+      description: "Look up an exact integer",
+      input: Schema.Struct({ encoded: Schema.BigIntFromString }),
+      output: Schema.Struct({ value: Schema.BigIntFromString }),
+      execute: () => Effect.succeed({ value: "9007199254740993" }),
+    })
+
+    expect(inputTypeScript(native)).toBe("{ value: bigint; values: Array<bigint> }")
+    expect(outputTypeScript(native)).toBe("bigint")
+    expect(inputTypeScript(decoded)).toBe("{ encoded: string }")
+    expect(outputTypeScript(decoded)).toBe("{ value: bigint }")
+    expect(CodeMode.make({ tools: { native, decoded } }).catalog().map((item) => item.signature)).toStrictEqual([
+      "tools.decoded(input: {\n  encoded: string,\n}): Promise<{\n  value: bigint,\n}>",
+      "tools.native(input: {\n  value: bigint,\n  values: Array<bigint>,\n}): Promise<bigint>",
+    ])
+  })
+
   test("nested objects recurse with increasing indent and their own JSDoc", () => {
     const pretty = jsonSchemaToTypeScript(
       {