diff --git a/packages/core/src/tool/runtime.ts b/packages/core/src/tool/runtime.ts
index d3d3a0cc465..b048f180691 100644
--- a/packages/core/src/tool/runtime.ts
+++ b/packages/core/src/tool/runtime.ts
@@ -31,7 +31,38 @@ export const execute = (tool: Tool.Info<any, any>, input: unknown, context: Tool
     }
   })
 
-const decodeInput = (schema: Tool.ValueSchema<any>, value: unknown) => {
+const decodeInput = (schema: Tool.ValueSchema<any>, value: unknown) =>
+  attemptDecodeInput(schema, value).pipe(
+    Effect.catchTag("Tool.Error", (error) => {
+      // JSON Schema derived from Effect schemas advertises `X | null` for optional
+      // fields because JSON cannot express undefined, so callers legitimately send
+      // null to mean "omitted". Retry with null properties removed: schemas that
+      // genuinely accept null succeed on the first attempt, and the original error
+      // is reported when the retry cannot help.
+      const stripped = withoutNullProperties(value)
+      if (stripped === value) return error
+      return attemptDecodeInput(schema, stripped).pipe(Effect.catchTag("Tool.Error", () => error))
+    }),
+  )
+
+// Removes null-valued object properties recursively. Array elements are positional
+// and stay untouched. Returns the input reference when nothing changed.
+const withoutNullProperties = (value: unknown): unknown => {
+  if (Array.isArray(value)) {
+    const items = value.map(withoutNullProperties)
+    return items.some((item, index) => item !== value[index]) ? items : value
+  }
+  if (typeof value !== "object" || value === null) return value
+  const entries = Object.entries(value).flatMap(([key, item]) =>
+    item === null ? [] : [[key, withoutNullProperties(item)] as const],
+  )
+  const changed =
+    entries.length !== Object.keys(value).length ||
+    entries.some(([key, item]) => (value as Record<string, unknown>)[key] !== item)
+  return changed ? Object.fromEntries(entries) : value
+}
+
+const attemptDecodeInput = (schema: Tool.ValueSchema<any>, value: unknown) => {
   if (Schema.isSchema(schema)) {
     if (isForeignSchema(schema)) return foreignSchemaPassthrough(value)
     return Schema.decodeUnknownEffect(schema)(value).pipe(
diff --git a/packages/core/test/tool-input-null.test.ts b/packages/core/test/tool-input-null.test.ts
new file mode 100644
index 00000000000..c5d9c41f50a
--- /dev/null
+++ b/packages/core/test/tool-input-null.test.ts
@@ -0,0 +1,110 @@
+import { expect, test } from "bun:test"
+import { Tool } from "@opencode-ai/core/tool"
+import { execute } from "@opencode-ai/core/tool/runtime"
+import { Agent } from "@opencode-ai/schema/agent"
+import { Session } from "@opencode-ai/schema/session"
+import { SessionMessage } from "@opencode-ai/schema/session-message"
+import type { Info } from "@opencode-ai/schema/tool"
+import { Effect, Schema } from "effect"
+
+const context = {
+  sessionID: Session.ID.make("ses_null"),
+  agent: Agent.ID.make("build"),
+  messageID: SessionMessage.ID.make("msg_null"),
+  id: Tool.CallID.make("call_null"),
+  progress: () => Effect.void,
+}
+
+// The JSON Schema advertised for these tools renders optional fields as `X | null`
+// (JSON cannot express undefined), so callers legitimately send null to mean
+// "omitted". The runtime must accept that without weakening schemas that
+// genuinely distinguish null.
+const collect = (input: Info["input"]) => {
+  let received: unknown
+  const tool: Info = {
+    name: "probe",
+    description: "Probe",
+    input,
+    execute: (value) => {
+      received = value
+      return Effect.succeed({ content: "ok" })
+    },
+  }
+  return {
+    tool,
+    run: (value: unknown) => Effect.runPromise(execute(tool, value, context)).then(() => received),
+    fail: (value: unknown) => Effect.runPromiseExit(execute(tool, value, context)).then((exit) => exit.toString()),
+  }
+}
+
+test("null optional properties decode as omitted", async () => {
+  const probe = collect(
+    Schema.Struct({
+      title: Schema.String,
+      agent: Schema.optional(Schema.String),
+    }),
+  )
+  expect(await probe.run({ title: "probe", agent: null })).toEqual({ title: "probe" })
+})
+
+test("nested null optional properties decode as omitted", async () => {
+  const probe = collect(
+    Schema.Struct({
+      worktree: Schema.optional(
+        Schema.Struct({
+          branch: Schema.String,
+          base: Schema.optional(Schema.String),
+        }),
+      ),
+    }),
+  )
+  expect(await probe.run({ worktree: { branch: "main", base: null } })).toEqual({ worktree: { branch: "main" } })
+})
+
+test("schemas that accept null keep it", async () => {
+  const probe = collect(Schema.Struct({ next: Schema.NullOr(Schema.String) }))
+  expect(await probe.run({ next: null })).toEqual({ next: null })
+})
+
+test("null array elements survive the retry", async () => {
+  const probe = collect(
+    Schema.Struct({
+      tags: Schema.Array(Schema.NullOr(Schema.String)),
+      agent: Schema.optional(Schema.String),
+    }),
+  )
+  expect(await probe.run({ tags: ["a", null], agent: null })).toEqual({ tags: ["a", null] })
+})
+
+test("unfixable nulls report the original error", async () => {
+  const probe = collect(Schema.Struct({ title: Schema.String }))
+  const message = await probe.fail({ title: null })
+  expect(message).toContain("Invalid tool input")
+  expect(message).toContain("Expected string")
+})
+
+test("standard schema inputs get the same retry", async () => {
+  const attempts: Array<unknown> = []
+  const input = {
+    "~standard": {
+      version: 1,
+      vendor: "test",
+      validate: (value: unknown) => {
+        attempts.push(value)
+        const record = value as Record<string, unknown>
+        if ("agent" in record && record.agent === null) return { issues: [{ message: "Expected string | undefined" }] }
+        return { value }
+      },
+      jsonSchema: {
+        input: () => ({ type: "object" }),
+        output: () => ({ type: "object" }),
+      },
+    },
+  } as unknown as Info["input"]
+  const probe = collect(input)
+  expect(await probe.run({ title: "probe", agent: null })).toEqual({ title: "probe" })
+  expect(attempts).toEqual([
+    { title: "probe", agent: null },
+    { title: "probe" },
+  ])
+})