diff --git a/packages/core/tools/src/py-types.ts b/packages/core/tools/src/py-types.ts
index 6e69541fb9..39d9644420 100644
--- a/packages/core/tools/src/py-types.ts
+++ b/packages/core/tools/src/py-types.ts
@@ -149,6 +149,19 @@ function allocateClassName(base: string, state: RenderState): string {
   return name
 }
 
+/**
+ * Append a child-name segment to a parent class-name base, capping the result
+ * at {@link MAX_CLASS_NAME_BASE}. Capping AT PROPAGATION (not only inside
+ * {@link allocateClassName}) keeps each level O(1): a deep `oneOf`- or
+ * object-chain would otherwise carry an ever-growing ConsString down the tree
+ * and re-materialize it (via `.length`/`.slice`) at every level — Θ(depth²).
+ * The bounded base plus the collision counter still yields unique names.
+ */
+function childClassName(base: string, segment: string): string {
+  const joined = `${base}${segment}`
+  return joined.length > MAX_CLASS_NAME_BASE ? joined.slice(0, MAX_CLASS_NAME_BASE) : joined
+}
+
 /**
  * Render one validated scalar as Python literal text (`True`/`False`,
  * JSON-quoted strings, bare numbers). `null` cannot reach here: the `null`
@@ -259,12 +272,12 @@ function renderType(schema: unknown, className: string, state: RenderState): str
           continue
         }
         if (frame.kind === 'oneOf') {
-          // Concatenate with `+` (not `Array.join`): V8 builds a lazy
-          // ConsString, so a deep oneOf chain materializes once at the root
-          // instead of re-materializing the accumulated string at every level
-          // (which `join` would, making it Θ(depth²)). This matches the array
-          // arm's template-literal laziness and ts-types' composable-document
-          // approach — the whole walk stays linear in schema depth.
+          // Concatenate incrementally (template literal, not `Array.join`): V8
+          // builds a lazy ConsString, so a deep oneOf chain materializes once
+          // at the root instead of re-materializing the accumulated string at
+          // every level (which `join` would, making it Θ(depth²)). This matches
+          // the array arm's template-literal laziness and ts-types' composable-
+          // document approach — the whole walk stays linear in schema depth.
           let union = ''
           for (const [index, childType] of frame.childTypes.entries()) {
             union = index === 0 ? childType : `${union} | ${childType}`
@@ -324,7 +337,7 @@ function renderType(schema: unknown, className: string, state: RenderState): str
       const node = frame.schema
       if (node.oneOf !== undefined) {
         frame.kind = 'oneOf'
-        frame.children = node.oneOf.map((branch, index) => ({ schema: branch, className: `${frame.className}${index + 1}` }))
+        frame.children = node.oneOf.map((branch, index) => ({ schema: branch, className: childClassName(frame.className, `${index + 1}`) }))
         continue
       }
       if (node.type === undefined) {
@@ -383,7 +396,7 @@ function renderType(schema: unknown, className: string, state: RenderState): str
           frame.entries = entries
           // frame.allocated was assigned two statements up; the ?? arm is for the type system only.
           /* v8 ignore next -- allocated is always set before children are built. */
-          frame.children = entries.map(([field, child]) => ({ schema: child, className: `${frame.allocated ?? ''}${camelCase(field)}` }))
+          frame.children = entries.map(([field, child]) => ({ schema: child, className: childClassName(frame.allocated ?? '', camelCase(field)) }))
           break
         }
         /* v8 ignore next 4 -- assertSupportedJsonSchema narrowed this closed type union. */
diff --git a/packages/core/tools/tests/py-types.spec.ts b/packages/core/tools/tests/py-types.spec.ts
index aa0bc30cff..3cea474e8d 100644
--- a/packages/core/tools/tests/py-types.spec.ts
+++ b/packages/core/tools/tests/py-types.spec.ts
@@ -471,18 +471,54 @@ describe('renderToolsSdkPy', () => {
 
   it('renders a deeply nested oneOf chain in linear time (no per-level re-materialization)', () => {
     // Each level is a two-branch oneOf whose first branch recurses; joining the
-    // accumulated union string at every level would be Theta(depth^2). The `+`
-    // (ConsString) concatenation keeps it linear, like the array arm.
-    const depth = 20000
+    // accumulated union string at every level would be Theta(depth^2). At this
+    // depth the quadratic path (~100,000^2 char copies) blows past vitest's 5s
+    // default, so this fails loud on a regression; the `+`/ConsString path is
+    // milliseconds. (Guard the depth explicitly so the assertions stay exact.)
+    const depth = 100000
     let deep: Record<string, unknown> = { type: 'string' }
     for (let i = 0; i < depth; i++) deep = { oneOf: [deep, { type: 'null' }] }
     const type = jsonSchemaToPy(deep)
-    // depth levels of ` | None` appended to the innermost `str`.
     expect(type.startsWith('str | None')).toBe(true)
     expect(type.endsWith(' | None')).toBe(true)
     expect(type.length).toBe('str'.length + ' | None'.length * depth)
   })
 
+  it('names a deep oneOf-of-object chain in linear time (bounded propagated class names)', () => {
+    // Every level is a oneOf whose first branch is a closed empty object (a
+    // named TypedDict) and recurses. Propagating the full ancestor path as the
+    // class name and slicing it in allocateClassName at every level would be
+    // Theta(depth^2); childClassName caps the propagated base so it stays
+    // linear. The quadratic path at this depth exceeds the 5s default.
+    const depth = 60000
+    let deep: Record<string, unknown> = { type: 'object', additionalProperties: false, properties: {} }
+    for (let i = 0; i < depth; i++) {
+      deep = { oneOf: [deep, { type: 'null' }] }
+    }
+    const tool: ToolSdkSchema = { name: 'deep', description: 'Deep oneOf-object chain.', parameters: { type: 'object', additionalProperties: false, properties: { root: deep }, required: ['root'] }, output: { type: 'string' } }
+    const text = renderToolsSdkPy([tool])
+    // No emitted class name exceeds the cap (plus a short collision suffix).
+    const longest = [...text.matchAll(/^class (\w+)\(TypedDict\):/gm)].reduce((max, m) => Math.max(max, m[1]?.length ?? 0), 0)
+    expect(longest).toBeLessThanOrEqual(140)
+    expect(text).toContain('class Tools(Protocol):')
+  })
+
+  it('caps the class name for a tool whose name exceeds the base length limit', () => {
+    // The root class base is `${CamelCase(name)}Args`; a very long tool name
+    // makes it exceed MAX_CLASS_NAME_BASE, so allocateClassName caps it.
+    const longName = `x_${'a'.repeat(200)}`
+    const tool: ToolSdkSchema = {
+      name: longName,
+      description: 'Long name.',
+      parameters: { type: 'object', additionalProperties: false, properties: { f: { type: 'string' } }, required: ['f'] },
+      output: { type: 'string' },
+    }
+    const text = renderToolsSdkPy([tool])
+    const longest = [...text.matchAll(/^class (\w+)\(TypedDict\):/gm)].reduce((max, m) => Math.max(max, m[1]?.length ?? 0), 0)
+    expect(longest).toBeLessThanOrEqual(140)
+    expect(text).toContain('class Tools(Protocol):')
+  })
+
   it('emits pass for a subscript-only tool set (comments are not statements)', () => {
     const t: ToolSdkSchema = {
       name: 'my-exotic.tool',