Workers AI: Tool call arguments produce broken control token instead of UUID across multiple models

[crisog]

Bug Report

Description

When using the Workers AI OpenAI-compatible /v1/chat/completions endpoint with tool calling, models produce a broken control token <|"| in tool call arguments where a UUID string should appear. This occurs identically across two unrelated models — @cf/google/gemma-4-26b-a4b-it and @cf/zai-org/glm-4.7-flash — strongly suggesting the issue is in the Workers AI inference/serving layer, not in the models themselves.

Routing the same model through OpenRouter (served by Parasail) with zero code changes produces correct UUIDs immediately. Only the base URL and API key were changed.

Reproduction

Setup: An agent running on a Durable Object calls the Workers AI API via the OpenAI-compatible endpoint. The agent has tools for looking up items (returning JSON with UUID fields) and performing actions using those IDs.

Flow:

1. Model calls a lookup tool → receives a JSON result containing valid UUIDs:

   {
     "items": [
       {"itemId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890", "name": "Item A", "price": 50},
       {"itemId": "f9e8d7c6-b5a4-3210-fedc-ba9876543210", "name": "Item B", "price": 80}
     ]
   }

2. Model calls an action tool → produces a broken argument:

   {"itemId": "<|\"|", "quantity": 1}

3. The model retries with the exact same broken token every time — it is deterministic, not random.

Evidence this is a platform issue

Provider	Model	UUID produced	Result
Workers AI	@cf/google/gemma-4-26b-a4b-it	<|"|	Broken
Workers AI	@cf/zai-org/glm-4.7-flash	<|"|	Broken (same token, different model)
OpenRouter (Parasail)	google/gemma-4-26b-a4b-it	a1b2c3d4-e5f6-...	Correct

• Two completely unrelated models produce the identical broken token on Workers AI: Gemma 4 26B (Google, MoE) and GLM 4.7 Flash (Zhipu AI). Different architectures, tokenizers, and training data.
• The model correctly extracts and uses other string values from tool results (e.g., slugs are passed correctly). Only UUIDs in tool call arguments are affected.
• Gemma 4 26B scores 85.5% on τ2-bench (agentic tool use benchmark) — it is highly capable at tool calling when served by other providers.

Logs (redacted)

Workers AI — Gemma 4 26B (@cf/google/gemma-4-26b-a4b-it):

{
  "toolCalls": [
    {"name": "list_items", "arguments": {}},
    {"name": "lookup_item", "arguments": {"identifier": "some-item-slug"}},
    {"name": "perform_action", "arguments": {"itemId": "<|\"|", "quantity": 1}},
    {"name": "perform_action", "arguments": {"itemId": "<|\"|", "quantity": 1}},
    {"name": "perform_action", "arguments": {"itemId": "<|\"|", "quantity": 1}}
  ]
}

Workers AI — GLM 4.7 Flash (@cf/zai-org/glm-4.7-flash):

{
  "toolCalls": [
    {"name": "list_items", "arguments": {}},
    {"name": "lookup_item", "arguments": {"identifier": "some-item-slug"}},
    {"name": "perform_action", "arguments": {"itemId": "<|\"|", "quantity": 1}},
    {"name": "perform_action", "arguments": {"itemId": "<|\"|", "quantity": 1}},
    {"name": "perform_action", "arguments": {"itemId": "<|\"|", "quantity": 1}}
  ]
}

OpenRouter (Parasail) — Same Gemma 4 26B model:

{
  "toolCalls": [
    {"name": "list_items", "arguments": {}},
    {"name": "lookup_item", "arguments": {"identifier": "some-item-slug"}},
    {"name": "perform_action", "arguments": {"itemId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890", "quantity": 1}}
  ]
}

Environment

• API endpoint: https://api.cloudflare.com/client/v4/accounts/{account_id}/ai/v1/chat/completions (OpenAI-compatible)
• Models tested: @cf/google/gemma-4-26b-a4b-it, @cf/zai-org/glm-4.7-flash
• Client library: @mariozechner/pi-ai v0.65.0 / @mariozechner/pi-agent-core v0.65.0 (pi-mono)
• API protocol: OpenAI-compatible /v1/chat/completions with tool definitions as JSON Schema (via TypeBox)
• Runtime: Cloudflare Worker with Durable Objects
• Wrangler: v4.71.0, compatibility_date: 2026-03-12, compatibility_flags: ["nodejs_compat"]
• Worker placement: aws:us-east-2
• Date: 2026-04-07

Expected behavior

The model should reproduce the UUID from the tool result verbatim in subsequent tool call arguments.

Actual behavior

The model emits a broken control token <|\"| — deterministic and identical across all models tested on Workers AI.

[vasinl124]

+1

[seangeng]

Hey also experienced this issue! Please resolve 🙏

[luis-herasme]

+1