[BUG] Kiro provider returns 400 "Improperly formed request" when used via Claude code

[hieunguyen4-sys]

9Router lastest version

Installation Method

npm (global)

Operating System

Windows

OS Version

Windows 10 (Build 19045)

Node.js Version

24.10.0

Provider(s) Involved

Kiro

Model(s) Involved

• kiro/claude-sonnet-4.5-thinking-agentic
• [Add other Kiro models you tested if applicable]

CLI Tool

• Claude code

Description

When using the Kiro provider, 9Router returns a 400 error with the message "Improperly formed request". The error occurs specifically when making requests to the kiro/claude-sonnet-4.5-thinking-agentic.

Steps to Reproduce

1. Configure 9router with Kiro provider credentials
2. Configure the claude code to use Kiro provider with model kiro/claude-sonnet-4.5-thinking-agentic
3. Observe the 400 error in the logs when the agent attempts to call the LLM
4. Error message: API Error: 400 [kiro/claude-sonnet-4.5-thinking-agentic] [400]: {"message":"Improperly formed request.","reason":null} (reset after 2m)

Expected Behavior

The agent should successfully make requests to the Kiro provider through 9router, with the request being properly formatted and accepted by the Kiro API. The LLM should process the conversation turn and return a valid response.

Actual Behavior

9router returns HTTP 400 error with "Improperly formed request" message.

Error Logs / Output

API Error: 400 [kiro/claude-sonnet-4.5-thinking-agentic] [400]: {"message":"Improperly formed request.","reason":null} (reset after 2m)

[SARAMALI15792]

Yes sounds same here also , Even from your new account at the fresh start give this same error after one response of greeting , after that it throws this error again and agiin within the loop cycle reset time

heres the sample error same As occureds:

[hieunguyen4-sys]

You can try it with the Codex; I only encountered this problem when using Claude Code

[hieunguyen4-sys]

The issue happens because Kiro limits the number of tool calls to under 120. To fix this, just type /mcp and disable some MCP servers to bring your total tool count below 120.

Now it working!!!!

The problem cause by kiro limit tool calls least than 120, just disable your mcps || plugins by @klong-dev

[swagatsourav]

It still happens mostly with Claude model. I don't have more tools. just 3 of it. But still I get the same issue in 9router log.

[13:35:34] ⚠️ [CHAT] [kiro/claude-opus-4.8] [400]: {"message":"Improperly formed request.","reason":"REQUEST_BODY_INVALID"} (reset after 1m 36s)
[13:35:34] ⚠️ [COMBO] Model kr/claude-opus-4.8 failed, trying next {"status":400}
[13:35:34] ℹ️ [COMBO] Trying model 4/15: kr/claude-opus-4.7
[13:35:34] ⚠️ [AUTH] kiro | all 2 accounts locked for claude-opus-4.7 (reset after 1m 36s) | lastError=[400]: {"message":"Improperly formed request.","re

[rivaldiarief21]

9Router v0.5.40 on Linux ARM64 still reproduces this with Kiro claude-opus-4.8, claude-opus-4.7, and claude-sonnet-5 when used by Hermes Agent through the Anthropic-compatible request path.

Additional controlled findings:

• The commonly cited <120 tools explanation is not sufficient. Controlled requests succeeded with 120 synthetic tools.
• Payload size alone is not the cause: controlled requests succeeded up to approximately 589 KiB.
• The models and Kiro credential are healthy: zero-tool requests to Opus 4.8, Opus 4.7, and Sonnet succeed with HTTP 200.
• 28 real Hermes core tool schemas (approximately 55 KiB) also succeed when replayed in a minimal clean request.
• The same provider/models fail with 400 REQUEST_BODY_INVALID for full real agent requests.
• After one 400, 9Router temporarily locks the Kiro account for roughly 68–120 seconds. Requests during that period can look like additional payload failures, so every experiment needs a zero-tool HTTP 200 control after cooldown.
• OpenAI/Codex fallback succeeds for the same agent session.

This points to a request-shape interaction in the full Anthropic/Claude-style payload—likely conversation/content blocks, thinking fields, streaming options, or their combination—not a simple tool-count or byte-size limit.

Observed upstream error:

[kiro/claude-opus-4.8] [400]: {"message":"Improperly formed request.","reason":"REQUEST_BODY_INVALID"}

Please consider reopening this issue. A useful regression test would replay a realistic multi-turn Anthropic request containing tool calls/results and thinking-related fields through the Claude-to-Kiro translator, while separating upstream 400 responses from the temporary account-lock cache.

[vuongtlt13]

The issue happens because Kiro limits the number of tool calls to under 120. To fix this, just type /mcp and disable some MCP servers to bring your total tool count below 120.

Now it working!!!!

The problem cause by kiro limit tool calls least than 120, just disable your mcps || plugins by @klong-dev

Check again plz, 32 tool (not more than 120), v0.5.50

[14:02:05] 🟤 ▶ POST claude-sonet → kiro/claude-sonnet-4.5 · FMT: claude→kiro · STREAM · 180 MSG · 32 TOOL · ACC:kiro-1

[RTK] saved 7416B / 519216B (1.4%) via [find,grep,dedup-log,git-status] hits=8

[14:02:09] 🟤 ✗ ERROR 400 · kiro/claude-sonnet-4.5 · 4312ms URL: https://runtime.us-east-1.kiro.dev/generateAssistantResponse 
[400]: {"message":"Improperly formed request.","reason":"REQUEST_BODY_INVALID"}```