Endpoint Constraints 🔌

Provider integrations are not interchangeable just because they speak an OpenAI-shaped HTTP protocol. A request is shaped by four layers at once:

1. endpoint family: openai-completions, openai-responses, openai-codex-responses, anthropic-messages, etc.
2. gateway/auth surface: OpenRouter, Vercel AI Gateway, Azure OpenAI, Copilot, Alibaba Coding Plan, Kimi Code, Fireworks/Firepass, and similar hosts
3. model metadata and compat overrides
4. request context: tools, images, reasoning mode, stateful session, service tier

Use this page when adding a provider, adding a compat flag, or moving logic out of a provider-specific branch. The goal is to encode endpoint constraints once, at the narrowest layer that actually owns the behavior.

Related references:

• Providers — provider availability, credentials, custom providers
• Model and Provider Configuration — models.yml, routing, and compat fields
• Provider streaming internals — stream event normalization
• Adding a provider — catalog/auth wiring for a new provider

Baseline rules

• Prefer compat metadata over provider-name branches when behavior is model or endpoint configurable.
• Keep transport mechanics transport-local. Codex websocket replay, Responses item routing, and Chat Completions SSE decoding are protocol behavior, not generic compat flags.
• Scope fallbacks to the failing capability. A strict-tool failure should not disable unrelated features. A stale Responses chain should reset chain state, not disable Responses entirely.
• Do not emit defaults that alter gateway routing. OpenRouter is the known case for default max_tokens, but any gateway can treat optional fields as routing hints.
• Stop retrying after visible side effects. Once text or a tool call is visible to the user/session, retry policy must avoid duplicate output and duplicate tool execution.

1. Choose the endpoint family first

OpenAI Chat Completions compatible

Preserve these differences instead of treating every host as stock OpenAI:

• stream_options.include_usage is only safe when compat says streaming usage is supported.
• store: false is accepted only by some hosts.
• max-output caps use either max_tokens or max_completion_tokens.
• stop sequences and frequency penalty live on this path among the current OpenAI-like endpoint set.
• OpenRouter-style reasoning and routing fields are not portable to other OpenAI-compatible hosts unless compat says so.

OpenAI Responses compatible

Responses request shape is its own dialect:

• uses input, instructions, store, prompt_cache_key, optional previous_response_id, and max_output_tokens
• can default official OpenAI requests to stateful chaining with previous_response_id plus store: true
• third-party Responses proxies may reject native reasoning history, encrypted reasoning replay, or previous_response_id
• stream completion is authoritative only after response.completed or response.incomplete; a stream close before either terminal event should fail for OpenAI Responses rather than surface partial output as success

OpenAI Codex Responses

Codex is not plain Responses with a different URL. Keep these as Codex transport policy:

• Codex account headers and beta headers
• x-codex-turn-state and x-models-etag
• optional websocket transport plus SSE fallback
• responsesLite
• prompt-cache/session ids used as transport state
• websocket-only previous_response_id chaining; SSE never chains
• Codex retry/replay rules, including reconnect and SSE replay boundaries
• provider retry only before user-visible content has been emitted
• whitespace-only tool-call argument loop breaker

Codex intentionally does not forward caller max-token caps because the backend rejects them.

Anthropic/OpenAI dual-surface providers

Kimi Code and Synthetic can be called as OpenAI-compatible or Anthropic-compatible. The shim may need to:

• switch format
• rebuild an Anthropic model when needed
• map internal reasoning to Anthropic thinking budgets
• delegate back to OpenAI Completions

Do not encode these as one-way provider migrations; they are runtime surface selection decisions.

2. Apply gateway and auth overlays

These constraints sit above the endpoint family. They affect auth, headers, routing, model ids, or usage accounting.

Azure OpenAI

• Chat Completions base URL reshapes to /deployments/{deployment}/chat/completions?api-version=....
• Deployment names may differ from model ids through AZURE_OPENAI_DEPLOYMENT_NAME_MAP.

GitHub Copilot

• The API key is parsed into an access token.
• Dynamic Copilot headers depend on messages/images.
• premiumRequests must survive usage population and replacement.
• Base URL may be resolved from the raw key.

OpenRouter

• Adds attribution/cache headers.
• Supports routing suffixes such as :nitro and :floor.
• Appends a routing suffix only when the model id has no explicit suffix after the last provider path segment.
• Uses nested reasoning request fields.
• Routes providers through the OpenRouter provider object.
• Has special cache-write usage accounting.
• Has strict-tool fallback for Anthropic grammar-size failures.
• Should omit catalog-default max_tokens unless the caller explicitly set a cap, so upstream routing is not biased.

Vercel AI Gateway

• Routing preferences go under providerOptions.gateway.only and providerOptions.gateway.order.
• Do not reuse OpenRouter’s provider object.

Alibaba Coding Plan

• API key bytes may be JSON carrying { token, enterpriseUrl }.
• Auth and base URL resolution are provider-specific.

Kimi Code

• The OpenAI-compatible path needs common Kimi headers.
• It also participates in the OpenAI/Anthropic dual-surface shim.

Fireworks and Firepass

• Wire model ids need provider-specific mapping.
• Fireworks can conflict when DeepSeek-style thinking and OpenAI-style reasoning_effort are both present after extra body fields are merged.

3. Serialize request parameters by dialect

Check these before adding or forwarding a field:

• Model id. Some models resolve a wire id from reasoning effort. Firepass/Fireworks transform ids. OpenRouter suffix handling is path-segment aware.
• Max output tokens. Kimi-family models may require a max-token field even when the caller did not set one. OpenRouter should omit catalog defaults unless explicit. Codex drops caller caps. Responses uses max_output_tokens; Chat Completions uses max_tokens or max_completion_tokens.
• Service tier. Completions, Responses, and Codex all handle service tiers, but allowed values and pricing multipliers differ. Codex has a special priority multiplier for gpt-5.5.
• Prompt cache/session. OpenAI Responses uses prompt_cache_key. OpenRouter Responses uses session_id. Codex uses prompt cache/session ids for transport state. Anthropic-style cache control requires cache_control on a text part.
• Stateful chaining. Official OpenAI Responses may chain by default. Third-party endpoints generally should not. Codex chains only on websocket response.create.

4. Map reasoning and thinking explicitly

Reasoning fields are not interchangeable.

OpenAI-style reasoning_effort

• Effort values come from compat/model metadata.
• If reasoning is disabled but the host has no real off switch, map to the lowest supported effort rather than inventing an unsupported value.

Responses reasoning

• Uses reasoning: { effort, summary }.
• Can include reasoning.encrypted_content for replay.
• xAI Grok models may require omitting reasoning.effort.
• Some compat paths inject the GPT-5 # Juice: 0 !important developer scaffold.

OpenRouter reasoning

• Uses nested reasoning: { effort }.
• Disabling reasoning must send reasoning: { enabled: false }; OpenRouter can otherwise default reasoning models into thinking.

Z.AI / GLM

• Uses thinking: { type: "enabled" } or thinking: { type: "disabled" }.
• GLM 5.2 reasoning-effort models may also receive reasoning_effort.
• Tool requests need tool_stream: true.

Qwen

• One dialect uses top-level enable_thinking.
• Another uses chat_template_kwargs.enable_thinking.

Anthropic-compatible format

• Reasoning maps to Anthropic thinking enablement and thinking-budget tokens, not OpenAI-style fields.

DeepSeek reasoning history

• DeepSeek-compatible reasoning models may require exact reasoning_content replay.
• Some variants require replay on every assistant turn, not only tool-call turns.
• Synthetic "." placeholders are acceptable for Kimi/OpenRouter-style compat, but not DeepSeek V4 exact replay.

Reasoning plus tool choice

• DeepSeek reasoning models can reject tool_choice while thinking is enabled.
• Kimi can reject forced tool choice while thinking is enabled.
• Compat needs both policies: disable reasoning for any tool choice, and disable reasoning only for forced tool choice.

xAI Grok through Responses/SuperGrok

Keep these independent:

• omit reasoning.effort
• include or drop encrypted reasoning replay
• filter reasoning-history wrappers

Some models reject only one of those fields; do not collapse them into one “Grok mode” branch.

5. Normalize tools and schemas per endpoint

Strict tools

Strict schemas are not a universal capability:

• some providers support strict tools
• some reject mixed strict/non-strict tools
• some reject strictified schemas
• OpenRouter Anthropic models can fail with “compiled grammar too large”

Retry-without-strict should be a compat recovery policy scoped to the current session/provider path.

Responses and Codex custom tools

Responses and Codex both support freeform custom grammar tools for apply_patch. Custom grammar tools do not force request-level parallel_tool_calls; Codex responsesLite separately disables request-level parallel tool calls whenever tools are present. Responses additionally:

• sanitizes schemas differently
• quarantines invalid enum/const schema contradictions
• repairs orphan tool outputs into assistant notes
• synthesizes placeholder outputs for orphan tool calls

Codex applies its own request transformation before sending.

Tool choice

Before emitting tool_choice:

• confirm the endpoint supports it
• downgrade forced choice to auto if forced choice is unsupported
• drop tool_choice: "none" when no tools are emitted
• drop forced named tool choice if that named tool was filtered out

Anthropic through LiteLLM/Bedrock

• If history contains tool calls/results and context.tools is undefined, send tools: [] as a sentinel.
• If context.tools = [], treat it as explicit opt-out and do not emit the sentinel.

Mistral / Devstral

• Tool-call ids must be exactly 9 alphanumeric characters.
• Some flows need a synthetic assistant bridge after tool results before the next user message.

Custom tool outputs

Responses/Codex must remember whether a call was custom_tool_call; the paired output must then be custom_tool_call_output, not function_call_output.

MiniMax-compatible streaming arguments

Tool arguments can stream as objects instead of JSON strings. Deep-merge object deltas, then emit one final concat-safe JSON delta.

6. Convert messages and replay history safely

• System/developer roles. Reasoning models may require developer. Some providers do not support developer and must downgrade to user. Some reject multiple system messages and need coalescing.
• Responses system prompts. Responses usually uses top-level instructions. Reasoning models that support developer put system prompts inline as developer messages.
• Assistant content. Some OpenAI-compatible backends mirror array content literally, so assistant content is normalized to a string. Tool-call replay may require content: "" or content: "." instead of null.
• Thinking replay. Some models want thinking as visible text. Others need a provider-specific reasoning field. Some permit synthetic placeholders; others need exact replay.
• Vision. If the model/provider cannot accept images, convert image input and tool-result images to placeholders. Some Qwen/Dashscope-compatible modes are text-only even when the high-level model is multimodal.
• Native Responses history. Native provider payload replay is model-bound. Strip or normalize foreign reasoning signatures. Shared code normalizes Responses pipe-separated tool ids, hashes foreign item ids, and can filter reasoning history.

7. Decode streams by provider behavior, not just schema

• Generic OpenAI-compatible streams. Keepalive chunks, role-only deltas, and empty choices: [] are not progress. Idle watchdogs must not sleep forever because of them.
• Mistral Medium 3.5-style content. delta.content can be an array/object of text parts, not a string; normalize it to text.
• DeepSeek via NVIDIA/native/proxies. Some endpoints leak chat-template markers like <｜...｜> into visible content. Buffering is required because markers can be split across chunks.
• DeepSeek/template-leak tool calls. Some providers leak tool-call markup in text while also producing structured tool calls. Markup healing belongs in the stream decoder policy, not endpoint business logic.
• MiniMax-M3 cumulative reasoning. Reasoning deltas may be cumulative snapshots. Deduplicate by reasoning field signature.
• Responses streams. Route parallel items by output_index, item_id, call-id aliases, and prefixed fc_ aliases. Tolerate missing content_part.added or output_item.added. Finalize pending tool calls at the terminal event.
• Terminal behavior. Chat Completions can break after finish_reason plus usage. Responses breaks on response.completed or response.incomplete. Tool calls with stop promote to toolUse. Codex/Responses end_turn:false maps to pause_turn.
• Ollama length failures. finish_reason: length with no visible content is treated as context-window failure and mapped to an error.

8. Preserve usage and cost semantics

• OpenRouter prompt_tokens_details.cache_write_tokens is billed differently: subtract it from input tokens and emit it as cache-write usage.
• DeepSeek native prompt_cache_miss_tokens is the billed input portion, not a separate cache-write charge. Do not double-count it.
• GitHub Copilot premiumRequests must survive when usage is populated or replaced.
• Responses and Codex both adjust cost by resolved service tier, but Codex uses different multipliers.

9. Implement recovery at the right boundary

• Strict tool fallback. 400/422 schema or strict-tool failures should disable strict tools for the appropriate session scope and retry non-strict.
• OpenAI Responses stateful fallback. Stale, invalid, or unsupported previous_response_id resets chain state and retries with full context. Zero Data Retention disables chaining immediately.
• Codex websocket fallback. Websocket connection errors, stale sockets, connection limits, retry-budget exhaustion, or unsafe partial output can trigger reconnect or SSE replay.
• Codex whitespace tool-loop breaker. Codex can stream whitespace-only tool-call argument deltas indefinitely. Cap events/chars, drop the degenerate partial tool call, and retry only when safe.
• Codex previous_response_id fallback. Stale or unsupported ids are chain breaks and retry with full context, but only for websocket because SSE never chains.
• Provider retry before content. Codex retries retryable provider stream errors only before user-visible content has been emitted.

10. Checklist for a new constraint

Before adding a branch or compat field, answer these in order:

1. Is this endpoint-family behavior, gateway behavior, model behavior, or request context behavior?
2. Can it be represented by existing compat metadata?
3. If not, is a new compat field better than a provider-name branch?
4. Does the field need provider-level defaults, model-level overrides, or both?
5. Does it interact with tools, images, reasoning, stateful Responses chains, or service tier?
6. Can retry happen before visible text/tool calls only?
7. Does usage accounting still preserve cache reads/writes, billed input, service tier multipliers, and provider-specific counters such as Copilot premiumRequests?