diff --git a/src/adapters/openai-chat.ts b/src/adapters/openai-chat.ts
index 99b62af5e..370706cb1 100644
--- a/src/adapters/openai-chat.ts
+++ b/src/adapters/openai-chat.ts
@@ -744,6 +744,18 @@ export function createOpenAIChatAdapter(provider: OcxProviderConfig): ProviderAd
         messages,
         stream: parsed.stream,
       };
+      // Preserve a caller-selected service tier for OpenAI-compatible chat gateways. The
+      // request pipeline deliberately does not inject fast mode for this adapter, but dropping
+      // an explicit value here makes the Responses parser's serviceTier projection ineffective.
+      //
+      // Opt-in, like `prompt_cache_key` directly below: `service_tier` is an OpenAI-specific
+      // extension and 66 registry providers share this adapter, several of which reject
+      // unknown body fields. Forwarding unconditionally would turn a caller-supplied
+      // `service_tier` into an upstream 400 on those routes. `supportsServiceTier` is the
+      // Responses-wire flag (applyServiceTierGate) and deliberately does not gate this path.
+      if (provider.chatServiceTier && parsed.options.serviceTier !== undefined) {
+        body.service_tier = parsed.options.serviceTier;
+      }
       if (modelInList(provider.reasoningSplitModels, parsed.modelId)) body.reasoning_split = true;
       const maxTokens = resolveMaxTokens(provider, parsed);
       const openRouterRouting = resolveOpenRouterRouting(provider, parsed.modelId);
diff --git a/src/providers/derive.ts b/src/providers/derive.ts
index 452eed56d..0aef21218 100644
--- a/src/providers/derive.ts
+++ b/src/providers/derive.ts
@@ -239,6 +239,7 @@ export function providerConfigSeed(entry: ProviderRegistryEntry): OcxProviderCon
     ...(entry.noPenaltyModels ? { noPenaltyModels: [...entry.noPenaltyModels] } : {}),
     ...(entry.parallelToolCalls !== undefined ? { parallelToolCalls: entry.parallelToolCalls } : {}),
     ...(entry.promptCacheKey !== undefined ? { promptCacheKey: entry.promptCacheKey } : {}),
+    ...(entry.chatServiceTier !== undefined ? { chatServiceTier: entry.chatServiceTier } : {}),
     ...(entry.responsesPath !== undefined ? { responsesPath: entry.responsesPath } : {}),
     ...(entry.statelessResponses !== undefined ? { statelessResponses: entry.statelessResponses } : {}),
     ...(entry.requiresAdjacentResponsesToolResults !== undefined
@@ -419,6 +420,7 @@ export function enrichProviderFromRegistry(name: string, prov: OcxProviderConfig
   if (!prov.noPenaltyModels && seed.noPenaltyModels) prov.noPenaltyModels = [...seed.noPenaltyModels];
   if (prov.parallelToolCalls === undefined && seed.parallelToolCalls !== undefined) prov.parallelToolCalls = seed.parallelToolCalls;
   if (prov.promptCacheKey === undefined && seed.promptCacheKey !== undefined) prov.promptCacheKey = seed.promptCacheKey;
+  if (prov.chatServiceTier === undefined && seed.chatServiceTier !== undefined) prov.chatServiceTier = seed.chatServiceTier;
   // Fill-only: a hand-edited path must survive, and a config saved before the registry
   // learned this route still gets backfilled.
   if (prov.responsesPath === undefined && seed.responsesPath !== undefined) prov.responsesPath = seed.responsesPath;
diff --git a/src/providers/registry.ts b/src/providers/registry.ts
index 42eef9369..6f0801119 100644
--- a/src/providers/registry.ts
+++ b/src/providers/registry.ts
@@ -237,6 +237,12 @@ export interface ProviderRegistryEntry {
   parallelToolCalls?: boolean;
   /** Opt this provider into forwarding prompt_cache_key (OpenAI-specific; strict backends reject it). */
   promptCacheKey?: boolean;
+  /**
+   * Opt-in: forward `service_tier` on the `/chat/completions` wire. Same hazard as
+   * `promptCacheKey` — an OpenAI-specific extension that strict gateways reject. Distinct from
+   * `supportsServiceTier`, which governs the Responses wire.
+   */
+  chatServiceTier?: boolean;
   autoToolChoiceOnlyModels?: string[];
   preserveReasoningContentModels?: string[];
   requiresReasoningPlaceholderModels?: string[];
diff --git a/src/router.ts b/src/router.ts
index 435148f3f..b01fee043 100644
--- a/src/router.ts
+++ b/src/router.ts
@@ -350,6 +350,7 @@ export function routedProviderConfig(providerName: string, provider: OcxProvider
     // opt-in, while an explicit user `false` keeps overriding registry `true`.
     ...(provider.parallelToolCalls === undefined && registryEntry.parallelToolCalls !== undefined ? { parallelToolCalls: registryEntry.parallelToolCalls } : {}),
     ...(provider.promptCacheKey === undefined && registryEntry.promptCacheKey !== undefined ? { promptCacheKey: registryEntry.promptCacheKey } : {}),
+    ...(provider.chatServiceTier === undefined && registryEntry.chatServiceTier !== undefined ? { chatServiceTier: registryEntry.chatServiceTier } : {}),
     ...(provider.reasoningWireFormat === undefined && registryEntry.reasoningWireFormat !== undefined
       ? { reasoningWireFormat: registryEntry.reasoningWireFormat }
       : {}),
diff --git a/src/types.ts b/src/types.ts
index 3dc5c3354..6de3fae29 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1438,6 +1438,15 @@ export interface OcxProviderConfig {
    * fields. Default off; only enable for providers that document this parameter.
    */
   promptCacheKey?: boolean;
+  /**
+   * Opt-in: forward `service_tier` to the upstream `/chat/completions` body.
+   * OpenAI-specific extension with the same hazard as `promptCacheKey` — strict backends
+   * reject unknown fields, and 66 registry providers share the `openai-chat` adapter, so a
+   * caller-supplied `service_tier` would otherwise turn working requests into upstream 400s.
+   * `supportsServiceTier` is the Responses-wire flag and does not apply here.
+   * Default off; only enable for providers that document this parameter on the chat wire.
+   */
+  chatServiceTier?: boolean;
   /**
    * Provider-local passthrough SSE repair for broken openai-responses gateways that reuse exact
    * placeholder message/reasoning ids or omit the terminal id after a stable added event.
diff --git a/tests/openai-chat-hardening.test.ts b/tests/openai-chat-hardening.test.ts
index b78c3102b..61de2920d 100644
--- a/tests/openai-chat-hardening.test.ts
+++ b/tests/openai-chat-hardening.test.ts
@@ -302,6 +302,37 @@ describe("openai-chat credential hardening", () => {
     expect(body).not.toHaveProperty("prompt_cache_key");
   });
 
+  test("preserves a caller-supplied service tier when the provider opts in", () => {
+    const adapter = createOpenAIChatAdapter(provider({ chatServiceTier: true }));
+    const req = parsed();
+    req.options.serviceTier = "priority";
+
+    const body = JSON.parse(adapter.buildRequest(req).body);
+
+    expect(body.service_tier).toBe("priority");
+  });
+
+  // `service_tier` is an OpenAI-specific extension and this adapter serves 66 registry
+  // providers, several of which reject unknown body fields. Forwarding it by default would
+  // turn a caller-supplied tier into an upstream 400 on those routes, so absence of the
+  // opt-in must mean the field is dropped — the same contract `prompt_cache_key` uses.
+  test("drops a caller-supplied service tier when the provider has not opted in", () => {
+    for (const p of [provider(), provider({ chatServiceTier: false })]) {
+      const req = parsed();
+      req.options.serviceTier = "priority";
+
+      const body = JSON.parse(createOpenAIChatAdapter(p).buildRequest(req).body);
+
+      expect(body).not.toHaveProperty("service_tier");
+    }
+  });
+
+  test("an opted-in provider without a caller tier still sends no service_tier", () => {
+    const body = JSON.parse(createOpenAIChatAdapter(provider({ chatServiceTier: true })).buildRequest(parsed()).body);
+
+    expect(body).not.toHaveProperty("service_tier");
+  });
+
   test("canonical Kimi Coding Plan routes forward Codex prompt_cache_key", () => {
     for (const [providerName, authMode] of [
       ["kimi", "oauth"],