fix(ai): learn the max_tokens refusal from the field the request carried

A call that ran beside the one that learned the lesson read today's
selection instead and failed with the 400. The matcher now also wants the
refusal to name max_tokens, so a refusal of another field that echoes the
request does not switch the token field.

Refs #1304

src/lib/ai/__tests__/azure-token-budget.test.ts

@@ -197,11 +197,40 @@ describe("an endpoint that refuses max_tokens", () => {
     expect(mock).toHaveBeenCalledTimes(1);
   });
 
+  it("asks again on every call that sent max_tokens, also beside one that learned it", async () => {
+    const mock = vi
+      .fn()
+      .mockResolvedValueOnce(refused(400, AZURE_REFUSAL))
+      .mockResolvedValueOnce(refused(400, AZURE_REFUSAL))
+      .mockResolvedValue(ok());
+    vi.stubGlobal("fetch", mock);
+    const client = gateway("prod-chat");
+    const results = await Promise.allSettled([
+      client.generateCompletion(turn()),
+      client.generateCompletion(turn()),
+    ]);
+    expect(results.map((r) => r.status)).toEqual(["fulfilled", "fulfilled"]);
+    expect(mock).toHaveBeenCalledTimes(4);
+  });
+
   it("reads only a 400 that names the field", () => {
     expect(isMaxTokensRejection(400, AZURE_REFUSAL)).toBe(true);
     expect(isMaxTokensRejection(422, AZURE_REFUSAL)).toBe(false);
     expect(
       isMaxTokensRejection(400, '{"error":"Unsupported parameter: top_k"}'),
     ).toBe(false);
+    // A refusal of another field that echoes the request.
+    expect(
+      isMaxTokensRejection(
+        400,
+        '{"error":"Unsupported parameter: reasoning_effort is not supported","request":{"max_tokens":1000}}',
+      ),
+    ).toBe(false);
+    expect(
+      isMaxTokensRejection(
+        400,
+        '{"error":"max_tokens is not supported with this model"}',
+      ),
+    ).toBe(true);
   });
 });

src/lib/ai/openai-capabilities.ts

@@ -99,14 +99,17 @@ export function resetMaxTokensRefusals(): void {
 
 /**
  * True when a 400 says the model takes `max_completion_tokens` and not
- * `max_tokens`. A complaint about the value ("max_tokens is too large") is
- * not a refusal of the field and does not match.
+ * `max_tokens`: it names the replacement, or it refuses `max_tokens` by
+ * name. A complaint about the value ("max_tokens is too large") or about
+ * another field that echoes the request is not a refusal of the field.
  */
 export function isMaxTokensRejection(status: number, body: string): boolean {
   if (status !== 400 || !/\bmax_tokens\b/.test(body)) return false;
   return (
     /max_completion_tokens/.test(body) ||
-    /unsupported[\s_-]*parameter|not supported/i.test(body)
+    /unsupported[\s_-]*parameter\W{0,4}max_tokens\b|\bmax_tokens\W{0,4}(is\s+)?(not\s+supported|unsupported)/i.test(
+      body,
+    )
   );
 }
 

src/lib/ai/openai-client.ts

@@ -206,11 +206,11 @@ export class OpenAIClient implements AIProvider {
       ? jsonModeDialectFor(this.config.baseUrl)
       : "response_format";
     const sendJsonFormat = useJsonFormat && jsonDialect === "response_format";
-    const capabilities = selectOpenAIChatCompletionsCapabilities(
+    const caps = selectOpenAIChatCompletionsCapabilities(
       this.config.baseUrl,
       this.config.model,
     );
-    const reasoningWire = this.planReasoning(params, hasTools, capabilities);
+    const reasoningWire = this.planReasoning(params, hasTools, caps);
     // #1126 — the gateway entry's own setting, for a call without a level,
     // unless the wire carries one or the endpoint refused the field (#1279).
     const entryEffort =
@@ -225,13 +225,13 @@ export class OpenAIClient implements AIProvider {
       reasoningWire.kind !== "none" ? reasoningWire.effort : entryEffort;
     const thinkingTokens = this.thinkingRoom(params, reasoningWire);
     const tokenBudget = (params.maxTokens ?? 1000) + thinkingTokens;
-    const capabilityParams = capabilities.supportsSamplingControls
+    const capabilityParams = caps.supportsSamplingControls
       ? {
-          [capabilities.tokenBudgetField]: tokenBudget,
+          [caps.tokenBudgetField]: tokenBudget,
           temperature: params.temperature ?? 0.3,
           ...(params.seed !== undefined ? { seed: params.seed } : {}),
         }
-      : { [capabilities.tokenBudgetField]: tokenBudget };
+      : { [caps.tokenBudgetField]: tokenBudget };
 
     // A gateway that needs no bearer (a LAN LiteLLM without a master key)
     // gets no `Authorization` header at all rather than an empty one — an
@@ -299,7 +299,7 @@ export class OpenAIClient implements AIProvider {
     );
 
     if (!res.ok) {
-      return this.failed(res, params, hops, { sent, sendJsonFormat });
+      return this.failed(res, params, hops, { sent, sendJsonFormat, caps });
     }
 
     // v1.33.1 (#470) — the standard `response_format` request succeeded: pin
@@ -512,7 +512,11 @@ export class OpenAIClient implements AIProvider {
     res: Response,
     params: CompletionParams,
     hops: RequestHops,
-    sent: { sent: string | null; sendJsonFormat: boolean },
+    sent: {
+      sent: string | null;
+      sendJsonFormat: boolean;
+      caps: OpenAIChatCompletionsCapabilities;
+    },
   ): Promise<CompletionResult> {
     const bodyExcerpt = sanitiseBodyExcerpt(await res.text().catch(() => ""));
     // The entry's own setting (#1126) included: a refused field is learned
@@ -535,13 +539,10 @@ export class OpenAIClient implements AIProvider {
         this.isGateway && sent.sendJsonFormat
           ? { endpoint: this.config.baseUrl, action: "ai.compat.jsonDialect" }
           : null,
-      // The request that failed carried `max_tokens`: nothing is learned
-      // between sending it and reading its refusal, so the selection holds.
+      // The field this request carried, not today's selection: a call
+      // running beside it may have learned the lesson meanwhile.
       budget:
-        selectOpenAIChatCompletionsCapabilities(
-          this.config.baseUrl,
-          this.config.model,
-        ).tokenBudgetField === "max_tokens"
+        sent.caps.tokenBudgetField === "max_tokens"
           ? { endpoint: this.config.baseUrl }
           : null,
     });