diff --git a/src/adapters/openai-chat.ts b/src/adapters/openai-chat.ts
index 99b62af5e5..b79d73fa9c 100644
--- a/src/adapters/openai-chat.ts
+++ b/src/adapters/openai-chat.ts
@@ -205,6 +205,98 @@ function isRecord(value: unknown): value is Record<string, unknown> {
   return value !== null && typeof value === "object" && !Array.isArray(value);
 }
 
+type InvalidToolCallReason =
+  | "tool_calls_not_array"
+  | "tool_call_not_object"
+  | "tool_call_id_invalid"
+  | "tool_call_function_not_object"
+  | "tool_call_function_name_invalid"
+  | "tool_call_function_name_blank"
+  | "tool_call_function_arguments_invalid";
+
+/**
+ * Explain only the rejected wire shape, never its values. This diagnostic exists so provider
+ * compatibility can be tightened from evidence without retaining tool arguments or credentials.
+ */
+function diagnoseInvalidToolCalls(
+  rawToolCalls: unknown,
+  mode: "stream" | "response",
+): { reason: InvalidToolCallReason; callIndex?: number; valueType: string } | undefined {
+  if (!Array.isArray(rawToolCalls)) {
+    return { reason: "tool_calls_not_array", valueType: rawToolCalls === null ? "null" : typeof rawToolCalls };
+  }
+  for (let callIndex = 0; callIndex < rawToolCalls.length; callIndex++) {
+    const rawToolCall = rawToolCalls[callIndex];
+    if (!isRecord(rawToolCall)) {
+      return {
+        reason: "tool_call_not_object",
+        callIndex,
+        valueType: rawToolCall === null ? "null" : Array.isArray(rawToolCall) ? "array" : typeof rawToolCall,
+      };
+    }
+    if (mode === "stream") {
+      // The streamed path validates the pieces it is about to store (#1531): a present
+      // `function` must be a record, and a present `name`/`arguments`/`id` must be a string.
+      // Blank names are caught later at flush, not here, so they are not diagnosed on this
+      // branch. Describe exactly that boundary rather than tightening compatibility in a
+      // diagnostic change.
+      const streamFunction = (rawToolCall as { function?: unknown }).function;
+      if (streamFunction !== undefined && streamFunction !== null) {
+        if (!isRecord(streamFunction)) {
+          return {
+            reason: "tool_call_function_not_object",
+            callIndex,
+            valueType: Array.isArray(streamFunction) ? "array" : typeof streamFunction,
+          };
+        }
+        if (streamFunction.name !== undefined && typeof streamFunction.name !== "string") {
+          return { reason: "tool_call_function_name_invalid", callIndex, valueType: typeof streamFunction.name };
+        }
+        if (streamFunction.arguments !== undefined && typeof streamFunction.arguments !== "string") {
+          return { reason: "tool_call_function_arguments_invalid", callIndex, valueType: typeof streamFunction.arguments };
+        }
+      }
+      if (rawToolCall.id !== undefined && typeof rawToolCall.id !== "string") {
+        return { reason: "tool_call_id_invalid", callIndex, valueType: typeof rawToolCall.id };
+      }
+      continue;
+    }
+    // Precedence must mirror the buffered validator below, or a payload with more than one
+    // problem is reported under the wrong reason and sends compatibility work after the wrong
+    // shape. That validator checks the `function` container first (`!isRecord(rawToolCall) ||
+    // !isRecord(rawToolCall.function)`), then id/name/arguments types together, and only then
+    // the blank name.
+    if (!isRecord(rawToolCall.function)) {
+      return {
+        reason: "tool_call_function_not_object",
+        callIndex,
+        valueType: rawToolCall.function === null ? "null" : Array.isArray(rawToolCall.function) ? "array" : typeof rawToolCall.function,
+      };
+    }
+    if (typeof rawToolCall.id !== "string") {
+      return { reason: "tool_call_id_invalid", callIndex, valueType: typeof rawToolCall.id };
+    }
+    if (typeof rawToolCall.function.name !== "string") {
+      return { reason: "tool_call_function_name_invalid", callIndex, valueType: typeof rawToolCall.function.name };
+    }
+    if (typeof rawToolCall.function.arguments !== "string") {
+      return { reason: "tool_call_function_arguments_invalid", callIndex, valueType: typeof rawToolCall.function.arguments };
+    }
+    // Last, matching the validator: #1531 also rejects a blank or whitespace-only name here,
+    // because such a call cannot select a dispatch target. Reporting it as `name_invalid`
+    // would claim a type problem for a correctly-typed value, so it gets its own code.
+    if (rawToolCall.function.name.trim().length === 0) {
+      return { reason: "tool_call_function_name_blank", callIndex, valueType: "string" };
+    }
+  }
+  return undefined;
+}
+
+function logInvalidToolCalls(mode: "stream" | "response", rawToolCalls: unknown): void {
+  const diagnostic = diagnoseInvalidToolCalls(rawToolCalls, mode);
+  if (diagnostic) debugProviderDiagnostic("openai-chat", "invalid-tool-calls", { mode, ...diagnostic });
+}
+
 function developerSystemText(message: OcxMessage): string | undefined {
   if (message.role !== "developer") return undefined;
   if (typeof message.content === "string") return message.content;
@@ -1018,10 +1110,12 @@ export function createOpenAIChatAdapter(provider: OcxProviderConfig): ProviderAd
             // through the adapter error channel instead of escaping as TypeError (#1325). Null is
             // tolerated as absent because OpenAI-compatible providers may emit it as stream padding.
             if (!Array.isArray(rawToolCalls)) {
+              logInvalidToolCalls("stream", rawToolCalls);
               return yield* terminateWithError(invalidToolCallsEvent(pendingUsage));
             }
             for (const rawToolCall of rawToolCalls) {
               if (!isRecord(rawToolCall)) {
+                logInvalidToolCalls("stream", rawToolCalls);
                 return yield* terminateWithError(invalidToolCallsEvent(pendingUsage));
               }
               const tc = rawToolCall as {
@@ -1036,16 +1130,19 @@ export function createOpenAIChatAdapter(provider: OcxProviderConfig): ProviderAd
               const rawFunction = (rawToolCall as { function?: unknown }).function;
               if (rawFunction !== undefined && rawFunction !== null) {
                 if (!isRecord(rawFunction)) {
+                  logInvalidToolCalls("stream", rawToolCalls);
                   return yield* terminateWithError(invalidToolCallsEvent(pendingUsage));
                 }
                 const rawName = rawFunction.name;
                 const rawArguments = rawFunction.arguments;
                 if ((rawName !== undefined && typeof rawName !== "string")
                   || (rawArguments !== undefined && typeof rawArguments !== "string")) {
+                  logInvalidToolCalls("stream", rawToolCalls);
                   return yield* terminateWithError(invalidToolCallsEvent(pendingUsage));
                 }
               }
               if (tc.id !== undefined && typeof tc.id !== "string") {
+                logInvalidToolCalls("stream", rawToolCalls);
                 return yield* terminateWithError(invalidToolCallsEvent(pendingUsage));
               }
               const key = typeof tc.index === "number"
@@ -1206,9 +1303,13 @@ export function createOpenAIChatAdapter(provider: OcxProviderConfig): ProviderAd
         if (typeof msg.content === "string") events.push({ type: "text_delta", text: msg.content });
         const rawToolCalls = msg.tool_calls;
         if (rawToolCalls !== undefined && rawToolCalls !== null) {
-          if (!Array.isArray(rawToolCalls)) return [invalidToolCallsEvent(usage)];
+          if (!Array.isArray(rawToolCalls)) {
+            logInvalidToolCalls("response", rawToolCalls);
+            return [invalidToolCallsEvent(usage)];
+          }
           for (const rawToolCall of rawToolCalls) {
             if (!isRecord(rawToolCall) || !isRecord(rawToolCall.function)) {
+              logInvalidToolCalls("response", rawToolCalls);
               return [invalidToolCallsEvent(usage)];
             }
             const id = rawToolCall.id;
@@ -1219,6 +1320,7 @@ export function createOpenAIChatAdapter(provider: OcxProviderConfig): ProviderAd
             // a whitespace-only function name is not a legitimate tool-call shape either.
             if (typeof id !== "string" || typeof name !== "string" || typeof args !== "string"
               || name.trim().length === 0) {
+              logInvalidToolCalls("response", rawToolCalls);
               return [invalidToolCallsEvent(usage)];
             }
             events.push({ type: "tool_call_start", id, name });
diff --git a/tests/openai-chat-hardening.test.ts b/tests/openai-chat-hardening.test.ts
index b78c3102b8..9a2117f4a4 100644
--- a/tests/openai-chat-hardening.test.ts
+++ b/tests/openai-chat-hardening.test.ts
@@ -1,5 +1,7 @@
-import { describe, expect, test } from "bun:test";
+import { afterEach, describe, expect, test } from "bun:test";
 import { createOpenAIChatAdapter as createOpenAIChatAdapterProduction } from "../src/adapters/openai-chat";
+import { getDebugLogEntries, resetDebugLogBufferForTests } from "../src/lib/debug-log-buffer";
+import { resetDebugSettingsForTests } from "../src/lib/debug-settings";
 import { routeModel } from "../src/router";
 import type { AdapterEvent, OcxConfig, OcxParsedRequest, OcxProviderConfig } from "../src/types";
 import { withTestTranslatorBudget } from "./helpers/translator-budget";
@@ -7,6 +9,15 @@ import { withTestTranslatorBudget } from "./helpers/translator-budget";
 const createOpenAIChatAdapter = (...args: Parameters<typeof createOpenAIChatAdapterProduction>) =>
   withTestTranslatorBudget(createOpenAIChatAdapterProduction(...args));
 
+const previousDebug = process.env.OCX_DEBUG;
+
+afterEach(() => {
+  resetDebugSettingsForTests();
+  resetDebugLogBufferForTests();
+  if (previousDebug === undefined) delete process.env.OCX_DEBUG;
+  else process.env.OCX_DEBUG = previousDebug;
+});
+
 function parsed(): OcxParsedRequest {
   return {
     modelId: "test-model",
@@ -148,6 +159,77 @@ describe("openai-chat non-stream response hardening", () => {
       }]);
     }
   });
+
+  test("debug mode records only the non-stream tool-call shape failure", async () => {
+    process.env.OCX_DEBUG = "1";
+    const secretArguments = "private-tool-arguments";
+    const adapter = createOpenAIChatAdapter(provider());
+    const events = await adapter.parseResponse!(new Response(JSON.stringify({
+      choices: [{ message: { role: "assistant", tool_calls: [{
+        id: "call_1",
+        function: { name: "tool", arguments: { secretArguments } },
+      }] } }],
+    })));
+
+    expect(events).toEqual([{ type: "error", message: "upstream response contained invalid tool calls" }]);
+    const lines = getDebugLogEntries().map(entry => entry.line).join("\n");
+    expect(lines).toContain("[ocx:openai-chat:invalid-tool-calls]");
+    expect(lines).toContain('"mode":"response"');
+    expect(lines).toContain('"reason":"tool_call_function_arguments_invalid"');
+    expect(lines).toContain('"valueType":"object"');
+    expect(lines).not.toContain(secretArguments);
+    expect(lines).not.toContain("call_1");
+  });
+
+  test("tool-call structural diagnostics stay disabled by default", async () => {
+    delete process.env.OCX_DEBUG;
+    const adapter = createOpenAIChatAdapter(provider());
+    await adapter.parseResponse!(new Response(JSON.stringify({
+      choices: [{ message: { role: "assistant", tool_calls: { privateArguments: "secret" } } }],
+    })));
+
+    expect(getDebugLogEntries()).toHaveLength(0);
+  });
+
+  // The diagnostic's job is to say WHICH check rejected the payload. If its precedence drifts
+  // from the validator's, a payload with more than one problem is reported under the wrong
+  // reason and sends provider-compatibility work after the wrong shape. These cases each carry
+  // two defects at once, so only the matching order produces the expected reason.
+  describe("diagnostic precedence matches the buffered validator", () => {
+    async function reasonFor(toolCall: unknown): Promise<string> {
+      process.env.OCX_DEBUG = "1";
+      const adapter = createOpenAIChatAdapter(provider());
+      await adapter.parseResponse!(new Response(JSON.stringify({
+        choices: [{ message: { role: "assistant", tool_calls: [toolCall] } }],
+      })));
+      const lines = getDebugLogEntries().map(entry => entry.line).join("\n");
+      const match = /"reason":"([a-z_]+)"/.exec(lines);
+      return match?.[1] ?? "";
+    }
+
+    test("a bad function container outranks a bad id", async () => {
+      // Validator checks `!isRecord(rawToolCall.function)` before it reads `id`.
+      expect(await reasonFor({ id: 7, function: "not-an-object" }))
+        .toBe("tool_call_function_not_object");
+    });
+
+    test("a bad id outranks a bad name", async () => {
+      expect(await reasonFor({ id: 7, function: { name: 9, arguments: "{}" } }))
+        .toBe("tool_call_id_invalid");
+    });
+
+    test("a bad arguments type outranks a blank name", async () => {
+      // Both are rejected by the same validator condition; arguments is checked first there,
+      // so a blank name must not shadow it.
+      expect(await reasonFor({ id: "call_1", function: { name: "   ", arguments: 5 } }))
+        .toBe("tool_call_function_arguments_invalid");
+    });
+
+    test("a blank name is reported as blank, not as a type problem", async () => {
+      expect(await reasonFor({ id: "call_1", function: { name: "   ", arguments: "{}" } }))
+        .toBe("tool_call_function_name_blank");
+    });
+  });
 });
 
 describe("openai-chat stream response hardening", () => {
@@ -227,6 +309,29 @@ describe("openai-chat stream response hardening", () => {
       }]);
     }
   });
+
+  test("debug mode classifies streaming tool-call structure without retaining values", async () => {
+    process.env.OCX_DEBUG = "1";
+    const privateName = "private-tool-name";
+    const adapter = createOpenAIChatAdapter(provider());
+    const response = new Response([
+      `data: ${JSON.stringify({ choices: [{ delta: { tool_calls: [{
+        privateName,
+        privateArguments: "private arguments",
+      }, null] } }] })}\n\n`,
+      "data: [DONE]\n\n",
+    ].join(""));
+
+    const events = await collect(adapter.parseStream(response));
+    expect(events).toEqual([{ type: "error", message: "upstream response contained invalid tool calls" }]);
+    const lines = getDebugLogEntries().map(entry => entry.line).join("\n");
+    expect(lines).toContain('"mode":"stream"');
+    expect(lines).toContain('"reason":"tool_call_not_object"');
+    expect(lines).toContain('"callIndex":1');
+    expect(lines).toContain('"valueType":"null"');
+    expect(lines).not.toContain(privateName);
+    expect(lines).not.toContain("private arguments");
+  });
 });
 
 describe("openai-chat credential hardening", () => {