diff --git a/changelog.d/fixes/7255-nonstreaming-gemini-family-projection.md b/changelog.d/fixes/7255-nonstreaming-gemini-family-projection.md
new file mode 100644
index 0000000000..8e0ce75dc1
--- /dev/null
+++ b/changelog.d/fixes/7255-nonstreaming-gemini-family-projection.md
@@ -0,0 +1 @@
+- fix(sse): project non-streaming JSON responses back to the Gemini/Antigravity `{response:{candidates}}` envelope instead of leaking the raw OpenAI `choices[]` shape, so tool calls are no longer dropped for Gemini-family clients on the JSON path (#7255) (thanks @warelik)
diff --git a/open-sse/handlers/responseTranslator.ts b/open-sse/handlers/responseTranslator.ts
index faaf50d84b..12f384c6b1 100644
--- a/open-sse/handlers/responseTranslator.ts
+++ b/open-sse/handlers/responseTranslator.ts
@@ -556,6 +556,20 @@ export function translateNonStreamingResponse(
     return convertOpenAINonStreamingToClaude(toRecord(intermediateOpenAI));
   }
 
+  // Gemini-family clients (Gemini, Antigravity): the streaming SSE path already
+  // projects OpenAI chunks into the `{ response: { candidates: [...] } }` envelope
+  // via the registered FORMATS.OPENAI -> FORMATS.ANTIGRAVITY translator
+  // (translator/response/openai-to-antigravity.ts), but this non-streaming path had
+  // no equivalent back-conversion step — it silently returned the raw OpenAI
+  // chat.completion shape (leaking `choices[]`/`tool_calls` instead of
+  // `candidates[]`/`functionCall`) to any non-streaming Gemini/Antigravity client.
+  if (
+    (sourceFormat === FORMATS.GEMINI || sourceFormat === FORMATS.ANTIGRAVITY) &&
+    sourceFormat !== targetFormat
+  ) {
+    return convertOpenAINonStreamingToGeminiFamily(toRecord(intermediateOpenAI));
+  }
+
   // Return intermediateOpenAI (which is either the raw response if unknown targetFormat, or an OpenAI compatible payload)
   return intermediateOpenAI;
 }
@@ -664,3 +678,92 @@ function convertOpenAINonStreamingToClaude(openaiResponse: JsonRecord): JsonReco
 
   return claudeResponse;
 }
+
+const OPENAI_TO_GEMINI_FINISH_REASON: Record<string, string> = {
+  stop: "STOP",
+  length: "MAX_TOKENS",
+  tool_calls: "STOP",
+  content_filter: "SAFETY",
+};
+
+/**
+ * Parse an OpenAI tool-call `arguments` payload into a Gemini `functionCall.args`
+ * object. Never throws: a provider emitting malformed/truncated JSON must not take
+ * down the whole non-streaming response path, so an unparseable payload degrades to
+ * `{}` (matching the streaming Gemini translator's behaviour).
+ */
+function parseFunctionCallArgs(args: unknown): Record<string, unknown> {
+  if (typeof args !== "string") return toRecord(args);
+  try {
+    return toRecord(JSON.parse(args || "{}"));
+  } catch {
+    return {};
+  }
+}
+
+/**
+ * Helper to convert an OpenAI chat.completion JSON object into the Gemini/Antigravity
+ * `{ response: { candidates: [...] } }` envelope for non-streaming clients. Mirrors the
+ * shape already produced for streaming by the registered
+ * FORMATS.OPENAI -> FORMATS.ANTIGRAVITY translator
+ * (translator/response/openai-to-antigravity.ts) so both paths agree.
+ */
+function convertOpenAINonStreamingToGeminiFamily(openaiResponse: JsonRecord): JsonRecord {
+  const choices = openaiResponse.choices as unknown[] | undefined;
+  const isChoicesArray = Array.isArray(choices);
+  if (!isChoicesArray && openaiResponse.object !== "chat.completion") {
+    return openaiResponse; // If it doesn't look like OpenAI, return as-is
+  }
+
+  const choice = isChoicesArray ? toRecord(choices[0]) : {};
+  const messageObj = toRecord(choice.message);
+
+  const parts: JsonRecord[] = [];
+  const reasoningText = resolveReasoningText(messageObj);
+  if (reasoningText) {
+    parts.push({ text: reasoningText, thought: true });
+  }
+  if (typeof messageObj.content === "string" && messageObj.content.length > 0) {
+    parts.push({ text: messageObj.content });
+  }
+  const toolCalls = Array.isArray(messageObj.tool_calls) ? messageObj.tool_calls : [];
+  for (const toolCall of toolCalls) {
+    const toolObj = toRecord(toolCall);
+    const fn = toRecord(toolObj.function);
+    parts.push({
+      functionCall: {
+        name: toString(fn.name),
+        args: parseFunctionCallArgs(fn.arguments),
+      },
+    });
+  }
+  if (parts.length === 0) parts.push({ text: "" });
+
+  const finishReason =
+    OPENAI_TO_GEMINI_FINISH_REASON[toString(choice.finish_reason, "stop")] ?? "STOP";
+
+  const usageSrc = toRecord(openaiResponse.usage);
+  const promptTokens = toNumber(usageSrc.prompt_tokens, 0);
+  const completionTokens = toNumber(usageSrc.completion_tokens, 0);
+
+  const geminiResponse: JsonRecord = {
+    response: {
+      candidates: [
+        {
+          content: { role: "model", parts },
+          finishReason,
+          index: 0,
+        },
+      ],
+      usageMetadata: {
+        promptTokenCount: promptTokens,
+        candidatesTokenCount: completionTokens,
+        totalTokenCount: toNumber(usageSrc.total_tokens, promptTokens + completionTokens),
+      },
+      modelVersion: toString(openaiResponse.model, "unknown"),
+      responseId: toString(openaiResponse.id, `resp_${Date.now()}`),
+    },
+  };
+
+  return geminiResponse;
+}
diff --git a/tests/unit/nonstreaming-gemini-family-response-projection.test.ts b/tests/unit/nonstreaming-gemini-family-response-projection.test.ts
new file mode 100644
index 0000000000..e495df853c
--- /dev/null
+++ b/tests/unit/nonstreaming-gemini-family-response-projection.test.ts
@@ -0,0 +1,165 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { FORMATS } from "../../open-sse/translator/formats.ts";
+import { translateNonStreamingResponse } from "../../open-sse/handlers/responseTranslator.ts";
+
+interface GeminiFamilyPart {
+  text?: string;
+  thought?: boolean;
+  functionCall?: { name: string; args: Record<string, unknown> };
+}
+
+interface GeminiFamilyResponse {
+  choices?: unknown;
+  response?: {
+    candidates: Array<{
+      content: { role: string; parts: GeminiFamilyPart[] };
+      finishReason: string;
+      index: number;
+    }>;
+    usageMetadata: {
+      promptTokenCount: number;
+      candidatesTokenCount: number;
+      totalTokenCount: number;
+    };
+  };
+}
+
+/**
+ * Regression guard for the projection drift ported from decolua/9router#2348.
+ *
+ * The streaming SSE path already projects an OpenAI-shaped chunk into the
+ * Antigravity/Gemini `{ response: { candidates: [...] } }` envelope via the
+ * registered `FORMATS.OPENAI -> FORMATS.ANTIGRAVITY` translator
+ * (open-sse/translator/response/openai-to-antigravity.ts).
+ *
+ * The non-streaming JSON path (`/v1/antigravity` with `stream:false`, or any
+ * combo target whose provider speaks a different wire format than the
+ * client) goes through `translateNonStreamingResponse` instead — a
+ * hand-rolled function whose "Phase 3: translate back to client format" step
+ * only special-cases FORMATS.CLAUDE. For every other non-OpenAI client format
+ * (Gemini, Antigravity) it silently falls through and returns the raw OpenAI
+ * chat.completion shape, leaking `choices[]`/`tool_calls` instead of the
+ * client's expected `candidates[]`/`functionCall` envelope — the exact
+ * "leaks OpenAI format to non-OpenAI clients, function calls dropped" bug
+ * class from the upstream report.
+ */
+test("translateNonStreamingResponse projects an OpenAI provider payload back to the Antigravity/Gemini envelope for antigravity clients", () => {
+  const openAICompletion = {
+    id: "chatcmpl-1",
+    object: "chat.completion",
+    created: 1700000000,
+    model: "gpt-4o",
+    choices: [
+      {
+        index: 0,
+        message: {
+          role: "assistant",
+          content: "",
+          tool_calls: [
+            {
+              id: "call_1",
+              type: "function",
+              function: { name: "lookup", arguments: '{"q":"x"}' },
+            },
+          ],
+        },
+        finish_reason: "tool_calls",
+      },
+    ],
+    usage: { prompt_tokens: 3, completion_tokens: 5, total_tokens: 8 },
+  };
+
+  const translated = translateNonStreamingResponse(
+    openAICompletion,
+    FORMATS.OPENAI,
+    FORMATS.ANTIGRAVITY
+  ) as GeminiFamilyResponse;
+
+  // The Antigravity/Gemini client expects a `{ response: { candidates: [...] } }`
+  // envelope with `functionCall` parts, never a raw OpenAI `choices[]`/`tool_calls`
+  // shape.
+  assert.ok(
+    translated?.response?.candidates,
+    `expected {response:{candidates:[...]}} envelope for an antigravity client, got: ${JSON.stringify(translated)}`
+  );
+  assert.equal(translated.choices, undefined);
+
+  const candidate = translated.response!.candidates[0];
+  assert.equal(candidate.content.role, "model");
+  assert.deepEqual(candidate.content.parts[0].functionCall, {
+    name: "lookup",
+    args: { q: "x" },
+  });
+  assert.equal(candidate.finishReason, "STOP");
+  assert.equal(translated.response!.usageMetadata.totalTokenCount, 8);
+});
+
+test("translateNonStreamingResponse projects a Claude provider payload back to the Gemini envelope for gemini clients", () => {
+  const claudeMessage = {
+    id: "msg_1",
+    type: "message",
+    role: "assistant",
+    model: "claude-sonnet",
+    content: [
+      { type: "thinking", thinking: "reasoning trace" },
+      { type: "text", text: "final answer" },
+    ],
+    stop_reason: "end_turn",
+    stop_sequence: null,
+    usage: { input_tokens: 4, output_tokens: 6 },
+  };
+
+  const translated = translateNonStreamingResponse(
+    claudeMessage,
+    FORMATS.CLAUDE,
+    FORMATS.GEMINI
+  ) as GeminiFamilyResponse;
+
+  assert.ok(
+    translated?.response?.candidates,
+    `expected {response:{candidates:[...]}} envelope for a gemini client, got: ${JSON.stringify(translated)}`
+  );
+  const parts = translated.response!.candidates[0].content.parts;
+  assert.deepEqual(
+    parts.find((p) => p.thought === true),
+    { text: "reasoning trace", thought: true }
+  );
+  assert.ok(parts.some((p) => p.text === "final answer"));
+});
+
+test("translateNonStreamingResponse degrades malformed tool-call arguments to {} instead of throwing", () => {
+  // A provider emitting truncated/invalid JSON in `arguments` must not take down the
+  // whole non-streaming response path with an uncaught SyntaxError.
+  const openAICompletion = {
+    id: "chatcmpl-2",
+    object: "chat.completion",
+    created: 1700000000,
+    model: "gpt-4o",
+    choices: [
+      {
+        index: 0,
+        message: {
+          role: "assistant",
+          content: "",
+          tool_calls: [
+            { id: "call_1", type: "function", function: { name: "lookup", arguments: '{"q":' } },
+          ],
+        },
+        finish_reason: "tool_calls",
+      },
+    ],
+  };
+
+  const translated = translateNonStreamingResponse(
+    openAICompletion,
+    FORMATS.OPENAI,
+    FORMATS.GEMINI
+  ) as GeminiFamilyResponse;
+
+  assert.deepEqual(translated.response!.candidates[0].content.parts[0].functionCall, {
+    name: "lookup",
+    args: {},
+  });
+});