diff --git a/changelog.d/fixes/6276-toolcall-args-logs.md b/changelog.d/fixes/6276-toolcall-args-logs.md
new file mode 100644
index 0000000000..90b6bde61b
--- /dev/null
+++ b/changelog.d/fixes/6276-toolcall-args-logs.md
@@ -0,0 +1 @@
+- fix(api): merge tool_call continuation deltas that carry only `id` (no `index`) so tool-call arguments are no longer split/lost in request/response logs (#6276)
diff --git a/open-sse/utils/streamPayloadCollector.ts b/open-sse/utils/streamPayloadCollector.ts
index 26e9f4418e..63cb901179 100644
--- a/open-sse/utils/streamPayloadCollector.ts
+++ b/open-sse/utils/streamPayloadCollector.ts
@@ -129,13 +129,29 @@ function buildOpenAISummary(events: StructuredSSEEvent[], fallbackModel?: string
     function: { name: string; arguments: string };
   };
   const toolCalls = new Map<string, ToolCall>();
+  // Aliases every `idx:N` key we've seen to the `id:X` it was first observed with (and
+  // vice versa), so a later delta chunk that only carries one of the two dimensions
+  // (e.g. a continuation chunk with `id` but no `index` — a known quirk of some
+  // OpenAI-compatible proxies) still resolves to the SAME accumulator entry instead of
+  // splitting one logical tool call into two (#6276).
+  const keyAliases = new Map<string, string>();
   let unknownToolCallSeq = 0;
   let finishReason = "stop";
   let usage: JsonRecord | null = null;
 
   const getToolCallKey = (toolCall: JsonRecord) => {
-    if (Number.isInteger(toolCall.index)) return `idx:${toolCall.index}`;
-    if (toolCall.id) return `id:${toolCall.id}`;
+    const idKey = typeof toolCall.id === "string" && toolCall.id ? `id:${toolCall.id}` : null;
+    const idxKey = Number.isInteger(toolCall.index) ? `idx:${toolCall.index}` : null;
+
+    const resolvedKey = (idKey && keyAliases.get(idKey)) || (idxKey && keyAliases.get(idxKey));
+    const key = resolvedKey || idKey || idxKey;
+
+    if (key) {
+      if (idKey) keyAliases.set(idKey, key);
+      if (idxKey) keyAliases.set(idxKey, key);
+      return key;
+    }
+
     unknownToolCallSeq += 1;
     return `seq:${unknownToolCallSeq}`;
   };
diff --git a/tests/unit/stream-payload-collector.test.ts b/tests/unit/stream-payload-collector.test.ts
index 1cb9e58b85..669edcdd1e 100644
--- a/tests/unit/stream-payload-collector.test.ts
+++ b/tests/unit/stream-payload-collector.test.ts
@@ -75,3 +75,143 @@ test("createStructuredSSECollector collector has expected methods", () => {
   const keys = Object.keys(c);
   assert.ok(keys.length > 0);
 });
+
+// #6276 — tool_call arguments lost in request/response logs when a continuation
+// delta omits `index` (some OpenAI-compatible proxies only send `index` on the
+// FIRST tool_call delta chunk, then only `id` on subsequent chunks).
+
+type ToolCallSummary = {
+  choices: Array<{
+    message: {
+      tool_calls: Array<{ function: { name: string; arguments: string } }>;
+    };
+  }>;
+};
+
+function toolCallEvent(delta: Record<string, unknown>, finishReason?: string) {
+  return {
+    index: 0,
+    data: {
+      id: "chatcmpl-1",
+      object: "chat.completion.chunk",
+      created: 1,
+      model: "deepseek-v4-flash-free",
+      choices: [{ index: 0, delta, ...(finishReason ? { finish_reason: finishReason } : {}) }],
+    },
+  };
+}
+
+test("buildStreamSummaryFromEvents merges tool_call deltas when every chunk carries `index` (happy path)", () => {
+  const events = [
+    toolCallEvent({
+      role: "assistant",
+      tool_calls: [
+        { index: 0, id: "call_a", type: "function", function: { name: "Bash", arguments: "" } },
+      ],
+    }),
+    toolCallEvent({
+      tool_calls: [{ index: 0, id: "call_a", type: "function", function: { arguments: '{"x":1}' } }],
+    }),
+    toolCallEvent({}, "tool_calls"),
+  ];
+
+  const summary = collector.buildStreamSummaryFromEvents(
+    events,
+    "openai",
+    "deepseek-v4-flash-free"
+  ) as ToolCallSummary;
+  const toolCalls = summary.choices[0].message.tool_calls;
+
+  assert.equal(toolCalls.length, 1);
+  assert.equal(toolCalls[0].function.name, "Bash");
+  assert.equal(toolCalls[0].function.arguments, '{"x":1}');
+});
+
+test("buildStreamSummaryFromEvents merges a continuation delta that carries only `id` (no `index`) into the initiating tool_call (#6276)", () => {
+  const events = [
+    toolCallEvent({
+      role: "assistant",
+      tool_calls: [
+        {
+          index: 0,
+          id: "call_00_xasdOvEWoeldzXAqFPQP2849",
+          type: "function",
+          function: { name: "Bash", arguments: "" },
+        },
+      ],
+    }),
+    // Continuation chunk omits `index`, carries only `id` + arguments fragment.
+    toolCallEvent({
+      tool_calls: [
+        {
+          id: "call_00_xasdOvEWoeldzXAqFPQP2849",
+          type: "function",
+          function: { arguments: '{"command": "date' },
+        },
+      ],
+    }),
+    toolCallEvent({
+      tool_calls: [
+        {
+          id: "call_00_xasdOvEWoeldzXAqFPQP2849",
+          type: "function",
+          function: { arguments: '"}' },
+        },
+      ],
+    }),
+    toolCallEvent({}, "tool_calls"),
+  ];
+
+  const summary = collector.buildStreamSummaryFromEvents(
+    events,
+    "openai",
+    "deepseek-v4-flash-free"
+  ) as ToolCallSummary;
+  const toolCalls = summary.choices[0].message.tool_calls;
+
+  assert.equal(
+    toolCalls.length,
+    1,
+    `expected 1 tool_call, got ${toolCalls.length}: ${JSON.stringify(toolCalls)}`
+  );
+  assert.equal(toolCalls[0].function.name, "Bash");
+  assert.equal(toolCalls[0].function.arguments, '{"command": "date"}');
+});
+
+test("buildStreamSummaryFromEvents keeps two genuinely different interleaved tool_calls separate", () => {
+  const events = [
+    toolCallEvent({
+      role: "assistant",
+      tool_calls: [
+        { index: 0, id: "call_a", type: "function", function: { name: "Bash", arguments: "" } },
+        { index: 1, id: "call_b", type: "function", function: { name: "Read", arguments: "" } },
+      ],
+    }),
+    toolCallEvent({
+      tool_calls: [
+        { index: 0, id: "call_a", type: "function", function: { arguments: '{"cmd":"a"' } },
+        { index: 1, id: "call_b", type: "function", function: { arguments: '{"path":"b"' } },
+      ],
+    }),
+    toolCallEvent({
+      tool_calls: [
+        { index: 0, id: "call_a", type: "function", function: { arguments: "}" } },
+        { index: 1, id: "call_b", type: "function", function: { arguments: "}" } },
+      ],
+    }),
+    toolCallEvent({}, "tool_calls"),
+  ];
+
+  const summary = collector.buildStreamSummaryFromEvents(
+    events,
+    "openai",
+    "deepseek-v4-flash-free"
+  ) as ToolCallSummary;
+  const toolCalls = summary.choices[0].message.tool_calls;
+
+  assert.equal(toolCalls.length, 2);
+  assert.equal(toolCalls[0].function.name, "Bash");
+  assert.equal(toolCalls[0].function.arguments, '{"cmd":"a"}');
+  assert.equal(toolCalls[1].function.name, "Read");
+  assert.equal(toolCalls[1].function.arguments, '{"path":"b"}');
+});