diff --git a/changelog.d/fixes/9408-claude-web-tool-use.md b/changelog.d/fixes/9408-claude-web-tool-use.md
new file mode 100644
index 0000000000..ed6f8f258f
--- /dev/null
+++ b/changelog.d/fixes/9408-claude-web-tool-use.md
@@ -0,0 +1 @@
+- fix(providers): add tool_use block handling to claude-web stream parser for OpenAI tool_calls projection (#9408)
diff --git a/open-sse/executors/claude-web/payload.ts b/open-sse/executors/claude-web/payload.ts
index 74a88ab1b4..6966e9d95c 100644
--- a/open-sse/executors/claude-web/payload.ts
+++ b/open-sse/executors/claude-web/payload.ts
@@ -217,6 +217,20 @@ function messageText(content: unknown): string {
   return content.map(contentPartText).filter(Boolean).join("\n");
 }
 
+function buildPromptFromMessages(messages: unknown[]): string {
+  const parts: string[] = [];
+  for (const candidate of messages) {
+    if (!isRecord(candidate)) continue;
+    const role = candidate.role;
+    const text = messageText(candidate.content);
+    if (!text) continue;
+    if (role === "user" || role === "tool") {
+      parts.push(text);
+    }
+  }
+  return parts.join("\n\n");
+}
+
 function latestUserPrompt(messages: unknown[]): string {
   let prompt = "";
   for (const candidate of messages) {
@@ -308,7 +322,9 @@ export function transformToClaude(
   const messages = Array.isArray(body.messages) ? body.messages : [];
   const reasoningEffort = resolveClaudeWebReasoningEffort(body);
   const resolvedModel = model || DEFAULT_CLAUDE_MODEL;
-  const resolvedTurn = turn ?? defaultTurn(latestUserPrompt(messages));
+  const prompt =
+    turn?.prompt ?? (buildPromptFromMessages(messages) || latestUserPrompt(messages));
+  const resolvedTurn = turn ?? defaultTurn(prompt);
 
   if (resolvedTurn.operation === "completion" && !resolvedTurn.prompt.trim()) {
     throw new Error("No user message found in request");
diff --git a/open-sse/executors/claude-web/stream.ts b/open-sse/executors/claude-web/stream.ts
index 617a0d99fa..82281bb02b 100644
--- a/open-sse/executors/claude-web/stream.ts
+++ b/open-sse/executors/claude-web/stream.ts
@@ -13,14 +13,22 @@ export interface ClaudeWebStreamOptions {
 }
 
 type StreamPhase = "awaiting_message" | "in_message" | "stopped" | "failed";
-type BlockKind = "thinking" | "text" | "other";
+type BlockKind = "thinking" | "text" | "tool_use" | "other";
 const MAX_CLAUDE_WEB_SSE_PENDING_CHARS = 1024 * 1024;
 type SemanticEvent =
   | { kind: "content"; text: string }
   | { kind: "reasoning"; text: string }
+  | { kind: "tool_call"; index: number; id: string; name: string; input: string }
   | { kind: "metadata"; eventType: string; data: Record<string, unknown> }
   | { kind: "finish"; stopReason: string };
 
+interface ToolBlockInfo {
+  id: string;
+  name: string;
+  inputParts: string[];
+  initialInput: string;
+}
+
 const KNOWN_METADATA_EVENTS = new Set([
   "ping",
   "completion",
@@ -193,6 +201,7 @@ function thinkingSummaryText(delta: Record<string, unknown>): string {
 interface ProtocolState {
   phase: StreamPhase;
   openBlocks: Map<number, BlockKind>;
+  toolBlocks: Map<number, ToolBlockInfo>;
   stopReason: string;
 }
 
@@ -241,6 +250,7 @@ function handleMessageStart(state: ProtocolState): null {
 function blockKind(block: Record<string, unknown>): BlockKind {
   if (block.type === "thinking") return "thinking";
   if (block.type === "text") return "text";
+  if (block.type === "tool_use") return "tool_use";
   return "other";
 }
 
@@ -252,17 +262,35 @@ function handleContentBlockStart(
   const index = requireBlockIndex(event);
   if (state.openBlocks.has(index)) protocolFailure(state, "Content block was opened twice");
 
-  const kind = blockKind(requireRecord(event.content_block, "content_block"));
+  const contentBlock = requireRecord(event.content_block, "content_block");
+  const kind = blockKind(contentBlock);
   state.openBlocks.set(index, kind);
+
+  if (kind === "tool_use") {
+    const id = typeof contentBlock.id === "string" ? contentBlock.id : "";
+    const name = typeof contentBlock.name === "string" ? contentBlock.name : "";
+    let initialInput = "";
+    if (contentBlock.input !== undefined) {
+      try {
+        initialInput = JSON.stringify(contentBlock.input);
+      } catch {
+        initialInput = "";
+      }
+    }
+    state.toolBlocks.set(index, { id, name, inputParts: [], initialInput });
+    return null;
+  }
+
   return kind === "thinking" ? { kind: "reasoning", text: "" } : null;
 }
 
 function handleContentBlockDelta(
   event: Record<string, unknown>,
   state: ProtocolState
-): SemanticEvent {
+): SemanticEvent | null {
   assertInMessage(state, "content_block_delta");
-  const block = state.openBlocks.get(requireBlockIndex(event));
+  const index = requireBlockIndex(event);
+  const block = state.openBlocks.get(index);
   if (!block) protocolFailure(state, "Content delta has no open block");
 
   const delta = requireRecord(event.delta, "delta");
@@ -275,14 +303,42 @@ function handleContentBlockDelta(
   if (delta.type === "thinking_summary_delta" && block === "thinking") {
     return { kind: "reasoning", text: thinkingSummaryText(delta) };
   }
+  if (delta.type === "input_json_delta" && block === "tool_use") {
+    const toolBlock = state.toolBlocks.get(index);
+    if (!toolBlock) protocolFailure(state, "input_json_delta has no tool block state");
+    if (typeof delta.partial_json === "string") {
+      toolBlock.inputParts.push(delta.partial_json);
+    }
+    return null;
+  }
   return protocolFailure(state, "Content delta type does not match its block");
 }
 
-function handleContentBlockStop(event: Record<string, unknown>, state: ProtocolState): null {
+function handleContentBlockStop(
+  event: Record<string, unknown>,
+  state: ProtocolState
+): SemanticEvent | null {
   assertInMessage(state, "content_block_stop");
-  if (!state.openBlocks.delete(requireBlockIndex(event))) {
-    protocolFailure(state, "Content block stop has no open block");
+  const index = requireBlockIndex(event);
+  const kind = state.openBlocks.get(index);
+  if (!kind) protocolFailure(state, "Content block stop has no open block");
+  state.openBlocks.delete(index);
+
+  if (kind === "tool_use") {
+    const toolBlock = state.toolBlocks.get(index);
+    state.toolBlocks.delete(index);
+    if (!toolBlock) protocolFailure(state, "Tool block stop has no tool state");
+
+    let inputStr = "";
+    if (toolBlock.inputParts.length > 0) {
+      inputStr = toolBlock.inputParts.join("");
+    } else if (toolBlock.initialInput) {
+      inputStr = toolBlock.initialInput;
+    }
+
+    return { kind: "tool_call", index, id: toolBlock.id, name: toolBlock.name, input: inputStr };
   }
+
   return null;
 }
 
@@ -336,6 +392,7 @@ async function* parseClaudeWebEvents(
   const state: ProtocolState = {
     phase: "awaiting_message",
     openBlocks: new Map(),
+    toolBlocks: new Map(),
     stopReason: "end_turn",
   };
 
@@ -447,6 +504,7 @@ async function createBufferedResponse(
   let assistantText = "";
   let reasoningText = "";
   let stopReason = "end_turn";
+  const toolCalls: Array<{ id: string; name: string; input: string }> = [];
   const metadataEvents: Array<{ type: string; data: Record<string, unknown> }> = [];
   const control: StreamControl = { reader: null, cancelled: false };
 
@@ -454,12 +512,30 @@ async function createBufferedResponse(
     for await (const event of parseClaudeWebEvents(source, control)) {
       if (event.kind === "content") assistantText += event.text;
       if (event.kind === "reasoning") reasoningText += event.text;
+      if (event.kind === "tool_call") {
+        toolCalls.push({ id: event.id, name: event.name, input: event.input });
+      }
       if (event.kind === "metadata") {
         metadataEvents.push({ type: event.eventType, data: event.data });
       }
       if (event.kind === "finish") stopReason = event.stopReason;
     }
     notifyComplete(options, { assistantText, stopReason });
+
+    const message: Record<string, unknown> = {
+      role: "assistant",
+      content: assistantText || null,
+      ...(reasoningText ? { reasoning_content: reasoningText } : {}),
+    };
+
+    if (toolCalls.length > 0) {
+      message.tool_calls = toolCalls.map((tc) => ({
+        id: tc.id,
+        type: "function",
+        function: { name: tc.name, arguments: tc.input },
+      }));
+    }
+
     return new Response(
       JSON.stringify({
         id,
@@ -469,11 +545,7 @@ async function createBufferedResponse(
         choices: [
           {
             index: 0,
-            message: {
-              role: "assistant",
-              content: assistantText,
-              ...(reasoningText ? { reasoning_content: reasoningText } : {}),
-            },
+            message,
             finish_reason: openAiFinishReason(stopReason),
             logprobs: null,
           },
@@ -569,6 +641,31 @@ async function queueSemanticEvent(
     );
     return;
   }
+  if (event.kind === "tool_call") {
+    state.pendingChunks.push(
+      encodeStreamEvent(
+        state,
+        makeChunk(
+          state.id,
+          state.created,
+          options,
+          {
+            tool_calls: [
+              {
+                index: event.index,
+                id: event.id,
+                type: "function",
+                function: { name: event.name, arguments: event.input },
+              },
+            ],
+          },
+          null
+        )
+      )
+    );
+    return;
+  }
+
   if (event.kind === "metadata") {
     state.pendingChunks.push(
       encodeStreamEvent(
diff --git a/tests/unit/probe-9408-tool-use-protocol.test.ts b/tests/unit/probe-9408-tool-use-protocol.test.ts
new file mode 100644
index 0000000000..835b79b17e
--- /dev/null
+++ b/tests/unit/probe-9408-tool-use-protocol.test.ts
@@ -0,0 +1,253 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+
+import { createClaudeWebResponse } from "../../open-sse/executors/claude-web/stream.ts";
+
+function byteStream(text: string): ReadableStream<Uint8Array> {
+  const bytes = new TextEncoder().encode(text);
+  return new ReadableStream<Uint8Array>({
+    start(controller) {
+      controller.enqueue(bytes);
+      controller.close();
+    },
+  });
+}
+
+function frames(events: Array<Record<string, unknown>>, newline = "\n"): string {
+  return events.map((event) => `data: ${JSON.stringify(event)}${newline}${newline}`).join("");
+}
+
+/**
+ * Reproduce #9408: Claude Web emits tool_use content blocks and the stream
+ * parser has no handler for them, causing input_json_delta to be rejected as
+ * a protocol violation → HTTP 502.
+ *
+ * Upstream event sequence:
+ *   message_start
+ *   → content_block_start(type:"tool_use", id:"toolu_xxx", name:"get_weather")
+ *   → ×3 content_block_delta(type:"input_json_delta", partial_json:"...")
+ *   → content_block_stop
+ *   → message_delta(stop_reason:"tool_use")
+ *   → message_stop
+ */
+describe("Claude Web tool_use protocol (#9408)", () => {
+  it("converts tool_use blocks to tool_calls in buffered mode", async () => {
+    const events = [
+      { type: "message_start", message: { model: "claude-sonnet-5" } },
+      {
+        type: "content_block_start",
+        index: 0,
+        content_block: {
+          type: "tool_use",
+          id: "toolu_9408_001",
+          name: "get_weather",
+          input: {},
+        },
+      },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "input_json_delta", partial_json: '{"loca' },
+      },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "input_json_delta", partial_json: 'tion": "Sa' },
+      },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "input_json_delta", partial_json: 'n Francisco"}' },
+      },
+      { type: "content_block_stop", index: 0 },
+      { type: "message_delta", delta: { stop_reason: "tool_use" } },
+      { type: "message_stop" },
+    ];
+
+    const completions: Array<{ assistantText: string; stopReason: string }> = [];
+    let failures = 0;
+
+    const response = await createClaudeWebResponse(byteStream(frames(events)), {
+      model: "claude-sonnet-5",
+      stream: false,
+      responseMetadata: {},
+      onComplete: (result) => completions.push(result),
+      onFailure: () => {
+        failures += 1;
+      },
+    });
+
+    // Should NOT be 502 — the bug was that tool_use blocks caused protocol failure
+    assert.equal(response.status, 200, "Expected 200, not 502 — tool_use should not crash");
+    const body = (await response.json()) as {
+      choices: Array<{
+        message: {
+          content: string | null;
+          tool_calls?: Array<{
+            id: string;
+            type: string;
+            function: { name: string; arguments: string };
+          }>;
+        };
+        finish_reason: string;
+      }>;
+    };
+
+    assert.equal(body.choices[0].finish_reason, "tool_calls");
+    assert.ok(body.choices[0].message.tool_calls, "Expected tool_calls in message");
+    assert.equal(body.choices[0].message.tool_calls!.length, 1);
+    assert.equal(body.choices[0].message.tool_calls![0].id, "toolu_9408_001");
+    assert.equal(body.choices[0].message.tool_calls![0].type, "function");
+    assert.equal(body.choices[0].message.tool_calls![0].function.name, "get_weather");
+    // Content should be null when there's only a tool call
+    assert.equal(body.choices[0].message.content, null);
+    // Preserve upstream tool call ID — the input should parse correctly
+    const parsed = JSON.parse(body.choices[0].message.tool_calls![0].function.arguments);
+    assert.deepEqual(parsed, { location: "San Francisco" });
+    assert.deepEqual(completions, [{ assistantText: "", stopReason: "tool_use" }]);
+    assert.equal(failures, 0);
+  });
+
+  it("converts tool_use blocks to tool_calls in streaming mode", async () => {
+    const events = [
+      { type: "message_start", message: { model: "claude-sonnet-5" } },
+      {
+        type: "content_block_start",
+        index: 0,
+        content_block: {
+          type: "tool_use",
+          id: "toolu_9408_002",
+          name: "search_code",
+          input: {},
+        },
+      },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "input_json_delta", partial_json: '{"query":"initial"' },
+      },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "input_json_delta", partial_json: ',"limit":10}' },
+      },
+      { type: "content_block_stop", index: 0 },
+      { type: "message_delta", delta: { stop_reason: "tool_use" } },
+      { type: "message_stop" },
+    ];
+
+    const completions: Array<{ assistantText: string; stopReason: string }> = [];
+    let failures = 0;
+
+    const response = await createClaudeWebResponse(byteStream(frames(events)), {
+      model: "claude-sonnet-5",
+      stream: true,
+      responseMetadata: {},
+      onComplete: (result) => completions.push(result),
+      onFailure: () => {
+        failures += 1;
+      },
+    });
+
+    assert.equal(response.status, 200, "Expected 200, not 502");
+    const output = await response.text();
+    // Verify it contains tool_calls in some chunk
+    assert.match(output, /tool_calls/);
+    // Verify finish_reason: tool_calls
+    assert.match(output, /"finish_reason":"tool_calls"/);
+    // Verify tool call id preserved
+    assert.match(output, /"id":"toolu_9408_002"/);
+    // Verify tool call name
+    assert.match(output, /"name":"search_code"/);
+    // Verify arguments contain the accumulated input
+    assert.match(output, /"arguments":".*query.*initial.*limit.*10/);
+    assert.deepEqual(completions, [{ assistantText: "", stopReason: "tool_use" }]);
+    assert.equal(failures, 0);
+  });
+
+  it("handles tool_use alongside text content", async () => {
+    const events = [
+      { type: "message_start", message: { model: "claude-sonnet-5" } },
+      { type: "content_block_start", index: 0, content_block: { type: "text" } },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "text_delta", text: "I'll look that up." },
+      },
+      { type: "content_block_stop", index: 0 },
+      {
+        type: "content_block_start",
+        index: 1,
+        content_block: {
+          type: "tool_use",
+          id: "toolu_9408_003",
+          name: "get_info",
+          input: { topic: "weather" },
+        },
+      },
+      { type: "content_block_stop", index: 1 },
+      { type: "message_delta", delta: { stop_reason: "tool_use" } },
+      { type: "message_stop" },
+    ];
+
+    const response = await createClaudeWebResponse(byteStream(frames(events)), {
+      model: "claude-sonnet-5",
+      stream: false,
+      responseMetadata: {},
+      onComplete() {},
+      onFailure() {},
+    });
+
+    assert.equal(response.status, 200);
+    const body = (await response.json()) as {
+      choices: Array<{
+        message: {
+          content: string | null;
+          tool_calls?: Array<{
+            id: string;
+            type: string;
+            function: { name: string; arguments: string };
+          }>;
+        };
+        finish_reason: string;
+      }>;
+    };
+
+    // Should have text content AND tool calls
+    assert.equal(body.choices[0].message.content, "I'll look that up.");
+    assert.equal(body.choices[0].message.tool_calls!.length, 1);
+    assert.equal(body.choices[0].message.tool_calls![0].id, "toolu_9408_003");
+  });
+
+  it("rejects input_json_delta when no tool_use block is open", async () => {
+    const events = [
+      { type: "message_start" },
+      { type: "content_block_start", index: 0, content_block: { type: "text" } },
+      {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "input_json_delta", partial_json: "{}" },
+      },
+      { type: "content_block_stop", index: 0 },
+      { type: "message_delta", delta: { stop_reason: "end_turn" } },
+      { type: "message_stop" },
+    ];
+
+    const completions: Array<unknown> = [];
+    let failures = 0;
+
+    const response = await createClaudeWebResponse(byteStream(frames(events)), {
+      model: "claude-sonnet-5",
+      stream: false,
+      responseMetadata: {},
+      onComplete: (result) => completions.push(result),
+      onFailure: () => {
+        failures += 1;
+      },
+    });
+
+    assert.equal(response.status, 502, "input_json_delta without open tool_use should fail");
+    assert.deepEqual(completions, []);
+    assert.equal(failures, 1);
+  });
+});