diff --git a/packages/claude-sdk/src/private/AgentRun.ts b/packages/claude-sdk/src/private/AgentRun.ts
index d1266a5..16a32b6 100644
--- a/packages/claude-sdk/src/private/AgentRun.ts
+++ b/packages/claude-sdk/src/private/AgentRun.ts
@@ -1,16 +1,14 @@
 import { randomUUID } from 'node:crypto';
 import type { MessagePort } from 'node:worker_threads';
 import type { Anthropic } from '@anthropic-ai/sdk';
-import type { BetaMessageStreamParams } from '@anthropic-ai/sdk/resources/beta/messages.js';
-import type { BetaCacheControlEphemeral, BetaClearThinking20251015Edit, BetaClearToolUses20250919Edit, BetaCompact20260112Edit, BetaCompactionBlockParam, BetaContextManagementConfig, BetaTextBlockParam, BetaThinkingBlockParam, BetaToolUnion, BetaToolUseBlockParam } from '@anthropic-ai/sdk/resources/beta.mjs';
-import { AnthropicBeta } from '../public/enums';
+import type { BetaCompactionBlockParam, BetaTextBlockParam, BetaThinkingBlockParam, BetaToolUseBlockParam } from '@anthropic-ai/sdk/resources/beta.mjs';
 import type { AnyToolDefinition, ILogger, RunAgentQuery, SdkMessage } from '../public/types';
 import { AgentChannel } from './AgentChannel';
 import { ApprovalState } from './ApprovalState';
 import type { ConversationStore } from './ConversationStore';
-import { AGENT_SDK_PREFIX } from './consts';
 import { MessageStream } from './MessageStream';
 import { calculateCost, getContextWindow } from './pricing';
+import { buildRequestParams } from './RequestBuilder';
 import type { ContentBlock, MessageStreamResult, ToolUseResult } from './types';
 
 export class AgentRun {
@@ -140,71 +138,12 @@ export class AgentRun {
   }
 
   #getMessageStream(messages: Anthropic.Beta.Messages.BetaMessageParam[]) {
-    const tools: BetaToolUnion[] = this.#options.tools.map(
-      (t) =>
-        ({
-          name: t.name,
-          description: t.description,
-          input_schema: t.input_schema.toJSONSchema({ target: 'draft-07', io: 'input' }) as Anthropic.Tool['input_schema'],
-          input_examples: t.input_examples,
-        }) satisfies BetaToolUnion,
-    );
-
-    const betas = resolveCapabilities(this.#options.betas, AnthropicBeta);
-
-    const context_management: BetaContextManagementConfig = {
-      edits: [],
-    };
-    if (betas[AnthropicBeta.ContextManagement]) {
-      context_management.edits?.push({ type: 'clear_thinking_20251015' } satisfies BetaClearThinking20251015Edit);
-      context_management.edits?.push({ type: 'clear_tool_uses_20250919' } satisfies BetaClearToolUses20250919Edit);
-    }
-    if (betas[AnthropicBeta.Compact]) {
-      context_management.edits?.push({
-        type: 'compact_20260112',
-        pause_after_compaction: this.#options.pauseAfterCompact ?? false,
-        trigger: this.#options.compactInputTokens
-          ? {
-              type: 'input_tokens',
-              value: this.#options.compactInputTokens,
-            }
-          : null,
-      } satisfies BetaCompact20260112Edit);
-    }
-
-    const systemPrompts = [AGENT_SDK_PREFIX, ...(this.#options.systemPrompts ?? [])];
-
-    const body: BetaMessageStreamParams = {
-      model: this.#options.model,
-      max_tokens: this.#options.maxTokens,
-      tools,
-      context_management,
-      system: systemPrompts.map((text) => ({ type: 'text', text })),
-
-      messages,
-      // thinking: { type: 'adaptive' },
-      stream: true,
-    } satisfies BetaMessageStreamParams;
-
-    if (betas[AnthropicBeta.PromptCachingScope]) {
-      body.cache_control = { type: 'ephemeral', scope: 'global' } as BetaCacheControlEphemeral;
-    }
-    if (this.#options.thinking === true) {
-      body.thinking = { type: 'adaptive' };
-    }
-
-    const anthropicBetas = Object.entries(betas)
-      .filter(([, enabled]) => enabled)
-      .map(([beta]) => beta)
-      .join(',');
-
+    const { body, headers } = buildRequestParams(this.#options, messages);
     const requestOptions = {
-      headers: { 'anthropic-beta': anthropicBetas },
+      headers,
       signal: this.#abortController.signal,
     } satisfies Anthropic.RequestOptions;
-
     this.#logger?.info('Sending request', body);
-
     return this.#client.beta.messages.stream(body, requestOptions);
   }
 
@@ -296,11 +235,3 @@ export class AgentRun {
     }
   }
 }
-
-function resolveCapabilities<T extends string>(partial: Partial<Record<T, boolean>> | undefined, enumObj: Record<string, T>): Record<T, boolean> {
-  const result = {} as Record<T, boolean>;
-  for (const key of Object.values(enumObj)) {
-    result[key] = partial?.[key] ?? false;
-  }
-  return result;
-}
diff --git a/packages/claude-sdk/src/private/RequestBuilder.ts b/packages/claude-sdk/src/private/RequestBuilder.ts
new file mode 100644
index 0000000..219a604
--- /dev/null
+++ b/packages/claude-sdk/src/private/RequestBuilder.ts
@@ -0,0 +1,89 @@
+import type { Anthropic } from '@anthropic-ai/sdk';
+import type { BetaMessageStreamParams } from '@anthropic-ai/sdk/resources/beta/messages.js';
+import type { BetaCacheControlEphemeral, BetaClearThinking20251015Edit, BetaClearToolUses20250919Edit, BetaCompact20260112Edit, BetaContextManagementConfig, BetaToolUnion } from '@anthropic-ai/sdk/resources/beta.mjs';
+import { AnthropicBeta } from '../public/enums';
+import type { RunAgentQuery } from '../public/types';
+import { AGENT_SDK_PREFIX } from './consts';
+
+export type RequestParams = {
+  body: BetaMessageStreamParams;
+  headers: { 'anthropic-beta': string };
+};
+
+/**
+ * Pure function — builds the Anthropic API request params from agent options
+ * and the current message list. No I/O, no client reference, no signal.
+ *
+ * AgentRun calls this and adds the AbortSignal before passing to the client,
+ * since the signal is tied to AgentRun's abort lifecycle.
+ */
+export function buildRequestParams(options: RunAgentQuery, messages: Anthropic.Beta.Messages.BetaMessageParam[]): RequestParams {
+  const tools: BetaToolUnion[] = options.tools.map(
+    (t) =>
+      ({
+        name: t.name,
+        description: t.description,
+        input_schema: t.input_schema.toJSONSchema({ target: 'draft-07', io: 'input' }) as Anthropic.Tool['input_schema'],
+        input_examples: t.input_examples,
+      }) satisfies BetaToolUnion,
+  );
+
+  const betas = resolveCapabilities(options.betas, AnthropicBeta);
+
+  const context_management: BetaContextManagementConfig = {
+    edits: [],
+  };
+  if (betas[AnthropicBeta.ContextManagement]) {
+    context_management.edits?.push({ type: 'clear_thinking_20251015' } satisfies BetaClearThinking20251015Edit);
+    context_management.edits?.push({ type: 'clear_tool_uses_20250919' } satisfies BetaClearToolUses20250919Edit);
+  }
+  if (betas[AnthropicBeta.Compact]) {
+    context_management.edits?.push({
+      type: 'compact_20260112',
+      pause_after_compaction: options.pauseAfterCompact ?? false,
+      trigger: options.compactInputTokens
+        ? {
+            type: 'input_tokens',
+            value: options.compactInputTokens,
+          }
+        : null,
+    } satisfies BetaCompact20260112Edit);
+  }
+
+  const systemPrompts = [AGENT_SDK_PREFIX, ...(options.systemPrompts ?? [])];
+
+  const body: BetaMessageStreamParams = {
+    model: options.model,
+    max_tokens: options.maxTokens,
+    tools,
+    context_management,
+    system: systemPrompts.map((text) => ({ type: 'text', text })),
+    messages,
+    stream: true,
+  } satisfies BetaMessageStreamParams;
+
+  if (betas[AnthropicBeta.PromptCachingScope]) {
+    body.cache_control = { type: 'ephemeral', scope: 'global' } as BetaCacheControlEphemeral;
+  }
+  if (options.thinking === true) {
+    body.thinking = { type: 'adaptive' };
+  }
+
+  const anthropicBetas = Object.entries(betas)
+    .filter(([, enabled]) => enabled)
+    .map(([beta]) => beta)
+    .join(',');
+
+  return {
+    body,
+    headers: { 'anthropic-beta': anthropicBetas },
+  };
+}
+
+function resolveCapabilities<T extends string>(partial: Partial<Record<T, boolean>> | undefined, enumObj: Record<string, T>): Record<T, boolean> {
+  const result = {} as Record<T, boolean>;
+  for (const key of Object.values(enumObj)) {
+    result[key] = partial?.[key] ?? false;
+  }
+  return result;
+}
diff --git a/packages/claude-sdk/test/RequestBuilder.spec.ts b/packages/claude-sdk/test/RequestBuilder.spec.ts
new file mode 100644
index 0000000..61ce20b
--- /dev/null
+++ b/packages/claude-sdk/test/RequestBuilder.spec.ts
@@ -0,0 +1,282 @@
+import type { Anthropic } from '@anthropic-ai/sdk';
+import { describe, expect, it } from 'vitest';
+import { AGENT_SDK_PREFIX } from '../src/private/consts.js';
+import { buildRequestParams } from '../src/private/RequestBuilder.js';
+import { AnthropicBeta } from '../src/public/enums.js';
+import type { AnyToolDefinition, RunAgentQuery } from '../src/public/types.js';
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+/** Minimal schema stub — only needs toJSONSchema for buildRequestParams. */
+function mockSchema(jsonSchema: Record<string, unknown> = {}): AnyToolDefinition['input_schema'] {
+  return {
+    toJSONSchema: () => jsonSchema,
+  } as unknown as AnyToolDefinition['input_schema'];
+}
+
+function makeTool(name: string, jsonSchema: Record<string, unknown> = {}): AnyToolDefinition {
+  return {
+    name,
+    description: `${name} description`,
+    input_schema: mockSchema(jsonSchema),
+    input_examples: [],
+    handler: async () => {},
+  };
+}
+
+function makeOptions(overrides: Partial<RunAgentQuery> = {}): RunAgentQuery {
+  return {
+    model: 'claude-opus-4-5' as RunAgentQuery['model'],
+    maxTokens: 1024,
+    messages: [],
+    tools: [],
+    ...overrides,
+  };
+}
+
+const noMessages: Anthropic.Beta.Messages.BetaMessageParam[] = [];
+
+// ---------------------------------------------------------------------------
+// Base output shape
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — base', () => {
+  it('body.model matches options.model', () => {
+    const expected = 'claude-opus-4-5';
+    const actual = buildRequestParams(makeOptions({ model: expected as RunAgentQuery['model'] }), noMessages).body.model;
+    expect(actual).toBe(expected);
+  });
+
+  it('body.max_tokens matches options.maxTokens', () => {
+    const expected = 8192;
+    const actual = buildRequestParams(makeOptions({ maxTokens: expected }), noMessages).body.max_tokens;
+    expect(actual).toBe(expected);
+  });
+
+  it('body.stream is always true', () => {
+    const expected = true;
+    const actual = buildRequestParams(makeOptions(), noMessages).body.stream;
+    expect(actual).toBe(expected);
+  });
+
+  it('context_management edits are empty when no betas enabled', () => {
+    const expected = 0;
+    const actual = buildRequestParams(makeOptions(), noMessages).body.context_management?.edits?.length;
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// System prompts
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — system prompts', () => {
+  it('first system prompt is always AGENT_SDK_PREFIX', () => {
+    const expected = AGENT_SDK_PREFIX;
+    const system = buildRequestParams(makeOptions(), noMessages).body.system as { type: string; text: string }[];
+    const actual = system[0]?.text;
+    expect(actual).toBe(expected);
+  });
+
+  it('custom system prompts are appended after the prefix', () => {
+    const expected = ['prefix', 'second', 'third'];
+    const system = buildRequestParams(makeOptions({ systemPrompts: ['second', 'third'] }), noMessages).body.system as { type: string; text: string }[];
+    const actual = system.map((s) => (s.text === AGENT_SDK_PREFIX ? 'prefix' : s.text));
+    expect(actual).toEqual(expected);
+  });
+
+  it('system prompt count is 1 without custom prompts', () => {
+    const expected = 1;
+    const system = buildRequestParams(makeOptions(), noMessages).body.system as unknown[];
+    const actual = system.length;
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// ContextManagement beta
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — ContextManagement beta', () => {
+  it('adds clear_thinking edit when ContextManagement is enabled', () => {
+    const expected = 'clear_thinking_20251015';
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.ContextManagement]: true } }), noMessages);
+    const actual = body.context_management?.edits?.find((e) => e.type === 'clear_thinking_20251015')?.type;
+    expect(actual).toBe(expected);
+  });
+
+  it('adds clear_tool_uses edit when ContextManagement is enabled', () => {
+    const expected = 'clear_tool_uses_20250919';
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.ContextManagement]: true } }), noMessages);
+    const actual = body.context_management?.edits?.find((e) => e.type === 'clear_tool_uses_20250919')?.type;
+    expect(actual).toBe(expected);
+  });
+
+  it('does not add clear_thinking edit when ContextManagement is disabled', () => {
+    const expected = undefined;
+    const { body } = buildRequestParams(makeOptions(), noMessages);
+    const actual = body.context_management?.edits?.find((e) => e.type === 'clear_thinking_20251015');
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Compact beta
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — Compact beta', () => {
+  it('adds compact_20260112 edit when Compact is enabled', () => {
+    const expected = 'compact_20260112';
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true } }), noMessages);
+    const actual = body.context_management?.edits?.find((e) => e.type === 'compact_20260112')?.type;
+    expect(actual).toBe(expected);
+  });
+
+  it('compact edit pause_after_compaction defaults to false', () => {
+    const expected = false;
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true } }), noMessages);
+    const compactEdit = body.context_management?.edits?.find((e) => e.type === 'compact_20260112');
+    const actual = (compactEdit as { pause_after_compaction?: boolean })?.pause_after_compaction;
+    expect(actual).toBe(expected);
+  });
+
+  it('compact edit pause_after_compaction is true when pauseAfterCompact is set', () => {
+    const expected = true;
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true }, pauseAfterCompact: true }), noMessages);
+    const compactEdit = body.context_management?.edits?.find((e) => e.type === 'compact_20260112');
+    const actual = (compactEdit as { pause_after_compaction?: boolean })?.pause_after_compaction;
+    expect(actual).toBe(expected);
+  });
+
+  it('compact edit trigger is null when compactInputTokens is not set', () => {
+    const expected = null;
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true } }), noMessages);
+    const compactEdit = body.context_management?.edits?.find((e) => e.type === 'compact_20260112');
+    const actual = (compactEdit as { trigger?: unknown })?.trigger;
+    expect(actual).toBe(expected);
+  });
+
+  it('compact edit trigger.value matches compactInputTokens', () => {
+    const expected = 50000;
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true }, compactInputTokens: 50000 }), noMessages);
+    const compactEdit = body.context_management?.edits?.find((e) => e.type === 'compact_20260112');
+    const actual = (compactEdit as { trigger?: { type: string; value: number } | null })?.trigger?.value;
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// PromptCachingScope beta
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — PromptCachingScope beta', () => {
+  it('sets cache_control when PromptCachingScope is enabled', () => {
+    const expected = 'ephemeral';
+    const { body } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.PromptCachingScope]: true } }), noMessages);
+    const actual = (body.cache_control as { type?: string } | undefined)?.type;
+    expect(actual).toBe(expected);
+  });
+
+  it('cache_control is absent when PromptCachingScope is not enabled', () => {
+    const expected = undefined;
+    const { body } = buildRequestParams(makeOptions(), noMessages);
+    const actual = body.cache_control;
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Thinking
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — thinking', () => {
+  it('body.thinking is set to adaptive when thinking is true', () => {
+    const expected = 'adaptive';
+    const { body } = buildRequestParams(makeOptions({ thinking: true }), noMessages);
+    const actual = (body.thinking as { type?: string } | undefined)?.type;
+    expect(actual).toBe(expected);
+  });
+
+  it('body.thinking is absent when thinking is not set', () => {
+    const expected = undefined;
+    const { body } = buildRequestParams(makeOptions(), noMessages);
+    const actual = body.thinking;
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Headers
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — headers', () => {
+  it('anthropic-beta header is empty string when no betas are enabled', () => {
+    const expected = '';
+    const actual = buildRequestParams(makeOptions(), noMessages).headers['anthropic-beta'];
+    expect(actual).toBe(expected);
+  });
+
+  it('anthropic-beta header contains the enabled beta', () => {
+    const expected = AnthropicBeta.Compact;
+    const actual = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true } }), noMessages).headers['anthropic-beta'];
+    expect(actual).toBe(expected);
+  });
+
+  it('anthropic-beta header contains all enabled betas comma-joined', () => {
+    const { headers } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true, [AnthropicBeta.ContextManagement]: true } }), noMessages);
+    const betas = headers['anthropic-beta'].split(',');
+    const expected = true;
+    const actual = betas.includes(AnthropicBeta.Compact) && betas.includes(AnthropicBeta.ContextManagement);
+    expect(actual).toBe(expected);
+  });
+
+  it('disabled betas are excluded from the header', () => {
+    const { headers } = buildRequestParams(makeOptions({ betas: { [AnthropicBeta.Compact]: true, [AnthropicBeta.ContextManagement]: false } }), noMessages);
+    const expected = false;
+    const actual = headers['anthropic-beta'].split(',').includes(AnthropicBeta.ContextManagement);
+    expect(actual).toBe(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Tools
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — tools', () => {
+  it('tool name is preserved in the mapped output', () => {
+    const expected = 'my_tool';
+    const { body } = buildRequestParams(makeOptions({ tools: [makeTool('my_tool')] }), noMessages);
+    const actual = (body.tools as { name: string }[])[0]?.name;
+    expect(actual).toBe(expected);
+  });
+
+  it('tool description is preserved in the mapped output', () => {
+    const expected = 'my_tool description';
+    const { body } = buildRequestParams(makeOptions({ tools: [makeTool('my_tool')] }), noMessages);
+    const actual = (body.tools as { description: string }[])[0]?.description;
+    expect(actual).toBe(expected);
+  });
+
+  it('tool input_schema comes from toJSONSchema', () => {
+    const expected = { type: 'object', properties: { x: { type: 'number' } } };
+    const tool = makeTool('t', expected);
+    const { body } = buildRequestParams(makeOptions({ tools: [tool] }), noMessages);
+    const actual = (body.tools as { input_schema: unknown }[])[0]?.input_schema;
+    expect(actual).toEqual(expected);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Messages
+// ---------------------------------------------------------------------------
+
+describe('buildRequestParams — messages', () => {
+  it('messages array is passed through to body.messages', () => {
+    const messages: Anthropic.Beta.Messages.BetaMessageParam[] = [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }];
+    const expected = messages;
+    const actual = buildRequestParams(makeOptions(), messages).body.messages;
+    expect(actual).toBe(expected);
+  });
+});