diff --git a/apps/server/src/services/inference/dcp/__tests__/deduplication.test.ts b/apps/server/src/services/inference/dcp/__tests__/deduplication.test.ts
new file mode 100644
index 0000000..1b7eb89
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/__tests__/deduplication.test.ts
@@ -0,0 +1,33 @@
+import { describe, it, expect } from 'vitest';
+import { deduplicate } from '../strategies/deduplication.js';
+import type { DcpMessage } from '../messages.js';
+
+describe('deduplicate', () => {
+  it('removes consecutive identical tool_call+tool_result pairs', () => {
+    const messages: DcpMessage[] = [
+      { role: 'user', content: 'search for x' },
+      { role: 'assistant', content: '', tool_calls: [{ id: '1', name: 'grep', arguments: '{}' }] },
+      { role: 'tool', content: 'result1', tool_call_id: '1' },
+      // Duplicate pair
+      { role: 'assistant', content: '', tool_calls: [{ id: '2', name: 'grep', arguments: '{}' }] },
+      { role: 'tool', content: 'result1', tool_call_id: '2' },
+    ];
+
+    const { messages: result, stats } = deduplicate(messages);
+    expect(result).toHaveLength(3); // user + first pair
+    expect(stats.removedCount).toBe(2);
+  });
+
+  it('preserves non-duplicate content', () => {
+    const messages: DcpMessage[] = [
+      { role: 'assistant', content: '', tool_calls: [{ id: '1', name: 'grep', arguments: '{}' }] },
+      { role: 'tool', content: 'result1', tool_call_id: '1' },
+      { role: 'assistant', content: '', tool_calls: [{ id: '2', name: 'grep', arguments: '{}' }] },
+      { role: 'tool', content: 'result2', tool_call_id: '2' }, // Different result
+    ];
+
+    const { messages: result, stats } = deduplicate(messages);
+    expect(result).toHaveLength(4);
+    expect(stats.removedCount).toBe(0);
+  });
+});
diff --git a/apps/server/src/services/inference/dcp/__tests__/messages.test.ts b/apps/server/src/services/inference/dcp/__tests__/messages.test.ts
new file mode 100644
index 0000000..2228a64
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/__tests__/messages.test.ts
@@ -0,0 +1,22 @@
+import { describe, it, expect } from 'vitest';
+import { toDcpMessages, fromDcpMessages } from '../messages.js';
+
+describe('toDcpMessages', () => {
+  it('converts user messages', () => {
+    const result = toDcpMessages([{ role: 'user', content: 'hello' }]);
+    expect(result[0].role).toBe('user');
+    expect(result[0].content).toBe('hello');
+  });
+
+  it('marks Error: content as isError', () => {
+    const result = toDcpMessages([{ role: 'tool', content: 'Error: file not found', tool_call_id: '1' }]);
+    expect(result[0].isError).toBe(true);
+  });
+});
+
+describe('fromDcpMessages', () => {
+  it('round-trips messages', () => {
+    const original = [{ role: 'user', content: 'hello' }];
+    expect(fromDcpMessages(toDcpMessages(original))).toEqual(original);
+  });
+});
diff --git a/apps/server/src/services/inference/dcp/__tests__/purge-errors.test.ts b/apps/server/src/services/inference/dcp/__tests__/purge-errors.test.ts
new file mode 100644
index 0000000..153c76a
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/__tests__/purge-errors.test.ts
@@ -0,0 +1,33 @@
+import { describe, it, expect } from 'vitest';
+import { purgeErrors } from '../strategies/purge-errors.js';
+import type { DcpMessage } from '../messages.js';
+
+describe('purgeErrors', () => {
+  it('removes tool results where content starts with Error:', () => {
+    const messages: DcpMessage[] = [
+      { role: 'tool', content: 'Error: file not found', tool_call_id: '1' },
+      { role: 'tool', content: '{"files":[]}', tool_call_id: '2' },
+    ];
+    const { messages: result, stats } = purgeErrors(messages);
+    expect(result).toHaveLength(1);
+    expect(stats.removedCount).toBe(1);
+  });
+
+  it('removes empty tool results', () => {
+    const messages: DcpMessage[] = [
+      { role: 'tool', content: '', tool_call_id: '1' },
+    ];
+    const { messages: result, stats } = purgeErrors(messages);
+    expect(result).toHaveLength(0);
+    expect(stats.removedCount).toBe(1);
+  });
+
+  it('preserves valid tool results', () => {
+    const messages: DcpMessage[] = [
+      { role: 'tool', content: '{"files":["a.ts"]}', tool_call_id: '1' },
+    ];
+    const { messages: result, stats } = purgeErrors(messages);
+    expect(result).toHaveLength(1);
+    expect(stats.removedCount).toBe(0);
+  });
+});
diff --git a/apps/server/src/services/inference/dcp/__tests__/transform.test.ts b/apps/server/src/services/inference/dcp/__tests__/transform.test.ts
new file mode 100644
index 0000000..1d6b0e6
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/__tests__/transform.test.ts
@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest';
+import { transformMessages } from '../transform.js';
+import type { DcpMessage } from '../messages.js';
+
+describe('transformMessages', () => {
+  it('applies dedup then purge in order', () => {
+    const input: DcpMessage[] = [
+      { role: 'user', content: 'hello' },
+      { role: 'assistant', content: '', tool_calls: [{ id: '1', name: 'grep', arguments: '{}' }] },
+      { role: 'tool', content: 'result', tool_call_id: '1' },
+      { role: 'assistant', content: '', tool_calls: [{ id: '2', name: 'grep', arguments: '{}' }] },
+      { role: 'tool', content: 'result', tool_call_id: '2' }, // Dup
+    ];
+
+    const { messages, stats } = transformMessages('test-chat', input);
+    expect(stats.removedCount).toBeGreaterThan(0);
+    expect(messages.length).toBeLessThan(input.length);
+  });
+
+  it('handles empty input', () => {
+    const { messages, stats } = transformMessages('empty', []);
+    expect(messages).toHaveLength(0);
+    expect(stats.removedCount).toBe(0);
+  });
+});
diff --git a/apps/server/src/services/inference/dcp/index.ts b/apps/server/src/services/inference/dcp/index.ts
new file mode 100644
index 0000000..7bc55a5
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/index.ts
@@ -0,0 +1,4 @@
+export { transformMessages } from './transform.js';
+export type { DcpMessage } from './messages.js';
+export { toDcpMessages, fromDcpMessages } from './messages.js';
+export { getDcpState, clearDcpState } from './state.js';
diff --git a/apps/server/src/services/inference/dcp/messages.ts b/apps/server/src/services/inference/dcp/messages.ts
new file mode 100644
index 0000000..b75389b
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/messages.ts
@@ -0,0 +1,34 @@
+// DCP message shape adapter.
+// Converts between BooCode MessagePart[] and the DCP internal shape.
+// Clean-room implementation — no AGPL source copied.
+
+export interface DcpMessage {
+  role: 'user' | 'assistant' | 'tool';
+  content: string;
+  tool_call_id?: string;
+  tool_calls?: Array<{ id: string; name: string; arguments: string }>;
+  isError?: boolean;
+}
+
+export function toDcpMessages(parts: any[]): DcpMessage[] {
+  return parts.map((p: any) => {
+    const msg: DcpMessage = { role: p.role, content: p.content ?? '' };
+    if (p.tool_call_id) msg.tool_call_id = p.tool_call_id;
+    if (p.tool_calls) msg.tool_calls = p.tool_calls;
+    if (p.isError) msg.isError = true;
+    if (p.role === 'tool' && p.content && p.content.startsWith('Error:')) {
+      msg.isError = true;
+    }
+    return msg;
+  });
+}
+
+export function fromDcpMessages(msgs: DcpMessage[]): any[] {
+  return msgs.map((m) => ({
+    role: m.role,
+    content: m.content,
+    ...(m.tool_call_id ? { tool_call_id: m.tool_call_id } : {}),
+    ...(m.tool_calls ? { tool_calls: m.tool_calls } : {}),
+    ...(m.isError ? { isError: true } : {}),
+  }));
+}
diff --git a/apps/server/src/services/inference/dcp/state.ts b/apps/server/src/services/inference/dcp/state.ts
new file mode 100644
index 0000000..487b831
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/state.ts
@@ -0,0 +1,27 @@
+// Per-chat session state for DCP.
+// Tracks last transform timestamp and message count to avoid re-processing.
+
+interface ChatDcpState {
+  lastTransformAt: number;
+  lastMessageCount: number;
+}
+
+const chatStates = new Map<string, ChatDcpState>();
+
+export function getDcpState(chatId: string): ChatDcpState | undefined {
+  return chatStates.get(chatId);
+}
+
+export function setDcpState(chatId: string, messageCount: number): void {
+  chatStates.set(chatId, { lastTransformAt: Date.now(), lastMessageCount: messageCount });
+}
+
+export function clearDcpState(chatId: string): void {
+  chatStates.delete(chatId);
+}
+
+export function shouldTransform(chatId: string, messageCount: number): boolean {
+  const state = chatStates.get(chatId);
+  if (!state) return true;
+  return state.lastMessageCount !== messageCount;
+}
diff --git a/apps/server/src/services/inference/dcp/strategies/deduplication.ts b/apps/server/src/services/inference/dcp/strategies/deduplication.ts
new file mode 100644
index 0000000..8c928bc
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/strategies/deduplication.ts
@@ -0,0 +1,50 @@
+import type { DcpMessage } from '../messages.js';
+
+export function deduplicate(messages: DcpMessage[]): { messages: DcpMessage[]; stats: { removedCount: number; freedTokens: number } } {
+  const result: DcpMessage[] = [];
+  let removedCount = 0;
+  let freedTokens = 0;
+  let i = 0;
+
+  while (i < messages.length) {
+    const current: DcpMessage = messages[i]!;
+    const next = messages[i + 1];
+
+    if (
+      current.role === 'assistant' &&
+      current.tool_calls &&
+      next &&
+      next.role === 'tool' &&
+      next.tool_call_id === current.tool_calls[0]?.id
+    ) {
+      const nextNext = messages[i + 2];
+      const nextNextNext = messages[i + 3];
+
+      if (
+        nextNext &&
+        nextNext.role === 'assistant' &&
+        nextNext.tool_calls &&
+        nextNextNext &&
+        nextNextNext.role === 'tool' &&
+        nextNextNext.tool_call_id === nextNext.tool_calls[0]?.id &&
+        nextNext.tool_calls[0]?.name === current.tool_calls[0]?.name &&
+        nextNext.tool_calls[0]?.arguments === current.tool_calls[0]?.arguments &&
+        nextNextNext.content === next.content
+      ) {
+        result.push(current, next);
+        i += 4;
+        removedCount += 2;
+        freedTokens += Math.ceil(nextNext.content.length / 4);
+        freedTokens += Math.ceil(current.content.length / 4);
+      } else {
+        result.push(current);
+        i++;
+      }
+    } else {
+      result.push(current);
+      i++;
+    }
+  }
+
+  return { messages: result, stats: { removedCount, freedTokens } };
+}
diff --git a/apps/server/src/services/inference/dcp/strategies/purge-errors.ts b/apps/server/src/services/inference/dcp/strategies/purge-errors.ts
new file mode 100644
index 0000000..195442a
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/strategies/purge-errors.ts
@@ -0,0 +1,34 @@
+// Purge-errors strategy — removes failed/empty tool_result entries.
+// Clean-room implementation.
+
+import type { DcpMessage } from '../messages.js';
+
+const ERROR_PREFIXES = ['Error:', 'error:', 'Error: '];
+const DEFAULT_WINDOW = 5;
+
+export function purgeErrors(
+  messages: DcpMessage[],
+  windowSize: number = DEFAULT_WINDOW,
+): { messages: DcpMessage[]; stats: { removedCount: number; freedTokens: number } } {
+  const result: DcpMessage[] = [];
+  let removedCount = 0;
+  let freedTokens = 0;
+
+  for (const msg of messages) {
+    if (msg.role === 'tool') {
+      const shouldRemove =
+        msg.isError ||
+        ERROR_PREFIXES.some((p) => msg.content.startsWith(p)) ||
+        msg.content.trim() === '';
+
+      if (shouldRemove) {
+        removedCount++;
+        freedTokens += Math.ceil(msg.content.length / 4);
+        continue; // Skip this message
+      }
+    }
+    result.push(msg);
+  }
+
+  return { messages: result, stats: { removedCount, freedTokens } };
+}
diff --git a/apps/server/src/services/inference/dcp/transform.ts b/apps/server/src/services/inference/dcp/transform.ts
new file mode 100644
index 0000000..6de0950
--- /dev/null
+++ b/apps/server/src/services/inference/dcp/transform.ts
@@ -0,0 +1,52 @@
+// Transform orchestrator — runs DCP strategies in sequence.
+// Clean-room implementation.
+
+import type { DcpMessage } from './messages.js';
+import { deduplicate } from './strategies/deduplication.js';
+import { purgeErrors } from './strategies/purge-errors.js';
+import { getDcpState, setDcpState, shouldTransform } from './state.js';
+
+export interface TransformStats {
+  removedCount: number;
+  freedTokens: number;
+  dedupRemoved: number;
+  purgeRemoved: number;
+}
+
+export interface TransformResult {
+  messages: DcpMessage[];
+  stats: TransformStats;
+}
+
+export function transformMessages(chatId: string, messages: DcpMessage[]): TransformResult {
+  if (!shouldTransform(chatId, messages.length)) {
+    return { messages, stats: { removedCount: 0, freedTokens: 0, dedupRemoved: 0, purgeRemoved: 0 } };
+  }
+
+  let m = messages;
+
+  // Step 1: Deduplicate
+  const dedupResult = deduplicate(m);
+  m = dedupResult.messages;
+  const dedupRemoved = dedupResult.stats.removedCount;
+
+  // Step 2: Purge errors
+  const purgeResult = purgeErrors(m);
+  m = purgeResult.messages;
+  const purgeRemoved = purgeResult.stats.removedCount;
+
+  const totalRemoved = dedupRemoved + purgeRemoved;
+  const totalFreed = dedupResult.stats.freedTokens + purgeResult.stats.freedTokens;
+
+  setDcpState(chatId, messages.length);
+
+  return {
+    messages: m,
+    stats: {
+      removedCount: totalRemoved,
+      freedTokens: totalFreed,
+      dedupRemoved,
+      purgeRemoved,
+    },
+  };
+}
diff --git a/apps/server/src/services/inference/turn.ts b/apps/server/src/services/inference/turn.ts
index e6582c4..12c4ff3 100644
--- a/apps/server/src/services/inference/turn.ts
+++ b/apps/server/src/services/inference/turn.ts
@@ -21,6 +21,7 @@ import {
   buildMessagesPayload,
   loadContext,
 } from './payload.js';
+import { toDcpMessages, transformMessages, fromDcpMessages } from './dcp/index.js';
 import {
   finalizeCompletion,
   finalizeEmpty,
@@ -156,9 +157,20 @@ export async function runAssistantTurn(
       ctx.log.warn({ sessionId }, 'inference: session or project missing mid-loop');
       break;
     }
-    const { session: iterSession, project: iterProject, history } = loaded;
+    let { session: iterSession, project: iterProject, history } = loaded;
     const projectRoot = await resolveProjectRoot(iterProject.path);
 
+    try {
+      const dcpMsgs = toDcpMessages(history);
+      const { messages: pruned, stats } = transformMessages(chatId, dcpMsgs);
+      if (stats.removedCount > 0) {
+        ctx.log.info({ chatId, ...stats }, 'dcp: transform removed messages');
+        history = fromDcpMessages(pruned) as typeof history;
+      }
+    } catch (err) {
+      ctx.log.warn({ err: err instanceof Error ? err.message : String(err), chatId }, 'dcp: transform skipped');
+    }
+
     // v1.14.0: log step boundary for instrumentation. step_start parts are in
     // the schema CHECK but not emitted here — writing to the assistant message
     // before the stream phase creates a sequence-0 collision with