diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index d75971c362..a3d8e48378 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -1452,12 +1452,20 @@ export async function handleChatCore({
     }
   }
 
+  // ── Proactive Context Compression (Phase 4) ──
+  // Check if context exceeds 85% of limit and compress proactively before sending to provider.
+  // This prevents "prompt too long" errors for large-but-not-full contexts.
   if (translatedBody && translatedBody.messages && Array.isArray(translatedBody.messages)) {
     const estimatedTokens = estimateTokens(JSON.stringify(translatedBody.messages));
     const contextLimit = getTokenLimit(provider, effectiveModel);
     const COMPRESSION_THRESHOLD = 0.85;
     const threshold = Math.floor(contextLimit * COMPRESSION_THRESHOLD);
 
+    log?.debug?.(
+      "CONTEXT",
+      `Checking compression: ${estimatedTokens} tokens vs ${threshold} threshold (${contextLimit} limit)`
+    );
+
     if (estimatedTokens > threshold) {
       log?.info?.(
         "CONTEXT",
@@ -1468,6 +1476,7 @@ export async function handleChatCore({
         provider,
         model: effectiveModel,
         maxTokens: contextLimit,
+        reserveTokens: 0,
       });
 
       if (compressionResult.compressed) {
@@ -1495,8 +1504,15 @@ export async function handleChatCore({
             layers: "layers" in stats ? stats.layers : undefined,
           },
         });
+      } else {
+        log?.debug?.("CONTEXT", `Compression not applied: context already fits within target`);
       }
     }
+  } else {
+    log?.debug?.(
+      "CONTEXT",
+      `Skipping compression check: translatedBody=${!!translatedBody}, messages=${!!translatedBody?.messages}, isArray=${Array.isArray(translatedBody?.messages)}`
+    );
   }
 
   // Resolve executor with optional upstream proxy (CLIProxyAPI) routing.
diff --git a/open-sse/services/contextManager.ts b/open-sse/services/contextManager.ts
index 722b9f4df9..25bf6cc033 100644
--- a/open-sse/services/contextManager.ts
+++ b/open-sse/services/contextManager.ts
@@ -6,7 +6,7 @@
  */
 
 import { REGISTRY } from "../config/providerRegistry.ts";
-import { getModelContextLimit } from "../../src/lib/modelCapabilities";
+import { getModelContextLimit } from "../../src/lib/modelCapabilities.ts";
 
 // Default token limits per provider (fallbacks when not in registry)
 const DEFAULT_LIMITS: Record<string, number> = {
@@ -34,6 +34,16 @@ function getEnvOverride(provider: string): number | null {
   return null;
 }
 
+// Reserve tokens override from environment variable
+function getReserveTokensOverride(): number | null {
+  const envValue = process.env.CONTEXT_RESERVE_TOKENS;
+  if (envValue) {
+    const parsed = parseInt(envValue, 10);
+    if (!isNaN(parsed) && parsed > 0) return parsed;
+  }
+  return null;
+}
+
 // Rough chars-per-token ratio for quick estimation
 const CHARS_PER_TOKEN = 4;
 
@@ -111,7 +121,7 @@ export function compressContext(
     options.maxTokens || getTokenLimit(provider, (body.model as string) || options.model || null);
   const defaultReserveTokens = Math.min(16000, Math.max(256, Math.floor(maxTokens * 0.15)));
   const reserveTokens = Math.min(
-    options.reserveTokens ?? defaultReserveTokens,
+    options.reserveTokens ?? getReserveTokensOverride() ?? defaultReserveTokens,
     Math.max(0, maxTokens - 1)
   );
   const targetTokens = Math.max(0, maxTokens - reserveTokens);
diff --git a/tests/integration/chatcore-compression-integration.test.ts b/tests/integration/chatcore-compression-integration.test.ts
new file mode 100644
index 0000000000..88391f89ed
--- /dev/null
+++ b/tests/integration/chatcore-compression-integration.test.ts
@@ -0,0 +1,331 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-compression-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.REQUIRE_API_KEY = "false";
+process.env.API_KEY_SECRET = process.env.API_KEY_SECRET || "test-compression-secret";
+
+const core = await import("../../src/lib/db/core.ts");
+const providersDb = await import("../../src/lib/db/providers.ts");
+const readCacheDb = await import("../../src/lib/db/readCache.ts");
+const { handleChatCore } = await import("../../open-sse/handlers/chatCore.ts");
+const { estimateTokens, getTokenLimit } = await import("../../open-sse/services/contextManager.ts");
+const { resetAllAvailability } = await import("../../src/domain/modelAvailability.ts");
+const { resetAllCircuitBreakers } = await import("../../src/shared/utils/circuitBreaker.ts");
+
+const originalFetch = globalThis.fetch;
+
+async function resetStorage() {
+  globalThis.fetch = originalFetch;
+  resetAllAvailability();
+  resetAllCircuitBreakers();
+  readCacheDb.invalidateDbCache();
+  await new Promise((resolve) => setTimeout(resolve, 20));
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  fs.mkdirSync(TEST_DATA_DIR, { recursive: true });
+}
+
+test.beforeEach(async () => {
+  await resetStorage();
+});
+
+test.after(async () => {
+  globalThis.fetch = originalFetch;
+  core.closeDbInstance();
+  try {
+    fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  } catch {}
+});
+
+test("chatCore integration: compressContext called proactively when context exceeds 85% threshold", async () => {
+  const provider = "openai";
+  const model = "gpt-4";
+
+  // Create multiple messages with history that can be compressed
+  // Use the same pattern as test 3 which successfully tests compression
+  const body = {
+    model,
+    messages: [
+      { role: "system", content: "You are helpful." },
+      { role: "user", content: "x".repeat(50000) },
+      { role: "assistant", content: "Response 1" },
+      { role: "user", content: "x".repeat(50000) },
+      { role: "assistant", content: "Response 2" },
+      { role: "user", content: "x".repeat(50000) },
+      { role: "assistant", content: "Response 3" },
+      { role: "user", content: "Final question" },
+    ],
+  };
+
+  // Create provider connection
+  const connectionId = await providersDb.createProviderConnection({
+    provider,
+    apiKey: "test-key",
+    isActive: true,
+  });
+
+  // Mock fetch to capture the request
+  let capturedBody: any = null;
+  globalThis.fetch = async (url: string | URL | Request, init?: RequestInit) => {
+    if (init?.body) {
+      capturedBody = JSON.parse(init.body as string);
+    }
+    return new Response(
+      JSON.stringify({
+        choices: [{ message: { role: "assistant", content: "test" } }],
+        usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 },
+      }),
+      {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      }
+    );
+  };
+
+  try {
+    const result = await handleChatCore({
+      body,
+      modelInfo: { provider, model },
+      credentials: { apiKey: "test-key" },
+      log: {
+        debug: () => {},
+        info: () => {},
+        warn: () => {},
+        error: () => {},
+      },
+      clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Map() },
+      connectionId,
+    });
+
+    assert.ok(result.success, "Request should succeed");
+    assert.ok(capturedBody, "Fetch should have been called");
+
+    // Verify that compression preserved the message structure
+    assert.ok(Array.isArray(capturedBody.messages), "Messages should remain an array");
+    assert.ok(capturedBody.messages.length > 0, "Messages should not be empty");
+
+    // Verify that the final question was preserved (compression keeps recent messages)
+    const lastMessage = capturedBody.messages[capturedBody.messages.length - 1];
+    assert.equal(lastMessage.content, "Final question", "Last user message should be preserved");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("chatCore integration: compressContext NOT called when context is below 85% threshold", async () => {
+  const provider = "openai";
+  const model = "gpt-4";
+  const contextLimit = getTokenLimit(provider, model);
+  const threshold = Math.floor(contextLimit * 0.85);
+
+  const smallMessage = "Hello, how are you?";
+  const body = {
+    model,
+    messages: [
+      { role: "system", content: "You are helpful." },
+      { role: "user", content: smallMessage },
+    ],
+  };
+
+  const estimatedTokens = estimateTokens(JSON.stringify(body.messages));
+  assert.ok(
+    estimatedTokens < threshold,
+    `Expected ${estimatedTokens} to be below threshold ${threshold}`
+  );
+
+  // Create provider connection
+  const connectionId = await providersDb.createProviderConnection({
+    provider,
+    apiKey: "test-key",
+    isActive: true,
+  });
+
+  // Mock fetch to capture the request
+  let capturedBody: any = null;
+  globalThis.fetch = async (url: string | URL | Request, init?: RequestInit) => {
+    if (init?.body) {
+      capturedBody = JSON.parse(init.body as string);
+    }
+    return new Response(
+      JSON.stringify({
+        choices: [{ message: { role: "assistant", content: "test" } }],
+        usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 },
+      }),
+      {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      }
+    );
+  };
+
+  try {
+    const result = await handleChatCore({
+      body,
+      modelInfo: { provider, model },
+      credentials: { apiKey: "test-key" },
+      log: { debug: () => {}, info: () => {}, warn: () => {}, error: () => {} },
+      clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Map() },
+      connectionId,
+    });
+
+    assert.ok(result.success, "Request should succeed");
+    assert.ok(capturedBody, "Fetch should have been called");
+
+    // Verify NO compression occurred
+    const originalTokens = estimateTokens(JSON.stringify(body.messages));
+    const finalTokens = estimateTokens(JSON.stringify(capturedBody.messages));
+
+    assert.equal(
+      finalTokens,
+      originalTokens,
+      `Context should NOT be compressed: ${finalTokens} === ${originalTokens}`
+    );
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("chatCore integration: compression preserves message structure", async () => {
+  const provider = "openai";
+  const model = "gpt-4";
+
+  const body = {
+    model,
+    messages: [
+      { role: "system", content: "You are helpful." },
+      { role: "user", content: "x".repeat(50000) },
+      { role: "assistant", content: "Response 1" },
+      { role: "user", content: "x".repeat(50000) },
+      { role: "assistant", content: "Response 2" },
+      { role: "user", content: "Final question" },
+    ],
+  };
+
+  // Create provider connection
+  const connectionId = await providersDb.createProviderConnection({
+    provider,
+    apiKey: "test-key",
+    isActive: true,
+  });
+
+  // Mock fetch to capture the request
+  let capturedBody: any = null;
+  globalThis.fetch = async (url: string | URL | Request, init?: RequestInit) => {
+    if (init?.body) {
+      capturedBody = JSON.parse(init.body as string);
+    }
+    return new Response(
+      JSON.stringify({
+        choices: [{ message: { role: "assistant", content: "test" } }],
+        usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 },
+      }),
+      {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      }
+    );
+  };
+
+  try {
+    const result = await handleChatCore({
+      body,
+      modelInfo: { provider, model },
+      credentials: { apiKey: "test-key" },
+      log: {
+        debug: (tag: string, msg: string) => console.log(`[DEBUG] ${tag}: ${msg}`),
+        info: (tag: string, msg: string) => console.log(`[INFO] ${tag}: ${msg}`),
+        warn: (tag: string, msg: string) => console.log(`[WARN] ${tag}: ${msg}`),
+        error: (tag: string, msg: string) => console.log(`[ERROR] ${tag}: ${msg}`),
+      },
+      clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Map() },
+      connectionId,
+    });
+
+    assert.ok(result.success, "Request should succeed");
+    assert.ok(capturedBody, "Fetch should have been called");
+    assert.ok(Array.isArray(capturedBody.messages), "Messages should remain an array");
+    assert.ok(capturedBody.messages.length > 0, "Messages should not be empty");
+
+    const hasSystem = capturedBody.messages.some((m: any) => m.role === "system");
+    assert.ok(hasSystem, "System message should be preserved");
+
+    const lastMessage = capturedBody.messages[capturedBody.messages.length - 1];
+    assert.equal(lastMessage.content, "Final question", "Last user message should be preserved");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("chatCore integration: compression handles tool messages", async () => {
+  const provider = "openai";
+  const model = "gpt-4";
+
+  const longToolOutput = "x".repeat(10000);
+  const body = {
+    model,
+    messages: [
+      { role: "system", content: "You are helpful." },
+      { role: "user", content: "Run the tool" },
+      { role: "assistant", content: "Running tool", tool_calls: [{ id: "t1", type: "function" }] },
+      { role: "tool", content: longToolOutput, tool_call_id: "t1" },
+      { role: "user", content: "What's the result?" },
+    ],
+  };
+
+  // Create provider connection
+  const connectionId = await providersDb.createProviderConnection({
+    provider,
+    apiKey: "test-key",
+    isActive: true,
+  });
+
+  // Mock fetch to capture the request
+  let capturedBody: any = null;
+  globalThis.fetch = async (url: string | URL | Request, init?: RequestInit) => {
+    if (init?.body) {
+      capturedBody = JSON.parse(init.body as string);
+    }
+    return new Response(
+      JSON.stringify({
+        choices: [{ message: { role: "assistant", content: "test" } }],
+        usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 },
+      }),
+      {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      }
+    );
+  };
+
+  try {
+    const result = await handleChatCore({
+      body,
+      modelInfo: { provider, model },
+      credentials: { apiKey: "test-key" },
+      log: { debug: () => {}, info: () => {}, warn: () => {}, error: () => {} },
+      clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Map() },
+      connectionId,
+    });
+
+    assert.ok(result.success, "Request should succeed");
+    assert.ok(capturedBody, "Fetch should have been called");
+
+    const toolMessage = capturedBody.messages.find((m: any) => m.role === "tool");
+    assert.ok(toolMessage, "Tool message should exist");
+
+    // Tool message should be truncated if compression was triggered
+    if (toolMessage.content.length < longToolOutput.length) {
+      assert.ok(
+        toolMessage.content.includes("[truncated]"),
+        "Tool message should have truncation marker"
+      );
+    }
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});