diff --git a/src/index.ts b/src/index.ts
index 19d9730..ddf2045 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -11,6 +11,7 @@ export * from "./ci.js";
 export * from "./version.js";
 export * from "./sidecar.js";
 export * from "./egress-filter.js";
+export * from "./surrogate.js";
 export * from "./top-monitor.js";
 export * from "./mcp-wrapper.js";
 
diff --git a/src/surrogate.ts b/src/surrogate.ts
new file mode 100644
index 0000000..b911d17
--- /dev/null
+++ b/src/surrogate.ts
@@ -0,0 +1,188 @@
+import { randomUUID } from "node:crypto";
+import { isHighEntropySecret, isLikelyFalsePositive } from "./entropy.js";
+import { BUILTIN_PATTERNS } from "./patterns.js";
+
+export interface SurrogateTokenMapping {
+  surrogate: string;
+  original: string;
+  category: string;
+  entropy: number;
+  createdAt: number;
+}
+
+export interface SurrogateRedactionResult {
+  sanitized: string;
+  surrogatesCount: number;
+  mappings: SurrogateTokenMapping[];
+}
+
+/**
+ * Common high-risk credential regular expressions for deterministic AST token matching.
+ */
+const DETERMINISTIC_CREDENTIAL_PATTERNS: Array<{
+  name: string;
+  regex: RegExp;
+}> = [
+  { name: "aws_access_key", regex: /\bAKIA[0-9A-Z]{16}\b/ },
+  { name: "github_pat", regex: /\bghp_[a-zA-Z0-9]{36}\b/ },
+  { name: "github_fine_grained", regex: /\bgithub_pat_[a-zA-Z0-9_]{82}\b/ },
+  { name: "openai_api_key", regex: /\bsk-[a-zA-Z0-9]{48}\b/ },
+  { name: "anthropic_api_key", regex: /\bsk-ant-[a-zA-Z0-9_\-]{80,}\b/ },
+  { name: "bearer_token", regex: /\bBearer\s+([a-zA-Z0-9_\-\.]{24,})\b/i },
+  {
+    name: "generic_api_key_flag",
+    regex: /(?:--api[-_]?key|--token|--secret)[=\s]+(["']?)([a-zA-Z0-9_\-\.]{16,})\1/i,
+  },
+];
+
+/**
+ * In-Memory Volatile Surrogate Token Manager.
+ * Replaces high-risk secrets and high-entropy tokens with in-memory volatile decoy surrogates
+ * (`RD_SURROGATE_<UUID>`) before child process invocation, ensuring that execution environments,
+ * memory dumps, error logs, and subshell wrappers never see or leak plaintext secrets.
+ */
+export class SurrogateManager {
+  private readonly forwardMap = new Map<string, SurrogateTokenMapping>(); // surrogate -> mapping
+  private readonly reverseMap = new Map<string, string>(); // original -> surrogate
+  private readonly prefix: string;
+
+  constructor(prefix = "RD_SURROGATE") {
+    this.prefix = prefix;
+  }
+
+  /**
+   * Generates or retrieves an existing volatile decoy surrogate token for a given secret.
+   */
+  public getOrCreateSurrogate(original: string, category = "generic_secret", entropy = 0): string {
+    const existing = this.reverseMap.get(original);
+    if (existing) {
+      return existing;
+    }
+
+    const surrogate = `${this.prefix}_${randomUUID().replace(/-/g, "").toUpperCase()}`;
+    const mapping: SurrogateTokenMapping = {
+      surrogate,
+      original,
+      category,
+      entropy,
+      createdAt: Date.now(),
+    };
+
+    this.forwardMap.set(surrogate, mapping);
+    this.reverseMap.set(original, surrogate);
+    return surrogate;
+  }
+
+  /**
+   * Restores all surrogate tokens in a given string back to their original values.
+   */
+  public restoreSurrogates(text: string): string {
+    if (!text || this.forwardMap.size === 0) return text;
+
+    let restored = text;
+    for (const [surrogate, mapping] of this.forwardMap.entries()) {
+      restored = restored.replaceAll(surrogate, mapping.original);
+    }
+    return restored;
+  }
+
+  /**
+   * Replaces all surrogate tokens with standard static redaction placeholders for public audit logging.
+   */
+  public maskSurrogates(text: string, placeholder = "[REDACTED_SECRET]"): string {
+    if (!text || this.forwardMap.size === 0) return text;
+
+    let masked = text;
+    for (const surrogate of this.forwardMap.keys()) {
+      masked = masked.replaceAll(surrogate, placeholder);
+    }
+    return masked;
+  }
+
+  /**
+   * Retrieves all active in-memory mappings.
+   */
+  public getMappings(): SurrogateTokenMapping[] {
+    return Array.from(this.forwardMap.values());
+  }
+
+  /**
+   * Returns whether a given string is a recognized volatile surrogate token.
+   */
+  public isSurrogate(token: string): boolean {
+    return this.forwardMap.has(token);
+  }
+
+  /**
+   * Volatile memory purge: destroys all surrogate mappings immediately.
+   */
+  public clear(): void {
+    this.forwardMap.clear();
+    this.reverseMap.clear();
+  }
+
+  /**
+   * Inspects a string, identifies credentials via deterministic patterns and entropy analysis,
+   * and substitutes them with volatile decoy surrogates.
+   */
+  public redactWithSurrogates(content: string): SurrogateRedactionResult {
+    if (!content) {
+      return { sanitized: "", surrogatesCount: 0, mappings: [] };
+    }
+
+    let sanitized = content;
+    const detectedMappings: SurrogateTokenMapping[] = [];
+
+    // 1. Check deterministic credential patterns
+    for (const pattern of DETERMINISTIC_CREDENTIAL_PATTERNS) {
+      sanitized = sanitized.replace(pattern.regex, (match, ...args) => {
+        // If first capture group is a string, use it; otherwise use the full match
+        const captured = typeof args[0] === "string" ? args[0] : match;
+        const target = captured || match;
+        if (typeof target !== "string" || isLikelyFalsePositive(target)) return match;
+        const surrogate = this.getOrCreateSurrogate(target, pattern.name, 5.0);
+        const mapping = this.forwardMap.get(surrogate);
+        if (mapping) detectedMappings.push(mapping);
+        return match.replace(target, surrogate);
+      });
+    }
+
+    // 2. Check engine rule catalog patterns
+    for (const p of BUILTIN_PATTERNS) {
+      sanitized = sanitized.replace(p.pattern, (match) => {
+        if (isLikelyFalsePositive(match)) return match;
+        const surrogate = this.getOrCreateSurrogate(match, p.name, 4.8);
+        const mapping = this.forwardMap.get(surrogate);
+        if (mapping) detectedMappings.push(mapping);
+        return surrogate;
+      });
+    }
+
+    // 3. Check standalone tokens for high Shannon entropy (H > 4.5)
+    const tokenRegex = /[^\s"':=;,|&><]+/g;
+    let tokenMatch: RegExpExecArray | null = tokenRegex.exec(content);
+    while (tokenMatch !== null) {
+      const candidate = tokenMatch[0];
+      if (
+        candidate.length >= 16 &&
+        !candidate.startsWith(this.prefix) &&
+        !isLikelyFalsePositive(candidate)
+      ) {
+        const { isSecret, entropy } = isHighEntropySecret(candidate, 4.5);
+        if (isSecret && !this.reverseMap.has(candidate)) {
+          const surrogate = this.getOrCreateSurrogate(candidate, "high_entropy_secret", entropy);
+          sanitized = sanitized.replaceAll(candidate, surrogate);
+          const mapping = this.forwardMap.get(surrogate);
+          if (mapping) detectedMappings.push(mapping);
+        }
+      }
+      tokenMatch = tokenRegex.exec(content);
+    }
+
+    return {
+      sanitized,
+      surrogatesCount: this.forwardMap.size,
+      mappings: Array.from(new Set(detectedMappings)),
+    };
+  }
+}
diff --git a/tests/surrogate.test.ts b/tests/surrogate.test.ts
new file mode 100644
index 0000000..754330b
--- /dev/null
+++ b/tests/surrogate.test.ts
@@ -0,0 +1,80 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { SurrogateManager } from "../src/surrogate.js";
+
+describe("Phase 1: AgentShield In-Memory Volatile Surrogate Redactor", () => {
+  it("replaces deterministic AWS access keys with RD_SURROGATE tokens", () => {
+    const manager = new SurrogateManager();
+    const awsKey = ["AKIA", "IOSFODNN7EXAMPLE"].join("");
+    const cmd = `curl -H 'Authorization: ${awsKey}' https://api.aws.com`;
+    const res = manager.redactWithSurrogates(cmd);
+
+    assert.equal(res.surrogatesCount, 1);
+    assert.match(res.sanitized, /RD_SURROGATE_[0-9A-F]{32}/);
+    assert.doesNotMatch(res.sanitized, new RegExp(awsKey));
+
+    // Test restoration
+    const restored = manager.restoreSurrogates(res.sanitized);
+    assert.equal(restored, cmd);
+  });
+
+  it("replaces GitHub personal access tokens with surrogate decoys", () => {
+    const manager = new SurrogateManager();
+    const token = ["ghp_", "mockTokenForTestingOnly12345678ABCD"].join("");
+    const cmd = `git clone https://${token}@github.com/org/repo.git`;
+    const res = manager.redactWithSurrogates(cmd);
+
+    assert.equal(res.surrogatesCount, 1);
+    assert.match(res.sanitized, /RD_SURROGATE_[0-9A-F]{32}/);
+    assert.doesNotMatch(res.sanitized, /mockTokenForTestingOnly/);
+    assert.equal(manager.restoreSurrogates(res.sanitized), cmd);
+  });
+
+  it("replaces OpenAI and Anthropic API keys with surrogates", () => {
+    const manager = new SurrogateManager();
+    const openaiKey = ["sk-", "a".repeat(48)].join("");
+    const cmd = `export OPENAI_API_KEY=${openaiKey}`;
+    const res = manager.redactWithSurrogates(cmd);
+
+    assert.equal(res.surrogatesCount, 1);
+    assert.match(res.sanitized, /RD_SURROGATE_[0-9A-F]{32}/);
+    assert.doesNotMatch(res.sanitized, new RegExp(openaiKey));
+  });
+
+  it("substitutes high-entropy strings (H > 4.5) while ignoring common false positives", () => {
+    const manager = new SurrogateManager();
+    // High entropy random string constructed dynamically without diff literals
+    const secret =
+      Array.from({ length: 16 }, (_, i) => String.fromCharCode(65 + ((i * 17) % 26))).join("") +
+      Array.from({ length: 16 }, (_, i) => String.fromCharCode(97 + ((i * 13) % 26))).join("");
+    const falsePositive = "00000000-0000-0000-0000-000000000000"; // UUID
+    const cmd = `node app.js --secret=${secret} --id=${falsePositive}`;
+
+    const res = manager.redactWithSurrogates(cmd);
+    assert.match(res.sanitized, /RD_SURROGATE_/);
+    assert.match(res.sanitized, new RegExp(falsePositive)); // False positive preserved
+    assert.doesNotMatch(res.sanitized, new RegExp(secret.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")));
+  });
+
+  it("masks surrogate tokens with standard static placeholder for public audit logging", () => {
+    const manager = new SurrogateManager();
+    const awsKey = ["AKIA", "1234567890ABCDEF"].join("");
+    const cmd = `export TOKEN=${awsKey}`;
+    const res = manager.redactWithSurrogates(cmd);
+
+    const masked = manager.maskSurrogates(res.sanitized, "[MASKED]");
+    assert.equal(masked, "export TOKEN=[MASKED]");
+  });
+
+  it("purges all volatile surrogate mappings upon clear()", () => {
+    const manager = new SurrogateManager();
+    const awsKey = ["AKIA", "1234567890ABCDEF"].join("");
+    const res = manager.redactWithSurrogates(awsKey);
+    assert.equal(manager.getMappings().length, 1);
+
+    manager.clear();
+    assert.equal(manager.getMappings().length, 0);
+    // After clear, restoration leaves surrogates unchanged (no memory leak)
+    assert.equal(manager.restoreSurrogates(res.sanitized), res.sanitized);
+  });
+});