diff --git a/session.ts b/session.ts
index 2e90290..3d3c5de 100644
--- a/session.ts
+++ b/session.ts
@@ -1,79 +1,145 @@
 /**
  * Session ID resolution for mcp-server-nerf.
  *
- * Resolves the Claude Code session ID from the environment, filesystem
- * artifacts, or generates a stable fallback.
+ * Resolves the Claude Code session ID from the environment, the project
+ * transcript directory, or generates a fallback identifier of last resort.
  */
 
-import { readdirSync } from "node:fs";
+import { existsSync, readdirSync, statSync } from "node:fs";
+import { join } from "node:path";
+import { homedir } from "node:os";
 import { createHash } from "node:crypto";
 import { log } from "./logger.ts";
 
+const PROJECTS_DIR = join(homedir(), ".claude", "projects");
+
+/**
+ * UUID-shaped basename (case-insensitive). Claude Code transcript filenames
+ * are lowercase v4 UUIDs in current builds, but the `i` flag costs nothing
+ * and protects against a hypothetical future generator that uses uppercase.
+ */
+const SESSION_UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
 /**
  * Resolve the session ID from the Claude Code environment.
  *
- * Strategy:
- * 1. Check CLAUDE_SESSION_ID env var (set by some Claude Code configurations)
- * 2. Scan /tmp for claude session artifacts (transcript/output files)
- * 3. Fallback: generate a stable ID from PID + process start time
+ * Strategy, in order:
+ * 1. Explicit `override` from tool params.
+ * 2. `CLAUDE_SESSION_ID` env var (set by some Claude Code configurations).
+ * 3. Most-recently-modified transcript in this project's transcript dir.
+ *    Claude Code writes per-project transcripts at
+ *      ~/.claude/projects/<slug>/<session_uuid>.jsonl
+ *    where <slug> = process.cwd() with `/` replaced by `-`. Scoping to the
+ *    calling CC's own project dir disambiguates correctly when the user has
+ *    multiple concurrent CC sessions in different projects.
+ * 4. Fallback: stable ID from `md5(pid-ppid)`. Logged at `warn` level so the
+ *    fleet sees this gap if it fires.
+ *
+ * Returns the resolved session ID. Never throws.
+ *
+ * `projectsDir` and `cwd` are exposed for testability; production callers pass
+ * neither and get the real Claude Code paths.
  */
-export function resolveSessionId(override?: string): string {
-  // 0. Explicit override from tool params
+export function resolveSessionId(
+  override?: string,
+  projectsDir: string = PROJECTS_DIR,
+  cwd: string = process.cwd(),
+): string {
   if (override) {
     log.debug("state_change", { what: "session", to: override }, "Resolved via explicit override");
     return override;
   }
 
-  // 1. Direct env var
   const envId = process.env.CLAUDE_SESSION_ID;
   if (envId) {
     log.debug("state_change", { what: "session", to: envId }, "Resolved via CLAUDE_SESSION_ID env var");
     return envId;
   }
 
-  // 2. Scan /tmp for session artifacts
-  const scanned = scanForSessionArtifacts();
-  if (scanned) {
-    log.debug("state_change", { what: "session", to: scanned }, "Resolved via artifact scan");
-    return scanned;
+  const fromTranscripts = resolveFromTranscripts(projectsDir, cwd);
+  if (fromTranscripts) {
+    log.debug("state_change", { what: "session", to: fromTranscripts }, "Resolved via newest transcript");
+    return fromTranscripts;
   }
 
-  // 3. Fallback: stable ID from PID + timestamp
   const stableId = generateStableId();
-  log.debug("state_change", { what: "session", to: stableId }, "Resolved via stable ID fallback");
+  log.warn(
+    "session_resolution",
+    { cause: "fallback_used", to: stableId },
+    "Resolved via md5(pid-ppid) fallback — no transcript found in project's transcript dir",
+  );
   return stableId;
 }
 
 /**
- * Scan /tmp for Claude session artifacts and extract a session ID.
- * Looks for files matching claude-session-* or similar patterns.
+ * Derive the per-project transcript directory name from a project root.
+ * Claude Code uses the absolute path with `/` replaced by `-`.
+ *
+ *   /home/bakerb/sandbox/github/foo  →  -home-bakerb-sandbox-github-foo
+ *
+ * Exported for testing.
+ */
+export function projectSlug(cwd: string): string {
+  return cwd.replace(/\//g, "-");
+}
+
+/**
+ * Find the newest UUID-named `.jsonl` transcript in this project's transcript
+ * directory. Scans only `<projectsDir>/<slug>/`, where `slug` is derived from
+ * `cwd` (defaults to `process.cwd()`).
+ *
+ * Returns the basename (without `.jsonl`) of the newest match, or null when
+ * the directory is missing/empty/unreadable. Exported for testing.
  */
-function scanForSessionArtifacts(): string | null {
+export function resolveFromTranscripts(
+  projectsDir: string = PROJECTS_DIR,
+  cwd: string = process.cwd(),
+): string | null {
+  const slug = projectSlug(cwd);
+  const projectDir = join(projectsDir, slug);
+
+  if (!existsSync(projectDir)) {
+    return null;
+  }
+
+  let newest: { sessionId: string; mtimeMs: number } | null = null;
+
+  let entries: string[];
   try {
-    const entries = readdirSync("/tmp");
-    // Look for nerf config files first (nerf-<session_id>.json)
-    for (const entry of entries) {
-      const match = entry.match(/^nerf-([a-f0-9-]+)\.json$/);
-      if (match) {
-        return match[1];
-      }
-    }
-    // Look for claude session markers
-    for (const entry of entries) {
-      const match = entry.match(/^claude-session-([a-f0-9-]+)/);
-      if (match) {
-        return match[1];
+    entries = readdirSync(projectDir);
+  } catch (err) {
+    // Directory existed (existsSync passed) but was unreadable — permissions,
+    // symlink loop, FS error. Log so an operator diagnosing a fallback can
+    // see the actual cause, not just "no transcript found."
+    log.debug(
+      "session_resolution",
+      { projectDir, err: String(err) },
+      "readdirSync failed on existing project dir",
+    );
+    return null;
+  }
+
+  for (const entry of entries) {
+    if (!entry.endsWith(".jsonl")) continue;
+    const sessionId = entry.slice(0, -".jsonl".length);
+    if (!SESSION_UUID_RE.test(sessionId)) continue;
+    try {
+      const mtimeMs = statSync(join(projectDir, entry)).mtimeMs;
+      if (!newest || mtimeMs > newest.mtimeMs) {
+        newest = { sessionId, mtimeMs };
       }
+    } catch {
+      // Unreadable file — skip silently
     }
-  } catch {
-    // /tmp not readable — fall through
   }
-  return null;
+
+  return newest?.sessionId ?? null;
 }
 
 /**
- * Generate a stable ID from process characteristics.
- * Uses PID and a fixed seed so the ID is deterministic within a process.
+ * Generate a stable ID from process characteristics. Last-resort fallback —
+ * never collides with the UUID shape used by real Claude Code sessions, so
+ * this ID cannot be confused for a transcript-derived one downstream.
  */
 function generateStableId(): string {
   const seed = `${process.pid}-${process.ppid ?? 0}`;
diff --git a/tests/session.test.ts b/tests/session.test.ts
new file mode 100644
index 0000000..0bde67e
--- /dev/null
+++ b/tests/session.test.ts
@@ -0,0 +1,162 @@
+/**
+ * Regression tests for session.ts — session ID resolution.
+ *
+ * Covers issue #24: the previous /tmp self-debris scan returned a fake
+ * 12-char-hex md5(pid-ppid) ID forever once written. The new resolver reads
+ * from the per-project transcript dir under ~/.claude/projects/ — Claude Code
+ * writes those files itself, so the source of truth cannot be self-poisoned.
+ */
+
+import { describe, test, expect, beforeEach, afterEach } from "bun:test";
+import { mkdtempSync, mkdirSync, writeFileSync, rmSync, utimesSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { resolveSessionId, resolveFromTranscripts, projectSlug } from "../session.ts";
+
+const SESSION_A = "991053c7-f9e4-4840-b906-d0252650793e";
+const SESSION_B = "44c9b97f-ac26-4fe8-be6f-45466b0b265e";
+const SESSION_C = "5044048d-1cf7-4497-b9d7-66b78bc9d188";
+
+function writeTranscript(projectsDir: string, project: string, sessionId: string, mtimeMs: number): void {
+  const projDir = join(projectsDir, project);
+  mkdirSync(projDir, { recursive: true });
+  const path = join(projDir, `${sessionId}.jsonl`);
+  writeFileSync(path, "");
+  const seconds = mtimeMs / 1000;
+  utimesSync(path, seconds, seconds);
+}
+
+describe("projectSlug", () => {
+  test("replaces all forward slashes with dashes", () => {
+    expect(projectSlug("/home/bakerb/sandbox/github/foo")).toBe("-home-bakerb-sandbox-github-foo");
+  });
+
+  test("handles single-component absolute path", () => {
+    expect(projectSlug("/foo")).toBe("-foo");
+  });
+
+  test("preserves dashes already in the path", () => {
+    expect(projectSlug("/home/user/my-project")).toBe("-home-user-my-project");
+  });
+});
+
+describe("resolveFromTranscripts (issue #24)", () => {
+  let projectsDir: string;
+  const CWD = "/home/test/project-a";
+  const SLUG = "-home-test-project-a";
+
+  beforeEach(() => {
+    projectsDir = mkdtempSync(join(tmpdir(), "nerf-session-projects-"));
+  });
+
+  afterEach(() => {
+    try { rmSync(projectsDir, { recursive: true, force: true }); } catch { /* ignore */ }
+  });
+
+  test("returns null when project dir does not exist for this cwd", () => {
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBeNull();
+  });
+
+  test("returns null when project dir is empty", () => {
+    mkdirSync(join(projectsDir, SLUG), { recursive: true });
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBeNull();
+  });
+
+  test("returns the session ID from a single transcript", () => {
+    writeTranscript(projectsDir, SLUG, SESSION_A, Date.now());
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBe(SESSION_A);
+  });
+
+  test("returns newest by mtime within this project", () => {
+    const now = Date.now();
+    writeTranscript(projectsDir, SLUG, SESSION_A, now - 30_000);
+    writeTranscript(projectsDir, SLUG, SESSION_B, now);
+    writeTranscript(projectsDir, SLUG, SESSION_C, now - 60_000);
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBe(SESSION_B);
+  });
+
+  test("ignores transcripts in other project directories (multi-session isolation)", () => {
+    // Critical regression check: when the user has concurrent CC sessions
+    // in different projects, the resolver MUST scope to its own cwd, not pick
+    // up the freshest transcript across the entire fleet.
+    const now = Date.now();
+    writeTranscript(projectsDir, SLUG, SESSION_A, now - 60_000);
+    writeTranscript(projectsDir, "-home-test-other-project", SESSION_B, now); // newer but wrong project
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBe(SESSION_A);
+  });
+
+  test("ignores non-UUID-shaped basenames (debris)", () => {
+    // Old fallback IDs were 12-char hex like 81457c8d97e2 — must not be picked up
+    // even if a stray file ends up in a project dir.
+    writeTranscript(projectsDir, SLUG, "81457c8d97e2", Date.now());
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBeNull();
+  });
+
+  test("ignores non-jsonl files", () => {
+    const projDir = join(projectsDir, SLUG);
+    mkdirSync(projDir, { recursive: true });
+    writeFileSync(join(projDir, `${SESSION_A}.json`), "");
+    writeFileSync(join(projDir, `${SESSION_A}.txt`), "");
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBeNull();
+  });
+
+  test("UUID-shaped match wins over debris with newer mtime", () => {
+    const now = Date.now();
+    writeTranscript(projectsDir, SLUG, SESSION_A, now - 60_000);
+    writeTranscript(projectsDir, SLUG, "abcd1234ef56", now); // newer debris, ignored
+    expect(resolveFromTranscripts(projectsDir, CWD)).toBe(SESSION_A);
+  });
+});
+
+describe("resolveSessionId (issue #24)", () => {
+  let savedEnv: string | undefined;
+
+  beforeEach(() => {
+    savedEnv = process.env.CLAUDE_SESSION_ID;
+    delete process.env.CLAUDE_SESSION_ID;
+  });
+
+  afterEach(() => {
+    if (savedEnv === undefined) {
+      delete process.env.CLAUDE_SESSION_ID;
+    } else {
+      process.env.CLAUDE_SESSION_ID = savedEnv;
+    }
+  });
+
+  test("explicit override beats env var", () => {
+    process.env.CLAUDE_SESSION_ID = SESSION_B;
+    expect(resolveSessionId(SESSION_A)).toBe(SESSION_A);
+  });
+
+  test("env var beats transcript scan", () => {
+    process.env.CLAUDE_SESSION_ID = SESSION_A;
+    expect(resolveSessionId()).toBe(SESSION_A);
+  });
+
+  test("fallback fires with 12-char hex ID when no transcript exists for this cwd", () => {
+    // Force the resolver into the fallback path with an empty projectsDir +
+    // a cwd that has no transcripts. This is the path that USED to be the
+    // only thing that fired (the bug); now it must be a true last resort.
+    const emptyProjectsDir = mkdtempSync(join(tmpdir(), "nerf-fallback-empty-"));
+    try {
+      const id = resolveSessionId(undefined, emptyProjectsDir, "/home/test/no-transcripts");
+      expect(id).toMatch(/^[0-9a-f]{12}$/);
+    } finally {
+      try { rmSync(emptyProjectsDir, { recursive: true, force: true }); } catch { /* ignore */ }
+    }
+  });
+
+  test("transcript scan beats fallback when a transcript is present", () => {
+    const projectsDir = mkdtempSync(join(tmpdir(), "nerf-resolve-"));
+    const cwd = "/home/test/project-x";
+    const slug = "-home-test-project-x";
+    try {
+      writeTranscript(projectsDir, slug, SESSION_A, Date.now());
+      const id = resolveSessionId(undefined, projectsDir, cwd);
+      expect(id).toBe(SESSION_A);
+    } finally {
+      try { rmSync(projectsDir, { recursive: true, force: true }); } catch { /* ignore */ }
+    }
+  });
+});