MCP JDWP Inspector

MCP server that gives AI agents full debugger control over running Java applications — inspect state, set breakpoints, evaluate expressions, and mutate values at runtime via JDWP/JDI.

Release notes — see CHANGELOG.md. 2.0.0 is a breaking release: several jdwp_clear_* tools were unified, and field watchpoints are new.

Built on the foundations of mcp-jdwp-java by Nicolas Vautrin — the original project that provided core JDI connectivity, thread/stack/variable inspection, stepping, and basic breakpoint management. Everything described below as "beyond standard JDWP" was built on top of that base.

What you get (TL;DR)

An MCP server that speaks JDWP — the same protocol IntelliJ / Eclipse / VS Code use to attach — and exposes the debugger to Claude Code as 47 tools + 2 MCP resources over STDIO. Once the agent attaches to your JVM, it can:

• See runtime state, not stack traces — read locals, fields, threads, the entire object graph
• Evaluate arbitrary Java in the suspended frame — compiled to real bytecode, full classpath, including private/package-private members
• Mutate the running program — set locals, write fields, test "what if?" without restart
• Stop on the bad case only — conditional breakpoints, exception breakpoints at the throw site, deferred BPs that arm before the class loads, chains that gate one BP on another
• Trace without stopping — line/exception/field logpoints write to an event log, the thread runs on
• Catch the writer of a mutating field — watchpoints suspend at the JVM-level store, including reflective Field.set that a line BP can never see
• Pure STDIO — no daemon, no extra port, no GUI

The agent gets the same power as IntelliJ's debugger, with extras that exist specifically because an agent isn't a human staring at a UI (filtering, blocking resume, one-shot context dumps, recursion guards, reconnect after a wedge).

Security & trust

This MCP server runs entirely on your local machine:

• No network calls — the server communicates with Claude Code over STDIO and with the target JVM over a local JDWP socket. It makes zero outbound HTTP/internet requests. No telemetry, no analytics, no phone-home.
• Built from source — the JAR is compiled locally on your machine from the source code in this repository (either via the plugin auto-build hook or manually with ./mvnw clean package). No pre-built binaries are downloaded or distributed.
• Auditable — the full source is here. The server is a standard Spring Boot application with no obfuscation or native code. Try asking Claude Code itself: "audit these sources for anything that touches network or filesystem beyond the JVM target".

Quick start

Prerequisites

• JDK 17+ on PATH to run the server (must be a JDK, not a JRE — JDI lives in jdk.jdi).
• JDK 21+ to build from source — the build toolchain pins to JDK ≥ 21 because Error Prone 2.48 ships Java-21 bytecode. The bytecode target stays at Java 17, so the resulting JAR still runs on a JDK 17 runtime.

No separate Maven install is required — the repository ships with the Maven Wrapper (./mvnw), which downloads a pinned Maven 3.9.x into ~/.m2/wrapper/ on first use. The SessionStart hook and every build command in this README use the wrapper.

1. Install the plugin

Option A: Plugin marketplace (recommended)

Installs the MCP server, the java-debug skill (debugging workflows, recipes, gotchas), and the .mcp.json configuration in one step:

/plugin marketplace add https://github.com/FgForrest/mcp-jdwp-java.git
/plugin install jdwp-debugging@mcp-jdwp-java

The server JAR is built automatically on first session start via the bundled ./mvnw wrapper (requires JDK 17+ on PATH — no Maven install needed). The hook is content-aware: it rebuilds when git HEAD moves, not just when the JAR is missing, so git pull + restart picks up updates without manual intervention. Restart Claude Code to pick up the plugin.

If you prefer to register the MCP server directly without the plugin (no skill, no auto-build):

1. Build the JAR:

git clone https://github.com/FgForrest/mcp-jdwp-java.git
cd mcp-jdwp-java
./mvnw clean package -DskipTests   # use mvnw.cmd on Windows

2. Register with Claude Code:

claude mcp add jdwp-inspector -s user \
  -e MCP_TIMEOUT=30000 \
  -e MCP_TOOL_TIMEOUT=120000 \
  -- java --add-modules jdk.jdi,jdk.attach -jar /path/to/mcp-jdwp-java.jar

To change the JDWP port (default 5005), add -DJVM_JDWP_PORT=12345 before -jar.

The MCP_TIMEOUT and MCP_TOOL_TIMEOUT environment variables are important — JVM startup is not instant (class loading, Spring context initialization), so the default MCP timeouts will cause Claude Code to give up before the server is ready. MCP_TIMEOUT=30000 gives the server 30 seconds to start, and MCP_TOOL_TIMEOUT=120000 allows up to 2 minutes for long-running tools like first-time expression evaluation (which discovers the target's classpath and compiles bytecode).

Re-installing requires removing first: claude mcp remove jdwp-inspector -s user

Drop -s user to scope to the current project only.

.mcp.json:

{
  "mcpServers": {
    "jdwp-inspector": {
      "command": "java",
      "args": [
        "--add-modules", "jdk.jdi,jdk.attach",
        "-jar", "/path/to/mcp-jdwp-java.jar"
      ],
      "env": {
        "MCP_TIMEOUT": "30000",
        "MCP_TOOL_TIMEOUT": "120000"
      }
    }
  }
}

The plugin-managed install also passes -DLOG_PATH=${CLAUDE_PLUGIN_ROOT}/logs/mcp-jdwp-inspector.log so the server's SLF4J log lands inside the plugin directory; add the same flag if you want a fixed log path here.

2. Launch your Java application with JDWP

Maven Surefire (test debugging):

mvn test -Dmaven.surefire.debug

Starts the JVM with JDWP on port 5005, suspended until a debugger connects.

Any Java application:

-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005

3. Attach from Claude Code

From there, just tell the agent what to do — "attach to JDWP, set a breakpoint in OrderService.createOrder line 42, run the failing test, dump the context when it hits". The bundled java-debug skill teaches the agent the right tool sequence; the next section gives you a smoke test that exercises the surface.

Find the Bug — test flights

The jdwp-sandbox module ships 9 deliberately broken Java classes. Each one compiles fine, looks reasonable at first glance, and fails its test with a confusing message. Your job: attach with the JDWP MCP server and find the root cause.

This doubles as a setup verification — if you can solve these, everything works.

Each flight is built so that one tool group is the path of least resistance, and lists a par — the minimum tool calls that cleanly reveal the root cause. Hitting par is the elegant solve; the suite as a whole exercises the full tool surface (expression eval, exception breakpoints, field watchpoints, event history, marks, logpoints, runtime mutation, and multi-thread inspection).

Just installed the plugin? Grab the sandbox zip

The fastest path for fresh plugin users is a self-contained zip — no clone, no reactor build:

curl -L -o jdwp-sandbox.zip \
  https://github.com/FgForrest/mcp-jdwp-java/releases/latest/download/jdwp-sandbox.zip
unzip jdwp-sandbox.zip && cd jdwp-sandbox

Inside, you get a parentless Maven project with src/, a pom.xml, a flight-game README.md, and a CLAUDE.md that briefs the agent on the game's house rules. Open the folder in Claude Code and ask it to play flight #1 — the bundled CLAUDE.md keeps it honest (no peeking at source, no spoiler-fetching). Continue with the workflow below.

How to launch a test flight

Start Claude Code from the sandbox folder (or this repo's root, if you cloned) and type:

Use JDWP to debug <TestClass> in the jdwp-sandbox module — the test is failing, find the root cause.

---

#1 The Phantom Session

Difficulty: Moderate | Test: SessionStoreTest | Package: session | Par: 4 | Exercises: expression eval

Symptom: retrieve() returned null — a session was stored, upgraded, and then... vanished from the map.

Hint: The session is still in the HashMap. The HashMap just can't find it anymore.

UserSession is used as a HashMap key, with both userId and role in hashCode(). upgradeUserRole() calls session.upgradeRole("PREMIUM"), which mutates role — changing the hashCode while the key is still in the map. The entry sits in the old hash bucket; lookups compute the new hash and search the wrong bucket.

Debug path: Breakpoint before and after upgradeRole(). Use jdwp_assert_expression("session.hashCode()", "<value-before>") after the upgrade — MISMATCH confirms the hash drifted.

---

#2 The Swallowed Exception

Difficulty: Hard | Test: EventBusTest | Package: events | Par: 4 | Exercises: exception breakpoint + trigger gate

Symptom: expected stock < 100 but was 100 and an empty error summary — the order was supposed to reserve inventory, but nothing happened and nobody complained.

Hint: The handler runs on a background thread. Nothing in your code catches its failure — so there's no catch block to break on. Catch the throw itself.

OrderEvent narrows the raw quantity through byte (200 → -56), so Inventory.reserve() throws IllegalStateException. EventBus.dispatch() runs each handler on a single-thread executor as a fire-and-forget task — the Future is never inspected, so the exception is captured inside java.util.concurrent.FutureTask.run (a JDK frame) and discarded. No sandbox frame ever holds the throwable, so a breakpoint-context dump has nothing to show, and getErrorSummary() stays empty.

Debug path: Because the exception is caught (by FutureTask), an uncaught-only breakpoint never fires. Set a line BP at the test/dispatch entry as a trigger, then jdwp_set_exception_breakpoint("java.lang.IllegalStateException", caught=true, triggerBreakpointId=<trigger>) so bootstrap exceptions don't drown the signal. The throw site suspends with qty = -56 in the frame's locals.

---

#3 The Time Traveler's Config

Difficulty: Hard | Test: ConfigurationProviderTest | Package: config | Par: 4 | Exercises: field watchpoint + event history

Symptom: expected timeout=5000 but was 0 — the timeout was set during construction, yet it reads back as the default.

Hint: The value was set correctly. Something wrote over it afterward. There's no half-built object to inspect — the damage is in the order of writes.

ConfigurationProvider's constructor calls init(5000), so the timeout is correct. Then runMaintenanceSweep() starts a background config-reaper thread that wrongly treats the live instance as stale and calls resetToDefaults(), writing the timeout back to 0. By the time the test reads it, the heap shows 0 and no local holds 5000 — a single context dump looks like the value was never set.

Debug path: jdwp_set_field_breakpoint(className="…config.Configuration", fieldName="timeout", mode="modification"), then jdwp_get_events shows the two stores in order — 0→5000 on the main thread, then 5000→0 on config-reaper. jdwp_get_stack on the second event names the reaper as the clobberer.

---

#4 The Audit That Lies

Difficulty: Hard | Test: TransferServiceTest | Package: bank | Par: 4 | Exercises: field watchpoint + stack narration

Symptom: expected discrepancy=0 but was non-zero — money is neither created nor destroyed, yet the audit says the books don't balance.

Hint: The transfer moves money in two steps. The audit snapshot is taken between them.

TransferService.transfer() is not atomic: it calls source.withdraw(), then auditService.snapshotBalances(), then destination.deposit(). The snapshot captures the intermediate state where money has left the source but hasn't arrived at the destination — showing a total of 1500 instead of 2000.

Debug path: jdwp_set_field_breakpoint on AuditService.lastTotalSnapshot (modification). It fires mid-transfer with the stack at TransferService.transfer between the debit and the credit; jdwp_get_stack plus a live balance read confirm the captured dip.

---

#5 The Field That Lies

Difficulty: Hard | Test: UserProfileTest | Package: userprofile | Par: 3 | Exercises: event history as evidence

Symptom: expected: <Alice> but was: <alice> — the welcome message rendered correctly, yet the user's stored display name has silently changed casing.

Hint: You will not find the write by ripgrep'ing for setDisplayName — the public setter is never called. A line BP on the setter never fires. Whatever path the write travels, the field's value still flips. Let the JVM tell you exactly when it changes, regardless of how the write reaches the field.

LoginNormalizer.welcomeMessage calls a private canonicalForm helper which delegates to a static DisplayNameMirror nested class. The mirror uses Field.setAccessible(true) + Field.set(profile, canonical) to write the lower-cased form straight into UserProfile.displayName — bypassing the public setter entirely. From the test's perspective the formatter is read-only; from JDI's perspective every reflective write is still a real field modification.

Debug path: jdwp_set_field_breakpoint(className="one.edee.jdwp.sandbox.userprofile.UserProfile", fieldName="displayName", mode="modification") — JDI watchpoints fire on every JVM-level field store, including stores issued through Field.set and Unsafe. The next write suspends the thread inside DisplayNameMirror.mirror; jdwp_get_stack walks back through canonicalForm to LoginNormalizer.welcomeMessage and names the culprit. A line BP on UserProfile.setDisplayName would never fire — the setter is genuinely unused.

---

#6 The Doppelgänger Cart

Difficulty: Moderate | Test: CheckoutTest | Package: cart | Par: 6 | Exercises: marked instances ($label)

Symptom: expected 45.0 but was 0.0 — the cart goes through pricing and discounting, yet its total never changes.

Hint: The object you get back from the pipeline is not the object you passed in. Prove it.

Checkout.process runs the cart through validate → price → discount. validate was changed to return a defensive snapshot — return new Cart(cart) — so every later stage mutates the copy. process reassigns its local to the copy as it threads the stages, so no single frame holds both the caller's cart and the copy at once. The test ignores the return value (assuming in-place mutation), so its cart stays at total 0. The copy's fields are identical to the original — only identity differs.

Debug path: Break in the test once the cart exists, jdwp_mark_instance(label="input", objectId=<id>) to pin it, then set a breakpoint in price (or applyDiscount) with condition cart != $input. It fires — the stage is working on a doppelgänger, not the caller's cart.

---

#7 The Heisenbug Race

Difficulty: Hard | Test: RaceCounterTest | Package: race | Par: 3 | Exercises: logpoints (non-stopping)

Symptom: expected 2 but was 1 — two threads each increment a counter once, but one increment vanishes.

Hint: Two threads, one lost update. Suspending a thread to look changes the timing — watch the reads without stopping anything.

RaceCounter.increment reads the count, waits at a CyclicBarrier so both threads have read before either writes, then writes back read + 1. Both threads read 0 and both write 1 — one increment is lost. A suspending breakpoint serializes the threads and makes you juggle two parked threads to reconstruct what happened.

Debug path: Set a non-suspending jdwp_set_logpoint at the read site logging the thread name and count (or a field logpoint on count). Let the test run, then jdwp_get_events — both racer-1 and racer-2 are recorded reading 0 before either writes, which is the lost update laid out in order.

---

#8 The Magic Patch

Difficulty: Warm-up | Test: DateParserTest | Package: parser | Par: 3 | Exercises: runtime mutation (set_local)

Symptom: NumberFormatException: For input string: "15 " — a date string that looks right fails to parse.

Hint: The input looks almost right. Rather than rebuild with a fix, patch the value in place and see if the test goes green.

DateParser.parse splits YYYY-MM-DD and Integer.parseInts each part. The feed delivers "2026-05-15 " with a trailing space, so parseInt("15 ") throws. The parser never trims its input.

Debug path: Break at the top of parse, observe input = "2026-05-15 ", then jdwp_set_local("input", "2026-05-15") and resume. The parse now succeeds and the test passes — confirming the trailing space is the whole story, with no rebuild.

---

#9 The Polite Standoff

Difficulty: Hard | Test: TransferDeadlockTest | Package: deadlock | Par: 4 | Exercises: multi-thread inspection (no breakpoints)

Symptom: The test hangs and then fails its join — both transfers never complete.

Hint: Nothing is executing. No exception is thrown. Find out what each thread is waiting for.

Account.transfer locks the source account, pauses, then locks the destination. Two transfers in opposite directions (A→B and B→A) each grab their first lock and then wait forever for the other's — a classic lock-ordering deadlock.

Debug path: No breakpoint can fire — the threads execute nothing. jdwp_get_threads shows transfer-A-to-B and transfer-B-to-A in MONITOR state; jdwp_dump_locks prints the lock graph with both monitor holders and the inverse-order cycle; jdwp_get_stack on each thread shows the lock-ordering inversion. (Trying jdwp_evaluate_expression / jdwp_to_string on these threads is refused — invoking a method on a monitor-blocked thread would hang the debugger, which is itself the tell.)

---

Scorecard

Flights solved — did you find the root cause?

Solved	Rating
0-2	The JVM is winning. Check your setup.
3-5	Solid start. You're getting the hang of breakpoint-driven debugging.
6-8	Impressive. You found bugs that would take hours with println.
9	Bug terminator. Nothing survives your debugger.

Style — how close to par? Award ⭐⭐⭐ for a flight solved at its par tool count, ⭐⭐ within 2× par, ⭐ solved at all. A flawless run is 27 stars across the nine flights.

Features beyond standard JDWP

These are the capabilities this server adds on top of raw JDI — the reason to use it instead of writing JDI calls directly.

Conditional breakpoints

jdwp_set_breakpoint(
  className="com.example.OrderService",
  lineNumber=42,
  condition="order.getTotal() > 1000"
)

The breakpoint fires on every hit, but the thread is only suspended when the condition evaluates to true. False hits are auto-resumed transparently. This is essential for AI agents — without conditions, a breakpoint in a hot loop would generate thousands of useless stops. Conditions (and expressions, and logpoint bodies) all accept block syntax — wrap in { ...; return X; } to use intermediate locals, try/catch, or early returns.

Logpoints (non-stopping breakpoints)

jdwp_set_logpoint(
  className="com.example.OrderService",
  lineNumber=42,
  expression="\"Processing order \" + order.getId() + \" total=\" + order.getTotal()",
  condition="order.getTotal() > 1000"
)

A logpoint evaluates an expression every time the line is hit, logs the result to the event history, and never suspends the thread. Combined with an optional condition, this gives the agent non-intrusive tracing without stopping execution. View results with jdwp_get_events().

Deferred breakpoints

By default, every line BP, logpoint, exception BP, and field BP is registered passively: if the target class isn't loaded when the breakpoint is set, the server registers a ClassPrepareRequest and promotes the breakpoint to active when the JVM loads the class on its own — same behaviour as IntelliJ / Eclipse / jdb.

jdwp_set_breakpoint(className="com.example.LazyService", lineNumber=15)
→ "Breakpoint deferred — class com.example.LazyService not yet loaded. Will activate on class load."

jdwp_overview() shows pending breakpoints with their status.

Pass forceLoad=true only when you need the breakpoint to bind immediately — the server will call Class.forName in the target VM to load the class up front. That runs the class's <clinit> (early static init, eager dependency loads), which can mask lazy-init / classloader-leak diagnostics. Use sparingly.

Exception breakpoints

jdwp_set_exception_breakpoint(
  exceptionClass="java.lang.NullPointerException",
  caught=true,
  uncaught=true
)

Catch exceptions at the throw site — before the stack unwinds. Supports deferred activation (if the exception class isn't loaded yet). Use jdwp_overview(types="exception_breakpoint") to see active and pending exception breakpoints.

Log-only mode — record the throw without suspending the thread, evaluating an expression with $exception bound to the thrown object. Use the dedicated jdwp_set_exception_logpoint tool:

jdwp_set_exception_logpoint(
  exceptionClass="java.sql.SQLException",
  expression="$exception.getSQLState() + \": \" + $exception.getMessage()"
)

Each hit records an EXCEPTION_LOG entry to jdwp_get_events; failures during evaluation surface as EXCEPTION_LOG_ERROR so the listener never throws. Use this to trace exception flows in long-running services without stopping them.

Field breakpoints (watchpoints)

jdwp_set_field_breakpoint(
  className="com.example.OrderState",
  fieldName="status",
  mode="modification"
)

Suspend whenever a field is read (access), written (modification), or both (both — IntelliJ-style, binds both directions to one synthetic ID). Conditions are evaluated against the firing frame with five synthetic bindings:

• $oldValue — value before the event (the value being read, or the value about to be overwritten)
• $newValue — value about to be written (modification events only)
• $object — the instance the field belongs to, or null for static fields
• $fieldName — the field name as a string
• $mode — "access" or "modification", identifying which direction fired

Log-only mode — record reads/writes without suspending, evaluating an expression with the same bindings:

jdwp_set_field_logpoint(
  className="com.example.OrderState",
  fieldName="status",
  mode="modification",
  expression="$oldValue + \" -> \" + $newValue"
)

Each hit records a FIELD_LOGPOINT entry to jdwp_get_events; failures surface as FIELD_LOGPOINT_ERROR. Filters (threadFilterId, objectFilterId) and trigger chaining (triggerBreakpointId, oneShot) work the same as for line and exception BPs. Deferred activation is supported — the watchpoint installs the moment the declaring class loads, so static-initializer writes are caught.

Hard errors (no silent fallback): invalid mode, ambiguous or missing field, objectFilterId on a static field. The deferred path can't validate static-ness until class load, so it surfaces a warning Note in the response.

Performance: field watchpoints fire on every read/write of the watched field. For hot fields they can dominate target-VM CPU — prefer narrow filters or short-lived sessions. jdwp_diagnose surfaces canWatchFieldAccess / canWatchFieldModification plus a perf warning when connected.

Use jdwp_overview(types="field_breakpoint") to see active and pending field breakpoints with chain status, mode, and any pending failure reason.

Chained breakpoints

jdwp_set_breakpoint(
  className="com.example.OrderProcessor",
  lineNumber=87,
  triggerBreakpointId=12,
  oneShot=true
)

Make one breakpoint depend on another. The dependent BP stays disabled until its trigger fires; from then on it's armed and behaves normally. Use this when a hit is uninteresting unless reached via a specific code path — e.g., suspend at the order-pricing line only after a "VIP customer" path has been entered, or break inside a generic utility only when called from a specific feature.

Two modes:

• Sticky (default, oneShot=false) — once the trigger fires, the dependent stays armed forever. Use when you want to filter by an early gate but observe every subsequent hit.
• One-shot (oneShot=true) — the dependent self-disarms after firing, so the next hit requires the trigger to fire again. Matches IntelliJ's "Remove once hit" behavior.

Chains compose: a trigger BP can itself be a dependent of an earlier trigger. Cycles are rejected at registration time with the offending path in the error message. Pending (not-yet-loaded) BPs are first-class participants — a chained dependent registered against a class that hasn't loaded yet is promoted with the chain bit intact, so the very first event after class load can't fire before the trigger has. Trigger fires that happen while the dependent is still pending are remembered, so a deferred dependent isn't penalised for arriving late.

Both line breakpoints and exception breakpoints can be chain dependents and chain triggers; mix them freely.

Manage chains with:

• jdwp_set_breakpoint_dependency(dependentId, triggerId, oneShot?) — add an edge between two existing BPs (active or pending)
• jdwp_clear_breakpoint_dependency(dependentId) — detach the edge and re-arm the dependent
• jdwp_disarm_until_trigger(dependentId) — manually re-disable a dependent without changing its trigger (e.g., after a one-shot fired and you want another round)

When the trigger BP is removed, every dependent is automatically armed and a CHAIN_BROKEN event is recorded for each — jdwp_get_events shows exactly which BPs lost their guard. jdwp_diagnose recognises the "every armed BP is waiting on a trigger that hasn't fired" state and tells you why no BP is currently firing, including any pending dependents waiting on class load.

Expression evaluation

jdwp_evaluate_expression(
  threadId=25,
  expression="request.getData().get(\"_domain\")",
  frameIndex=0
)
→ "self.operationTypeSelect = 3"

Compiles arbitrary Java expressions to bytecode using Eclipse JDT, injects them into the target JVM via ClassLoader.defineClass(), and executes them in the context of the suspended frame. Full classpath is discovered automatically (including container classloaders like Tomcat). Results are cached for performance. Handles Guice/CGLIB proxies automatically.

Two input modes:

• Single expression — order.getTotal(), x + y, list.size() == 5. The compiler treats the input as the body of return ...;.
• Block mode — wrap the input in { ... } to send a multi-statement body with try/catch, intermediate locals, or early return. The agent writes explicit return X; to yield a value. Available everywhere expressions are accepted: jdwp_evaluate_expression, jdwp_assert_expression, breakpoint conditions, logpoint expressions, watchers.

See docs/expression-evaluation.md for the compilation pipeline details, or docs/index.md for the full developer reference.

Assertions

jdwp_assert_expression(
  expression="order.getStatus()",
  expected="CONFIRMED",
  threadId=25
)
→ "OK" or "MISMATCH: actual='PENDING', expected='CONFIRMED'"

Evaluate and compare in one call — useful for agents running verification sequences.

Value mutation

jdwp_set_local(threadId=25, frameIndex=0, varName="retryCount", value="3")
jdwp_set_field(objectId=26886, fieldName="limit", value="100")

Change local variables and object fields at runtime. The agent can test hypotheses ("what if this value were different?") without restarting the application.

Marked instances

jdwp_mark_instance(label="input", objectId=27531)

Pin a cached object in the target heap and label it as $label, so the agent can reference it across many events from inside expressions — conditions, logpoints, watchers, assertions. Reserved bindings ($exception, $oldValue, $newValue, $object, $fieldName, $mode, _this) and label collisions are rejected. Pinned by default via disableCollection; pass pin=false to allow natural GC.

Watchers

Attach persistent expressions to breakpoints that are evaluated automatically on every hit:

jdwp_attach_watcher(breakpointId=27, label="request data", expression="request.getData()")
jdwp_attach_watcher(breakpointId=27, label="user context", expression="request.getUser().getName()")

# Later, when breakpoint 27 fires:
jdwp_evaluate_watchers(threadId=25, scope="current_frame", breakpointId=27)

Watchers are MCP-side state — they survive across breakpoint hits and are cleaned up when the breakpoint is deleted.

Recursive breakpoint protection

When an expression evaluation at a breakpoint re-enters the breakpointed line (e.g., this.compute(n - 1) evaluated inside compute), JDI would re-suspend the thread and deadlock the server. This server wraps every invokeMethod chain in a per-thread reentrancy guard:

1. Recursive breakpoint/exception/step events are auto-resumed instead of suspending
2. A BREAKPOINT_SUPPRESSED / EXCEPTION_SUPPRESSED / STEP_SUPPRESSED entry is recorded in event history
3. The outer breakpoint context is preserved

Covered invocation sites: jdwp_evaluate_expression, jdwp_assert_expression, jdwp_evaluate_watchers, logpoint evaluation, conditional breakpoint evaluation, jdwp_to_string, classpath discovery, and deferred class loading via Class.forName.

Smart filtering

Threads: jdwp_get_threads() hides JVM internals (Reference Handler, Finalizer, surefire workers) by default. Pass includeSystemThreads=true to see everything. Defaults to a compact 1-line table per thread (a 200-thread Tomcat renders in ~25 lines); pass verbose=true for the legacy block-per-thread format.

Stack frames: jdwp_get_stack() collapses junit/surefire/reflection noise frames by default. Pass includeNoise=true to see the full stack.

Blocking resume & soft-wait

jdwp_resume_until_event(timeoutMs=30000)
→ blocks until next breakpoint/step/exception, returns context immediately

Replaces the manual "resume → poll events → poll events" pattern. The agent resumes and gets the next stop in one synchronous call. Returns one of:

• Event fired … — a BP / step / exception suspended a thread; the message includes thread + stop site
• [TIMEOUT] … — the wait expired without an event; the message offers wait_more / reconnect / abort and reports JDI health so the agent never blindly hangs
• [VM_DEATH] … — the target VM died before any event
• [NO_EVENT] … — the wait was released without a stop (e.g., a passive disarm); the message states whether the session is still armed or was cleared

The same soft-wait envelope wraps jdwp_wait_for_attach, so bootstrap and resume share one "wait sensibly, decide deliberately" idiom.

Lock-graph dump for deadlock investigations

jdwp_dump_locks()
→ shows monitor holders, blocked threads, and any deadlock cycles

Takes a transient VM-wide suspend/resume snapshot to enumerate Java monitor ownership — who is blocked on what, who holds it, and any deadlock cycle (lock-ordering inversions are surfaced explicitly). Genuine deadlocks stay deadlocked after the snapshot resumes (the probe is read-only in practice). Only synchronized-monitor contention shows; Object.wait() and java.util.concurrent locks are out of scope by design.

Reconnect without losing your BPs

jdwp_reconnect()
→ re-attaches to the last target; BP IDs, conditions, logpoints, chains, watchers all survive

When the target VM is restarted, or the JDI session wedges and you need a fresh transport, jdwp_reconnect re-attaches to the most recent endpoint and replays every BP spec onto the new VM. Synthetic BP IDs are preserved — BP #7 stays #7 — so the agent doesn't have to re-derive its bookkeeping. The marks registry, object cache, last-thread pointer, and classpath cache do not survive vm.dispose() and are cleared by design.

One-shot breakpoint context

jdwp_get_breakpoint_context(maxFrames=5, includeThisFields=true)

Returns thread info, top stack frames, locals at frame 0, and this fields in a single call — replaces the four-call sequence get_current_thread → get_stack → get_locals → get_fields(this) that an agent would otherwise need at every breakpoint hit.

Stepping: when it's worth it

jdwp_step_over / jdwp_step_into / jdwp_step_out are wired through the same event-and-latch pipeline as breakpoints — the step resumes the thread, the next STEP event suspends it again, and jdwp_resume_until_event blocks on it. threadId is optional; omitted, the step targets the thread of the last breakpoint hit.

Each step is one round-trip, so prefer a breakpoint at the destination + jdwp_resume_until_event whenever you can predict where execution will go. Stepping pays off in three narrow situations:

• step_into — polymorphic dispatch is unclear; you can't tell from source which override will run.
• step_out — an exception/early-abort left you deep in a frame you don't care about and finding the caller line for a breakpoint is awkward.
• step_over — the single next line, when observing a state mutation is faster than predicting it. More than ~3 in a row → set a breakpoint instead.

Tool reference (47 tools)

Connection (4)

Tool	Parameters	Description
jdwp_connect	—	Connect to JDWP on configured host:port
jdwp_disconnect	—	Disconnect (sends JDWP Dispose) and clears session state
jdwp_reconnect	—	Re-attach to the last target; BP IDs, conditions, logpoints, chains, watchers replay onto the new VM
jdwp_wait_for_attach	host?, port?, timeoutMs?	Poll until JVM is listening, then attach (soft-wait envelope at the ceiling)

Inspection (9)

Tool	Parameters	Description
jdwp_get_version	—	JVM version info
jdwp_get_threads	includeSystemThreads?, verbose?	List threads with status and frame counts (compact 1-line table by default)
jdwp_dump_locks	includeSystemThreads?	Monitor lock graph: holders, blocked threads, deadlock cycles
jdwp_get_stack	threadId, maxFrames?, includeNoise?	Stack trace (noise frames collapsed by default)
jdwp_get_locals	threadId, frameIndex	Local variables at a frame (includes this)
jdwp_get_fields	objectId	Object fields, collection elements, or array contents
jdwp_to_string	objectId, threadId?	Invoke toString() on a cached object
jdwp_get_breakpoint_context	maxFrames?, includeThisFields?	One-shot context dump at current breakpoint
jdwp_get_current_thread	—	Thread ID of the last breakpoint hit

Execution control (7)

Tool	Parameters	Description
jdwp_resume	—	Resume all threads
jdwp_resume_thread	threadId	Resume a specific thread
jdwp_suspend_thread	threadId	Suspend a specific thread
jdwp_resume_until_event	timeoutMs?	Resume and block until next breakpoint/step/exception (soft-wait envelope at the ceiling)
jdwp_step_over	threadId?	Step over current line; follow with resume_until_event
jdwp_step_into	threadId?	Step into method call; follow with resume_until_event
jdwp_step_out	threadId?	Step out of current frame; follow with resume_until_event

Breakpoints (7)

Tool	Parameters	Description
jdwp_set_breakpoint	className, lineNumber, suspendPolicy?, condition?, triggerBreakpointId?, oneShot?, forceLoad?	Set line breakpoint (conditions, deferred-by-default, trigger chaining; forceLoad=true binds immediately and runs <clinit>)
jdwp_set_logpoint	className, lineNumber, expression, condition?, forceLoad?	Non-stopping line breakpoint that logs expression result
jdwp_clear_breakpoint	breakpointId	Remove a breakpoint by ID — routes by kind across line, exception, and field BPs
jdwp_set_exception_breakpoint	exceptionClass, caught?, uncaught?, triggerBreakpointId?, oneShot?	Suspend on exception throw (supports deferred and trigger chaining)
jdwp_set_exception_logpoint