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.

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.

Why this exists

Raw JDWP/JDI gives you threads, stack frames, and variables. That's enough for a human with IntelliJ — but an AI agent needs more:

• Conditional breakpoints — stop only when a Java expression is true, so the agent isn't flooded with irrelevant hits
• Logpoints — evaluate an expression and log the result without stopping, for non-intrusive tracing
• Deferred breakpoints — set breakpoints on classes that haven't loaded yet; they activate automatically
• Exception breakpoints — catch exceptions at the throw site, not after they've unwound the stack
• Chained breakpoints — disable a breakpoint until another fires first, so a hit is only meaningful when reached via a specific code path (one-shot or sticky)
• Expression evaluation — compile and execute arbitrary Java at any breakpoint, with full classpath
• Value mutation — change local variables and object fields at runtime to test hypotheses
• Recursive breakpoint protection — expression evaluation is safe even when it re-enters the breakpointed line
• Smart filtering — JVM internal threads and framework noise frames are hidden by default
• Blocking resume — resume_until_event eliminates the "resume → poll → poll" dance

46 MCP tools, exposed over STDIO. Your agent gets the same power as IntelliJ's debugger.

Quick start

Prerequisites

• JDK 17+ (must be a JDK, not a JRE — JDI lives in jdk.jdi)

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). Restart Claude Code to pick up the plugin.

Alternative: manual MCP registration (without 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"
      }
    }
  }
}

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

Find the Bug — test flights

The jdwp-sandbox module ships 6 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.

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 Vanishing Pennies

Difficulty: Warm-up | Test: OrderProcessorTest | Package: order

Symptom: expected 71.982 but was 71.0 — the order total loses its decimal part somewhere between calculation and return.

Hint: The calculation is correct. Something after it changes the total. Who would mutate an order during logging?

Reveal root cause

AuditLogger.log(order) calls cacheFormattedTotal(order), which silently mutates the order: order.setTotal((double)(int)(order.getTotal())) — truncating 71.982 to 71.0. The caller assumes log() is read-only, but it isn't.

Debug path: Set a breakpoint in OrderProcessor.process(), step over the log() call, and eval order.getTotal() before and after. The value changes across what should be a side-effect-free call.

---

#2 The Phantom Session

Difficulty: Moderate | Test: SessionStoreTest | Package: session

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.

Reveal root cause

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.

---

#3 The Swallowed Exception

Difficulty: Moderate | Test: EventBusTest | Package: events

Symptom: expected stock < 100 but was 100 and no error summary — the order was supposed to reserve inventory, but nothing happened and nobody complained.

Hint: There are actually two bugs. One is hiding the other. Start with the exception — why isn't the error summary showing anything?

Reveal root cause

Bug 1 (hidden): OrderEvent casts the raw quantity through byte: a quantity of 200 overflows to -56. Inventory.reserve() throws IllegalStateException("Cannot reserve negative quantity: -56").

Bug 2 (hiding bug 1): EventBus.dispatch() catches the exception and wraps it in CompletionException -> EventHandlerException -> original cause. But getErrorSummary() only reports top-level messages ("Async task failed"), losing the root cause entirely.

Debug path: Set an exception breakpoint on IllegalStateException. The throw site reveals the -56 quantity. Then inspect the OrderEvent construction to see the byte cast.

---

#4 The Time Traveler's Config

Difficulty: Hard | Test: ConfigurationProviderTest | Package: config

Symptom: expected timeout=5000 but was 0 — the configuration exists but its timeout field is still at the default value.

Hint: The config object is assigned to the shared field before it's fully initialized. A reader thread sees the reference but reads a half-constructed object.

Reveal root cause

ConfigurationProvider.getConfig() assigns instance = new Configuration(...) and then signals a latch — but calls instance.init() (which sets timeout=5000) after the latch release. A reader thread waiting on that latch sees instance != null and returns the un-initialized object with timeout=0.

Debug path: Set breakpoints in both the initializer and reader paths of getConfig(). Use jdwp_get_threads() to see both threads, then inspect the Configuration object from each thread's perspective. The initializer hasn't called init() yet when the reader returns.

---

#5 The Audit That Lies

Difficulty: Hard | Test: TransferServiceTest | Package: bank

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.

Reveal root cause

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: Set breakpoints on withdraw(), snapshotBalances(), and deposit(). Step through and eval source.getBalance() + destination.getBalance() at each stop. The total drops after withdraw and recovers after deposit — the snapshot catches the dip.

---

#6 The Field That Lies

Difficulty: Hard | Test: UserProfileTest | Package: userprofile

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.

Reveal root cause

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.

---

Scorecard

Solved	Rating
0-1	The JVM is winning. Check your setup.
2-3	Solid start. You're getting the hang of breakpoint-driven debugging.
4-5	Impressive. You found bugs that would take hours with println.
6	Bug terminator. Nothing survives your debugger.

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.

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

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

If the target class isn't loaded when the breakpoint is set, the server registers a ClassPrepareRequest and automatically promotes the breakpoint to active when the JVM loads the class. Works for line breakpoints, logpoints, and exception breakpoints. jdwp_overview() shows pending breakpoints with their status.

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.

See docs/EXPRESSION_EVALUATION.md for the compilation pipeline details.

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.

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.

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

Blocking resume

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.

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 (46 tools)

Connection (3)

Tool	Parameters	Description
jdwp_connect	—	Connect to JDWP on configured host:port
jdwp_disconnect	—	Disconnect (sends JDWP Dispose)
jdwp_wait_for_attach	host?, port?, timeoutMs?	Poll until JVM is listening, then attach

Inspection (8)

Tool	Parameters	Description
jdwp_get_version	—	JVM version info
jdwp_get_threads	includeSystemThreads?	List threads with status and frame counts
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
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?	Set line breakpoint (supports conditions, deferred, and trigger chaining)
jdwp_set_logpoint	className, lineNumber, expression, condition?	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	exceptionClass, expression, condition?, caught?, uncaught?, triggerBreakpointId?, oneShot?	Non-stopping exception breakpoint with $exception bound
jdwp_set_field_breakpoint	className, fieldName, mode, condition?, threadFilterId?, objectFilterId?, triggerBreakpointId?, oneShot?	Suspend on field access/modification/both (supports conditions, filters, deferred, chaining)
jdwp_set_field_logpoint	className, fieldName, mode, expression, condition?, threadFilterId?, objectFilterId?, triggerBreakpointId?, oneShot?	Non-stopping field watchpoint with $oldValue/$newValue/$object/$fieldName/$mode bound

(For listing or bulk-clearing breakpoints across any combination of kinds, see jdwp_overview and jdwp_clear in the Debug state section below.)

Breakpoint chains (3)

Tool	Parameters	Description
jdwp_set_breakpoint_dependency	dependentId, triggerId, oneShot?	Make dependentId depend on triggerId firing first; cycles are rejected
jdwp_clear_breakpoint_dependency	dependentId	Remove the chain edge and re-arm the dependent
jdwp_disarm_until_trigger	dependentId	Re-disable a dependent that already has a chain (e.g., after a one-shot fired)

Expression evaluation and mutation (4)

Tool	Parameters	Description
jdwp_evaluate_expression	threadId, expression, frameIndex?	Evaluate Java expression at suspended frame
jdwp_assert_expression	expression, expected, threadId, frameIndex?	Evaluate and compare against expected value
jdwp_set_local	threadId, frameIndex, varName, value	Set a local variable's value
jdwp_set_field	objectId, fieldName, value	Set a field's value on a cached object

Events (2)

Tool	Parameters	Description
jdwp_get_events	count?	Recent events (breakpoints, steps, exceptions, logpoints, exception logs, chain events)
jdwp_clear_events	—	Clear event history

Diagnostics (1)

Tool	Parameters	Description
jdwp_diagnose	inspectAll?	Three-block "state of the world" snapshot: (1) MCP server (PID/uptime/configured target), (2) JDWP connection — last-attempt error when disconnected, breakpoints+events report when connected, (3) Local JVMs visible to the user with their JDWP ports (confirmed via handshake). Pass inspectAll=true to attach briefly to every same-user JVM whose port could not be read from /proc, to discover the port via sun.jdwp.listenerAddress (default false — attaches are visible to targets). Recognises the chain-stuck state where every armed BP is WAITING on a non-fired trigger. Run this first when nothing seems to work.

Watchers (4)

Tool	Parameters	Description
jdwp_attach_watcher	breakpointId, label, expression	Attach expression watcher to a breakpoint
jdwp_detach_watcher	watcherId	Remove a watcher
jdwp_list_watchers_for_breakpoint	breakpointId	List watchers on a specific breakpoint
jdwp_evaluate_watchers	threadId, scope, breakpointId?	Evaluate watchers (current_frame or full_stack)

Marked instances (3)

Tool	Parameters	Description
jdwp_mark_instance	label, objectId, pin?	Label a cached object as $label so expressions (conditions, logpoints, watchers) can reference it; pinned by default (disableCollection)
jdwp_unmark_instance	label	Remove a mark and release its pin
jdwp_rename_mark	oldLabel, newLabel	Rename a mark, preserving its pin and underlying object

Debug state (2)

Tool	Parameters	Description
jdwp_overview	types?, filter?	Unified read-only listing of breakpoints, exception breakpoints, field breakpoints, logpoints, watchers, and marks. Filter by type subset and/or case-insensitive substring (class/label/expression/type).
jdwp_clear	types, filter?	Bulk-delete by type and/or substring filter. types is REQUIRED (use 'all' to clear every supported kind). To preview, call jdwp_overview with the same args first.

Session (1)

Tool	Parameters	Description
jdwp_reset	—	Clear all state (breakpoints, watchers, marks, cache, events) without disconnecting

Resources (2)

In addition to tools, the server exposes two read-only MCP resources. In Claude Code, type @ in the prompt and pick them from the autocomplete (URI form: @jdwp-inspector:jdwp://...). Attaching a resource pulls its rendered text straight into the prompt without spending a model turn on a tool call — handy for "is my target up, and on which port?" without involving the agent.

Resource	URI	Content
JDWP diagnose	jdwp://diagnose	Same three-block snapshot as the jdwp_diagnose tool with inspectAll=false: MCP-server status, JDWP connection (or last-attempt error), local-JVM inventory with detected ports.
Local JVMs	jdwp://jvms	Local-JVM inventory only — which Java processes are running, which expose a JDWP agent, and the state of each port (LISTENING / SUSPENDED / UNREACHABLE / …). Cheaper than jdwp://diagnose when you only need a port list.

Note: resource updates are not pushed to the client — re-attach the URI to see fresh content. For probe-the-world style refreshes (briefly attach to every same-user JVM to learn its port), use the jdwp_diagnose tool with inspectAll=true; the resources do not run those attaches.

Usage workflows

Debugging a REST request

1. Launch your app with JDWP enabled
2. In Claude Code:
   "Set a breakpoint in OrderService.createOrder line 42 and wait for a hit"

3. Claude:
   jdwp_connect()
   jdwp_set_breakpoint("com.example.OrderService", 42)
   jdwp_resume_until_event(timeoutMs=60000)
   jdwp_get_breakpoint_context()
   → "Thread http-nio-8080-exec-3 stopped at OrderService:42.
      Local 'order' has total=0.0, status=null — looks like
      the order wasn't initialized before reaching this line."

Non-intrusive tracing with logpoints

1. "Add a logpoint to trace every order over $1000"

2. Claude:
   jdwp_set_logpoint(
     "com.example.OrderService", 42,
     "\"order=\" + order.getId() + \" total=\" + order.getTotal()",
     "order.getTotal() > 1000"
   )
   jdwp_resume()

3. Later:
   jdwp_get_events(count=20)
   → Shows LOGPOINT entries with evaluated expressions, no thread was ever stopped

Catching exceptions at the throw site

1. "I'm getting a NullPointerException somewhere in the order flow"

2. Claude:
   jdwp_set_exception_breakpoint("java.lang.NullPointerException", caught=true, uncaught=true)
   jdwp_resume_until_event(timeoutMs=60000)
   jdwp_get_breakpoint_context()
   → "NullPointerException thrown at OrderValidator:87.
      Local 'customer' is null — the order was submitted without a customer reference."

Test flight: recursive breakpoint scenario

The jdwp-sandbox module includes a deterministic scenario (one.edee.jdwp.sandbox.recursion package) that reproduces the recursive breakpoint case:

# Terminal 1 — launch sandbox, suspended on port 5005
./mvnw -pl jdwp-sandbox test -Dtest=RecursiveCalculatorTest -DskipTests=false -Dmaven.surefire.debug

# From Claude Code:
jdwp_wait_for_attach()
jdwp_set_breakpoint("one.edee.jdwp.sandbox.recursion.RecursiveCalculator", 22)
jdwp_resume_until_event()
# → BP fires inside compute(5)
jdwp_evaluate_expression(threadId, "this.compute(3)")
# → returns 2 without deadlock
jdwp_get_events()
# → shows BREAKPOINT_SUPPRESSED entries for each recursive hit

Architecture

Claude Code ──MCP/STDIO──> Spring Boot MCP Server ──JDI──> Target JVM (port 5005)

The server is SYNC mode, web-application-type=none — JSON over STDIO, no HTTP.

Core components

Component	Role
JDWPTools	44 @McpTool methods — the MCP surface. Thin orchestration over services below.
JDIConnectionService	Singleton VirtualMachine connection. Object cache (ConcurrentHashMap<Long, ObjectReference>), smart collection rendering, classpath discovery.
BreakpointTracker	Breakpoint registry with synthetic IDs. Tracks pending/deferred state, conditions, logpoint expressions, exception breakpoints, and chain dependencies (with cycle detection and trigger-fire memory across pending → active promotion).
JdiEventListener	Daemon thread consuming the JDI event queue. Routes events, evaluates conditions/logpoints, handles recursive suppression.
EvaluationGuard	Per-thread reentrancy guard preventing deadlocks during expression evaluation.
EventHistory	Ring buffer of the last 100 JDWP events (including suppressed).

Expression evaluation pipeline (evaluation/)

1. JdiExpressionEvaluator — Analyzes the stack frame, generates a wrapper class with a UUID name, delegates compilation, caches results.
2. ClasspathDiscoverer — Walks target JVM classloader hierarchy (including Tomcat/container) to find all JARs. Uses JdkDiscoveryService to locate a local JDK matching the target version.
3. InMemoryJavaCompiler — Compiles Java source to bytecode using Eclipse JDT (ECJ), entirely in memory.
4. RemoteCodeExecutor — Injects bytecode via ClassLoader.defineClass() and invokes it.

Watcher system (watchers/)

• WatcherManager — CRUD, dual-indexed by watcher UUID and breakpoint ID. Auto-cleans when breakpoint is deleted.
• Watcher — Immutable model: id, label, breakpointId, expression.

Project structure

mcp-jdwp-java/
├── pom.xml                              # Parent POM (reactor)
├── mvnw / mvnw.cmd                      # Maven wrapper
├── README.md
├── WORKFLOW.md                          # Development guide
├── .mcp.json                            # MCP server configuration
├── .claude-plugin/
│   ├── plugin.json                      # Claude Code plugin metadata
│   └── marketplace.json                 # Plugin marketplace registry
├── hooks/
│   └── hooks.json                       # SessionStart hook: auto-builds JAR if missing
├── skills/
│   └── java-debug/
│       ├── SKILL.md                     # Debugging skill (workflows, recipes, gotchas)
│       └── references/
│           ├── prerequisites.md         # Build-system-specific JDWP launch details
│           └── troubleshooting.md       # MCP server troubleshooting
├── docs/
│   └── EXPRESSION_EVALUATION.md         # Expression evaluation pipeline docs
│
├── jdwp-mcp-server/                     # The MCP server
│   ├── pom.xml
│   └── src/main/java/one/edee/mcp/jdwp/
│       ├── JDWPMcpServerApplication.java
│       ├── JDWPTools.java               # 44 @McpTool methods
│       ├── JDIConnectionService.java    # JDI connection + object cache
│       ├── BreakpointTracker.java       # Breakpoint registry + deferred state
│       ├── JdiEventListener.java        # JDI event consumer
│       ├── EvaluationGuard.java         # Recursive breakpoint protection
│       ├── EventHistory.java            # Event ring buffer
│       ├── ThreadFormatting.java        # Thread/frame noise filtering
│       ├── evaluation/
│       │   ├── JdiExpressionEvaluator.java
│       │   ├── RemoteCodeExecutor.java
│       │   ├── InMemoryJavaCompiler.java
│       │   ├── ClasspathDiscoverer.java
│       │   └── JdkDiscoveryService.java
│       └── watchers/
│           ├── WatcherManager.java
│           └── Watcher.java
│
└── jdwp-sandbox/                        # Debugging targets (test flights)
    ├── pom.xml                          # Tests skipped by default
    └── src/                             # Deliberately broken scenarios

Dependencies

• Spring Boot 4.0 — Framework
• Spring AI MCP 2.0.0-M4 — MCP protocol integration
• JDI (jdk.jdi module) — Java Debug Interface
• Eclipse JDT Compiler (ECJ) — In-memory expression compilation
• JSpecify + NullAway — Compile-time nullness enforcement

Troubleshooting

Problem	Solution
tools.jar not found / jdk.jdi not available	Ensure JAVA_HOME points to a JDK, not a JRE. Launch with --add-modules jdk.jdi.
Connection refused	Verify target JVM has -agentlib:jdwp=...address=*:5005. Check port matches -DJVM_JDWP_PORT.
MCP server doesn't respond	Rebuild: ./mvnw clean package -DskipTests. Check jar path. Restart Claude Code.
MCP server times out on startup	JVM startup takes several seconds. Ensure MCP_TIMEOUT=30000 (or higher) is set in the MCP registration — the default is too short for a Spring Boot Java process.
"Thread is not suspended"	The thread must be stopped at a breakpoint for stack/locals/expression tools.
Expression evaluation timeout	First evaluation is slow (classpath discovery). Increase MCP_TOOL_TIMEOUT. Subsequent evaluations use cache.

License

MIT