[BUG] Breaking Change in Claude Code SDK v1.0.124 Session Resume Behavior

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary

Claude Code v1.0.124 introduced a breaking change that fundamentally alters how session resume functionality works:

• v1.0.123 and earlier: Creates a NEW session ID when using --resume
• v1.0.124 and later: Reuses the SAME session ID when using --resume

This change breaks systems that rely on the previous behavior of creating new session IDs on resume, causing issues with session tracking, conversation branching, and event storage systems.

What Should Happen?

when new sessions are created with --resume, they should get a new session_id

Error Messages/Logs

Steps to Reproduce

Prerequisites

1. Docker and Docker Compose installed (optional, recommended)
2. Anthropic API Key
3. Set your API key:

``bash
export ANTHROPIC_API_KEY="your-api-key-here"
``

Method 1: Using Docker (Recommended)

1. Create a new directory for testing:

``bash
mkdir claude-session-test && cd claude-session-test
``

1. Save the test files provided below
2. Build the Docker image:

``bash
docker compose build
``

1. Test version 1.0.123 (creates new session IDs):

``bash
docker compose run --rm claude-test ./test.sh 1.0.123
``

1. Test version 1.0.124 (reuses session IDs):

``bash
docker compose run --rm claude-test ./test.sh 1.0.124
``

Method 2: Direct Installation (Local)

1. Save the test script below as test.sh
2. Make it executable:

``bash
chmod +x test.sh
``

1. Run tests:

``bash
./test.sh 1.0.123 # Old behavior
./test.sh 1.0.124 # New behavior
``

Expected Results

Version 1.0.123 (Old Behavior)

Session IDs are DIFFERENT (creates_new_id)
Initial: 66658f92-2cd1-48a5-a904-9acbbf0197cb
Resumed: dc038920-1cb0-4d21-b417-98fcb753b54c

Version 1.0.124 (New Behavior)

Session IDs are IDENTICAL (reuses_same_id)
Initial: b41d4917-26d4-4dcd-84bf-dc429457ed39
Resumed: b41d4917-26d4-4dcd-84bf-dc429457ed39

Test Files

docker-compose.yaml

services:
  claude-test:
    build: .
    environment:
      - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY}

Dockerfile

FROM ubuntu:22.04

# Install dependencies
RUN apt-get update && apt-get install -y \
    curl \
    bash \
    jq \
    ca-certificates \
    && rm -rf /var/lib/apt/lists/*

# Set up working directory
WORKDIR /app

# Copy test script
COPY test.sh /app/test.sh
RUN chmod +x /app/test.sh

CMD ["/bin/bash", "/app/test.sh"]

test.sh

#!/bin/bash
set -e

VERSION="${1:-1.0.124}"

echo "================================================"
echo "Testing Claude Code version: $VERSION"
echo "================================================"

# Install specific Claude version
echo "Installing Claude $VERSION..."
echo "Downloading installer..."

# Download and run installer with timeout and error checking
if ! timeout 60 bash -c "curl -fsSL https://claude.ai/install.sh | bash -s -- '$VERSION'" 2>&1; then
    echo "ERROR: Failed to install Claude $VERSION"
    exit 1
fi

export PATH="$HOME/.local/bin:$PATH"

# Verify installation
INSTALLED_VERSION=$(claude --version 2>/dev/null || echo "Not installed")
echo "Installed: $INSTALLED_VERSION"

if [ "$INSTALLED_VERSION" = "Not installed" ]; then
    echo "ERROR: Claude installation failed"
    exit 1
fi

# Check API key
if [ -z "$ANTHROPIC_API_KEY" ]; then
    echo "ERROR: ANTHROPIC_API_KEY not set"
    exit 1
fi
echo "API key is set"

# Test 1: Create initial session
echo -e "\n1. Creating initial session with 'say foo'"
echo "----------------------------------------"
if ! timeout 30 claude --print --output-format=stream-json --verbose "say foo" > session1.jsonl 2>&1; then
    echo "ERROR: Failed to create initial session"
    cat session1.jsonl
    exit 1
fi

SESSION1_ID=$(grep -o '"session_id":"[^"]*"' session1.jsonl | head -1 | cut -d'"' -f4)
if [ -z "$SESSION1_ID" ]; then
    echo "ERROR: No session ID found in response"
    cat session1.jsonl
    exit 1
fi
echo "Initial session ID: $SESSION1_ID"

# Test 2: Resume session
echo -e "\n2. Resuming session with 'say bar'"
echo "----------------------------------------"
if ! timeout 30 claude --print --output-format=stream-json --verbose --resume "$SESSION1_ID" "say bar" > session2.jsonl 2>&1; then
    echo "ERROR: Failed to resume session"
    cat session2.jsonl
    exit 1
fi

SESSION2_ID=$(grep -o '"session_id":"[^"]*"' session2.jsonl | head -1 | cut -d'"' -f4)
if [ -z "$SESSION2_ID" ]; then
    echo "ERROR: No session ID found in resume response"
    cat session2.jsonl
    exit 1
fi
echo "Resumed session ID: $SESSION2_ID"

# Compare results
echo -e "\n3. Results"
echo "----------------------------------------"

if [ "$SESSION1_ID" = "$SESSION2_ID" ]; then
    BEHAVIOR="reuses_same_id"
    IDENTICAL=true
    echo "✓ Session IDs are IDENTICAL (reuses_same_id)"
else
    BEHAVIOR="creates_new_id"
    IDENTICAL=false
    echo "✓ Session IDs are DIFFERENT (creates_new_id)"
fi

# Extract turn counts
TURNS1=$(grep -o '"num_turns":[0-9]*' session1.jsonl | tail -1 | cut -d: -f2)
TURNS2=$(grep -o '"num_turns":[0-9]*' session2.jsonl | tail -1 | cut -d: -f2)
echo "Conversation turns: $TURNS1 -> $TURNS2"

# Save summary
cat > summary.json <<EOF
{
  "version": "$VERSION",
  "installed_version": "$INSTALLED_VERSION",
  "behavior": "$BEHAVIOR",
  "session_ids": {
    "initial": "$SESSION1_ID",
    "resumed": "$SESSION2_ID",
    "identical": $IDENTICAL
  },
  "conversation_turns": {
    "initial": $TURNS1,
    "resumed": $TURNS2
  }
}
EOF

echo -e "\nSummary:"
cat summary.json | jq -c .

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

1.0.123

Claude Code Version

1.0.124

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

Please either:

1. Revert to the v1.0.123 behavior (create new session IDs on resume), OR
2. Document this breaking change clearly in the release notes and provide a migration guide, OR
3. Add a flag to control this behavior (e.g., --resume-same-session vs --resume-new-session)

Potentially related to https://github.com/anthropics/claude-code/issues/8176